Prepared and Published with the Support of COUNTY ATLAS SERIES THE BENTON COUNTY BOARD OF COMMISSIONERS AND ATLAS C-23, PART A MINNESOTA GEOLOGICAL SURVEY the Minnesota Environment and Natural Resources Trust Fund Plate 3—Surficial Geology Harvey Thorleifson, Director as recommended by the Legislative-Citizen Commission on Minnesota Resources
94°15' W. 94° W. R. 32 W. R. 31 W. R. 30 W. R. 29 W. Qp 350 370 R. 28 W. MORRISON COUNTY 390 390 1 1 6 Qcf Qci Qp Qcd 340 1 6 Qp Qe 6 390 340 Qci 1 River Creek 1 370 6 SURFICIAL GEOLOGY Qe 390 Qa 330 l Qe Hil 380 Qct MORRISON COUNTY Qwh er 330 k 390 360 Bun 360 380 Qa Creek 370 340 380 West MILLE LACS COUNTY Platte 10 Qcd Qp By 390 A 370 A' Branch 320 Creek Qp 370 380 Qp Qwl Qa 350 Zuleger Gary N. Meyer 380 360 r 310 LANGOLA GRAHAM Creek ALBERTA e Qcd GRANITE 380 iv Qci R T. 38 N. Qcs LEDGE 390
+ 2010 Qct 360 STEARNS Qcf 370 A Qct 340+ COUNTY T. 38 N.
380 Qp + Rock Qcs Riv Mayhew ++ er Qwr Rum Qe + ++ 340 Little + 350 370 Qco
25 St. Francis 370 lk 350 E Qcs Rice Qp 350 45°45' N. 370 B B' CORRELATION OF MAP UNITS Mississippi 45°45' N. 31 Qe 36 Qe Qf Little 31 350 31 31 310 36 36 Qct Qcs Hudson Ql 360 360 36 Rock 360 Qco Qa Qf Ql Qe Qp 350 360 Episode Qwl Qp Qa Lake Qp 94°15' W. E Qwl s West Campus Ql 6 te 1 1 k s Qcf 6 1 e 1 350 formation re 6 Digital base modified from the Minnesota Department of C 360 River B Qwr QUATERNARY 350 roo Qwh Transportation BaseMap data; digital base annotation by 330 k Michigan Wisconsin Bailey 360 Qco Qcs the Minnesota Geological Survey. New Ulm Subepisode Episode ek Qp Qno Qnw Qns Qnd Qnl Qwl Qf re Qcd 340 formation Elevation contours were derived from the U.S. Geological 310 C Survey 30-meter Digital Elevation Model (DEM) by the Cromwell River Sucker 23 Qco Qcs Qci Qcd Qcf Qct Minnesota Geological Survey. Qa 350 Formation 330 Universal Transverse Mercator Projection, grid zone 15 Mayhew 340 340 350 PALEOPROTEROZOIC 1983 North American Datum Lake Qp 350 Qct A and NEOARCHEAN Qwh 340 340 Oak Qwr Qcs 360 Park Qco T. 37 N. 340 Qcs Creek Qns Qp 350 T. 37 N. 350 Qwl WATAB Qe MAYHEW Qco GILMANTON MAYWOOD Qcf 340 Qco 340 LAKE Qns Qct Qcd 340 Qcd of the Mississippi River, and preserved in terraces above the Qp Foley INTRODUCTION 340 Qco modern floodplain. During these early stages the river served as 320 Qe Qcd A The surficial geology map of Benton County shows the earth material expected an outlet stream for melting glacial ice to the north. The West C 340 C' to be encountered below the topsoil, generally about 3 feet (1 meter) below the Campus formation is mapped at two terrace levels upstream of Qp Qp 36 Qns land surface. This map was modified from recently published maps (Meyer and the confluence with the Minnesota River at Fort Snelling. 31 Qnl 31 36 31 36 others, 2001; Ellingson, 2002; Meyer, 2008) on the basis of an expanded water Qwh A 36 Qwl Langdon terrace—The surface is 10 to 30 feet (3 to 9 meters) above 320 Collner well data base compiled for this study (Plate 1, Data-Base Map); 18 shallow auger 330 the recent floodplain level (prior to damming of the Mississippi 340 350 holes and 5 deeper rotary-sonic core holes carried out for this study; examination Qe Qct River), ranging in elevation from about 1,000 feet (305 meters) Qco Qa of new exposures (mostly in gravel pits); bridge boring records from the Benton Qp 6 1 Qns 330 at Sauk Rapids to about 1,050 feet (320 meters) upstream near 1 Qns E 6 l 1 6 330 County Highway Department and from the Minnesota Department of Transportation k 1 the northwest corner of the county. Shells are present in places. Qcs Qct Qct (2009); and evaluation of the updated soil survey (Natural Resources Conservation Qp 340 Most contacts with other map units are scarps. Qco Service, 2009). In Figure 1, the surficial sediments are grouped into more permeable A ++ + Qwr Richfield terrace—The surface is 30 to 70 feet (9 to 21 meters) above Qe BrookQco (bedded sand to gravel) and less permeable (bedded silt to clay and diamicton) Sartell SAUK + + Qci + Qa units. The topographic expression created by glacial sculpturing and subsequent the recent floodplain level (prior to damming of the Mississippi RAPIDS 330 + Qct 320 + River), ranging in elevation from about 1,025 feet (312 meters) at 340 meltwater and wