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Document - Map – scan only - Geologic maps of Dakota County. - 1/1/1990MINNESOTA GEOLOGICAL SURVEY GEOLOGIC ATLAS DAKOTA COUNTY, MINNESOTA Edited by N.H. Balaban and Howard C. Hobbs COUNTY ATLAS SERIES ATLAS C-6 Plate 1, Data -base map Plate 2, Bedrock geology Plate 3, Surficial geology Plate 4, Depth to bedrock and bedrock topography Plate 5, Quaternary hydrogeology Plate 6, Bedrock hydrogeology Plate 7, Sensitivity of the Prairie du Chien —Jordan aquifer to pollution Plate 8, Geology and well construction Plate 9, Geologic resources UNIVERSITY OF MINNFSOTA UNIVERSITY OF MINNESOTA MINNESOTA GEOLOGICAL SURVEY Priscilla C. Grew, Director Prepared and Published with the Support of the DAKOTA COUNTY SOIL AND WATER CONSERVATION DISTRICT AND BOARD OF COMMISSIONERS AND MINNESOTA DEPARTMENT OF NATURAL RESOURCES, WATERS DIVISION COUNTY ATLAS SERIES ATLAS C-6, PLATE 1 OF 9 DATA BASE MAP Port gill T115i`1 T 114N LOCATION DIAGRAM a* R21 W T113N T 112 N Radio --- Tower T 27 L ekef- COUNTY WELL INDEX - MINNESOTA GEOLOGICAL SURVEY. MN. UN.NO. : 127263 LOCAL ID.NO: NAME : APPLE VALLEY 6 PERMIT NO: COUNTY : DAKOTA T/R/SEC. : 115/20/26BBCB ELEVATION: 990 FT. DPTH BDRK: 49 FT. OPEN HOLE: PRAIRIE DU AQUIFER : PRAIRIE DU QUAD : FARMINGTON POTENTIAL POLLUTION SOURCE USE : MUNICIPAL DEPTH: 507 FT. CASED: 426 FT. GROUT: 426 YDS. BEDRK:PLATTEVILLE CHIEN GROUP-JORDAN CHIEN GROUP-JORDAN 400 FT. Aida lndu trialAirport Vermulii©n • R2 CWI DRILLED: 76/08/27 DIAM. : 16 IN. CASING : STEEL DRLR/DS: DATE NITRATE BACTERIA SOURCE SWL ELEV SOURCE 78/02 <0.4 MDH 129 861 62012 COMMENTS: 700 FT. S. OF 140TH ST. & CO. RD. 40 CASING: 30 TO 50; 16 TO 426. M.G.S. NO.1144 MGS WATER WELL DATA BASE LOG IS AVAILABLE. *************************************************************************** MINNESOTA GEOLOGICAL SURVEY DAKOTA COUNTY WELLOG DATA BASE. UNIQUE NO.: 127263 WELL NAME : APPLE VALLEY 6 COUNTY : DAKOTA ADDRESS QUADRANGLE: FARMINGTON TOWNSHIP : 115 NORTH RANGE 20 WEST SECTION 26/BBCBBB LATITUDE : 44:44:40 N LOCATED BY: INFO. FROM ELEVATION DEPTH COMPLETED 990 FT. 507 FT. 76/08/27 DNR PA NO.: 74-5229 7.5 MINUTE OWNER WELLOG DATE ENTERED: 89/07/12. APPLE VALLEY UTM-EASTING : UTM-NORTHING: UTM-ZONE LONGITUDE 484415 4954384 15 93:11:48 W WATER LEVEL : 129 FT. (EL. 861 FT.) DATE : 76/07/07 AQUIFER(S) : PRAIRIE DU CHIEN GROUP- : JORDAN WELL USE : MUNICIPAL DRILLER : (AND/OR DATA SOURCE) KEYS WELL CO. LIC. NO.: 62012 CASING : STEP DOWN 30 INCH TO 16 INCH TO 50 FEET 426 FE SOURCE OF POSSIBLE CONTAMINATION FEET: 400 DIRECTION: NORTH SCREEN MAKE/TYPE: NONE PUMP : DATA UNAVAILABLE REMARKS : M.G.S. NO. 1144 TYPE: CITY GARAGE PUMPAGE TEST DATE: 76/07 TEST 1 TEST 2 TEST 3 TEST 4 TEST 5 TEST 6 HOURS RATE(GPM) DRAWDOWN(FT) 17 2010 99 10 1230 66 GEOLOGIC LOG R 23 W 7 28 N arthy ke oke My'ii rd Mem,' iHosp ' i L �.. ennrk arrd n• Str;�- R1 eB^NDONE O CD Quarry' < • 00 ems R 22 W INTRODUCTION The public health and economic development of Dakota County are directly dependent upon the wise use and management of its land and water resources. Geologic and hydrologic information is essential before decisions are made that affect natural resources. Although the amount of geologic information required for making specific decisions can vary, the information will not be used at all if it is not available when it is needed, or if it is available only in a highly technical form or scattered in many different maps and reports. The Minnesota Geological Survey county atlases present detailed geologic and hydrologic information in an interpretive as well as descriptive form. Maps and texts either summarize basic geologic and hydrologic conditions at a county scale or interpret these conditions in terms of the impacts of possible land -use and water -use decisions. Site -specific information also is available at a greater level of technical detail than shown on the maps of this atlas. These data are too voluminous to present in the atlas, but have been incorporated into readily accessible files housed at the Minnesota Geological Survey. Several sources commonly provide information about an area or an individual property, but they may use different classification schemes to describe the same geologic materials. As a result, dis- crepancies in interpreting the data may arise or the different sources may appear to contradict each other. For example, water -well contractors may describe glacial till as "clay," but engineering records will describe it as a "clayey sand." Both descriptions are acceptable for their original purposes. "Clay" defines the general inability of the DEPTH INTERVAL (IN FEET) LITHOLOGY STRATIGRAPHIC UNIT SYSTEM/GROUP/FORMATION AGE HARDNESS COLOR 0 49 49 67 67 75 75 220 220 417 417 441 441 507 DRIFT LIMESTONE SHALE SANDSTONE DOLOMITE SANDSTONE SANDSTONE QUATERNARY UNDIFF. PLATTEVILLE GLENWOOD ST.PETER PRAIRIE DU CHIEN JORDAN JORDAN QUA ORD ORD ORD GROUP ORD CAM CAM DRILLER'S DESCRIPTION DRIFT PLATTEVILLE SHALE ST. PETER SANDROCK SHAKOPEE SHAKOPEE JORDAN *****************************************************************.********************************** ************************************************************************************* MINNESOTA GEOLOGICAL SURVEY DAKOTA COUNTY TESTHOLE DATA BASE. UNIQUE NO. 1504 COUNTY : DAKOTA DATA SOURCE : MINNESOTA HIGHWAY DEPARTMENT PROJECT NO. : 1986-02 BRIDGE NO. . DATE BORED : 70/05/21 ELEVATION 828 FT. DEPTH 41.0 FT. SOIL CLASS. : MINNESOTA HIGHWAY DEPT. UTM EASTING UTM NORTHING UTM ZONE LATITUDE LONGITUDE WATER LEVEL DRY HOLE HAMMER 48 65 61 : 4967294 : 15 : 44:51:39 N : 93:10:12 W 40 FT. : NO : 140/30/2 DEPTH INTERVAL BORING LOG (IN FEET) LITHOLOGY STRATIGRAPHY 0.0 9.0 11.5 13.5 16.0 18.5 21.0 23.0 30.0 35.0 9.0 11.5 13.5 16.0 18.5 21.0 23.0 30.0 35.0 41.0 SAND SAND SAND SAND LOAMY SAND SAND SAND SILTY SILTY SAND LOAM LOAM PLEISTOCENE PLEISTOCENE PLEISTOCENE PLEISTOCENE PLEISTOCENE PLEISTOCENE PLEISTOCENE PLEISTOCENE PLEISTOCENE PLEISTOCENE TESTS MS C CR MC TESTHOLE DRILLER'S DESCRIPTION M D 10 SAND BROWN M A 22 SAND BROWN M B 14 SAND BROWN M A 12 SAND BROWN M A 22 LOAMY SAND W C 18 SAND BROWN W E 13 SAND BROWN W E 20 SAND BROWN M E 22 SILTY LOAM W E 17 SILTY LOAM ABBREVIATED COLUMN HEADINGS (TESTS) MS=MOISTURE C =CONE TEST CR=CORE RECOVERED MC=MOISTURE CONTENT COLUMN VALUES D=DRY, M=MOIST, W=WET A=0-4,B=5-9,C=10-15,D=16-30,E>30;IN BLOWS/FT. PER CENT RECOVERED X10 PER CENT ************************************************************************************** BROWN GRAY BROWN DEPTH (FEET) 0 100 200 300 400 500 507TD DRIFT T 28 N GRADE Radio Tow T 27 N R 18 W R18W Base modified from U.S. Geological Survey, Faribault, Hastings, and St. Paul, 1985. T 112 N Cartography by Richard B. Darling 411e&b Park DATA BASE MAP By Jane M. Cleland, Timothy E. Wahl, Bruce A. Bloomgren, and Howard C. Hobbs 1990 8 9 10 11 42 13 14 49 15 46 g. KILOMETERS 1 0 1 2 3 H H H - 20 2i 17 Gavin, Gagi Sta 18 • • Nit P • H DAKOTA CQ ow trip R 17 VI Every possible effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Geological Survey in St. Paul. In addition, every effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering -scale decisions without site -specific verification. • Sandpit INDEX TO 1:24,000-SCALE MAPS 1. St. Paul West 2. St. Paul East 3. Bloomington 4. St. Paul SW 5. Inver Grove Heights 6. St. Paul Park 7. Prescott 8. Orchard Lake 9. Farmington 10. Coates 11. Vermillion T115N N 12. Hastings 13. Diamond Bluff West 14. New Market 15. Castle Rock 16. Randolph 17. Cannon Falls 18. Miesville 19. Little Chicago 20. Northfield 21. Dennison T SCALE 1:100,000 1 INCH ON THE MAP REPRESENTS NEARLY 1.6 MILES ON THE GROUND 4 5 6 7 8 9 10 11 12 13 14 15 R 16 W 16 17 EXPLANATION Record of water -well construction Cutting samples of glacial drift and bedrock 0 Borehole geophysical log (combined with other symbols) A Soil boring record -` Bedrock exposure T Textural analysis of glacial drift S Seismic study Water -quality data 0 Sinkhole 0 Spring S Seep BLof ext. 4th P.M. Several townships in Dakota County were surveyed from the 4th principal meridian and base line established in 1831 for Wisconsin. Most of the county -and most of Minnesota - were surveyed later from the 5th principal meridian and base line in Arkansas that was established in 1815. Because the two surveys had considerably different starting points, township numbers are very different in the northern and southern parts of the county. Map modified from Thompson, M.M., 1979, Maps for America: U.S. Geological Survey. 18 19 20 1-I 1-I H MILES 1 0 2 till to yield ground water to a well. "Clayey sand" defines the physical composition of the till relative to particle size and engineering properties. A geologic interpretation of both descriptions defines the material in terms of how it formed rather than in terms of how it is to be used. In this example, "till" is the unsorted jumble of rock frag- ments ranging in size from clay to cobbles and boulders that was deposited directly by glacial ice. All of the types of data described on this plate had to be interpreted by geologists or hydrogeologists before they were meaningful for mapping purposes. The 1:100,000 scale and smaller scales of the maps in this atlas were chosen because they can show the geologic and topical studies of the county while keeping the physical size of each plate to a manageable level. As a result, some detailed information that was gained by data interpretation and mapping cannot be shown on these maps or discussed in the texts. Whether to use the atlas alone, or to use the data bases, depends on the amount of detail needed. Generally, information in the data bases must be used to evaluate site -specific conditions, and test drilling may be necessary. THE DATA BASE MAP The types and locations of information used to prepare this atlas are shown on the map above. The map is current as of November 1989. The data are described below to aid the user in assessing which types of information may be useful for a particular need. The map DRILLER'S LOG CUTTINGS SET PLATTEVILLE SHALE ST. PETER SANDROCK SHAKOPEE JORDAN SAND & GRAVEL LIMESTONE SHALE SANDSTONE, PYRITE (P) NEAR BASE DOLOMITE SANDSTONE 0 'o'• o.00 0 • •0 P •• . / / / / / / / / / l /, f / / / / / / / / DRIFT LIMESTONE SHALE SANDSTONE GAMMA LOG MGS INTERPRETATION SHALEY SANDSTONE SANDSTONE, SHALE AND SILTSTONE DOLOMITE SANDSTONE FELDSPATHIC ZONE SANDSTONE WELL LOG CODE STRATIGRAPHY 0• o 0. o;• 'o 0. .o 1 1 l l P // /� /1 /// // / / / / / /// / / I / / / / / /// • F.F• • F F SUPERIOR OUTWASH �ORDOVICIAN PLATTEVILLE LIMESTONE ORDOVICIAN GLENWOOD SHALE ORDOVICIAN ST. PETER SANDSTONE ORDOVICIAN PRAIRIE DU CHIEN GROUP (INCLUDES SHAKOPEE & ONEOTA FORMATIONS) MGS interpretation of data from three different sources Comparison of geologic data obtained from a driller's well log, a cuttings set, and a gamma log. Information from all three sources generally agrees, but a geologist can learn something different from each source. The well log gives a list of rock units as described by the driller, recorded as the well was drilled. A cuttings set is a group of samples obtained from the well as it is being drilled. Cuttings provide much information, but drilling action can obscure clays and shales and mix samples. Gamma logs are obtained by measuring low-level radiation in rock CAMBRIAN JORDAN SANDSTONE units; the signal is printed on graph paper. From a gamma log, a geologist can pick contacts between formations and also describe the hydrologic features of the units. The log indicates that the basal St. Peter Sandstone contains approximately 20 - 25 feet of shale, which acts as a confining layer above the underlying Prairie du Chien - Jordan aquifer. A hard zone in the Jordan is probably due to feldspathic cement which results in a peak on the gamma log. Whenever possible, a geologist uses all three sources to make conclusions about various formations. 3 4 5 6 8 9 shows where data are sparse or lacking, and interpretation and extrapolation were required to prepare a map. Therefore, the data base map is a guide to the precision of the other maps in the atlas. Drill -Hole Information A record of water -well construction, or well driller's log, is a water -well contractor's description of the geologic formations penetrated during drilling and the construction materials used to complete the well. Hydrologic data, such as the static water level and test -pumping results, are commonly included. Before any driller's log can be used, the location of the well must be verified, and a geologist must interpret the log. Drillers' logs are the primary source of subsurface geologic and hydrologic data for Dakota County, and about 3200 of the estimated 3500 logs available were used for this atlas. On the map, water -quality points also represent well locations. Cutting samples are collected at set intervals (usually every 5 feet) during drilling from wells selected by the Minnesota Geological Survey on the basis of the need for additional data. They can be washed and studied under a microscope to determine rock type, grain size, color, and impurities. They provide physical evidence of subsurface geologic materials and are the principal means of establishing the nature of the glacial materials and bedrock. Borehole geophysical logging measures differences in the electrical and natural radioactive properties of subsurface materials. The gamma log in the column on Plate 2 is an example. It was obtained by lowering an instrument into an existing well to measure the low-level radiation given off by the rock. The signal was transmitted to a receiver which printed the log on graph paper. Peaks on the log reflect layers rich in potassium, which is found in some clays and feldspars. The graphic logs are correlated with cutting samples from the same hole or with information obtained from nearby outcrops or another nearby geophysical log. Each bedrock formation has a characteristic graphic configuration, and therefore these logs permit the acquisition of high -quality subsurface geologic and hydrologic information from wells for which little or no other information is available. Soil borings are test holes drilled to obtain information about the physical properties of subsurface materials for engineering, map- ping, or exploration purposes. Most terminate at very shallow depths or where bedrock is encountered. They are logged by an engineer or a geologist using a variety of classification schemes based upon particle sizes, penetration rate, moisture content, and color. Soil borings data are most useful in determining the composition of unconsolidated deposits. Some logs include the depth to bedrock and the lithology of the first bedrock encountered. Other Information Bedrock outcrops are exposures of solid rock at the land surface. The inventory of outcrops includes about 95 percent of the total exposed bedrock. The remainder are mostly small exposures created during construction. Some exposures of bedrock that are described in historical records are no longer visible. Although in much of Dakota County they are limited in distribution, they serve as reference points for mapping and for checking the accuracy of subsurface data. Bedrock at or near the surface must be considered in some land -use planning decisions, such as pipeline routing, sewage - system design, and excavations. Textural analyses express the proportion of sand-, silt-, and clay -size particles that make up a sample. They are helpful in iden- tifying and mapping unconsolidated materials, such as glacial deposits. The samples analyzed were taken from selected areas where other data were sparse or lacking. Seismic soundings are a geophysical method that measures the time required for sound or pressure waves to travel from a source to a receiver. The density and rigidity of the material through which the waves must travel affects their travel time. Bedrock such as limestone or dolomite exhibits seismic velocities 3 to 4 times those of unconsolidated deposits because bedrock is much more dense. The spacing of the receivers (geophones) and the arrival times (measured in milliseconds) are used to calculate the depth to bedrock. Seismic soundings are labor intensive but can provide high -quality data when no other sources of subsurface information are available. Water -quality data are obtained from records at the Dakota County Public Health Department and the Minnesota Department of Health. These data are incorporated in WELLOG in order to pair water -quality values with information on aquifers and well construction. Only water -quality data matched with a well log were plotted on the map. Sinkholes, springs, and seeps were located by sighting and also by talking with local residents. Sinkholes occur where the surface is underlain by carbonate bedrock that can be dissolved by mildly acidic ground water to form circular to elliptical depressions. These depressions range in size from less than 3 feet to more than 50 feet in diameter and from 1 to about 50 feet in depth. Springs are ground water issuing onto the surface. Seeps are places where the surface is saturated with ground water. 10 11 12 13 DATA BASE MANAGEMENT All of the data shown on the map were plotted on 7.5-minute topographic quadrangle maps, half -section maps, or highway alignment maps, and inventory numbers were assigned to all except bedrock exposures and some soil borings. Manual files and automated data bases were developed to provide easy access and rapid retrieval of these site -specific data. The data may be obtained from the Minnesota Geological Survey. Computer storage and retrieval systems are better than manual files for manipulating large amounts of data. Automated geologic data bases may be designed to interact with other computer files, such as land -use data. Such interaction permits more efficient assessment of cause -and -effect relationships concerning natural resources than is commonly possible with manual files. Several computer files were developed for point -source data in Dakota County. All of them use Public Land Survey coordinates as location criteria, and thus they are compatible with the natural -resource data bases housed at the Minnesota Land Management Information System. The automated data bases for Dakota County are: (1) water - well logs or WELLOG, (2) a county well index of aquifer usage and ground -water quality data or CWI, (3) soil borings and engineering test holes or TESTHOLE, and (4) sinkholes, springs, and seeps data. Information from water -well logs is entered into the Minnesota Geological Survey's WELLOG data base, which is a statewide file. Each log is assigned a six -digit unique number that also is used by state agencies and the Water Resources Division of the U.S. Geological Survey. Well locations are described in terms of Public Land Survey coordinates and also are digitized from 7.5-minute topo- graphic maps and half -section maps to obtain Universal Transverse Mercator and latitude -longitude coordinates. Elevations, expressed in feet above sea level, are determined from these topographic maps (see the index to 1:24,000-scale maps for quadrangle names). Software at the Minnesota Geological Survey is used to display and tabulate many of the data elements contained on the original well log. An example of WELLOG output is shown to describe these elements. Well -repair and abandonment records are not directly applicable for entry into WELLOG, but they contain useful information on ground water. In addition, community testing programs and the Minnesota Department of Health collect data on ground -water quality (mostly nitrate and coliform bacteria counts) and include some depth and construction data from wells that are sampled. These data, along with any historic static water -level measurements, are stored by unique number in the CWI (county well index) data base. CWI combines the water -quality data with location and selected depth data from WELLOG to form a catalog of aquifer use within the county. The street address of each well is also included whenever possible in order to provide data users with a well -location system that is compatible with local regulatory programs. A comparison of the example of CWI output with the WELLOG output shows specific differences between the two files. Information from soil borings and engineering test holes is stored in an automated file called TESTHOLE. Descriptions of the rock types penetrated and the soils classification system used are entered, together with specific field and laboratory tests. The most common tests are blow counts, liquid and plastic limits, water content, and dry density. The depth to the water table is entered if available. Each test hole receives a unique number, and the location is digitized from the site plan. An example of TESTHOLE output is shown to illustrate these elements. Sinkholes, springs, and seeps in Dakota County are assigned a five -digit code and their UTM coordinates entered into the Minnesota Geological Survey data base. Additional information, such as type of feature, formation in which it occurs, elevation, and land owner, if any, is available in manual files. FUTURE DATA COLLECTION The map on this plate was current as of November 1989. A data base map is out of date even before it is printed, because additional information is continually generated as new water wells are drilled, construction activities expose more bedrock, or another well is tested for water quality. The library of geologic information prepared for Dakota County is flexible so that old data can be reevaluated in light of new information, and new forms of data can be added if required. The need to manage ground water and other natural resources wisely will never become outdated. Future demands on these resources will require current data to assess the impacts. ACKNOWLEDGMENTS Staff members of the Minnesota Geological Survey who con- tributed to the development of the data bases include: Cynan Benedikt, Christopher Bratsch, Jacqueline Duley, Philip Heywood, Dirk Korth, Richard Marsh, Peter Moody, and Joyce Meints. We thank local water -well contractors and landowners for their valuable assistance. GEOLOGIC ATLAS OF DAKOTA COUNTY, MINNESOTA UNIVERSITY OF MINNESOTA MINNESOTA GEOLOGICAL SURVEY Priscilla C. Grew, Director Prepared and Published with the Support of the DAKOTA COUNTY SOIL AND WATER CONSERVATION DISTRICT AND BOARD OF COMMISSIONERS AND MINNESOTA DEPARTMENT OF NATURAL RESOURCES, WATERS DIVISION 022W COUNTY ATLAS SERIES ATLAS C-6, PLATE 2 OF 9 BEDROCK GEOLOGY T115r B Ti 4N LOCATION DIAGRAM R24V R�'`� T112N £Ig £m 3,r lr ,wl i; w�r� Vr ;'6�%'1i�is�vlh;,,1,^, ,l *I/,' T 28 N Opg £ INTRODUCTION R 20 0 pg BEDROCK GEOLOGY By John H. Mossler 1990 CASTLE ROCK UNTIL 1883 AND ALL THAT REMAINS OF IT A CENTURY LATER The singular pillar in Dakota County, known as Castle rock, consists of St. Peter sandstone.... [It is] an outlier from which most of the formation has been removed over an area of some miles about ... Near the base of the tower is a somewhat argillaceous layer, or one less firmly cemented, of a few inches, which weathers away faster than the rest, making the diameter there considerably less than above. Hence the tower has a threatening aspect, and the first A bedrock geologic map shows the geographic distribution of the geologic units that crop out or are covered only by unconsolidated materials. In the northern and western parts of Dakota County, the bedrock is covered by a mantle of unconsolidated Quaternary deposits that range to more than 500 feet in thickness in deeper buried valleys, but are thin or absent in some places along the valley of the present Mississippi River. In the southeastem and southern parts of the county, they are very thin and bedrock is exposed in many places; hills whose flat tops reflect a bedrock core capped by resistant Platteville Formation are common on the uplands. The cross sections add the dimension of depth and illustrate stratigraphic and structural relationships among units. The geologic formations are relatively thin in relation to their geographic extent and would be only a tenth as thick as shown, if the vertical scale were the same as the horizontal. This vertical exaggeration distorts the regional, as well as the local slope or dip of the formations. The considerable variations in thickness of unconsolidated formations overlying the bedrock and variations in elevation of the first bedrock formation are other features of the county's geology illustrated on the cross sections. Plate 4 of the atlas shows these variations in map form for the entire county. Knowledge about bedrock conditions generally decreases with depth below the land surface, because most of the geologic data were acquired during the drilling of water wells. In areas of thick overburden, it commonly is not necessary for well contractors to drill the entire thickness of unconsolidated overburden to find sufficient ground water. Therefore, the bedrock plate shows bedrock conditions within 200 feet more accurately than it shows conditions below 200 feet. Where little information is available on the lower formations, the geologic contacts on the cross sections are dashed. Virtually no information is available below the limits shown on the cross sections. Plates 5 through 10 of this atlas demonstrate how information from the bedrock plate, combined with information about other geologic or hydrologic aspects of Dakota County, can be used to construct derivative maps that relate to problems of resource management in Dakota County. These plates are intended to assist citizens and county officials who are not geologists. The bedrock map is a valuable source of basic data that can be used to prepare additional interpretive maps as needs arise. The bedrock geologic map is based partly on tabulated locations and lithology of bedrock outcrops in MGS files, and partly on the distribution of shallow bedrock areas as mapped by the Dakota County Soil Survey (Hundley, 1983). Much of the subsurface information used for this map was acquired during drilling of water wells or bore holes for engineering projects. These data are described on Plate 1 of this atlas. Seismic soundings by Vick and others (1980, 1980) were supplemented at selected localities by soundings done for this atlas by Andrew Streitz of the Minnesota Department of Natural Resources. The gamma log in the geologic column illustrates typical signatures of the formations; many wells were logged specifically for this atlas. Acquisition of subsurface data is a continuing process and each new data point adds to the store of knowledge about the subsurface geology of Dakota County. For this reason, as well as for greater detail, it is best to examine current data bases at MGS for site -specific geologic studies. GEOLOGIC HISTORY AND STRUCTURE All the bedrock units shown on the map are marine sedimentary rocks of Early Paleozoic age, a time when shallow seas covered southeastern Minnesota and parts Je• xko Nhr 0 R19W Je• J ti Nea NI)` Iam. Osp I Osp eQb. co Qb ,`e 44o QP Opg -Cm Osp 1 0 1 SCALE 1:100 000 2 3 4 5 6 1 0 5000 0 0pc of many adjacent states. Sand accumulated in near -shore bars, on beaches, and in sand dunes; silt and clay formed mud flats or settled out in quiet water farther from shore; and carbonate derived from remains of invertebrate shells and algae accumulated in small banks or reefs and as tabulate layers on the seafloor. These sediments were later lithified to form the sandstone, shale, and dolomitic limestone of today. After deposition of the Prairie du Chien Group, the marine waters withdrew from the area long enough for dry land to form and for significant erosion to occur. However, deposition of St. Peter Sandstone on sedimentary rocks older than the Prairie du Chien, as observed in Hennepin County (Olsen and Bloomgren, 1989), cannot be demonstrated in Dakota County. The regional dip of the Paleozoic strata toward the west and north reflects the position of Dakota County on the southeastern margin of the Twin Cities basin (Fig. 1). The Twin Cities basin developed in Middle Ordovician time over an older basin that formed along a part of the Midcontinent rift. The rift now is a large geologic feature composed of thick lava flows and red clastic sedimentary rocks. Large-scale block faulting in these Proterozoic rocks caused the formation of an elongate northeast -trending basin, as a down -faulted block or graben, beneath what was to become the Twin Cities Metropolitan Area. Much of the movement that deformed the Paleozoic rocks of the Twin Cities basin must have taken p1acu after deposition of the Middle Ordovician rocks ati depositedat about because the Platteville Formation. which was the same depth within the shallow ocean, as inferred from its lithic uniformity, is found at elevations of 800 feet near the center of the basin in St. Paul and crops out at elevations higher than 1000 feet in southern Dakota County. The Paleozoic Twin Cities basin, rather than a single down -faulted structure, is the result of many small folds and faults in step fashion. Individually they have small displacements of about 100 feet for folds and 50 to 150 feet for faults. Many of these faults and folds occur in eastern Dakota County along the margin of the Twin Cities basin. None are exposed there in outcrop, but they are inferred from subsurface data. They are shown on the map with standard geologic symbols. Two major structures are the Vermillion anticline and Empire fault (Jirsa and others, 1986). These structures and other folds are shown on cross section A -A'. Although faults are not visible in outcrop in Dakota County, in neighboring Washington County, they cut outcrops of Prairie du Chien dolostone and Jordan sandstone in bluffs on the Mississippi River about a mile above Hastings (SE1/4 sec. 1, T. 26 N., R. 21 W.). Maximum displacement (throw) is nearly 100 feet in the larger of the two major faults exposed (Schwartz, 1936, p. 92). SELECTED BIBLIOGRAPHY Chandler, V.W., McSwiggen, P.L., Morey, G.B., Hinze, W.J., and Anderson, R.R., 1989, Interpretation of seismic reflection, gravity, and magnetic data across the Middle Proterozoic Mid -Continent Rift System, northwestern Wisconsin, eastern Minnesota, and central Iowa: American Association of Petroleum Geologists Bulletin, v. 73, p. 261-275. Hundley, J.J., 1983, Soil Survey of Dakota County, Minnesota: U.S. Dept. of Agriculture, Soil Conservation Service, 272 p. Jirsa, M.A., Olsen, B.M., and Bloomgren, B.A., 1986, Bedrock geologic and topographic maps of the seven -county Twin Cities Metropolitan Area, Minnesota: Minnesota Geological Survey Miscellaneous Map M-55, scale 1:125,000, 2 plates. Morey, G.B., 1977, Revised Keweenawan subsurface stratigraphy, southeastern Minnesota: Minnesota Geological Survey Report of Investigations 16, 67 p. 'Opg impression of the beholder is the certainty that the first severe blast of wind will throw it from its place.... [It has so] invited the ambitious, but sacrilegious, carvings of visitors that a hole has been made through the body of the rock. — N.H. Winchell Reprinted from the First Annual Report in volume II of the Final Report of the Geological and Natural History Survey of Minnesota (1888), p. 78-79. T115N T Base modified from U.S. Geological Survey, Faribault, Hastings, and St Paul, 1985. Cartography by Richard B. Darling 7 8 9 10 KILOMETERS 2 3 MILES 5000 10000 FEET Opg £Ig £sf 4 5 15000 20000 25000 -Se_ Ally £m Opg B' Every possible effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Geological Survey in St. Paul. In addition, every effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering -scale decisions without site -specific verification. 1200 1000 800 600 0 —400 T, 200 0 -200 w e• a 8� 1000 800 600 — 400 — 200 —0 -200 MILES Conversion of Apparent Dip to True Dip Vertical Exaggeration x10 Mossler, J.H., 1987, Paleozoic lithostratigraphic nomenclature for Minnesota: Minnesota Geological Survey Report of Investigations 36, 36 p., 1 plate. Ojakangas, R.W., and Matsch, C.L., 1982, Minnesota's geology: Minneapolis, University of Minnesota Press, 255 p. Olsen, B.M., and Bloomgren, B.A., 1989, Bedrock Geology, Plate 2 in Balaban, N.H., ed., Geologic Atlas of Hennepin County, Minnesota: Minnesota Geological Survey Atlas C-4, 9 plates. Schwartz, G.M., 1936, The geology of the Minneapolis -St. Paul Metropolitan Area: Minnesota Geological Survey Bulletin 27, 267 p., 7 plates. Vick , T.D., Greilich, G., and Seltzer, G.O., 1980, Seismic survey of buried bedrock topography in the Cannon River valley between Northfield and Cannon Falls, Minnesota: Unpublished report, Carleton College, Northfield. 1980, Seismic survey of buried bedrock topography in the Cannon River valley: Minnesota Academy of Science Journal, v. 46, no. 3, p. 6-9. Sirmy. OF IN° Amaintir iitier ,ifiii■r� 0 .10 MILES 700 PA\, \ —650 5p0") ` \ -• ; 1 h5 0 o,1 / h°c 90 Figure 1. Elevations in feet above sea level of the base of the Prairie du Chien Group show the configuration of the Twin Cities basin. Elevation (feet) N T Opg 0sp £t £sf £ig PALEOZOIC ERA MIDDLE ORDOVICIAN SYSTEM/ (458-453 m.y. ago) SERIES I FORMATION OR GROUP NAME MAP SYMBOL GENERAL LITHOLOGY NATURAL GAMMA LOG increasing count THICKNESS (in feet) DECORAH SHALE 9t 28 to 31 128 to 160 145 to 70 to 125 187 tO 240 21 to 63 78 to 118 155 to 275 = - - . PLATTEVILLE & GLENWOOD FORMATIONS Opg - J - } _3 /7 / I/ = TT _ ST. PETER SANDSTONE Osp • - . .:.' -- LOWER ORDOVICIAN (505-478 m.y. ago) PRAIRIE DU CHIEN GROUP Van AMON ¢' ..�.... / �_� / / / ./'308 = , / / / AV £ j UPPER C A M B R I A N (523-505 m.y. ago) JORDAN SANDSTONE 1/////1://7 ST.IAWRENCF FORMATION sf -- - /T? G G G G G G T T G G—r-G FRANCONIA FORMATION IRONTON & GALESVILLE SANDSTONES -C,o _ G•• -G _ - EAU CLAIRE FORMATION z _ -D - - _ o3 --a G G•.G GGO GG. G O z MT. SIMON SANDSTONE £m .. — :. •O '•x W : I— O - - -- _ MIDDLE PROTEROZOIC, UNDIVIDED (1200-900 m.y. ago) NOT SHOWN — — — — — I / I / I / L LIMESTONE A Fossils Calcareous / DOLOSTONE T / / / I T Dolomitic / SANDY S SHALE c Glauconitic SANDY Quartz pebbles, SILTSTONE granules SANDSTONE Oolites MEDIUM TO COARSE FINE TO VERY FINE v Chert ///// Cross bedded ' \ '- / -- / BASALT Contact marks a major erosional surface DESCRIPTION OF BEDROCK UNITS DECORAH SHALE —Green, calcareous shale with thin interbeds of limestone. Largely restricted to the extreme northern part of county where it attains a maximum thickness of 90 feet PLATTEVILLE AND GLENWOOD FORMATIONS —Fine-grained dolostone and limestone of the Platteville Formation is underlain by green, sandy shale of the Glenwood Formation. They underlie the Decorah Shale and are distributed throughout much of Dakota County. Many small flat-topped mesas capped with Platteville occur in southeastern Dakota County. Platteville rocks range in thickness from 28 feet at the northem border to 18 feet in southeastem Dakota County. Shale of the Glenwood Formation is 2.5 to 3.0 feet thick in northern Dakota County and is reported to be 10 feet thick near the southeastern border ST. PETER SANDSTONE —Upper half to two thirds is fine- to medium - grained quartzose sandstone that generally is massive to very thick bedded. The lower part contains multicolored beds of sandstone, siltstone, and shale with interbeds of very coarse sandstone. The base is a major erosional contact. The St. Peter subcrops throughout much of Dakota County. Outcrops, which are limited to beds of the upper part of the formation, occur chiefly on the flanks of small mesas that are capped by the Platteville Formation in southeastern Dakota County and in cliffs along the Mississippi River in Mendota Heights. The St. Peter ranges in thickness from 160 feet in the north to 128 feet in the southern part of Dakota County (see Fig. 2) PRAIRIE DU CHIEN GROUP—Dolostone of the Shakopee Formation forms the upper two thirds to half. It is commonly thin bedded and sandy or oolitic, and contains thin beds of sandstone and chert. Dolostone in the lower part —the Oneota Dolomite —is commonly massive to thick bedded, and generally is not oolitic or sandy, except for a transitional zone just above the Jordan Sandstone. Dolostone of both formations is karsted, and the upper part, where the overlying formations have been removed by erosion, may be rubbly. The Prairie du Chien crops out along the Vermillion River in and near Hastings. There are some outcrops in low bluffs and roadcuts, and in ravines by the Mississippi River from near Nininger to west of Sedil and from Inver Grove Heights south. Many outcrops too small to show at the scale of this map occur in southeastern Dakota County. In northern Dakota County, east of Savage, the upper part of the Prairie du Chien is exposed in a large quarry. The Prairie du Chien ranges in thickness from 145 feet in extreme northern Dakota County by St. Paul to over 300 feet along the southern border of the county JORDAN SANDSTONE —The upper part is medium- to coarse -grained, friable, quartzose sandstone that is trough cross -bedded The lower part is primarily fine-grained sandstone that commonly is feldspathic, massively bedded, and bioturbated. The upper contact with the overlying Prairie du Chien Group, which is sharp compared to its gradational nature farther south in Minnesota, is exposed in the bank of the Mississippi River at Nininger and at the outlet of Spring Lake ST. LAWRENCE AND FRANCONIA FORMATIONS —The St. Lawrence consists of dolomitic shale and siltstone that is generally thin bedded. The contact between it and overlying Jordan Sandstone is gradational. The Franconia is composed of thin -bedded, very fine grained glauconitic sandstone and minor shale. Some sandstone beds are massive and bioturbated; others are cross -bedded. The Franconia does not crop out in Dakota County, nor do underlying formations IRONTON AND GALESVILLE SANDSTONES —Silty, fine -to coarse - grained, poorly sorted, quartzose sandstone underlain by better sorted, fossiliferous, fine- to medium -grained sandstone. The upper contact with the Franconia is sharp 1111 EAU CLAIRE FORMATION—Siltstone, very fine sandstone, and greenish -gray shale. Some sandstone beds are glauconitic. Minor dolomitic cement at the top of the formation. The contact with the overlying Galesville Sandstone is gradational. Shown only on the cross sections MT. SIMON SANDSTONE Chiefly fine to coarse, quartzose sandstone. The upper third contains many thin beds of well -sorted siltstone and very fine sandstone and is fossiliferous. The lower two thirds has fewer layers of fine-grained sandstone 'and consists primarily of medium- to coarse -grained sandstone. The basal part has poor to moderate sorting and contains thin layers of quartz and chert granules and pebbles. The basal contact is a major erosional surface. The upper contact with the Eau Claire is sharp and marked by a band of ferroan oolites in places. At the upper contact, the grain size increases markedly from the very fine of the Eau Claire, to fine to medium, to the very coarse of the Mt. Simon MIDDLE PROTEROZOIC ROCKS, UNDIVIDED —The Solor Church Formation is the principal geologic unit underlying the Paleozoic rocks. It consists of reddish -brown shale interbedded with reddish -brown lithic and feldspathic sandstone. About 760 feet of Solor Church was penetrated in an exploratory test hole west of Rosemount. However, geophysical modeling indicates that as much as 4 km (13,200 feet) of Solor Church Formation may underlie the Twin Cities basin (Chandler and others, 1989) in northern and western Dakota County. East of the Empire fault, the Fond du Lac Formation is the first Proterozoic unit (Morey, 1977). It is a light -red to dark -reddish -brown, poorly sorted feldspathic sandstone containing interbeds of moderate to very dusky red shale. Along the Empire fault and the Vermillion anticline, Proterozoic basaltic and rhyolitic volcanic rocks are present beneath thin layers of Solor Church Formation that are only a few feet thick in some places. The Hinckley Sandstone, which is the uppermost Proterozoic sedimentary formation in the region, is absent in most of Dakota County. In the past, the basal part of the Mt. Simon Sandstone was sometimes mistaken for the Hinckley £m MAP SYMBOLS Geologic contact —Approximately located Fault Upthrown and downthrown blocks are labeled - Anticline —Showing crestline and direction of plunge 1 - Syncline —Showing axis of the trough and direction of plunge + Dome GEOLOGIC ATLAS OF DAKOTA COUNTY, MINNESOTA UNIVERSITY OF MINNESOTA MINNESOTA GEOLOGICAL SURVEY Priscilla C. Grew, Director Prepared and Published with the Support of the DAKOTA COUNTY SOIL AND WATER CONSERVATION DISTRICT AND BOARD OF COMMISSIONERS AND MINNESOTA DEPARTMENT OF NATURAL RESOURCES, WATERS DIVISION COUNTY ATLAS SERIES ATLAS C-6, PLATE 4 OF 9 DEPTH TO BEDROCK LOCATION DIAGRAM a p TILL 0 Base modified from U.S. Geological Survey, Faribault, Hastings, and St. Paul, 1985. WATER FINE SAND SILT & CLAY o a SAND & GRAVEL a C O 0 Q 0 o { TILL 4 � o OLDER DEPOSITS y .00.VN'N•AY• Figure 3. Sketch of the development of an idealized ice -walled lake plain. INTRODUCTION The most obvious feature on the map is the buried bedrock valley that crosses the county from the northwest. The unconsolidated materials are thickest over this valley and are shown by the strongest colors. The valley has no surface expression, and the hills of the St. Croix moraine have been built above its northern segment. Probably until the most recent glaciation, which began about 25,000 years ago, that part of the valley was occupied by the area's master stream. Topographic contours superimposed on the colors, which show the depth to bedrock, show the elevation of the bedrock surface. In Dakota County, the bedrock ranges from about 1000 feet above sea level in places across T.113 N. down to 350 feet in the major buried valley. The bedrock formations in this former valley are also the oldest —the St. Lawrence and Franconia Formations, shown as a medium green on Plate 2, with a narrow strip of Ironton and Galesville Sandstones eroded more deeply in the middle of the valley at the eastem edge of the county. Bedrock is the chief influence on the modem landscape in areas where the surficial overburden is thinnest, which are shown uncolored on this map. Places where the bedrock is especially close to the surface are shown on Plate 3 by a light -blue color and by a speckled pattern on terrace units. Both the glacial deposits and the bedrock are important sources of ground water. Understanding the relationship between the bedrock topography and the surface topography is essential for understanding the occurrence and movement of ground water in Dakota County. The depth to bedrock is an important consideration for many decisions affecting land use, such as determining excavation costs, designing sewer tunnels, and planning landfills. MAP PREPARATION AND AUTHORSHIP The bedrock topography was compiled as a step in making the map of bedrock geology (Plate 2). It is based on the elevation of the bedrock surface in boreholes, mostly water wells, and the elevation of bedrock outcrops. Not all of the water wells shown on the data base map (Plate 1) are relevant to this map: the deeper the buried valley, the less likely it is that water wells will be drilled to bedrock, as shallower sources can provide sufficient water. The deepest valleys are defined by only a few points, some of which are outside the county. The greater thicknesses that were chosen for the two deepest intervals reflect our increasingly obscure view of the bedrock surface. Much of the bedrock topography on this map was based on data compiled for the map of the seven -county Metropolitan Area (Jirsa and others, 1986). Their contours were modified to reflect more recent data and some differences in interpretation by the authors. A seismic T 28 N DEPTH TO BEDROCK AND BEDROCK TOPOGRAPHY By Bruce A. Bloomgren, Howard C. Hobbs, John H. Mossler, and Carrie J. Patterson 1990 KILOMETERS 1 H H H H 0 1 2 3 4 5 6 SCALE 1:100 000 7 8 9 10 11 �-I • ST. PETER'. SANDSTONE DECORAH PLATTEVILLE GLENWOOD SEQUENCE COLLUVIUM ESCARPMENT SLOPE WASH PRAIRIE du CHIEN GROUP /VALLEY SIDE •'JORDAN•' SANDSTONE Figure 1. Schematic erosional ST. LAWRENCE profile of the major bedrock units of FORMATION • • TT. —7-- Dakota County; vertical exaggeration is about x20, and the Prairie du Chien FRANCONIA-T FOORMATIRMATI ON plateau is broader than shown. 12 13 14 15 16 17 R 1 Cartography by Philip Heywood. Every possible effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Geological Survey in St. Paul. In addition, every effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering -scale decisions without site -specific verification. 16 19 20 MILES 1 0 1 2 3 4 5 6 �QQER �ERRPCE , ,� S Q P' ,7//: BURIED BEDROCK VALLEY Nq`F \�Gt�PNOS TERRACE FAOF V��� RIVERLLS WARREN P SIP I 5 ifs A. River Warren Falls Glacial River Warren cut a bedrock valley Cj between downtown St. Paul and the confluence with Glacial River Mississippi. Both rivers flowed over resistant bedrock upstream from the River Warren falls, and each created a broad floodplain. investigation in Greenvale Township (T. 112 N., R. 20 W.) was conducted for this atlas by Andrew Streitz of the Minnesota Department of Natural Resources to supplement seismic data near the Cannon River obtained by Vick and others, as cited on Plate 2. In general, the bedrock landscape beneath the glacial deposits is similar to the surface landscape in extreme southeastern Minnesota adjacent to the Mississippi River, where the sediment cover is very thin. In particular, we assumed that the low points in the bedrock are not closed depressions, but reflect parts of stream valley systems. The depth to bedrock was determined by subtracting contoured bedrock elevations from the contoured surface elevations, which have been mapped in great detail at the much larger scale of 1:24,000 (see Index to these maps on Plate 1). In places where the surface topography is very complex and irregular, such as the St. Croix moraine, the surface topography was somewhat generalized before subtracting bedrock elevations. The authors are listed in alphabetical order. Mossler drew the bedrock topographic contours. Drift thickncas contours were drawn by Mossler in the southern part of the county; by Patterson in the northern and west -central part; and by Bloomgren in the east -central part; Patterson transferred them to the 1:100,000 scale. Hobbs wrote the text. BEDROCK TOPOGRAPHY The rate at which stream erosion removes different types of rock has been the chief factor in shaping the bedrock topography of Dakota County. The Decorah Shale weathers rapidly to soft clay and is soon grassed over where exposed. The Platteville Formation is mainly limestone, and although thin bedded, is fairly resistant. The underlying shale of the Glenwood Formation is weak. In most places the St. Peter and the Jordan Sandstones are poorly cemented and easily weathered to loose sand. They are separated by the Prairie du Chien Group, composed mostly of dolostone. The Prairie du Chien is highly resistant to erosion, both because of its thickness and its relatively massive character. The St. Lawrence is more resistant than the Franconia Formation, but neither one crops out in Dakota County and their topography in the valley floor is not well known. The Decorah Shale is preserved as remnants only north of the largest buried bedrock valley. This part of the county is structurally low near the center of the Twin Cities basin. The lower elevation of the shale, relative to its elevation where it was stripped away, apparently helped protect it from erosion. Mesas and buttes of Platteville -capped St. Peter Sandstone that are only thinly or partially mantled by glacial deposits (Fig. 1) occur in T.113 N., Rs. 17-19 W. They 7 8 9 10 11 12 13 ST ANTHONY FALLS MINNEHAHA FALLS DRAINAGE OUTLET _ PRE-EXISTING BEDROCK VALLEY RE -OCCUPIED BY GLACIAL RIVER MISSISSIPPI,__ S �O LIMIT OF THE FALLS OF RIVER WARREN RIVER WARREN VALLEY WIDENS UPSTREAM AS IT CUT INTO SOFT GLACIAL DEPOSITS RATHER THAN RESISTANT BEDROCK B. St. Anthony Falls and Minnehaha Falls These two waterfalls started after the River Warren falls had advanced past its confluence with Glacial River Mississippi. St. Anthony Falls progressed upstream more slowly than the River Warren falls, because Glacial River Mississippi was a much smaller stream. Minnehaha Falls was fed by a drainage outlet now occupied by Minnehaha Creek. Figure 4. Retreat of the falls of Glacial River Warren —hypothetical aerial view, looking north from Eagan or Burnsville. range in height from about 60 to 120 feet. Their escarpments are mantled by the windblown fine sediment called loess, which in turn mantles colluvium of broken flags of Platteville limestone in a matrix of sand. The colluvium thickens downslope; in places, it interfingers with thin slopewash sand (reworked St. Peter sandstone) over St. Peter bedrock. Farther down this gently sloping bedrock surface, the Prairie du Chien dolomite crops out at the surface. The topographic contours surrounding the bedrock plateaus in southeastern Dakota County are intricate because they essentially are the contours shown on topographic quadrangle maps, and therefore more is known about them. The topography of these formations is presumably similar in the areas covered by thick drift. There the distribution of bedrock units is known mostly from water -well records, rather than surface exposures, and the details of the topography cannot be observed. The Platteville -St. Peter contacts on the bedrock geologic map (Plate 2) also are generalized in areas of thick glacial overburden. Below the mesas and buttes, the eroded surface of the Prairie du Chien Group forms a rolling plain, capped in places by remnants of the lower part of the St. Peter Sandstone. Many small outcrops of sandstone may occur in these areas, but they can be identified only if penetrated by a water well or other type of drill hole. Although flat in a gross sense, the surface of the Prairie du Chien is dissected by numerous shallow valleys. Some of these were eroded in Ordovician time, when the Prairie du Chien was exposed before the deposition of the St. Peter Sandstone. Others were cut into the dolomite after the St. Peter was eroded off. Because of the resistance of the dolomite to erosion, only a few valleys are cut completely through the Prairie du Chien Group. The underlying Jordan Sandstone is readily eroded, and tends to form thin subcrop bands along steep walls of valleys cut through it into the St. Lawrence and Franconia Formations. The main buried valley is largely filled with sand and gravel (vfs on cross section A -A', Plate 3) underneath late Wisconsinan outwash and till of the Superior lobe. No till was encountered deep in the valley, and this suggests that most of the valley fill is the deposit of a single event, perhaps long-distance outwash from the advancing late Wisconsinan Superior lobe. If so, this valley must have been the pre -late Wisconsinan Mississippi River, as suggested by Schwartz (1936). The sediment below about 500 feet in this valley (not shown on A -A') is presumably older. The southern segment of the main buried valley may have been filled and abandoned before Wisconsinan time, with the pre-Wisconsinan master stream joining a different bedrock valley near Pine Bend. That valley was incised to about 500 feet in elevation before the late Wisconsinan Glaciation, and the river may have been flowing on bedrock at this elevation north of Hastings at that time. Other bedrock valleys (Fig. 2) also underlie areas where the modem drainage bears no relationship to the older drainage. A major tributary to the Vermillion bedrock valley is now buried beneath a modern lowland south of the town of Vermillion that is not occupied by any significant stream. Farther south, where the tributary stream changed direction from westward to northward, this buried valley underlies higher land of the so-called Hampton moraine, mapped as unit psd on Plate 3 (section B-B'). This part of the buried valley was obliterated by glacial drift in Illinoian time — possibly the same time as the deposition of the fill in the bottom of the deepest valleys. The modern drainage across T.113 N., R.17 W. is eastward to the Cannon River —the opposite direction from the stream that cut the buried bedrock valley. In the southwestern corner of the county, another buried drainage system flowed to the southwest into Rice County. The modern drainage, however, is chiefly to the east. Shallower buried bedrock valleys are fairly common in the county, cut into the Platteville and St. Peter Formations and the surface of the Prairie du Chien Group. Some are occupied by modern streams, such as the Cannon and Vermillion Rivers, although the details of the modem river courses are different. For example, the Cannon River flows over its associated bedrock valley from Northfield to about 2 miles west of Randolph, where its modem course diverges to the south. In this same area, a buried bedrock valley also underlies the North Branch of Chub Creek. Where modern streams approximately follow the underlying bedrock valleys, it is because the valleys were incompletely filled by unconsolidated sediment. Subsequent drainage followed the same general path, but not exactly the same, because the streams did not "know" where the original valley was. In some cases, the modem stream cuts across a bedrock high that had been covered by outwash. The site of Byllesby Dam on the Cannon River is a good example. Outwash from the advancing ice sheet of the Superior lobe filled the valleys and spread over the lower parts of the upland. The surface of the Rosemount outwash plain now is about 950 feet above sea level where it merges with the St. Croix moraine. It declines to the south and east, down the direction of flow, to about 850 feet in T.114 N., Rs. 16 and 17 W. Thus the Mississippi bedrock valley, which was about 450 feet above sea level, probably was filled with about 400 feet of meltwater sediment from the Superior lobe. As the Superior lobe retreated, meltwater sediments accumulated in large depressions where the ice melted fastest. Finer sediment was washed into the middle of these ice -walled lakes, and sand and gravel accumulated at the edge (Fig. 3). As the surrounding ice melted down, the lake became shallower, and the sediment in the center became coarser. The St. Croix moraine is pockmarked with depressions where buried —and insulated —ice blocks COLLUVIUM VALLEY FILL EXPLANATION Bedrock Topography 700-- Topographic contours in feet above sea level. Contour interval 50 feet Depth to Bedrock Less than 50 feet 51-100 feet 101-150 feet 151-200 feet 201-250 feet 251-300 feet 301-350 feet 351-400 feet 401-500 feet More than 500 feet Figure 2. Pre -late Wisconsinan bedrock valleys (black) are largely buried in the seven -county metropolitan area. The three largest modern rivers (blue) occupy the older valleys only in places. The older valleys are sketched on the basis of scattered well logs. Figure 5. Stone line on drift of the River Falls Formation in Marshan Township. A pebble lag like this is a very common feature on pre-Wisconsinan glacial deposits in Dakota County. It is overlain here by a thin layer of loess. remained longest. The former ice -walled lakes are now elevated above the surrounding moraine. The combined valley of the Minnesota and Mississippi Rivers was entrenched by Glacial River Warren, the southem outlet of Glacial Lake Agassiz. Downstream from downtown St. Paul, where River Warren began to follow the older Mississippi channel, erosion to the lower terrace level took place rapidly through unconsolidated late Wisconsinan sediment, creating the middle and lower terraces shown on Plate 3. Upstream from St. Paul, the channel cut by River Warren is new (Fig. 4). It is a gorge created by retreat of a large waterfall across a resistant floor of Platteville limestone. Water falling over the lip of limestone to the newly excavated lower valley eroded the soft St. Peter Sandstone and undercut the Platteville cap, which eventually collapsed under its own weight. The waterfall then moved upstream and the process began anew. The River Warren falls extinguished in thick glacial deposits at the edge of the Platteville Formation subcrop on the Minnesota River south of Bloomington west of Dakota County. When Lake Agassiz found lower outlets to Lake Superior, less than 10,000 years ago, Glacial River Warren shrank to the modem Minnesota River. Near St. Paul, River Warren had cut down to an elevation of about 600 feet. Its successor streams have been unable to carry all the sediment supplied by their tributaries down the low gradient suited to a much larger stream, and the floodplain has 'risen to an elevation of about 700 feet. The sediment fill of the Minnesota River is chiefly organic silt and clay like the sediment accumulating today. Mississippi River alluvium is relatively sandy, but it is underlain south of St. Paul by lake clay from an early stage of Lake Pepin (Wright, 1972). Lake Pepin exists because the Mississippi south of Dakota County is partly dammed by a fan of sediment from the Chippewa River of southwestern Wisconsin. Deposition of sediment by the Mississippi River near Hastings has in tum dammed the St. Croix River, forming Lake St. Croix. Although the southeastern part of Dakota County was not covered by ice during the last glaciation, it was indirectly affected. Lowlands were filled by outwash. The climate became cold, dry, and windy. Ice wedges, indicative of permafrost, occur in the Sangamon soil near Hampton. Reduction in vegetation allowed erosion to accelerate —a lag of pebbles, cobbles, and boulders is widespread on near - surface bedrock and Illinoian and older glacial materials (Fig. 5). In places, these stones are polished and faceted by wind abrasion. Deposits related to this erosion are slopewash sand, colluvium, and loess. Relatively little of Dakota County is covered by thick deposits of windblown sediment; presumably most areas experienced net deflation rather than accumulation, because they were so near the ice. Thin loess, however, is widespread over the pre-Wisconsinan units and the Rosemount outwash. SELECTED BIBLIOGRAPHY Balaban, N.H., ed., 1989, Geologic atlas of Hennepin County, Minnesota: Minnesota Geological Survey County Atlas Series C-4, scale 1:100,000, 9 pls. Balaban N.H., and McSwiggen, P.L., eds., 1982, Geologic atlas of Scott County, Minnesota: Minnesota Geological Survey County Atlas Series C-1, scale 1:100,000, 6 pls. Bowen, D.Q., and others, 1986, Correlation of Quaternary glaciations in the Northern Hemisphere, chart 1 in Sibrava, V., Bowen, D.H., and Richmond, G.M., eds., Quaternary glaciations in the northern hemisphere; Quaternary Science Reviews, v. 5. Hundley, S.J., and others, 1980, Soil survey of Dakota County, Minnesota: U.S. Department of Agriculture, Soil Conservation Service, 272 p. Jirsa, M.A., Olsen, B.O., and Bloomgren, B.A., 1986, Bedrock geologic and topographic maps of the seven -county Twin Cities Metropolitan Area, Minnesota: Minnesota Geological Survey Miscellaneous Map Series M-55, scale 1:125,000. Leverett, Frank, 1932, Quaternary geology of Minnesota and parts of adjoining states: U.S. Geological Survey Professional Paper 161, 149 p. Meyer, G.N., 1985, Quaternary geologic map of the Minneapolis -St. Paul urban area, Minnesota: Minnesota Geological Survey Miscellaneous Map M-54, scale 1:48,000. Ruhe, R.V., and Gould, L.M., 1954, Glacial geology of the Dakota County area, Minnesota: Geological Society of America Bulletin, v. 65, p. 769-792. Schwartz, G.M., 1936, The geology of the Minneapolis -St. Paul Metropolitan Area: Minnesota Geological Survey Bulletin 37, 267 p., 7 pls. Swanson, L., and Meyer, G.N., 1990, Geologic atlas of Washington County, Minnesota: Minnesota Geological Survey County Atlas Series C-5, scale 1:100,000, 7 pls. Todd, J.H., 1942, A contribution to the study of the Pleistocene history of the upper Mississippi River: Unpublished Ph.D. thesis, University of Minnesota, 71 p. Wright, H.E., Jr., 1972, Quaternary history of Minnesota, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 515-547. GEOLOGIC ATLAS OF DAKOTA COUNTY, MINNESOTA UNIVERSITY OF MINNESOTA MINNESOTA GEOLOGICAL SURVEY Priscilla C. Grew, Director Prepared and Published with the Support of the DAKOTA COUNTY SOIL AND WATER CONSERVATION DISTRICT AND BOARD OF COMMISSIONERS AND MINNESOTA DEPARTMENT OF NATURAL RESOURCES, WATERS DIVISION COUNTY ATLAS SERIES ATLAS C-6, PLATE 3 OF 9 QUATERNARY GEOLOGY LOCATION DIAGRAMS A 1000 — 900 — a) — c 800 0 cu 700 w 600 500 1100 — 'di 1000— c 900 — o 7-0 800 - 0 — 700 — 600 si B,z,,, Scott Co. Orchard Lake PLEISTOCENE GEOLOGIC HISTORY Dakota County was covered several times by continental ice sheets during the last 2 million years. The Great Ice Age, or Pleistocene, plus the 10,000 years of the Recent or Holocene, make up the Quaternary Period. The ice sheets, which advanced and receded in response to fluctuations in global climate, originated in two source areas in northern Canada — the Keewatin ice center to the northwest and the Labradorean to the northeast. Their flowpaths crossed different rock types, and the resulting difference in sediment is useful in reconstructing Quaternary history. Glacial Sediments in Dakota County Keewatin Labradorean Till texture (see Figure 1) loam to clay loam sandy loam Oxidized color yellow brown to olive brown to reddish brown Fresh color gray to dark gray reddish gray Paleozoic carbonate common to abundant rare to common Dark -gray to gray -green rocks uncommon to common abundant Red felsite and sandstone rare to common common to abundant Cretaceous shale absent to abundant absent Tills from the Keewatin center are the oldest glacial evidence in Dakota County. Till is the unsorted sediment, ranging in size from clay particles to boulders, that was deposited by a glacier with little or no reworking by meltwater. Unit pkt can be divided into two main groups on the basis of texture (Fig. 1A), one containing more than about 55 percent sand, and the other less than 55 percent, although the plots show some overlap. The two groups correspond to the upper and lower tills mentioned in the description of map units. Till that is less clayey than the lower till was observed below the lower till in cuttings samples. It appears to be a third, separate till, because it has been oxidized beneath unoxidized till. It is possible that more pre-Illinoian gray tills exist but are very similar to the others or are so patchy that no samples have been recovered. The two main tills of pkt have a subtle difference in rock composition that can be seen in the 1- to 2-mm sand fraction under a binocular microscope. The sandier upper till contains a small proportion of rock fragments typical of the Labradorean Superior lobe. The clayey lower till has very few such grains, and most samples have none at all.. However, the lower till contains a few Cretaceous shale grains, which the upper one wholly lacks, and also more Cretaceous limestone, pyrite, and lignite than the upper till; its provenance is more western than that of the upper till. This distinction is similar to the upper two pre- Illinoian gray tills in Olmsted County (Atlas C-3), where R23W T 28 A R22Vi KILOMETERS 1 0 1 2 3 4 5 fd T27N R 18 W sd RAND ICAGO AND SCALE 1:100,000 1 INCH ON THE MAP REPRESENTS NEARLY 1.6 MILES ON THE GROUND 7 8 9 10 11 12 SURFICIAL GEOLOGY By Howard C. Hobbs, Saul Aronow, and Carrie J. Patterson 1990 CORRELATION OF MAP UNITS ws so unconformity (Sangamon) t1 pso unconformity pko unconformity 7W Illinoian Pre-Illinoian Cambrian/ Ordovician T 115N B' Base modified from U.S. Geological Survey, Faribault, Hastings, and St. Paul, 1985 Gartography by Richard B. Darling True Slope S ope exaggeration for 10 - percent slope Hampton moraine 13 14 15 16 17 18 19 20 H I -I I -I. MILES 1 0 2 the overlying till contains inclusions of reddish sandy till. In Dakota County, no inclusions of Labradorean till were observed in unit pkt. At least two of the pkt tills appear to have covered the whole county. Information from other counties indicates that a period of weathering and erosion separated their deposition. After a long period of weathering and erosion, the Labradorean Superior lobe advanced into Dakota County during the Illinoian Glaciation, depositing reddish till and meltwater sediments of the River Falls Formation of Baker and others (1983). Only samples of till from map unit psd are plotted on Figure 1A. About half of these samples are sandier than the most sandy pkt samples, and an equal number plot within the sandy side of the pkt cluster. The three that are most clayey probably are local incorporations of clay into the till. No Keewatin sediments of the Illinoian Glaciation are known from Dakota County. The River Falls Formation appears to correlate with the Hawk Creek till of Matsch (1972) in the subsurface along the Minnesota River Valley and eastern South Dakota. Thus its extent was much greater than the late Wisconsinan advance of the Superior lobe, which terminated in the western suburbs of Minneapolis. To the southeast, however, it extended only about 15 miles farther than its late Wisconsinan counterpart. Very little of the River Falls drift remains in western Dakota County, but a large remnant of red drift that may be River Falls Formation is exposed in the New Market -Elko area in eastern Scott County. During the long interglacial warm period — about 70,000 years — between the Illinoian and Wisconsinan Glaciations, the Sangamon soil developed on the Illinoian and older deposits. The Sangamon soil was thicker and more deeply leached and oxidized than the modem soil. Little is known of the early part of the Wisconsinan Glaciation in the Midwest. If glaciers entered Minnesota, they were less extensive than the late Wisconsinan glaciers, and their deposits are buried. At some time after the Illinoian, but before the late Wisconsinan Glaciation, the Mississippi River cut down below its present level. Most if not all of the roughly 200 feet of sediment above the bedrock floor of the Mississippi is late Wisconsinan and Holocene. Late Wisconsinan Events Deposits of the Superior lobe — the Cromwell Formation of Wright and others (1970) — are the first evidence of late Wisconsinan ice in Dakota County. Its initial advance was somewhat more extensive than its later equilibrium position. The isolated patch of unit st just south of Rich Valley is a mantle of till of this early advance over a highland of bedrock and Keewatin till. As A -A' shows, Superior till both underlies and overlies Superior outwash in northern Dakota County. Therefore, the early advance was followed by a retreat and creation of an outwash plain. Renewed advance of the ice to an equilibrium position — where melting at the ice front kept pace with the flow of ice — built the St. Croix moraine on part of the outwash plain. The 3 4 5 6 7 unburied outwash, called the Rosemount outwash plain, continued to build higher from meltwater sediments. Deep undrained depressions on the outwash plain are common north of the islands of bedrock and Keewatin till near the Rosemount Experiment Station, but are fewer and shallower to the south. Only depressions deeper than about 30 feet are good evidence of buried glacial ice. Shallower depressions could have formed by sediment compaction or wind blow-out. Retreat of active ice allowed the Grantsburg sublobe of the Des Moines lobe to advance into the area once occupied by the Superior lobe. In northern Dakota County, much of the St. Croix moraine was so high that it remained free of Keewatin ice, and only the inner, lower part of it was overridden. The large kame complex on the boundary with Scott County was mapped si because it is only thinly mantled by Grantsburg sublobe deposits. An advance of the Des Moines lobe several miles east of its equilibrium position covered part of the Chub Creek outwash plain and the eroded hills of Keewatin drift. The thin mantle of till does not change the landforms, although this advance created some undrained depressions on unit dto. With retreat of the Des Moines lobe back to its equilibrium position, the Chub Creek outwash plain became inactive. Meltwater continued to flow from the ice margin south of Dakota County, north and east into the county. Unit do can be traced in the narrow part of the Cannon River valley, but it is very thin, and the map shows alluvium (fd) and near -surface bedrock (b). To the north, Des Moines lobe meltwater was cutting into Superior lobe sediment and laying down the mixed outwash of unit dso. It created the modern valley of the Vermillion River, and its Grantsburg sublobe meltwater cut a channel through the St. Croix moraine and across the Rosemount outwash plain. Downcutting of this outwash appears to have taken place in stages with erosion and deposition going on at the same time, because some dso occurs on the scarp separating so and dso above the final depositional surface of dso (Savina and others, 1979). Another example of a preserved intermediate stage of downcutting is the scarp within dso north of the Vermillion River in sections 2 and 11, Vermillion Township (T. 114 N., R. 18 W.). The higher area was protected from further downcutting by the bedrock island in section 11. As the active ice of the Des Moines lobe receded up the Minnesota River valley, outwash continued to flow to the Mississippi, following the path of the Minnesota River. This Glacial River Minnesota cut into the outwash that had been deposited, and formed the upper terrace (unit t3). The St. Croix moraine was breached from downtown St. Paul past South St. Paul over an old valley. Because Glacial River Minnesota flowed through this breach, it cut down more rapidly than the Rich Valley meltwater channel, beheading it and drying it up. With the retreat of the Des Moines lobe north of the continental divide, Glacial Lake Agassiz formed in northern Minnesota, North Dakota, and Canada. Its southern outlet followed the path of Glacial River 8 9 10 11 12 13 Minnesota, but it has been given a separate name — Glacial River Warren — to denote its much larger size and erosive capacity. River Warren cut its valley in stages. The first stage was the cutting of the middle terrace (unit t2) into the surface of the upper terrace. The middle terrace, in turn, consists of two distinct levels downstream from the confluence with the St. Croix River. The St. Croix River was also carrying a large volume of meltwater from Glacial Lake Superior at about this same time. Continued downcutting by River Warren created lower terraces (unit tl) below the middle terrace. The lowest level cut by River Warren is not shown on the map because it is everywhere covered by thick alluvium (unit fd). Several terrace levels may exist below the "lower terrace," because soil boring logs show greatly different thicknesses of alluvium in different places. THE SURFICIAL MAP Field mapping by Saul Aronow in the summers of 1979 and 1980. Howard Hobbs made field observations and collected additional samples, mostly in the summer of 1989. Hobbs compiled the map and wrote the text. Carrie Patterson described and interpreted subsurface samples and compiled the cross sections. The Quaternary map was compiled from the county soil survey, exposures of sediment in excavations, logs of soil borings and water wells, and geomorphic interpretation of 1:24,000-scale topographic maps. Much of the detail is based on the soil survey (Hundley and others, 1980), because exposures and subsurface logs are sparse in many areas. The map is compatible with the Quaternary maps of Hennepin County (Atlas C-4) and Washington County (Atlas C-5). The Quaternary map of Scott County (Atlas C-1) did not recognize the large kame complex on the border with Dakota County, and some dtp and dst was interpreted on that map as the equivalent of pkt. REFERENCES CITED Baker, R.W., Diehl, J.F., Simpson, T.W., Zelazny, L.W., and Beske-Diehl, S., 1983, Pre-Wisconsinan glacial stratigraphic chronology, and paleomagnetics of west -central Wisconsin: Geological Society of America Bulletin, v. 94, p. 1442- 1449. Hundley, S.J., and others, 1980, Soil survey of Dakota County, Minnesota: U.S. Department of Agriculture, Soil Conservation Service, 272 p. Matsch, C.L., 1972, Quaternary geology of southwestern Minnesota, in Sims, P.K., and Morey, G.B., eds., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 548-560. Savina, M., Jacobson, R., and Rodgers, D., 1979, Outwash deposits of central Dakota County, Minnesota.: Unpublished report, Department of Geology, Carlton College, Northfield, Minnesota. Wright, H.E., Jr., Mattson, L.A., and Thomas, J.A., 1970, Geology of the Cloquet quadrangle, Carlton County, Minnesota: Minnesota Geological Survey Geologic Map Series GM-3, 30 p., 1 plate, scale 1:24,000. 90 Sand 90 Sand Sand 80 80 70 70 60 60 Clay 50 50 Vertical exaggeration x 20 40 40 30 30 20 20 0 Silt 0 Silt + PSo + x SO O DO o T • DSO Figure 1. Textures of (A) pre-Wisconsinan tills; all of the psd points are till samples. (B) late Wisconsinan tills; and (C) outwash and terrace deposits. Note the enlarged scale of diagram C, which shows only the sand corner, but includes all the data points. fd ws DESCRIPTION OF MAP UNITS NONGLACIAL DEPOSITS ORGANIC DEPOSITS —Peat and organic -rich silt and clay; includes small bodies of open water. Largely drained and filled where built over FLOODPLAIN ALLUVIUM —Poorly bedded, moderately well sorted sediments deposited by modern streams during flood stage. Chiefly sand in the valleys of the Mississippi, Vermillion, and Cannon Rivers; chiefly clayey silt in the valley of the Minnesota River. Typically interbedded with organic -rich layers and buried soil. Much thicker in the valleys of the Minnesota and Mississippi Rivers than elsewhere. Some alluvium mapped in small tributaries to the Vermillion River may have accumulated as slackwater sediment related to outwash from the Des Moines lobe (dso) COLLUVIUM—Hillslope deposits derived from bedrock and loess upslope. Typically consists of two units —a rocky lower unit of angular carbonate clasts in a silty to sandy matrix, and an upper unit primarily of silt, which contains a few carbonate clasts. The composition of the lower unit reflects the bedrock upslope; the upper unit is largely reworked loess. Typically thickest at the bottom of the slope, and thin and patchy near the top. Colluvium and bedrock form intimate complexes, and their representation on the map has been considerably simplified DEPOSITS INDIRECTLY RELATED TO GLACIATION LOESS —Uniform unbedded silt and fine sand mixed with some clay. Shown only where thicker than 5 feet. The underlying material is shown, but the precision of mapping is less under the loess cover. Outwash plains, terraces, and pre-Wisconsinan drift areas generally have an unmapped cover of loess or windblown sand thinner than 5 feet SLOPEWASH SAND—Unbedded to poorly bedded sand deposited in valleys and on gently sloping plains above the level of Wisconsinan outwash. Derived from glacial drift and St. Peter Sandstone. The slopewash deposits commonly head upstream in bedrock escarpments and eroded hills of pre-Wisconsinan drift; they merge downslope into outwash plains or alluvium along modem streams. Where the slopewash merges downstream into outwash, the area of junction may contain flat -bedded silt and clay, deposited in a lake. Unit is gradational with outwash and the boundaries on the map are therefore arbitrary TERRACE DEPOSITS —Terraces represent T 114 N periods of stability separated by periods of downcutting by the large stream which carved the valley now occupied by the Mississippi and Minnesota Rivers. The lower and middle terrace units (tl and t2) each have two distinct levels, separated on the map by scarp symbols. In many places, terrace materials overlie outwash, and the boundary is not well defined. In all terrace units, a speckled pattern signifies that bedrock is generally 10 feet or less below the surface LOWER TERRACES —Clean sand and gravel, crudely flat bedded. The lower level of this terrace is 5 to 20 feet above the modern floodplain; the upper level is 40 to 70 feet above t. MIDDLE TERRACES —Similar material to that of the lower terraces. The lower level is 70 to 90 feet above the present floodplain; the higher level is 100 to 130 feet above. South St. Paul and Hastings are largely built on these terraces UPPER TERRACE Similar material to that of the middle terraces. The elevation is generally 150 to 170 feet above the floodplain DES MOINES LOBE DEPOSITS do OUTWASH—Sand, loamy sand, and gravel. Typical Des Moines lobe stone assemblage MIXED OUTWASH—Sand, loamy sand, and gravel; coarser texture near the edge of the lobe. Stone assemblage contains a considerable admixture of rocks typical of the Superior lobe. In places, distinguishable from Superior lobe outwash, only by its shale content ICE -CONTACT STRATIFIED DEPOSITS— Cobbly sand and gravel, locally interbedded with till (unit dst); stone assemblage is mixed Des Moines -Superior TILL —Chiefly loam texture; few beds and lenses of stratified sediment. Oxidized yellowish to olive brown above unoxidized gray. Calcareous except for a leached zone extending a few feet below the surface MIXED TILL —Complexly mixed yellowish -brown to gray and reddish -brown to reddish -gray, loam to sandy loam. Stone assemblage contains a considerable admixture of rocks typical of the Superior lobe THIN -MANTLED TILL —Thin Des Moines lobe till over pre-Wisconsinan drift; landforms same as pkt. Composition similar to that of dt south of the area of dto; similar to that of dst north of it THIN -MANTLED OUTWASH—Des Moines lobe outwash mantled by thin Des Moines lobe till. Till cover is especially thin north of Chub Creek. Composition of the till mantle is similar to dt; the underlying outwash is do SUPERIOR LOBE DEPOSITS (Cromwell Formation) OUTWASH—Gravel and sand. More cobbles and undrained depressions near the ice margin (the boundary with unit st). They diminish in number and depth to the south and east ICE -CONTACT STRATIFIED DEPOSITS — Gravel and sand; cobbles and boulders common. Locally intermixed with till (unit st). Many small ice -contact deposits are not distinguished from st on the map TILL —Reddish -brown sandy loam; cobbles and boulders common. Stringers and masses of poorly sorted sand and gravel very common, mixed on a scale ranging from inches to tens of feet. Most of the area mapped st can be regarded as a complex of st and si GLACIAL LAKE SAND —Reddish -brown sand at the surface, grading down into varved silt, clay, and very fine sand. Forms the tops of plateaus that were ice -walled lakes. The steep sides of these plateaus are commonly composed of till (st) and poorly sorted sand and gravel (si), both of which probably flowed or slumped off the ice into the water VALLEY FILL SEDIMENT —(cross section only). Chiefly fine sand, filling buried bedrock valleys. Age uncertain —either pre -Late Wisconsinan or very early Late Wisconsinan PRE -LATE WISCONSINAN DEPOSITS DRIFT OF THE RIVER FALLS FORMATION OF BAKER AND OTHERS (1983)—Outwash, ice - contact stratified drift, and till, undivided. Typically reddish brown to yellowish red. Deeply leached —most exposures noncalcareous. Predominantly stratified —where till is present, it is generally one or more layers a few feet thick near the top of the section. In most of its extent, psd is thin and patchy over bedrock and older till. The mapped boundaries of psd are considerably simplified in places OUTWASH OF RIVER FALLS FORMATION — Similar to unit psd, but flatter, and lacking till. The surface appears to be graded to a level considerably higher than the modern drainage system. Units psd and pso have traditionally been correlated to the Illinoian glaciation. This interpretation still seems reasonable, but no hard data are available "OLD GRAY" TILL Gray calcareous till which is leached and oxidized to yellowish brown near the surface. Consists of at least two tills, undivided. The upper till is friable loam to fine sandy loam; the lower one is firm loam to clay loam. Because of extensive erosion, the lower till is at the surface in much of the area mapped pkt "OLD GRAY" OUTWASH—Sand and gravel; not studied in detail. Sand was attributed to pko by stratigraphic position on the cross sections, by geomorphic association on the map. Units pkt and pko were traditionally correlated with the Kansan glaciation. "Kansan" drift is now known to have been deposited in many different glaciations, and units pkt and pko are here considered pre-Illinoian, undivided BEDROCK —Outcrops and thinly covered bedrock; mapped where bedrock is generally within 5 feet of the surface, exclusive of loess. Small areas of thicker sediment occur in areas mapped bedrock, but even in these, sediment is generally less than 10 feet thick A' Every possible effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Geological Survey in St. Paul. In addition, every effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering -scale decisions without site -specific verification. t3 dso dt dst so pso pko MAP SYMBOLS AIL ORGANIC DEPOSITS —Selected occurrences too small to map as areas .r,( -1SCARP—Mapped within an outwash or terrace unit; most of the contacts between different outwash or terrace units are scarps as well. Indicates an episode of erosion that affected only part of the unit. Hachures point down slope GEOLOGIC ATLAS OF DAKOTA COUNTY, MINNESOTA UNIVERSITY OF MINNESOTA MINNESOTA GEOLOGICAL SURVEY Priscilla C. Grew, Director Prepared and Published with the Support of the DAKOTA COUNTY SOIL AND WATER CONSERVATION DISTRICT AND BOARD OF COMMISSIONERS AND MINNESOTA DEPARTMENT OF NATURAL RESOURCES, WATERS DIVISION R22W 23 N WATER -TABLE MAP EXPLANATION COUNTY ATLAS SERIES ATLAS C-6, PLATE 5 OF 9 QUATERNARY HYDROGEOLOGY BURIED CONFINED CONDITIONS 115 i\ T115N LOCATION DIAGRAM T 27 R2371 114 N 11 21 V,' T113N T112N WATER TABLE Approximate boundary of Quaternary water -table aquifer Hachures toward Quaternary materials that are confined or yield little water Boundary of geologic unit that contains the water table Opg, Platteville; Osp, St. Peter; Opc-Cj, Prairie du Chien —Jordan; Q, Quaternary materials <5 R18W 5-50 Water yield from Quaternary aquifers In gallons per minute o Observation well, MGS 1988-89 x Observation well, MGS 1980 o Observation well, DNR-USGS-SWCD Data base for water -table elevations — 975 A Selected soil boring • Selected well record Water -table elevation In feet above mean sea level; contour interval 25 feet; arrows indicate general direction of ground -water movement R 17'W d WATER TABLE AQUIFER R 20 W UNSATURATED CONDITIONS IN WATER TABLE AQUIFER SPRING ONFINING: = _- LAYER DEEPER AQUIFER BURIED UNCONFINED CONDITIONS I I Figure 1. Schematic diagram relationship between buried buried unconfined conditions aquifer. Port I Kia giO 114 r° 0 O o • - R24 R 21 W 1113 -112N T 27 N ell ,r UNSATURATED CONDITIONS IN DEEPER AQUIFER RIVER WATER TABLE AQUIFER showing the confined and in a deeper trim Airport O co R 23 V/ �t�yk �3or - od Cask Rock R 19 W RW 2_ ve- Gtiav SowiejlFJ� . }hng Stri Oar SCIOTA T 112 N T 28 N RadJO Tew- T 112 N TA} CO c 8 W T 113 N THE WATER TABLE IS A POTENTIOMETRIC SURFACE The top of the zone that is completely saturated with water is called the water table. It may occur either in an aquifer or in material that yields little water. The potentiometric surface of an aquifer represents the elevation to which water in wells open to the aquifer will rise. It does not represent the top of the sediment or rock that contains the aquifer. It may be below the top of the aquiferabove it, or even above land surface elevation. An aquifer in which the top of the saturated zone is below the top of the sediment or rock is under water -table conditions; its potentiometric surface is the water table. An aquifer which underlies a less permeable stratum or confining bed and has a potentiometric surface above the top of the aquifer is under confined conditions. R16W The potentiometric surface is determined from elevations of static, or nonpumping, water levels which are reported on drillers' logs and are measured in observation wells. The static water level is expressed as feet below the surface. Therefore, the elevation of the static water level is the land surface elevation minus the static water level. The locations of wells must be accurately plotted on topographic maps to estimate land surface elevation. When the static water level elevations are plotted and contoured, the contours represent lines of equal hydraulic potential or head. Arrows perpendicular to the contours and pointing toward lower potential represent the flow path of water in the aquifer. SATURATED THICKNESS AND TRANSMISSIVITY OF QUATERNARY SAND AND GRAVEL AQUIFERS EXPLANATION —125 Saturated thickness Contour interval 25 feet; some intervals omitted in areas of steep gradient <500 >500- 25,000 Transmissivity, in gallons per day per foot Uncolored areas represent Quaternary materials that are either very thin, dewatered, or of low permeability 27 N R18W Hampton I1AN9f'ION Bg Nesb, R 18 W A x Selected well records —Buried confined, buried unconfined • Pumping test data used for yield and transmissivity Nes.J,Trier DAKOTA R 17 71 CO r R1(W Cartography by Philip Heywood. SCALE 1:125000 4 Mresvi3le 115N 0 1 36 1,;ENy'A 0 R167/ QUATERNARY HYDROGEOLOGY By Barbara M. Palen 1990 QUATERNARY AQUIFERS Glacial deposits of sand and gravel are a source of water for domestic and irrigation wells in Dakota County. Because of their susceptibility to pollution, they are not used for municipal or public supply wells. Their moderate yield capacity in the northern suburbs is adequate for some nonpotable industrial uses. In addition to the Rosemount outwash plain, outwash sand and gravel occurs in the valleys of the Vermillion and Cannon Rivers and their tributaries, and is buried by till of the St. Croix moraine north of Rich Valley. Some buried aquifers are hydrologically isolated from surface sand. Large differences in head between shallow soil borings and deeper water wells probably indicate perched water overlying a shallow clay or loamy till layer. The St. Croix moraine forms a leaky confining layer. In places, surface and buried sand layers have similar static water levels, and are probably connected by large inclusions of sand and gravel within the sandy loam till. In other places, water levels are lower in the underlying outwash. Buried unsaturated conditions common under the moraine are caused by lateral discharge into the Mississippi River valley (Fig. 1) and into a buried bedrock valley connected to the Mississippi. Drillers' records indicate that clay layers at the base of the outwash are not continuous, and so the outwash is hydraulically connected to the Prairie du Chien -Jordan except where the St. Peter overlies the Prairie du Chien. The Prairie du Chien -Jordan and the St. Lawrence - Franconia discharge into the sand in the bedrock valleys. The high yield of Quaternary wells in the deep buried valley southwest of Hastings indicates that confining layers are not extensive or thick enough to impede flow. WATER -TABLE MAP The water table is in bedrock in parts of Dakota County. Where it is in the Prairie du Chien -Jordan aquifer, the contours on the water -table map are identical with the potentiometric surface shown on Plate 6. Wells bottoming in the St. Peter were used to determine the water -table elevation in the St. Peter aquifer. Where no records of wells in the St. Peter were available, the elevation of the water -table in the St. Peter was inferred from elevations of the static water level in the Prairie du Chien -Jordan. Water -table elevations in the Platteville were taken from soil borings and a few drillers' records. The water table in outwash was determined from the static water levels reported in drillers' records and observation wells. Outwash deposits on the moraine in western and northern Dakota County are typically low points in the water -table surface, and they probably serve as localized recharge zones for buried outwash. Reports of water levels in till are scarce because the till is not used for water supply in Dakota County. Water - level reports from soil borings were used (where available) along major highways. Some soil borings in till showed locally perched water tables where clay layers occur within sandy till. The water table in the till also was inferred from the elevations of lakes and wetlands and the slope of the surface topography. However, investigations of several small lakes and ponds in northern Dakota County (Allred and others, 1971) show that such inferences may be doubtful in some areas. They found some ponds where the water table slopes away on all sides and others where the water -table slope opposes the regional slope of the surface topography. Lake levels that are much higher than the water levels in nearby soil borings and water wells were not used to define the water table. Because of the sparse data base of static water levels in the moraines, and complex conditions including local variations in the texture of sediments and possible perched water tables, the water -table map should not be used to infer ground -water flow directions for site -specific problems, such as leaking underground storage tanks. MAP OF SATURATED THICKNESS Although the map of saturated thickness of sand and gravel deposits implies that they act as a single aquifer, more than one aquifer can be present, separated from each other by relatively impermeable clay or till. The saturated thickness of the Quaternary deposits was determined by subtracting the elevation at the bottom of the well from the static water level, regardless of whether the well is drawing from a confined or unconfined Quaternary aquifer. For unconfined wells, this procedure is more accurate than for wells that penetrate a confining layer, which is recognized as a layer of clay 10 feet or thicker. Clay layers thicker than 5 feet were subtracted from the saturated thickness. The procedure underestimates the saturated thickness because the wells rarely extend to bedrock. To compensate for this underestimation, the saturated thickness was also estimated from sand and gravel intervals reported in the records of bedrock wells. The static water levels of neighboring Quaternary wells were used for the upper limit of the saturation zone for these wells. TRANSMISSIVITY Transmissivity is a measure of the rate at which water can move through an aquifer. On this map, transmissivity was determined from the specific capacity of high - capacity wells and some domestic wells, corrected for partial penetration (Walton, 1970), extended and generalized by estimates of hydraulic conductivity and saturated thickness. The hydraulic conductivity of the Quaternary deposits was estimated, based on textures reported in drillers' logs. The figure most commonly used was 1000 gallons per day per square foot. YIELD Yields were calculated for pumping test sites in confined aquifers according to the Theis nonequilibrium method (Davis and DeWiest, 1966) and were estimated for sites in unconfined aquifers from the graphs in Miller (1982). The results were generalized based on saturated thickness. Yield estimates require assumptions about well diameter, full penetration of the aquifer, lack of interference from nearby wells, and sustained duration of pumping. Because these assumptions are usually not met in individual design and operation, and because high - capacity pumping test data are sparse, the yield results are only approximate. The saturated thickness and transmissivity can be used to estimate the depth and diameter that will be needed for a well to produce a desired yield. WATER CHEMISTRY The ground water in the Quaternary drift is of the calcium -magnesium bicarbonate type. Although it is hard water and high in iron content, it is suitable for most purposes (Ruhl, 1987). There are fewer recent chemical analyses of ground water in the Quaternary deposits of Dakota County than in the Prairie du Chien -Jordan aquifer, which is used for municipal wells (MPCA, 1985). Most of the data are from a sampling program in 1960-61 (Maderak, 1963), and so it is impossible to tell if the water chemistry has changed over time. In that investigation most samples had nitrates greater than drinking water standards, but all the chloride concentrations were less than 9 ppm. Apparently factors in land -use patterns that cause nitrate pollution preceded those causing a rise in chlorides. Sulfate concentration was about 20 ppm in most wells. In the 1970s several studies of lake -water chemistry in Dakota County were conducted (Ayers, and others, 1980; Have, 1980; Payne, 1980; Tomes and Have, 1980) to document baseline conditions and the effect of urbanization. Some of the results are pertinent to ground- water quality concerns. Feedlots, fertilizer, and storm - sewer runoff were documented as reasons for increased concentration of nutrients, particularly nitrogen and phosphorus. Development of wetlands at lake inlets and outlets was recommended to control water quality, particularly to trap phosphorous. Elevated chloride concentration was attributed to road salt for deicing. The lack of a corresponding increase in sodium was thought to be the result of adsorption of sodium and release of calcium and magnesium ions in clayey soils at the sides of roads where salt is applied. The Vermillion River is monitored monthly for pH, nitrate, and specific conductivity, among other parameters. It receives discharge from the Quaternary, St. Peter, and Prairie du Chien -Jordan aquifers. Specific conductivity is proportional to total dissolved solids. Between 1974 and 1989, the pH remained stable, but increases in nitrate and specific conductivity confirm changes observed directly from chemical analyses. Nonpoint source pollution from these ground -water aquifers contributes to this increase in nitrates. RELATIONSHIP OF LAKE LEVELS TO GROUND -WATER LEVELS Ground -water levels in shallow sand aquifers rise in the spring with infiltration from rainfall and snowmelt (DNR, 1983-1988). Water levels usually decline the rest of the year as the recharge drains away. Water levels of some lakes also fluctuate. For example, Crystal Lake and a nearby observation well in sandy till were monitored between 1970 and 1979 (DNR, unpublished, 1980, 1990). Crystal Lake recharges the ground water, as shown by the slope of the water table away from the lake. Fluctuations of the water level of the lake and the well were nearly identical except at the end of the 1976-1977 drought, when the lake recovered more quickly. The lake was monitored into 1990, with a total fluctuation of only 4 feet between 1970 and 1990. Ground -water levels in deeper aquifers such as the Prairie du Chien -Jordan change over multiyear periods, usually lagging behind trends in precipitation. Precipitation was high in the early and mid-1980s, and many observation wells show a steady increase in static water levels from 1983 to the end of 1986. High water levels lessened the impact of the 1988-1989 drought. The water level in Marion Lake has been monitored since 1946. The total fluctuation of Marion Lake was 13 feet, with the low in the fall of 1964 and the high in the spring of 1985. The long-term behavior of this lake, although it is shallow, is more like that of the Prairie du Chien -Jordan than that of the Quaternary aquifers. Neither Crystal Lake nor Marion Lake has a surface outlet; they are close together and occupy a similar position in the Quaternary landscape. Their dissimilar history of water -level fluctuations cannot easily be explained with the data available. RELATIONSHIPS OF WETLANDS TO GROUND WATER Wetlands in Dakota County are shown on the surficial geologic map (Plate 3) as organic deposits and with the wetland symbol. There are two types in relationship to ground water: those that are discharge areas, and those that are recharge areas, at least part of the year. Discharge areas occur in the floodplains of the Minnesota and Mississippi Rivers, along the upper reaches of the Vermillion River and its tributaries, and in isolated areas along the Cannon River. Wetlands in the floodplains and river valleys purify surface water by using nitrogen and phosphorus in plant growth and absorbing heavy metals. Wetlands discharge ground water by evapotranspiration, because the water table is in or above the plant root zone. Organic deposits build up because the high water table inhibits decomposition of dead plant debris. Purification of surface water by wetlands in the Vermillion River watershed is important because several towns discharge treated sewage to the Vermillion River or its tributaries . Wetlands also contribute indirectly to maintenance of ground -water quality in the watershed because high - capacity wells near the river induce recharge. The high transmissivity and yield of wells near the river compared to wells farther away is evidence of this. The second type of wetland is in the moraine in northern and western Dakota County. In this area, the elevations of many lakes and swamps are above the water table, as shown by static water levels in nearby wells. The role of these wetlands in ground -water recharge is equivalent to small lakes in closed depressions on the moraine (Allred and others, 1971). Recharge to the Quaternary aquifer occurs when the wetland fills up after rain or spring snowmelt and expands beyond its normal boundaries. Although the bottom of the wetland may be sealed by accumulations of organic matter, recharge occurs around its edges where sand or sandy till is temporarily inundated. A slow, steady leakage must also take place through the organic sediments as well, even though they are not very permeable. REFERENCES Allred, E.R., Manson, P.W., Schwartz, G.M., Golany, P., and Reinke, J.W., 1971, Continuation of studies on the hydrology of ponds and small lakes: University of Minnesota, Agricultural Experiment Station Technical Bulletin 274, 62 p.. Ayers, M.A., Payne, G.A., and Have, M.R., 1980, Effects of urbanization on the water quality of lakes in Eagan, Minnesota: U.S. Geological Survey Water - Resources Investigations Report 80-71, 42 p. Davis, S.N., and DeWiest, R.J.M., 1966, Hydrogeology: John Wiley, 463 p. Have, M.R., 1980, Water quality of Rogers Lake, Dakota County, Minnesota: U.S. Geological Survey Water - Resources Investigations Report 80-5, 35 p. Maderak, M.L., 1963, Quality of waters, Minnesota: A compilation, 1955-62: Minnesota Conservation Department, Division of Waters Bulletin 21, 104 p. Miller, R.T., 1982, Appraisal of the Pelican River sand - plain aquifer, western Minnesota: U.S. Geological Survey Open -file Report 82-347, 44 p., 3 pls. Minnesota Department of Natural Resources, Division of Waters, 1990, Unpublished lake level records of Dakota County. Minnesota Department of Natural Resources, Division of Waters, Observation Well Data, Annual Summary, 1983-1988. Minnesota Department of Natural Resources, Division of Waters, Unpublished Observation Well Water Level Data for Dakota County, 1980, 1989. Minnesota Pollution Control Agency, 1985, Ground water quality monitoring program volume 6: A compilation of analytical data collected from 1978 to 1984. Payne, G.A., 1980, Baseline water quality of Schmidt, Hornbeam and Horseshoe Lakes, Dakota County, Minnesota: U.S. Geological Survey Water - Resources Investigations Report 80-3, 38 p. Ruhl, J.F., 1987, Hydrogeologic and water -quality characteristics of glacial -drift aquifers in Minnesota: U.S. Geological Survey Water -Resources Investigations Report 87-4224, 3 pls. Tornes, L.H., and Have, M.R., 1980, Water quality of four lakes in Lakeville, Minnesota: U.S. Geological Survey Water Resources Investigations Report 80- 66, 42 p. Walton, W.C., 1970, Ground -water resource evaluation: McGraw-Hill, 664 p. Every possible effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Geological Survey in St. Paul. In addition, every effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering -scale decisions without site -specific verification. R 20 W Base modified from U.S. Geological Survey, Faribault, Hastings, and St. Paul, 1985. R19W 2 0 2 6 8 10 MILES 2 0 2 4 6 8 10 KILOMETERS GEOLOGIC ATLAS OF DAKOTA COUNTY, MINNESOTA UNIVERSITY OF MINNESOTA MINNESOTA GEOLOGICAL SURVEY Priscilla C. Grew, Director Prepared and Published with the Support of the DAKOTA COUNTY SOIL AND WATER CONSERVATION DISTRICT AND BOARD OF COMMISSIONERS AND MINNESOTA DEPARTMENT OF NATURAL RESOURCES, WATERS DIVISION COUNTY ATLAS SERIES ATLAS C-6, PLATE 6 OF 9 BEDROCK HYDROGEOLOGY 1 1'5 LOCATION DIAGRAM it21V 21 1° T 1'2 T 27 29 ROCK UNIT AQUIFER SYSTEM HYDROLOGIC CONDITION WATER -LEVEL RELATIONSHIPS DECORAH SHALE CONFINING LAYER MOSTLY NON•AOUIFER' PLATTEVILLE FM GLENWOOD FM ST PETER SANDSTONE / / ' ST PETER 7 ' / // AQUIFER , / / UNCONFINED/ CONFINED N 'Abort: 10 to 25 feet CONFINING LAYER NON -AQUIFER SHAKOPEE FORMATION PRAIRIE DU CiIEN GROUP] ii PRAIRIE DU CHIEN-// JORDAN / / AQUIFER / A CONFINED MOST AREAS ONEOT/ DOLOMITE JORDAN SANDSTONE ST LAWRENCE FORMATION CONFINING LAYER NON -AQUIFER' FRANCONIA FORMATION ������/ FRANCONIA- / IRONTON- //GALESVILLE / AQUIFER //�///// , IN MOST AREAS IRONTON & SANDSTONES EAU CLAIRE FORMATION CONFINING LAYER NON -AQUIFER MT SIMON & HINCKLEY SANDSTONES 1 /2// / HINCKLEY AQUIFER/ et/ CONFINED PRECAMBRIAN UNKNOWN''' UNKNOWN T;15 * The Platteville and St Lawrence Formations locally supply limited amounts of water T 27 N 2_0r R 19 ' R22W 28 I0 PRAIRIE DU CHIEN—JORDAN AQUIFER POTENTIOMETRIC SURFACE AND YIELD EXPLANATION 840 — Potentiometric contour Shows inferred elevation in feet above mean sea level of static water in wells in the Prairie du Chien —Jordan aquifer. Contour interval 20 feet; arrows indicate general direction of ground -water movement Boundary between water -table and confined (artesian) conditions Hachures in direction of confined condition o Observation well, MGS 1988-1989 x Observation well, MGS 1980 o Observation well, DNR-USGS-SWCD • Selected well record 2 0 A Selected soil boring Data base for static water levels Potential yield in gallons per minute Uncolored where the Jordan Sandstone is absent /ter \ r,� R 17 W R 15 4^. CHEMISTRY 112 N The ground water of all sedimentary aquifers beneath Dakota County is of the calcium -magnesium bicarbonate type — hard to very hard — but is suitable for most uses. The ground -water chemistry is quite stable because bicarbonate from dissolved dolomite buffers the pH to about 7.5, and because of a relative lack of other reactive minerals in the rock. The dissolved Fe and Mn content is variable, with some samples having concentrations higher than drinking water standards of 0.3 ppm Fe and 0.05 ppm Mn. Reeder (1976) calculated from measurements of oxidation potential and pH that the ground -water was supersaturated in ferrous iron, a condition that apparently occurs widely for both Fe and Mn in the Prairie du Chien -Jordan aquifer in Dakota County. Water of this type commonly has an iron taste, and produces rust stains on fixtures. Sulfate varies geographically, from less than 5 ppm in Burnsville to 30 ppm in Hastings and Randolph and 80 ppm near Coates, but is below the drinking wafer standard of 250 ppm. With some exceptions, no trend over time was observed in sulfate concentration, and so the variation is probably R 22 W natural. In contrast, total dissolved solids have increased by about 20 percent, to an average of 320 ppm in the 1980s. For example, the chloride concentration, which is naturally low (<5 ppm), increased throughout the aquifer in most of the T 28 N T;'N R 21 V/ T113 T 1*2N R Base modified from U S Geological Survey, Farlbault, Hastings, and St Paul, 1985 R 190 T1120 2 T115N ,4 N T BEDROCK HYDROGEOLOGY By Barbara M. Palen 1990 R'6, water -quality observation wells in Dakota County from 1960 to the present, although present levels, up to 120 ppm, are still below the drinking -water standard of 250 ppm. There is no natural mineral source of chloride in the Prairie du Chien - Jordan aquifer. Human sources may be chlorination of wells, fertilizer, recharge from chlorinated municipal water, or road salt. A natural source might be upwelling of water from deeper aquifers, in response to pumping stress in the Prairie du Chien -Jordan. Increasing nitrate values over time were measured in over half of the observation wells. Like chloride, nitrate has no natural mineral origin in the aquifer, but unlike chloride, it has occurred at concentrations above drinking water standards (10 ppm NO3- as N) in 25 percent of the observation wells sampled in the 1980s. Nitrates may be recharging into the aquifers from fertilizers, manure in feed lots, or human waste in septic tank drain fields. Localized contamination may also occur through improperly constructed wells or wells with damaged casings. Recent tritium tests of ground water in the Prairie du Chien -Jordan show that recharge from precipitation has occurred within the last 35 years in some areas of Burnsville and Apple Valley. The presence of nitrate and chloride may also be indicators of relatively recent recharge to the Prairie du Chien -Jordan aquifer in Dakota County. PRAIRIE DU CHIEN—JORDAN AQUIFER SATURATED THICKNESS AND TRANSMISSIVITY R18w R18W 0 EXPLANATION <25,000 Transmissivity In gallons per day per foot; uncolored where the Jordan Sandstone is absent _125 — Saturated thickness Contour interval 25 feet; some intervals omitted in areas of steep gradient Pumping test site for transmissivity and yield calculation 1. 2 R 17 0/ Cartography by Philip Heywood. SCALE Cam. T115N 0 POTENTIOMETRIC SURFACE The potentiometric surface of an aquifer represents the elevation to which water in wells open to the aquifer will rise. In parts of Dakota County, the Prairie du Chien -Jordan aquifer is under water -table conditions, and the potentiometric contours on this map are the same as on the water -table map. See Plate 5 for discussion. When the static water -level elevations are plotted and contoured, the contours represent lines of equal hydraulic potential or head. Arrows perpendicular to the contours and pointing toward lower potential represent the flow path of water in the aquifer. SATURATED THICKNESS Under confined conditions, the saturated thickness of an aquifer is its total thickness. Under water -table conditions, it is the thickness of the aquifer below the water table. The saturated thickness of the Prairie du Chien -Jordan aquifer is measured from the base of the Jordan Sandstone to the top of the saturated zone in the Jordan or Prairie du Chien Group, or to the top of the aquifer if all of it is saturated. This map shows how the saturated thickness of the aquifer varies across the county. The saturated thickness of the aquifer is greatest (340 feet) where it is confined by the overlying St. Peter Sandstone. The saturated thickness is less where the aquifer is unconfined, partly eroded away, or both. The average saturated thickness of the Prairie du Chien -Jordan aquifer in Dakota County is about 275 ft. The saturated thickness of the aquifer must be known in order to estimate transmissivity and yield. Provided the storage coefficient is known, a map of saturated thickness gives an idea of the potential of the aquifer as a water supply. TRANSMISSIVITY Transmissivity is a measure of the rate at which water is transmitted through a unit width of an aquifer or confining bed under a unit hydraulic gradient. Transmissivity was estimated from specific capacity and saturated thickness (Walton, 1970) for about 150 high - capacity wells in Dakota County. No correction was made for possible recharge from an overlying aquifer during the pumping test. It is likely that significant recharge occurs during pumping of the Prairie du Chien -Jordan below partially saturated sand, and possibly under saturated St. Peter Sandstone. Furthermore, the sand may allow recharge by water from the Mississippi River and the Vermillion River. The transmissivity of the Prairie du Chien -Jordan is low where the aquifer thins toward bedrock valleys. High values are associated with maximum saturated thickness of the aquifer, proximity to rivers or buried bedrock valleys filled with saturated sand, and possibly proximity to fault zones. Fault zones may provide greater vertical hydraulic conductivity, a factor that greatly affects transmissivity. POTENTIAL YIELD The potential yield of an aquifer is the rate of withdrawal the aquifer can sustain without unacceptable changes in storage, water quality, or flow patterns. No limits to acceptable change in the Prairie du Chien -Jordan aquifer have been established. In the absence of a particular requirement for the aquifer, standard methods developed by Theis and Jacobs for confined and water -table aquifers were used to estimate yield from transmissivity, storage coefficient, and saturated thickness (see Miller (1982) for methods). A storage coefficient of 5x10-4, from an aquifer test in Dakota County (Norvitch and others, 1973), was used to estimate yield from pumping tests under confined conditions. A storage coefficient of 5x10-3 was assumed for water -table conditions. A drawdown of two thirds saturated thickness after 30 days continuous pumping at a steady rate was assumed. The results must be regarded as providing trends and relative differences in water -yielding capacity, rather than as exact estimates of yield in the aquifer, because the assumptions used in the method are unlikely to be met in actual conditions. The areas of highest yield (>2500 gpm) appear to depend in part on recharge from surface water and overlying aquifers. Consequently, continued high yield depends on continued availability of water from these sources. In addition, more recent pumping tests in some well fields, such as Apple Valley, show reduced specific capacity through time. Interference by nearby wells is a probable factor. The effect of well interference is to decrease the attainable yield from the mapped potential yield. USES OF THE MAPS Maps of the potentiometric surface, transmissivity, and saturated thickness can be used to estimate the average flow velocity of ground water in the aquifer and the rate of discharge across a given cross sectional area of the aquifer. However, these estimates cannot replace site - specific investigations of particular problems of contamination and supply. It is known, for example, that some contaminants can travel faster in ground water than the average velocity of the water itself. Furthermore, pumping rates of high -capacity wells vary seasonally, as well as over longer periods of time, so that ground -water levels are always changing. The average values represented on these maps are approximations, but are useful as guides. PLATTEVILLE AQUIFER The Platteville Formation is a creviced limestone, with a maximum thickness of 35 feet. It is overlain in places by the Decorah Shale confining bed. It is used for domestic wells in Mendota Heights, South St. Paul and Inver Grove Heights. Most of these wells were completed before drillers' records were required; consequently, little is known about specific capacity, transmissivity, or yield. Soil borings along Highway 13 and I-494 and water -level measurements during well abandonments provide information about static water levels. The static water level is about 985 feet in elevation near the Ramsey County border in West St. Paul, and 855 feet north of Sunfish Lake in Mendota Heights. Flow direction is to the south. The Platteville Formation occurs discontinuously in Burnsville, Apple Valley, Lakeville, and Rosemount, where several drillers' records report Platteville as dry. Several domestic wells around Crystal Lake have used the Platteville or underlying St. Peter, but the static water levels indicate that the formations recharge from the lake. Without such local recharge, the Platteville Formation is not a reliable source of water in this area. The Glenwood Formation, a confuting bed, separates the Platteville from the underlying St. Peter aquifer. This shale is less than 5 feet thick and is not mentioned in many drillers' logs. South and east of Castle Rock, the Platteville is generally dewatered, except where a perched water table occurs above the Glenwood. In that area, springs indicated on the data base map (Plate 1) occur at elevations of 965 to 1000 feet along the base of the Platteville. ST. PETER AQUIFER The St. Peter Sandstone is poorly cemented and granular (Plate 2). Wells completed in it usually require screening. The aquifer is widely used for domestic wells in the northern part of the county. It also is used in combination with the Prairie du Chien in high -capacity wells of less than 150 gpm (gallons per minute), including some older public supply wells. Construction of new multiaquifer wells is no longer permitted under the state well construction code (Plate 9), but many existing multiaquifer wells are still used. In general, ground -water flow directions in the St. Peter are similar to the underlying Prairie du Chien -Jordan aquifer. Local recharge is greatest where the aquifer is covered by sand, and not covered by either Glenwood shale or thick till. Lakes overlying the St. Peter may enhance local recharge. Shale at or near the base of the St. Peter Sandstone can retard infiltration downward into the Prairie du Chien. The shale may be thin or discontinuous in Dakota County, partly because the top of the Prairie du Chien is an uneven erosional surface. The shale layer has been recorded in drillers' logs in South St. Paul, Mendota Heights, and Inver Grove Heights. Static water levels recorded while drilling through the St. Peter and into the deep aquifers in Mendota Heights and South St. Paul confirm that the St. Peter has a static water level 10 to 30 feet higher than the Prairie du Chien in this area. In the area of Mendota Heights, South St. Paul, and Inver Grove Heights, the St. Peter is nearly continuous, together with extensive overlying Platteville and Glenwood. This area has high hydraulic gradient (slope of the potentiometric surface) because of the low elevations of the discharge boundaries in the valleys of the Mississippi and Minnesota Rivers. The difference in head between the St. Peter and the Prairie du Chien is probably increased by the high hydraulic gradients in both aquifers. In western Dakota County, drillers report shaly layers at various intervals in the St. Peter, not just at the base. Static water levels range from the same as in the underlying Prairie du Chien to as much as 40 feet higher. Areas of greatest head difference are local recharge areas to the St. Peter, such as the vicinity of Crystal Lake, Lakeville, and the Murphy-Hanrahan Park Reserve. Areas of no head difference indicate possible absence of confining shale beds, or flow around them where they are breached by sand -filled bedrock valleys. Records of St. Peter wells are too sparse in the remainder of the county to make inferences about the static water levels. The St. Peter probably is dry in parts of Castle Rock, Hampton, and Douglas Townships. PRAIRIE DU CHIEN-JORDAN AQUIFER The Prairie du Chien -Jordan aquifer is mapped on 31° this Plate. It is the major high -capacity aquifer for Dakota County, and is absent only in the buried bedrock valleys north of Pine Bend, south of Hastings, and in the Mississippi River valley (Plate 2). The Prairie du Chien -Jordan aquifer is composed of two units with different hydrologic characteristics. They act as one aquifer over a large region because there is no confining bed between them. The Prairie du Chien Group 13 N is a thin- to thick -bedded sandy dolomite in which water flows along joints, fractures and bedding planes. Calculations from an injection test of the Prairie du Chien in West St. Paul showed that these characteristics result in a high rate of mixing and dispersion of solutes in the ground water (Reeder, 1976). Vertical hydraulic conductivity, transmissivity, and yield increase where there is vertical jointing and fracturing. This allows more rapid recharge from the surface or overlying aquifers. Where the aquifer is at depth, yields of wells that penetrate only the upper part of the Prairie du Chien are unpredictable. This is because Every possible effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Geological Survey in St. Paul. In addition, every effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering -scale decisions without site -specific verification. 1 125 000 4 6 8 10 MILES 2 0 2 4 6 8 10 KILOMETERS I they depend on chance intersection of productive joints and bedding planes. The mapped yield is based only on wells that penetrate both the Prairie du Chien Group and the Jordan Sandstone. The Jordan Sandstone is a medium- to thick -bedded quartzose sandstone, which may have shaly beds at its base. Flow is primarily intergranular, and so transmissivity and yield are more directly related to saturated thickness than in the Prairie du Chien (Ruhl and others, 1983). In general, variations in hydraulic conductivity, transmissivity, and yield of the aquifer are chiefly the result of variations in the Prairie du Chien. The Jordan is of fairly constant thickness, possesses fairly uniform hydraulic properties, and is generally confined. In contrast, the Prairie du Chien ranges in saturated thickness from zero to 250 feet; it contains the water table in the eastern part of the county, and the top of the dolomite is an erosional surface. ST. LAWRENCE-FRANCONIA AQUIFER In northeastern Dakota County the St. Lawrence shaly dolomite serves as a low -yield (<50 gpm) aquifer which is used for domestic wells. It has also been used to supplement the yield of the Prairie du Chien -Jordan aquifer in multiaquifer wells. In other parts of the county, the St. Lawrence Formation acts as a leaky confining bed between the Jordan and the Franconia. The St. Lawrence Formation, ranging in saturated thickness from 30 to 100 feet, grades into the underlying shaly sandstone of the Franconia, having 95 to 225 feet of saturated thickness. They may be indistinguishable, in which case they are called the St. Lawrence -Franconia aquifer and have a combined saturated thickness of 175 to 275 feet. This aquifer has a low to moderate yield (<200 gpm) and is typically used to supplement the overlying Prairie du Chien -Jordan or underlying Ironton -Galesville and Mt. Simon -Hinckley aquifers. The St. Lawrence - Franconia extends throughout the county, except at the east end of the buried bedrock valley in Ravenna and Marshan townships. The head differences between the Prairie du Chien - Jordan and St. Lawrence -Franconia are minor, usually less than 30 feet. In contrast, the static water level in the Mt. Simon is as much as 100 feet lower than in the St. Lawrence so that a multiaquifer well open between the St. Lawrence and Mt. Simon recharges the latter. In Dakota County this has occurred at the high -capacity wells in South St. Paul, Pine Bend, and Burnsville. Static water levels in the St. Lawrence -Franconia were available primarily from domestic wells in the southeast corner of Inver Grove Heights and near Hastings, where the Prairie du Chien -Jordan is thin or missing. Industrial wells near Pine Bend and irrigation wells in Marshan and Vermillion Townships that provided static water levels are multiaquifer wells, so that their reported static water levels are less exact. Ground water in the St. Lawrence -Franconia aquifer recharges from the Prairie du Chien -Jordan southwest of Vermillion and flows to the northeast. It discharges into the Mississippi River and the bedrock valleys north of Pine Bend and south of Hastings. Ground water flows from the St. Lawrence -Franconia into the Prairie du Chien -Jordan near the bedrock valley, and directly into the Quaternary deposits in the bedrock valley where the Prairie du Chien -Jordan is absent. IRONTON-GALESVILLE AQUIFER The Ironton and Galesville Sandstones have a combined thickness of only about 50 feet in Dakota County, and so this aquifer has been used almost exclusively in high -capacity multiaquifer wells. There is only one drillers' record of a well open only to the Ironton -Galesville aquifer. Ruhl and others (1982) provide the most recent description and maps of the aquifer in southeast Minnesota. The Eau Claire Formation underlies the Galesville. It is a confining bed about 200 feet thick, consisting of shale interspersed with low -yielding layers of shaly sandstone. MT. SIMON-HINCKLEY AQUIFER The Mt. Simon -Hinckley is the deepest high -yield aquifer available to Dakota County. It underlies the entire county and, under natural conditions, is hydraulically isolated from the Prairie du Chien -Jordan aquifer. The Hinckley Sandstone may be absent or indistinguishable from the Mt. Simon in Dakota County, so the aquifer usually referred to as the Mt. Simon may include the Hinckley. The saturated thickness of the combined formations ranges from 215 feet in South St. Paul to 255 feet in Burnsville. Specific capacity ranges from 10 to 20 gpm/ft. With an approximate storage coefficient of 10-4 (Norvitch and others, 1973), transmissivity ranges from 104 to 3x104 gallons per day per foot and yield ranges from 650 to 1800 gallons per minute. In Dakota County the potentiometric surface of the Mt. Simon slopes toward high -capacity wells in Minneapolis and the suburbs of Hennepin County. The static water level of the Mt. Simon -Hinckley is about 650 feet in elevation in Eagan and 708 feet in Vermillion. Near some multiaquifer wells in Burnsville, Pine Bend, and South St. Paul, the static water level in the Mt. Simon has been raised to the level in the Franconia. Such artificial recharge from overlying aquifers changes the flow direction locally and may modify the natural water chemistry. REFERENCES Maderak, M.L., 1963, Quality of Waters, Minnesota: A Compilation, 1955-62: Minnesota Conservation Department, Division of Waters Bulletin 21, 104 p. Miller, R.T., 1982. Appraisal of the Pelican River sand -plain aquifer, western Minnesota: U.S. Geological Survey Open - file Report 82-347, 44 p., 3 pls. Minnesota Department of Health, Division of Environmental Health, 1977, 1989, Public Water Supply Data, Biennial Series. Minnesota Department of Natural Resources, Division of Waters, Observation Well Data, Annual Summary, 1983- 1988. Norvitch, R.F., Ross, T.G., and Brietkrietz, A., 1973, Water resources outlook for the Minneapolis -St. Paul Metropolitan Area, Minnesota: U.S. Geological Survey and Twin Cities Metropolitan Council, 219 p. Reeder, H.O., 1976, Artificial recharge through a well in fissured carbonate rock, West St. Paul, Minnesota: U.S. Geological Survey Water -Supply Paper 2004, 80 p.. Ruhl, J.F., Wolf, R.J., and Adolphson, D.G., 1982, Hydrogeologic and water -quality characteristics of the Ironton -Galesville aquifer, southeast Minnesota: U.S. Geological Survey Water -Resources Investigations Report 82-4080, 2 pls. 1983, Hydrogeologic and water -quality characteristics of the Prairie du Chien -Jordan aquifer, southeast Minnesota: U.S. Geological Survey Water -Resources Investigations Report 83-4045, 2 pls. Walton, W.C., 1970, Ground -water resource evaluation: McGraw-Hill, 664 p. GEOLOGIC ATLAS OF DAKOTA COUNTY, MINNESOTA UNIVERSITY OF MINNESOTA MINNESOTA GEOLOGICAL SURVEY Priscilla C. Grew, Director Prepared and Published with the Support of the DAKOTA COUNTY SOIL AND WATER CONSERVATION DISTRICT AND BOARD OF COMMISSIONERS AND MINNESOTA DEPARTMENT OF NATURAL RESOURCES, WATERS DIVISION R COUNTY ATLAS SERIES ATLAS C-6, PLATE 7 OF 9 POLLUTION SENSITIVITY LOCATION DIAGRAM T 27N R24V R23VI T 28 N T 28 N p R 21 `J T 113 N T 112 N 1 Airport aott - .akes ! ' C1 SENSITIVITY OF THE PRAIRIE DU CHIEN-JORDAN AQUIFER TO POLLUTION By Howard C. Hobbs 1990 MATRIX RATING SENSITIVITY OF THE PRAIRIE DU CHIEN—JORDAN AQUIFER (WHERE ABSENT, FIRST BEDROCK AQUIFER) BEDROCK CONFINING LAYERS ABOVE AQUIFER COMPOSITION AND THICKNESS OF MATERIAL OVERLYING BEDROCK Group 1 Greatest drift protection Loam to clay loam till <50 ft 50-100 ft >100 ft Group 2 Moderate drift protection Sandy loam till, glacial lake sand, colluvium, thick alluvium <50 ft 50-100 ft >100 ft Group 3 Minimal drift protection Chiefly sand and gravel <50 ft Decorah, Glenwood, base of St. Peter Base of St. Peter only LM No bedrock confining layer HM LM LM HM 50-100 ft >100 ft LM Group 4 No drift protection Terraces <10 ft Other <5 ft L L M HM LM Group 1 includes surficial mapping units (Plate 3) pkt, dt, and dtp. Group 2 includes unit dto south of Chub Creek, unit fd in the Minnesota and Mississippi River valleys, and units st, dst, sls, psd, and wc. Group 3 includes unit dto north of Chub Creek, terrace deposits thicker than 10 feet, and units ws, si, dsi, dso, do pko, and pso. Group 4 areas are shown on Plate 3 as a speckled pattern on terrace units and as unit b for near -surface bedrock. R18W R17 T115N T 113 N R 20 W Base modified from U.S. Geological Survey, Faribault, Hastings, and St. Paul, 1985. INTRODUCTION This plate shows areas that have different degrees of susceptibility to ground -water pollution. The ratings are based on characteristics of rock and sediment known to overlie the Prairie du Chien -Jordan bedrock aquifer. This aquifer underlies almost all of the county, and is the most heavily used source of ground water in the county. Where this aquifer has been removed by erosion, the map shows the susceptibility of the uppermost bedrock aquifer. However, the same criteria may be used to rate the pollution susceptibility of the St. Peter, Platteville, and drift aquifers. This may be done by the use of Plates 2, 3, and 4 of this atlas, and the appropriate matrix in the box below. This map is derived from information from other maps in the atlas, and from the data bases described on Plate 1. The various natural bodies of geologic material found in the county (Plates 2, 3, and 4) were classified according to their estimated permeability and then rated as shown in the matrix. Susceptibility to pollution can be rated on the basis of the ability of geologic material to 1) absorb and hold contaminants; 2) transform contaminants into benign substances; 3) dilute con- taminants to levels below some standard; or 4) control the rate that contaminated water flows to or through aquifers. The ratings on the map on this plate were based on the fourth of these: the estimated travel time for water-soluble, geologically inert contaminants released at the surface to reach the Prairie du Chien -Jordan aquifer. The boundaries between map units do not portray absolute differences in ground -water susceptibility. The map units themselves are not absolute; each portrays the predominant rating within its boundaries. A common folly is to enlarge, sometimes greatly, an existing map. Simply expanding this map will'falsely represent the complexity and susceptibility at any particular site. Projects that require more detail than is shown here require additional site -specific investigation. Water is called the universal solvent because it can dissolve many substances. Water can also transport suspended solids and immiscible liquids. Each of the large variety of contaminants moves to and through ground -water systems in different ways. Hence, these susceptibility ratings are not contaminant specific; no workable susceptibility rating system can specifically address all known contaminants. This map ignores changes in susceptibility caused by human activities. Improperly constructed or abandoned water wells offer ways for contaminants to enter aquifers. Thus, the numbers (Plate 1) and construction (Plate 8) of active and abandoned water wells can increase the threat to ground -water systems. Proper construction of new wells and sealing of abandoned wells are especially important in areas of otherwise low natural susceptibility. In these areas poorly constructed wells can ruin an otherwise protected source of water. Large withdrawals of ground water may change the directions and rates of ground -water movement. Improper mcthods of using and disposing of hazardous substances at the surface increase the danger of contaminants entering ground- water systems. RESIDENCE TIME OF GROUND WATER Susceptibility of ground water to pollution is best understood if it is rated relative to the age of the ground water. The age of the water — or its residence time — is the length of time that elapses from when a drop of water soaks into the ground until it is discharged or pumped from an aquifer. The rates of vertical and horizontal movement of ground water vary, depending on local geologic conditions. Water may have entered the aquifer along many different pathways; some pathways require longer travel times than others. Radiometric dating using isotopes of carbon and hydrogen is an effective means for determining the residence time of ground water in an aquifer (Alexander and Alexander, 1989). Residence times of years to at most a few decades were obtained for the Prairie du Chien -Jordan and shallower aquifers in Washington County. Ages greater than a few centuries were obtained only for water pumped from deeper aquifers. Alexander and Alexander (1989) found that significant nitrate contamination was confined to relatively young water. Preliminary results from a study in Dakota County showed evidence of recent recharge to the prairie du Chien -Jordan aquifer but not in all wells. At the time of this writing, only six wells have been sampled (Robert Tipping, personal communication). Such a small number of samples is inconclusive, but is consistent with the results obtained from Washington County. R 19 W Most ground water in Dakota County probably enters aquifers through areas assigned Very High and High ratings. Water infiltrating through less sensitive areas reaches aquifers at a slower rate and in smaller amounts. Short-term protection of ground -water quality in Dakota County can be accomplished by controlling contaminant sources in sensitive areas. However, because ground water moves laterally for considerable distances, only a regional effort can provide long-term protection. THE SUSCEPTIBILITY MATRIX Water moves generally downward from the surface to the Prairie du Chien -Jordan aquifer in Dakota County. From the surface to the top of the saturated zone (water table), the percolation is almost vertical, controlled by gravity. Upon reaching the water table the ground water moves mostly in a horizontal direction, but it also continues to move downward into the Prairie du Chien -Jordan aquifer. This downward component is controlled by the discharge of ground water from the aquifer into the valleys of the Mississippi, Minnesota, Vermillion, and Cannon Rivers. These discharge areas are lower than the top of the Prairie du Chien -Jordan aquifer. The relative travel time of contaminants to the Prairie du Chien - Jordan aquifer was estimated by considering the number and effectiveness of confining layers between it and the surface at any point, the depth to bedrock, and the composition of overlying glacial deposits. Where the aquifer has been removed by erosion, the susceptibility of the uppermost remaining bedrock aquifer is shown. The Decorah Shale is the best confining layer over the Prairie du Chien Group. However, it occurs only in small areas in the northern part of the county (Plate 2), and therefore does not protect most of the Prairie du Chien -Jordan aquifer. Next in degree of effectiveness is the shale of the Glenwood Formation present beneath the Platteville Formation across a good portion of the county. The least effective confining unit is the base of the St. Peter Sandstone, which contains a series of discontinuous, overlapping shale beds. The occurrence and effectiveness of this confining layer cannot be reliably predicted, although it is effective in places, as shown by higher water levels in some St. Peter wells than in nearby Prairie du Chien wells. In general, the thicker the overlying unconsolidated sediment, the better the protection for bedrock aquifers. Where less than 5 feet of sediment overlies bedrock in Dakota County, bedrock is mapped on Plate 3. These areas are considered to have no drift protection, regardless of sediment type. Terrace deposits are so permeable that, where they are less than 10 feet thick, they also afford no protection to the bedrock. Thicker terrace deposits and glacial drift units were divided into depth increments of less than 50 feet, 50 to 100 feet, and greater than 100 feet. The drift units from the surficial map (Plate 3) are grouped into three classes, based on their expected average permeability. Group 1, comprising tills with a loam to clay loam texture, contains the least permeable material. Water moves very slowly through this material, at least below the upper zone containing joints and burrows. Group 2 consists of moderately permeable materials, including sandy tills, glacial lake sediments, and colluvium. Materials in this group show a wide range of permeability, but are intermediate between materials of Group 1 and Group 3. Group 3 consists of outwash, ice -contact, and terrace deposits thicker than 10 feet, which are primarily sand and gravel, and slopewash sediment, which is primarily sand. These materials are very permeable. Exceptions and Complications Organic deposits are excluded from the matrix. Little information as to their thickness and composition is available, but it is assumed that they are relatively thin in Dakota County. Organic areas are classified by the assumed underlying material, which in most places is in Group 3 (minimal protection). Organic deposits may offer some protection, but not enough to change the rating. However, where they occur directly over Prairie du Chien dolomite, they reduce the rating from Very High to High. Similarly, loess (stipple pattern on Plate 3) was not used in the matrix. The relatively thin loamy till over outwash sand and gravel (dto on Plate 3) is included in Group 3 north of Chub Creek. The till cover in that area is very thin and discontinuous. It is included in Group 2 south of Chub Creek, where the till cover is thicker and almost continuous. This distinction changes the rating where the drift cover is generally greater than 50 feet: Groups 2 and 3 are rated High in areas where the drift is less than 50 feet, and no bedrock confining units are present above the aquifer. T 112 N RAN o SCALE 1:100 000 0 1 2 3 4 5 6 7 8 9 10 1 0 1 2 3 4 5 5000 0 5000 25000 FEET Alluvium (fd) is rated in Group 2 in the valleys of 'the Mississippi and Minnesota Rivers, where it can be as thick as 100 feet. In all other valleys, alluvium is assumed to be too thin to materially affect the ratings, and areas of it are rated the same as the material assumed to be under it, which in most places is outwash (Group 3). Geologic Complexity of the Glacial Drift The distribution of surficial materials is known in fairly good detail from soil maps. However, the surficial map units are known chiefly from shallow excavations, which do not tell us much about the lower part of the drift. In some places, the entire drift section is made up of the same material that is mapped at the surface, but in most places, the drift is more complex. The cross sections on Plate 3 represent some of this complexity, but not all of it, both for reasons of scale and the limitations of the data. For example, in the bottom row of the matrix showing ratings where no bedrock confining layer overlies the aquifer, sediments of Group 1 decrease in sensitivity from High -Moderate to Low - Moderate with increasing depth to bedrock. This is based on the observation that sediments of Group 1 are generally on bedrock highs, and are not underlain by sediments of higher permeability. Where the surficial deposits offer only moderate protection (Group 2), it appears unlikely that much retardation of flow will occur where the drift is less than 50 feet thick. Of course, in places where the drift is almost 50 feet thick the real sensitivity will be lower than the sensitivity in areas where the depth to bedrock is scarcely more than 5 feet. The ratings are intended to be fairly conservative; a given combination is rated according to its most sensitive part. Where the drift is 50 to 100 feet thick, Group 2 is rated High - Moderate. This lower rating reflects both a greater thickness of moderate permeability and a greater possibility that Group 2 sediments are underlain by sediments of Group 1. The opposite situation is also possible; Group 2 sediments may be underlain by sediments of Group 3, in which case greater depth to bedrock offers little additional protection. For this reason, the ratings must be considered approximate. Similar reasoning applies to the Moderate rating for Group 2 sediments in areas of drift thicker than 100 feet. In areas where the surface sediment offers minimal protection (Group 3), both the less than 50 and the 50-100 groups are rated High. Only where the drift is thicker than 100 feet is the sensitivity reduced to High -Moderate to reflect the possibility that buried layers of lower permeability may be present. But in part of this area, the rating was raised to High, as explained below, based on similar water levels in the outwash and in the underlying Prairie du Chien - Jordan aquifer. Drift thickness greater than 100 feet was not subdivided. In most of the county, drift much thicker than 100 feet is restricted to buried valleys, which are largely filled with sand and gravel. This material provides little additional protection. Adjustment Based on Water Levels In most of the area rated High in the central part of Dakota county, the water table in the Quaternary aquifer (so and dso, Plate 3) is no more than 20 feet higher than the potentiometric surface of the Prairie du Chien -Jordan aquifer. This close relationship means that the outwash and the bedrock function as a single aquifer. Some of the outwash area mapped High would have been rated from the matrix High -Moderate or Moderate, due to drift thickness greater than 100 feet or confining beds between the outwash and the Prairie du Chien -Jordan aquifer. But in areas where the water levels did not demonstrate a separation between aquifers, the susceptibility was revised upward to High, regardless of the ratings indicated by the matrix. The area revised upward roughly coincides with the main body of the outwash. Only where the two water surfaces differ by more than 20 feet, or where they have a different shape or slope, can a definite separation be demonstrated. Lines of equal potentiometric pressure are drawn from widely spaced water -level measurements (shown on Plates 5 and 6), and each point contains some uncertainty of land elevation (plus or minus 5 feet, generally). In addition, measurements were taken at different times, and were not corrected for seasonal and multi -year fluctuations of water level. So there may be some aquifer separation that could not be documented. Nitrate data partially support classification of the Rosemount outwash plain as uniformly High. Almost all the wells tested in the outwash plain east of Highway 52 show detectable nitrates, though generally within drinking -water standards. These include both drift R 17 W Cartography by Philip Heywood. N T Every possible effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Geological Survey in St. Paul. In addition, every effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering -scale decisions without site -specific verification. wells and Prairie du Chien -Jordan wells. Fewer wells were tested west of Highway 52. Most of them show no detectable nitrates. In this area, till and bedrock confining layers may be effective, though their influence is not shown in contrasting water levels. INTERPRETING THE MAP Most adjacent boxes in the matrix have ratings that are one step apart. Some adjacent boxes are rated the same, and others are rated two steps apart. For example, the presence of the Glenwood and Decorah confining beds lowers the estimated sensitivity ratings by two steps. Most adjacent areas on the map are thus only one or two steps apart. Greater contrasts occur where the depth to bedrock changes over a short distance and intermediate ratings are omitted. Similarly, a change in material at about the same place as a change in depth to bedrock may amplify a ratings change, when both are in the same direction, or reduce it when they are in opposite directions. In places where the boundaries of bedrock confining layers intersect lines of drift thickness or boundaries of surficial mapping units, a pattern commonly develops, as shown in Figure 1. This consists of a highest, a lowest, and two intermediate ratings, all meeting at the point of intersection. This probably does not represent the real -world situation, but is a consequence of the system used for drawing the map boundaries. Where adherence to HM R 16 W HM NOTE TO USERS OF THIS MAP The information on this plate is a guide for assessing the risk that particular activities and land uses present to ground -water quality. The map was prepared using assumptions discussed in the text. Different assumptions based on more or better data could change the maps considerably; therefore, the user should recognize the subjective nature of the map and use it accordingly. The classifications are not absolute. High susceptibility does not indicate that water quality has been or will become degraded; low susceptibility does not guarantee that ground water will remain pristine. SENSITIVITY RATINGS Estimated travel time for water -borne surface contaminants to reach the aquifer Very High Hours to months High Weeks to years HM High -Moderate Years to a decade Moderate Several years to decades LM Low -Moderate Several decades Low Several decades to a century Very Low More than a century MAP SYMBOL Areas where first bedrock is the St. Lawrence, Franconia, Ironton, or Galesville formation the matrix results in map units too small to show at this scale, only the dominant rating is mapped. The ratings scheme used in this atlas can be applied to a small area at a larger scale, if enough additional data are available, or can be obtained. The bedrock confining units can then be mapped more precisely, as can the surficial materials and the depth to bedrock. Then these maps can be combined, using the same matrix, to produce a more detailed sensitivity map. REFERENCE CITED Alexander, S.C., and Alexander, C.E., Jr., 1989, Residence times of Minnesota groundwaters: Minnesota Academy of Sciences Journal, v. 55, no. 1, p. 48-52. Figure 1. Intersections of sensitivity -rating criteria. RATING SENSITIVITY OF SHALLOWER AQUIFERS MATRIX FOR RATING THE SENSITIVITY OF THE ST. PETER AQUIFER BEDROCK CONFINING LAYERS ABOVE ST. PETER GLACIAL DRIFT PERMEABILITY AND THICKNESS Group Greatest <50' 1 materials drift protection 50-100' >100' Group Moderate <50' 2 materials drift protection 50-100' >100' Minimal <50' Group 3 drift protection 50-100' >100' Group 4 No drift protection <10' materials <5' Decorah and Glenwood L LM L LM LM L LM Y __ Glenwood only M LM L M LM L HM HM M y I.° , No bedrock confining layer f1M ' ' M LM H HM M H H HM MATRIX FOR RATING THE SENSITIVITY OF THE PLATTEVILLE AQUIFER BEDROCK CONFINING LAYER ABOVE PLATTEVILLE GLACIAL DRIFT PERMEABILITY AND THICKNESS Group Greatest <50' 1 materials drift protection 50-100' >100' Group Moderate <50' 2 materials drift protection 50-100' >100' Group Minimal <50' 3 materials drift protection 50-100' >100' Group 4 No drift <10' materials protection <5' Decorah Shale LM L M LM L M M LM HM 1111.111111111 that of more sensitive rating bedrock aquifer. same by the to relate that is, of topographic Plate 5. be no more factor Chien -Jordan as HM.. the with But way, full the the the The well, layer No bedrock confining y =" M"`W M LM H = M M H H HM This map shows areas having varying degrees of risk that the sensitivity of the Platteville should not be lower than surface events could degrade the county's principal bedrock aquifer. St. Peter at the same place, and the Platteville is never Similar maps of overlying aquifers can be constructed for a given than the drift above it. location by using the relevant matrix. Assessing the sensitivity of drift aquifers can be attempted For the Platteville and St. Peter aquifers, first trace the area of the use of the bottom row in the matrices used for the respective aquifer from the bedrock map (Plate 2). Within the aquifers, where no bedrock confining layer overlies the area of each aquifer, trace the overlying bedrock confining layer(s) the drift thickness groupings cannot be used in the from Plate 2. Then trace lines separating surficial materials of inasmuch as drift wells are not generally protected different permeability from the map on Plate 4. The resulting map thickness of the drift. A workable solution would be will have many areas defined by the lines you have drawn. Label depth intervals to the thickness of the unsaturated zone; each area according to the appropriate matrix, and simplify by difference between the surface elevation and the elevation absorbing very small areas into adjacent larger areas. The water table. A generalized map can be constructed from adjustment of ratings based on relative water levels is shown on this maps (see index on Plate 1) and the water table map on plate for the Prairie du Chien -Jordan aquifer. Any area rated High map of the thickness of the unsaturated zone can on the map above must be rated High on your St. Peter or Platteville accurate or detailed than the water table map. One simplifying map, inasmuch as the Prairie du Chien -Jordan aquifer is unlikely to is that all areas rated High on the Prairie du be more sensitive to pollution than any overlying aquifer. Similarly, susceptibility map will have High drift susceptibility reducing the area where a separate drift map may be wanted. GEOLOGIC ATLAS OF DAKOTA COUNTY, MINNESOTA UNIVERSITY OF MINNESOTA MINNESOTA GEOLOGICAL SURVEY Priscilla C. Grew, Director Prepared and Published with the Support of the DAKOTA COUNTY SOIL AND WATER CONSERVATION DISTRICT AND BOARD OF COMMISSIONERS AND MINNESOTA DEPARTMENT OF NATURAL RESOURCES, WATERS DIVISION R 22 W COUNTY ATLAS SERIES ATLAS C-6, PLATE 8 OF 9 WELL CONSTRUCTION T 115 LOCATION DIAGRAM T 27 N 21 '' T 113 N GEOLOGY AND WELL CONSTRUCTION By James F. Walsh and Bruce M. Olsen Minnesota Department of Health T 27 N Jenne Laqtfirt 1990 EXPLANATION Platteville Formation FOR.,":?,°;°, Permitted Not permitted for potable supply R 18 W Prairie du Chien Group Prairie du Chien absent Permitted Not permitted for potable supply \R17W T115N 36 sf R 16 W 2,' Base modified from U.S. Geological Survey, Faribault, Hastings, and St. Paul, 1985 2 0 2 0 SCALE 1:125000 4 6 2 4 4 H 6 8 R160AI DAKy1TA CO; 12 N 8 10 MILES 1 i 10 KILOMETERS I uesbv R18W CRITERIA FOR WATER -WELL CONSTRUCTION AND SEALING IN APPROVED METHODS OF NEW WELL CONSTRUCTION WELL CASING SHOULD BE GROUTED TO THE AQUIFER USED AND ALL OTHER FORMATIONS SEALED OFF A NOMINAL 2 INCHES OF CEMENT GROUT MUST BE PUMPED BETWEEN THE BOREHOLE WALL AND THE CASING OR BETWEEN STRINGS OF CASING OUTER CASINGS MUST BE AT LEAST 4 INCHES LARGER IN DIAMETER THAN INNER CASINGS DECORAH SHALE OR UNCONSOLIDATED DEPOSITS F T PLATTEVILLE I I I I I AQUIFER I , -I BASAL ST. PETER SHALE (CONFINING LAYER LOCALLY) L PUMPING LEVEL- CASING EXTENDS AT 6- LEAST 10 FEET BELOW PUMPING LEVEL PRAIRIE DU CHIEN AQUIFER ALL CASINGS MUST EXTEND TO 1 FOOT ABOVE LAND SURFACE CASING MUST BE GROUTED AT LEAST 10 FEET BELOW THE PUMPING LEVEL IN PRAIRIE DU CHIEN WELLS CASING SHOULD BE GROUTED AT LEAST 15 FEET INTO ANY AQUIFER USED FRANCONIA/ IRONTON- GALESVILLE WELL CASING EXTENDS AT LEAST 15 FEET ---- INTO JORDAN CASING EXTENDS AT LEAST 15 FEET INTO FRANCONIA CASING EXTENDS AT LEAST 15 FEET INTO MT. SIMON /?/7-7////7///7/ 7 OLDER BEDROCK WITH UNDETERMINED HYDROLOGIC PROPERTIES /////////////////// BEDROCK AQUIFERS BELOW THE ST. PETER SANDSTONE HOW IMPROPERLY CONSTRUCTED WELLS CONTAMINATE GROUND WATER Proper Sealing of Abandoned Wells is Essential 1 WHEN SEALING ABANDONED WELLS, UNGROUTED 3 CASING IN CONFINING LAYERS MUST BE REMOVED OR OPENED TO ALLOW GROUT TO PROPERLY SEAL OFF THESE FORMATIONS 2. ALL DEBRIS MUST BE REMOVED FROM WELL PRIOR TO SEALING WELLS MUST BE SEALED BY PUMPING GROUT OR OTHER APPROVED MATERIALS SO THEY ARE IN DIRECT CONTACT WITH CONFINING LAYERS TREMIE PIPE MUST FIRST BE LOWERED TO THE BOTTOM OF THE WELL MODIFIED OPEN DECORAH SHALE OR BOREHOLE ACROSS UNCONSOLIDATED CONFINING LAYERS CONTAMINATION ENTERS DEPOSITS / RUST HOLE IN CASING 'PLATTEVILLoE- ' AQUIFER CONTAMINATION FROM NEARBY WELLS WATER y1 LEVEL UNGROUTED LINER ACROSS A CONFINING LAYER UNGROUTED TELESCOPING CASING D /SURFACE WATER ri ENTERS WELL I ALONG CASING / DOWNWARD MOVEMENT OF WATER GLENWOOD CONFINING LAYER RUST HOLE ,UNGROUTED IN CASING CASING BASAL ST. PETER (CONFININGLOCALLY)YER PRAIRIE DU CHIENAQUIF JORDAN SANDSTONE AQUIFER ST. LAWRENCE CONFINING LAYER SECTIONS OF PUMP DROPPED INTO WELL AND LODGED IN THE OPEN BOREHOLE FRANCONIA/ IRONTON-GALESVILLE AQUIFER EAU CLAIRE CONFINING LAYER MT. SIMON AQUIFER 1. o0 o WATER LEVEL UNGROUTED CASING THROUGH A CONFINING LAYER WATER MOVES DOWNWARD ALONG CASING CASING OVERLAPPED BUT NOT GROUTED CASING NOT PROPERLY SEALED INTO CONFINING LAYER CASING NOT PROPERLY SEALED INTO CONFINING V LAYER 7 / /f////j/h/7//%T/ OLDER BEDROCK WITH UNDETERMINED HYDROLOGIC PROPERTIES //////////////////// BEDROCK WELLS Wells that penetrate only the uppermost bedrock aquifer must have the casing firmly sealed by using a hardened steel collar called a drive shoe or by cement grouting. Wells that penetrate more than one bedrock aquifer must have all casings grouted, and the finished casing should extend at least 15 feet into the aquifer being used. Wells completed in bedrock aquifers must not be extended into underlying confining layers. For example, wells completed in the Platteville Formation must not extend into the Glenwood Formation, because it hydrologically separates the Platteville and the St. Peter Sandstone. The Platteville Formation and the Prairie du Chien Group are karsted bedrock aquifers. Ground water moves in them through complex passageways of solution -widened joints and cavities. There is little filtering of contaminants in karst aquifers, and they are very susceptible to contamination (Plate 6). The map shows where the Platteville Formation and the Prairie du Chien Group may not be used for potable supply because the minimum 50 feet of cover within one mile of the well is lacking. In areas where the static water level is within 10 feet of the top of the Platteville or the Prairie du Chien, wells using either must have casing installed at least 10 feet below the pumping level and must be cement grouted. This requirement improves the sanitary condition of the well by preventing, or at least reducing, the direct infiltration of contaminants caused by pumping. GROUTED WELLS Wells drilled through bedrock confining layers must have the casing sealed into the borehole with cement grout. This ensures that the well does not interconnect aquifers along the annulus between the casing and the borehole. New wells and reconditioned wells that have altered casings may use only one aquifer. Well casings should be installed at least 15 feet into any aquifer below a bedrock confining layer. Figures B and C summarize the methods of well construction for properly completing wells in specific bedrock aquifers and show previous method $ that are no longer allowed. IMPROPERLY CONSTRUCTED WELLS No countywide standards for the type of casing or other materials used to construct wells were enforced before the Minnesota water -well code was implemented in 1974. Well -drilling methods and construction practices were not standardized according to geologic conditions. Exceptions to this general lack of regulation were the standards for municipal wells enforced by the Minnesota Department of Health. Some communities had well -permitting programs, but the level of inspection for new wells was not always consistently thorough. In short, most well -construction practices before 1974 were directed by cost, rather than by the need to protect ground -water resources. Ungrouted wells provide pathways for contaminated ground water to pass through bedrock confining layers into deeper aquifers. The use of multiple casings that were neither grouted into the borehole nor grouted between each other was common in wells constructed before 1974. Smaller casings were sleeved into larger casings with only a small section of overlap, so that the resulting construction has the configuration of an extended telescope. This is not permitted because such casings cannot be adequately sealed to prevent the movement of water between them. Multiple strings of casing in new wells must be separated from each other by at least two nominal inches of annular space to accommodate cement grout. Each casing must extend to the land surface and the grout must be installed under pressure. Other wells were cased only to the uppermost bedrock and left as open boreholes to the bottom. Many high -capacity wells, and even domestic wells in some areas, were constructed in this way and connect two or more bedrock aquifers. Some of these wells were later reconstructed because soft sandstone and shale caved in and interfered with pumping. The downward movement of ground water caused by the interconnection of aquifers is a cause of borehole erosion and subsequent well failure. Sections of casing that were installed in eroding areas to remedy this situation were rarely grouted in place. n17W Cartography by Richard B. Darling 113 N Every possible effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Geological Survey in St. Paul. In addition, every effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering -scale decisions without site -specific verification. INTRODUCTION Residents of Dakota County obtain their drinking water from the ground. Wells are used by all public water supplies and most single family residences. Furthermore, the county's major industries rely on ground water to supply their needs. Scores of high - capacity wells are used in the southern half of the county for irrigating row crops. Heavy industry and commercial enterprises dominate nonpotable use of ground water in the north. The future development of Dakota County is directly related to the availability and quality of this natural resource. The almost total dependence on ground water to supply Dakota County's water needs has resulted in the construction of thousands of water wells since settlement first began. Also, thousands of new wells have been drilled within the past decade as urban development from the Minneapolis -St. Paul area has extended into Dakota County. Most high -capacity wells draw water from bedrock aquifers, particularly the Prairie du Chien - Jordan or the Mt. Simon -Hinckley. However, some irrigation wells draw water from sand -plain aquifers in the Vermillion River and Cannon River valleys. Most domestic wells draw from the uppermost aquifer present, which varies throughout the county due to changes in the bedrock geology (Plate 2) and in the glacial geology (Plate 3). All aquifers have been used since the turn of the century, although the uppermost aquifer present locally is usually the one that is most heavily pumped. Large supplies of ground water exist (Plates 5 and 6) but are sensitive to contamination (Plate 7) from unwise land or water uses. The aquifers usually affected are those closest to the land surface, i.e., the ones now being used by most wells. Confining layers isolate deeper aquifers from surface contamination by preventing, or reducing, the rapid transfer of water from above. Water wells that are improperly constructed, maintained, or abandoned add to problems of ground- water contamination by serving as conduits for pollution to enter aquifers. Especially bad are multiple -aquifer wells that have open boreholes across confining layers. The goal of comprehensive land- and water -use planning is the effective use and protection of our natural resources. The proper construction of new wells, together with the sealing of abandoned or improperly constructed wells, is an essential element of any ground- water protection strategy. This plate describes the types of well construction and abandonment procedures now required in Dakota County and discusses problems associated with methods used before state regulations went into effect. WATER -WELL REGULATION The Minnesota Department of Health (MDH) was directed by state law in 1971 to license water -well contractors and to regulate the construction, repair, and sealing of wells. Previously, well -construction practices were unregulated and highly variable; they did not always protect ground -water resources or meet sanitation requirements. Well -construction standards were implemented in 1974 and revised by the Groundwater Protection Act, which was passed by the Minnesota Legislature in 1989. Dakota County received delegated authority from MDH to administer the state water -well code in 1989. A county water well construction and sealing ordinance was implemented that is based on the state well code but more fully addresses local geologic and ground -water conditions. County permits and site inspection are required for the construction of all new wells and for the reconstruction and sealing of existing wells. Both the state water well code and the Dakota County well construction and abandonment ordinance require that new wells must meet minimum standards of depth, isolation distances from sources of contamination, construction materials, and installation procedures. A water analysis is required to confirm potability (suitability for drinking). Licensing for well contractors requires proof of drilling experience and knowledge of the regulations governing well construction and sealing. Licensing is also required for repairing wells, installing monitor wells, drilling geologic and engineering test holes, and for constructing elevator shafts. Records of well construction and sealing must be submitted to the Dakota County Health Department. Property owners are permitted to install their own wells, but all regulations governing well construction still apply. WELL -CONSTRUCTION PRACTICES Variations in drilling conditions occur throughout Dakota County because of differences in the bedrock and Quaternary (glacial) geology . For most of the county, wells may be completed either in glacial sediment or in individual bedrock aquifers. Therefore, water -well construction depends on local geologic and hydrogeologic conditions. The data bases described on Plate 1 should be used in determining how a well should be constructed on a specific property. All new wells must be sited with respect to isolation distances from sources of contamination. Any future changes in the locations of such sources, such as the installation of a new on -site sewage disposal system, require that these isolation distances from the well still be maintained. Furthermore, all wells (including monitoring wells) that will be constructed through zones of contamination require special construction practices to ensure that they do not serve as conduits to spread contaminants to deeper ground -water supplies. UNCONSOLIDATED DEPOSITS Some wells do not use bedrock aquifers but rather obtain water from unconsolidated layers of sand and gravel. These wells are usually constructed with a slotted section of pipe or screen attached to the bottom of the casing, which prevents sand from being pumped into the well. Screened wells in unconsolidated aquifers need not be grouted with cement if (1) the casing is tightly sealed when driven into the borehole, or (2) the unconsolidated material, such as sand, collapses in around it, or (3) the annular space between the casing and the borehole can be sealed with fine-grained material such as clay to retard the vertical movement of potential contaminants (Fig. A). Cased wells that are entirely in unconsolidated deposits smd less than 50 feet deep must have double the normally required isolation distances from cesspools, subsurface disposal fields, manure storage piles, or similar sources of contamination penetrate at least 10 feet of impermeable material such as clay. Small -sized lots or existing features such as septic systems may dictate that the well be completed at a depth greater than 50 feet. A sand point well is a type of driven well that consists of a drive point attached to a screen. Sections of pipe are added as the point and screen are forced into the ground using a weight such as a fence -post driver. The pipe and screen form the well and are typically connected directly to the pump —a practice in compliance with the Minnesota well code only if the upper 10 feet of the well is cemented into an oversized casing at least 4 inches larger in diameter than the suction pipe. Sand point wells are usually installed by property owners rather than by water -well contractors. A record of their construction must be submitted to the Dakota County Health Department just as for any other type of water well. A CRITERIA FOR WELLS USING UNCONSOLIDATED DEPOSITS 1. WELL USING SAND AND GRAVEL AQUIFER MUST HAVE THE ANNULAR SPACE BETWEEN THE CASING AND BOREHOLE WALL SEALED BY DRIVING THE CASING OR BY FILLING THE SPACE WITH HEAVY DRILLING MUD, CEMENT GROUT, OR FINE-GRAINED MATERIAL. ANY ANNULAR SPACE REMAINING SHOULD BE GROUTED WITH NEAT CEMENT OR CONCRETE (ABOVE THE WATER LEVEL) USING A TREMIE PIPE AND PUMPING UNDER PRESSURE 2 SAND POINT WELLS ARE ALLOWED BUT REQUIRE GROUTED CONSTRUCTION IF THE CASING IS USED FOR THE SUCTION LINE FINE- GRAINED SCREENED WELLS SEALING MATERIAL • . v o•O / 'UNCONSOLIDATED' / DEPOSITS 7 WATER Q • LEVELO o 4 : o / /• /—iBOREHO0LE WALL • v .� . WATER LEVEL SAND OR GRAVEL AQUIFER • CASING . SCREEN OVERSIZED CASING TO AT LEAST 10 FEET SAND -POINT WELLS WHERE CASING WHERE A IS USED FOR THE SEPARATE SUCTION SUCTION LINE LINE IS USED CEMENT GROUT GROUT SAND POINT WATER LEVEL SUCTION LINE _ BEDROCK —SEE OTHER DIAGRAMS FOR DETAILS C w w u- Z CRITERIA FOR NEW WELL CONSTRUCTION AND SEALING IN THE PLATTEVILLE AND ST. PETER AQUIFERS APPROVED METHODS OF NE 1. WHERE THE PUMPING LEVEL IS ABOVE THE TOP OF THE PLATTEVILLE, CASING MUST BE FIRMLY SEATED INTO THE PLATTEVILLE USING A DRIVE SHOE 2. WHERE THE PUMPING LEVEL IS BELOW THE TOP OF THE PLATTEVILLE, THE CASING MUST BE GROUTED AT LEAST 10 FEET BELOW THE PUMPING LEVEL PLATTEVILLE WELLS ST. PETER WELL DRIVE SHOE GROUTED W WELL CONSTRUCTION ° G CASING ° o • (PUMPING • . LEVEL .UNCONSOLIDATED,' • ' DEPOSITS OR ° ° DECORAH SHALE Oc I M I DRIVE SHOE 2 a !PLATTEVILLE AQUIFER Ln AS MUCH AS 60 3. CASING SHOULD BE GROUTED AT LEAST 15 FEET INTO THE ST. PETER SANDSTONE IF IT IS USED 4. OPEN BOREHOLE IN THE ST. PETER SANDSTONE SHOULD NOT EXTEND THROUGH THE BASAL SHALY PORTION AND INTO THE PRAIRIE DU CHIEN GROUP '•0 p , • OQ o• 4 p p) .0; °. O • p CEMENT p GROUTS p cc PUNG • LEVEMPL III Iif 4 GLENWOOD / CONFINING LAYER ST. PETER SANDSTONE AQUIFER CASING EXTENDS AT LEAST 15 FEET INTO ST. PETER • 4 p A 6 OPEN BOREHOLE n p D p p.o •Q•o• 'CASING' 0 0 o • o. 6 DWATER LEVEL • BASAL ST. PETER SHALE CONFINING LAYER LOCALLY) PRAIRIE DU CHIEN AQUIFER Oo PACKER. •°e 0 • . WATER• LEVEL IMPROPER CONSTRUCTION AND PROPER SEALING 1. OPEN BOREHOLE AND UNGROUTED CASING ACROSS THE GLENWOOD INTERCONNECT AQUIFERS AND ARE NOT PERMITTED BY THE MINNESOTA DEPARTMENT OF HEALTH OPEN BORE- HOLE O• • Q• o UNCONSOLIDATED( DEPOSITS OR U • DECORAH SHALE • = PLATTEVILLE IZ AQUIFER 1, GLENWOOD' CONFINING LAYER DOWNWARD MOVEMENT OF WATER 1 I I - WATER LEVEL 2. ALL UNGROUTED CASINGS OR TELE- SCOPED CASINGS MUST BE REMOVED, PERFORATED, OR RIPPED BEFORE SEALING OF WELL WITH CEMENT GROUT UNGROUTED OPEN TELESCOPING BOREHOLE UNGROUTED CASING WITH WITH LEADER SINGLE OPTIONAL PIPE AND CASING SCREEN SCREEN 00. UN - GROUTED CASING OPEN BOREHOLE OPEN BOREHOLE. ST. PETER SANDSTONE AQUIFER 0 e• SMALL OVER- LAP o 0 SCREEN SCREEN, CASING, AND LEADER PIPE SAND -LOCKED IN ST. PETER SANDSTONE BASAL ST. PETER SHALE (CONFINING LAYER LOCALLY SEALING ABANDONED WELLS Wells that are no longer used must be properly sealed according to procedures specified by the Dakota County Health Department. An abandoned -well record must be submitted to the Minnesota Department of Health by a licensed water -well contractor following the sealing of each well. The Groundwater Protection Act of 1989 also requires that the locations of all wells be disclosed at the time of a property transfer. Abandoned wells must be properly sealed or the new owner must accept responsibility for them. The state water -well construction code requires that unsealed, inoperable wells have maintenance permits and that an annual renewal fee be paid. Many neighborhoods that once used individual wells are now serviced by municipal systems, but the wells remain —commonly in a state of disrepair. In rural areas, a new well may have been drilled to replace one that had failed. The old well was almost never properly sealed. Elsewhere, unused commercial, industrial, and irrigation wells remained after land use changed, and many neither meet the construction standards of today nor are properly sealed. Some wells were covered during construction projects and were abandoned by bulldozing the casing. Improperly abandoned wells may have an impact on the quality of ground water because of their potential to funnel contamination underground. Abandoned wells also present problems to property owners, because great expense and inconvenience may be required to seal them properly. Debris fills some; the pumping equipment was dropped into others. All materials blocking a well must be removed prior to sealing. Cleaning out a well is labor intensive and adds significantly to the costs of sealing; it sometimes adds thousands of dollars for a large -diameter well. Many domestic wells were constructed in basements or basement offsets. Building additions can cover them. Wells were sometimes constructed in pits or were covered so that today they can be found only by excavating yards and driveways. Property transfers may be held up while responsibility for well abandonment is determined, or new construction must cease until all abandoned wells are sealed. Determining how a well is to be sealed can be as difficult as deciding how a new well should be constructed. Geologic conditions and the type of well construction used must be considered. For example, ungrouted casings must be removed or perforated to permit proper sealing of bedrock confining layers. Merely filling these casings with grout will not suffice. Information describing the well may be sketchy or nonexistent. Site -specific work, such as televising the well with a downhole camera or conducting borehole geophysical logging, may be required to document well construction and local hydrogeologic conditions. Some general types of well construction are described for reference. PLATTEVILLE AND ST. PETER WELLS Generally, the Platteville Formation is used only in the western half of the county and there only by domestic wells. Elsewhere, it is dewatered or not present. Proper sealing of abandoned Platteville wells depends on the construction methods that were used. Open boreholes in the Platteville, and those that extend into the St. Peter Sandstone, can be sealed directly by cement grouting. However, some wells have casing or pipe that must be removed or punctured to permit sealing of the Glenwood Formation. Where the Platteville did not supply enough water, it was common practice in precode wells to extend the borehole into the poorly cemented St. Peter Sandstone. Here extra casing —and even a screen —was added to hold the borehole open and to prevent sand from being pumped. The extra casing or screen may now be difficult to remove because it has deteriorated or become sandlocked in the St. Peter; the casing must then be ripped or perforated to let grout enter the borehole annulus. OTHER TYPES OF BEDROCK WELLS Prior to 1974, wells completed in aquifers below the St. Peter Sandstone had no standardized method of construction (Fig. C). Wells completed in the Jordan Sandstone either were cased to the top of the Prairie du Chien Group or, rarely, were grouted if the casing extended to the Jordan. Wells drilled to deeper aquifers also tended to be cased only into the Prairie du Chien Group and were left as open boreholes below or were of telescoping construction. Wells having an open borehole construction can be sealed in a relatively straightforward manner by pressure grouting from bottom to top. Wells in which casing was installed but not grouted present technical as well as financial problems. The annulus between the borehole wall and the casing or between multiple casing strings must be filled with cement grout to seal these wells completely. If this is not possible, these casings must be removed or at least adequately punctured. To do .this commonly involves considerable time and expense. Furthermore, former problems with the caving in of sandstone or shale may reoccur and interfere with grouting once the casing is removed or opened. Such wells may have to be abandoned by sealing each aquifer and confining layer individually. Proper well drilling and sealing should result in the maintenance or improvement of ground -water quality in the long run. For information about this or about abandonment or construction of wells, contact: Minnesota Department of Health Well Management Unit 925 S.E. Delaware St. Minneapolis, MN 55440 Dakota County Public Health Department Suite 345 33 East Wentworth Avenue West St. Paul, MN 55075 GEOLOGIC ATLAS OF DAKOTA COUNTY, MINNESOTA UNIVERSITY OF MINNESOTA MINNESOTA GEOLOGICAL SURVEY Priscilla C. Grew, Director Prepared and Published with the Support of the DAKOTA COUNTY SOIL AND WATER CONSERVATION DISTRICT AND BOARD OF COMMISSIONERS AND MINNESOTA DEPARTMENT OF NATURAL RESOURCES, WATERS DIVISION R22W COUNTY ATLAS SERIES ATLAS C-6, PLATE 9 OF 9 GEOLOGIC RESOURCES LOCATION DIAGRAM ' Port T115N T 114 N R 24 W R 21 W T113N T 112 N T 27 N R234^ C- tty IQ 3 R 20 .T Base modified from U.S. Geological Survey, Faribault, Hastings, and St. Paul, 1985. INTRODUCTION Dakota County has about half of the significant and potentially significant rock aggregate and sand and gravel resources of the seven -county Twin Cities Metropolitan Area. It ranks second after Hennepin County in projected growth of demand for aggregate, because urbanization will continue to increase in the northern part of the county. However, its potential resources should suffice to meet the estimated demand for the near future. For example, Schenk and Jouseau (1983) estimated that for the 25 years until 2006, Dakota County had 1700 million tons of sand and gravel and 798 7 million tons of crushed rock resources to meet a demand of about 65.3 million tons. For comparison, in the entire seven -county metropolitan area, nearly 12 million tons of sand and gravel were sold or used by producers in 1988 (Hill, 1989), and about 4.8 million tons of crushed rock in 1987 (Hill, 1988). Principal uses for aggregate in the Twin Cities area are for concrete for buildings, bridges, and road pavement construction; road base and fill; bituminous road pavement; and lesser uses, such as riprap, agricultural lime, and railroad ballast (Hill, 1989). The maps on this plate reflect only geologic criteria. Urban development and zoning laws obviously have a major impact on availability of resources for mining. However, a map that attempted to take these factors into account would soon be outdated. This kind of rapidly changing demographic information is best portrayed and managed with a computerized Geographic Information System (GIS). SAND AND GRAVEL RESOURCES The vast sand and gravel outwash plain that borders the south and east side of the St. Croix moraine in Dakota and Washington Counties is the most significant aggregate resource in the seven -county metropolitan area and is by far the largest sand and gravel resource in Dakota County (Table 1). The second most significant resource is represented by outwash of the Des Moines lobe and its Grantsburg sublobe. Ice -contact and terrace deposits are comparatively minor. Figure 2. Outwash forms broad plains that contain some lenses of gravel. It is commonly capped by a thin layer of clay or silt. It is coarsest and contains the most gravel nearest the source (upper end of diagram). Resource Classification To be a primary resource, a sand and gravel resource must be at least 20 feet thick, have less than 10 feet of overburden, and contain at least 35 percent gravel -size material. Primary resources are by definition in areas where subsurface data are sufficient to identify them. Subsurface data in the county range from very good around some gravel pits, where intensive subsurface exploration has been done, to limited in rural areas, where the only subsurface data are scattered drillers' logs for water wells. Primary resources are further subdivided on the basis of presence of a high water table or presence of shallow bedrock that can be quarried for crushed rock aggregate. Secondary resources fail to meet one or more of the criteria for primary resources. Most fail to contain enough gravel -size material. In other cases, data may simply be inadequate to demonstrate the presence of a good gravel resource. Resource Quality and Geologic Units The map distinguishes among geologic units. Genetic classification is important because the quality of the resource is influenced by its origin. For example, many Des Moines ice -contact and outwash deposits contain material of suitable size, but they also contain appreciable shale which spalls or pops out when used in asphalt or concrete, and limits their use for aggregate (Table 2). The gravels of the Superior lobe, by contrast, are more acceptable for aggregate because they contain an abundance of pebbles from igneous and metamorphic rocks and very little shale. The River Falls drift is similar to the younger Superior lobe gravels. Mixed Des Moines and Superior lobe outwash has intermediate spall values. Chert and iron oxide are other undesirable materials in aggregate. Chert is particularly undesirable because it reacts with free alkali in cement to form sodium silicate, which is water soluble and forms cavities in concrete when it dissolves. Iron oxide particles are soft and nonresistant to weathering. Table 2 shows approximate percentages of spall materials in Dakota County sand and gravel. Figure 3. Ice -contact deposits generally form prominent hills or knobs (kames) in Dakota County. They generally contain abundant gravel -size material close to the surface. Table 1. Estimated tonnage of aggregate resources in Dakota County. [Estimates by Meyer and Jirsa (1984); values in millions of short tons; *, none identified. Includes resources currently unavailable for mining due to land -use constraints.] Primary resources Secondary % of Metro resources Area total Sand and gravel deposits Terrace and floodplain deposits (T, fd) Des Moines outwash (do, dto, dso) Des Moines ice -contact (dsi, dst) Superior outwash (so) Superior ice -contact (si, st) Pre -late Wisconsinan outwash (pso, psd) Undifferentiated Totals 24 316 38 1154 30 87 15 1664 Carbonate bedrock resources Dolomite Probable (reliability 1) 167 Possible (reliability 2, 3) 814 Dolomitic limestone Probable (reliability 1) Possible (reliability 2, 3) 5 396 <0.5 * * 3 404 4 62 27 59 28 94 90 49 29 49 10 25 T 112 N T 27 N 1 0 R1 1 R 18 W 1 0 5000 0 GEOLOGIC RESOURCES By John H. Mossler 1990 ECONOMIC RANKING OF RESOURCES SAND AND GRAVEL RESOURCES MIXED (Shown where estimated as aggregate source; gravel is present sporadically in uncolored areas) Primary Water table Water table >20 feet deep <20 feet deep More than 35 percent gravel (particles larger than 2 mm in diameter); deposit thicker than 20 feet; generally less than 10 feet of cover; good to limited subsurface data. SCALE 1:100 000 2 3 4 5 6 7 8 9 10 KILOMETERS 1 2 3 4 MILES 5000 10000 15000 20000 25000 LAR values, signifying resistance of gravel to abrasion, are lower in Superior lobe gravels by virtue of the higher percentage of hard igneous and metamorphic clasts (Table 3). Conversely, the Des Moines lobe gravels have higher LAR values because of their higher content of softer clasts (shale and carbonate). The origin of deposits may influence their potential as aggregate resources in other ways. For example, ice - contact deposits are poorly sorted and contain boulders and cobbles, as well as lenses of finer material. Outwash, terrace, and lacustrine deposits tend to be more uniformly sorted and more uniform in distribution. Because different size mixtures are required for different uses, a range of sizes may be desirable. A deposit may be less valuable if more than 20 percent consists of very large or very small particles. Further discussion of the geologic origin of the sands and gravels is included in the explanation. BEDROCK RESOURCES The bedrock formations of Dakota County have been mined in the past for various uses. However, they currently are quarried chiefly as sources of limestone and dolomite that can be crushed and used for rock aggregate. Crushed Rock The Prairie du Chien Group is classed as a primary resource because of its superior physical properties (Meyer and Jirsa, 1984) and greater thickness. The Platteville Formation has greater potential for deterioration, resulting primarily from freezing and thawing, and so the Minnesota Department of Transportation restricts its use in many types of roadway construction (Schenk and Jouseau, 1983). The Platteville Formation is relatively thin, generally less than 30 feet thick, and that also diminishes its value as a resource. As with sand and gravel resources, mapping of areas with potential for bedrock resources is based on data of varying reliability, as indicated on the map. Resources rated 1 can be considered as probable resources. Those rated 2 or 3 are best described as possible resources. Figure 4. Small deposits of gravel within glacial till generally bear no obvious relationship to the landforms in which they occur. During downwasting of the glacier that contained the till, meltwater washed fine material from some of the till, leaving only stones and gravel as small lenses within the till. FEET imImarArAmbl VA Mr/ itrA, Estimates of tonnages of probable and possible resources by Meyer and Jirsa (1984) indicate that Dakota County ranks second only to Washington County in estimated quantity (Table 1) in the metropolitan area. Building Stone In earlier days the limestones and dolostones were exploited for rough building stone. Two well-known pioneer structures are built from Platteville stone quarried at Mendota: The Faribault house has two walls of Platteville limestone and the H.H. Sibley home is built entirely of Platteville (Bowles, 1918). During pioneer days, Oneota dolostone (the lower formation of the Prairie du Chien Group) was quarried at Hastings for building foundations, and extensively quarried at Nininger for riprap used in river -bank protection (Bowles, 1918). Cemented and hard (indurated) St. Peter Sandstone from a quarry on a small island in the Minnesota River near Fort Snelling (SW1/4 sec. 33, T. 28 N., R. 23 W.) was used for piers of the old Fort Snelling bridge and in two walls of the Faribault House (Winchell, 1888; Bowles, 1918). The St. Peter Sandstone could be used as industrial sand -for example, for foundry sand (Knapp, 1923), if the demand arose. However it currently is not exploited in Dakota County. Ice contact Secondary Less than 35 percent gravel; or deposit less than 20 feet thick; and or more than 10 feet of cover. Includes areas with insufficiant sub- surface data that may contain good gravel resources. R 17 W 3 Cartography by Philip Heywood. Primary Water table >20 feet deep Sand and gravel resource underlain by Prairie du Chien dolostone that also may be an aggregate resource. T 113 N N T Every possible effort has been made to ensure the accuracy of the factual data on which this map interpretation is based; however the Minnesota Geological Survey does not warrant or guarantee that there are no errors. Users may wish to verify critical information; sources include both the references listed here and information on file at the offices of the Geological Survey in St. Paul. In addition, every effort has been made to ensure that the interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpretation shown is rigorously correct, however, and it should not be used to guide engineering -scale decisions without site -specific verification. Figure 5. Some Superior lobe ice -contact deposits in Eagan accumulated, at least in part, in ice -walled lakes. After the surrounding ice melted, the former lake bottoms became the flat to gently sloping tops of large rounded hills. Silt overlies thick deposits of fine to medium sand in the central parts of these hills. Gravel is commonly present along the steep side slopes and may be a source of aggregate (Meyer and Jirsa, 1984). Table 2. Spall* content in sand and gravel for aggregate in Dakota County. [Results in percentage of total weight. Numbers in parentheses are number of samples tested; T, trace amount; -, not counted. Source: Minnesota Department of Transportation gravel pit sheets] % shale % Fe in in oxide sand gravel unsound chert Pits sampled Reworked Superior outwash on floodplain Terrace deposits Des Moines outwash Des Moines ice -contact Mixed Des Moines - Superior outwash Superior outwash Superior ice -contact in till River Falls drift T(3) 0.0(3) 0.5(3) 0.3(7) 0.2(6) 0.6(6) 1.4(26) 0.8(26) 0.7(26) 2.6(1) 1.3(1) 0.1(1) 0.8(43) 0.5(38) 0.5(43) 0.2(39) T(39) 0.3(38) 0.1(11) 0.0(8) 0.2(10) 0.1(2) T(2) T(2) 1 1 T(8) 7 1 T(2) 8 T(13) 9 T(2) 4 3 ST BEDROCK RESOURCES (Shown where estimated to be within 10 feet of land surface) Primary Sandy dolostone with thin beds of quartzose sandstone of the Prairie du Chien Group. Some thin layers of chert, mainly in upper part. Generally 145 to 300 feet thick. Secondary Thin -bedded dolomitic limestone of the Platteville Formation; 10 to 30 feet thick. Reliability of bedrock data 1. Excellent, on the basis of four criteria. a. Bedrock outcrops present. b. Water wells and soil borings indicate shallow bedrock. c. Soil type mapped by U.S. Soil Conservation Service is characteristic of shallow bedrock. d. Surface topography indicates bedrock is shallow. 2. Good. Only 3 of the 4 criteria are met. Outcrops or well and soil borings commonly not available. 3. Fair, only 2 of the 4 criteria are met. R 1S`0 Figure 1. Terraces are benches that follow stream courses. Gravel may be covered by finer sediments, because stream competency generally is highest during early phases of deposition of terrace sediments. The bench -like form is caused when a stream is rejuvenated, cuts down below the earlier valley floor, and establishes its course at a lower elevation. Clay Resources The Decorah Shale was mined from 1890 to 1973 for manufacture of brick and tile in Lilydale near the boundary between Dakota and Ramsey counties (NEl/4 sec. 13, T. 28 N., R. 22 W.). The Twin City Brick and Tile Company was one of the major manufacturers of clay products in the state. Clay from glacial drift was formerly utilized at a small brickyard in West St. Paul (Grout and Soper, 1914). REFERENCES CITED Bowles, 0., 1918, The structural and ornamental stones of Minnesota: U.S. Geological Survey Bulletin 663, 225 p. Grout, Frank F., and Soper, E.K., 1914, Preliminary report on the clays and shales of Minnesota: Minnesota Geological Survey Bulletin 11, 175 p. Hill, J.J., 1988, The mineral industry of Minnesota, in Minerals yearbook 1987: U.S. Bureau of Mines, p. 209-221. Hill, J.J., 1989, Minnesota minerals yearbook 1988: U.S. Bureau of Mines, 13 p. Hundley, S.J., and others, 1980, Soil survey of Dakota County, Minnesota: U.S. Department of Agriculture, Soil Conservation Service, 272 p. Knapp, G.N., 1923, The foundry sands of Minnesota: Minnesota Geological Survey Bulletin 18, 205 p. Marshall, L.G., 1963, Sand and gravel operations and costs, Minneapolis -St. Paul area, Minnesota: U.S. Bureau of Mines Information Circular 8157, 66 p. Meyer, G.N., and Jirsa, M.A., 1984, Aggregate resources inventory, Twin Cities Metropolitan Area, Minnesota: Minnesota Geological Survey Information Circular 20, 16 p., 2 pls, scale 1:250,000. Schenk, C., and Jouseau M., 1983, Aggregate resources in the Twin Cities Metropolitan Area: Metropolitan Council of the Twin Cities Area Publication 10-83- 019, 100 p., 2 pls. Winchell, N.H. 1888, The geology of Dakota County, in N.H. Winchell and Warren Upham, eds., The Geology of Minnesota, v. II of the Final Report: Geological and Natural History Survey of Minnesota, p. 62-101. MAP SYMBOLS X X Gravel and borrow pits -Active or intermittently active; inactive Quarry -Active or intermittently active; inactive Boundary of large pit or quarry Resource has been depleted in inactive pits and is largely depleted in pits and quarries that are still active GEOLOGIC ORIGIN OF SAND AND GRAVEL fd, Gravel pits shown in areas mapped fd are generally developed in outwash and terrace deposits that underlie the modern alluvium. The only significant gravel deposit currently being mined is in section 5 of Ravenna Township southeast of Hastings where there is a large gravel pit in the Mississippi River floodplain. It and others nearby in the Mississippi floodplain may be the erosional remnants of low gravelly terraces that are covered by modern alluvium. Gravel pits in a small stream valley due east of Hastings probably are in Superior outwash that underlies thin mixed outwash and alluvium. Those at the mouth of the valley in the alluvial fan are reworked outwash gravels. The modem floodplain alluvium in the Minnesota River is chiefly silt; in the Mississippi, Vermillion, and Cannon Rivers it is sand. ws, Slopewash material derived from St. Peter Sandstone and pre-Wisconsinan glacial drift. Predominantly sand and generally poor in gravel. Subsurface information is limited, and areas were mapped as a potential gravel resource chiefly on the basis of the soils mapping by Hundley and others (1980). T, Terraces (Fig. 1). Formed along the courses of Glacial Rivers Mississippi, Minnesota, and Warren. They follow the present water courses of the Mississippi and Minnesota Rivers, but are 5 to 170 feet above the present floodplain. In places the underlying deposits are glacial outwash sand and gravel ,and both can be mined. The higher terraces commonly have thick gravel -poor sand deposits at the surface that could make extracting the underlying gravel uneconomic. In Inver Grove Heights, South St. Paul, Burnsville, and Eagan many of the terrace deposits have been depleted or pre-empted by urban construction. do, Des Moines lobe outwash (Fig. 2) in south-central Dakota County. It is characterized by higher shale and other spall content than Superior lobe sand and gravel. dso, Mixed Des Moines and Superior outwash. Coarsest near the northern and western edges of the county. Although it contains a high percentage of Superior lobe igneous and metamorphic pebbles, it contains more shale than Superior lobe outwash. dsi, Des Moines ice -contact stratified deposits (Fig. 3). Cobbly sand and gravel with some interbedded till. In addition to the three areas in Burnsville shown, many small hills nearby are too small to show on this map. Many are no longer available because of urban construction. dst, Mixed Des Moines and Superior till. Lenses of sand and gravel deposited by meltwater during the melting of glacial ice are interbedded in the till (fig. 4). They are similar in origin to deposits labeled dsi, but these are smaller and may have no topographic expression. dtp, Thin Des Moines ice -contact deposits over pre- Wisconsinan drift. Some of the gravel within this mapping unit is probably from the older drift. dto, Des Moines outwash mantled by thin Des Moines till. Shown as a resource mainly where sand and gravel soils are mapped. so, Superior lobe outwash (Fig. 2). Largest single potential source for natural aggregate in the Twin Cities Metropolitan Area. Thickness generally decreases from the northwest to southeast but is quite variable due to irregularity of the underlying bedrock and older till surface. In most places, it is more than 50 feet thick. Average grain size also decreases to the southeast. Near Rosemount, next to the St. Croix moraine, it contains much gravel and many cobbles; farther away from the moraine it is chiefly sand and fine gravel. It characteristically has only trace amounts of shale and very low spall values. si, Superior ice -contact deposits associated with the St. Croix moraine (Figs. 3 and 5). Some quite large pits have been opened in these deposits. Sand and gravel in them can be quite variable in thickness and quality over short distances. Gravel occurs along the steep sides of some kames in Eagan Township, where it may have accumulated on the beaches of ice -walled lakes during the melting of the glacier. st, Superior lobe till (Fig. 4). Lenses of sand and gravel deposited by meltwater during melting of the glacier are common in Superior lobe till. Their origin is similar to many deposits labeled si, but these lenses are smaller in size, and they generally do not have topographic expression. pso, River Falls Formation. Chiefly outwash and ice - contact sand and gravel with minor glacial till (psd). Several large pits have been opened in these deposits. Because these deposits have a Superior lobe origin, they have high percentages of igneous and metamorphic grains and low shale content. Gravel in the upper part of some deposits is clayey, and may have to be washed. pko, "Old gray" outwash sand and gravel. Not studied in detail, and the small, patchy occurrences along the southern boundary of the county are quantitatively of very minor importance. Table 3. Summary of Los Angeles Rattler (LAR)* tests of sand and gravel for aggregate in Dakota County. [Source: Minnesota Department of Transportation gravel pit sheets.] Size A (3/8-1-1/2 inch; 9.5-37.5 mm) Mean Range No of value tests Size B (3/8-3/4 inch; 9.5-19.0 mm) Mean Range No of value tests Size C (1/5-3/8 inch; 4.75-9.5 mm) Mean Range No of value tests Pits sampled Reworked Superior out - wash on floodplain Terrace deposits Des Moines outwash Des Moines ice -contact Mixed Des Moines - Superior outwash Superior outwash Superior ice -contact in till River Falls drift 18.4 19.8 27.3 28.2 20.0 20.5 16.6-20.1 18.9-23.1 22.4-32.1 16.0-25.0 16.8-24.7 2 18.1 17.9-18.4 3 7 21.6 20.9-22.6 7 17 26.4 24.0-30.2 20 1 40 20.3 19.4-28.2 29 42 20.7 17.4-23.6 20 25.1 1 21.4 18.7-24.8 7 1 1 7 1 24.7 21.2-31.8 8 8 9 26.6 3 1 1 11 67 *Spall materials are rock particles that will cause a pop -out in hardened concrete or bituminous pavement. Maximum permissible spall materials in coarse aggregate for concrete used in highway construction, by weight percent of total sample, are: 0.7 percent shale, 0.25 percent soft iron oxide particles, and total spall materials (shale and iron oxide plus unsound chert, coal, and clayey limestone) not more than 1.5 percent (Marshall, 1963). *LAR is a standard method for testing resistance of aggregate to abrasion. Coarse aggregate is rotated in a steel cylinder with steel balls for a specified time. The percentage of fine material abraded from the coarse aggregate originally put in the cylinder, expressed as percentage of loss, is the LAR value. The more resistant the aggregate, the lower the LAR values. Maximum permissible Toss for aggregate used in highway construction (total sample) is 42 percent (Marshall, 1963). GEOLOGIC ATLAS OF DAKOTA COUNTY, MINNESOTA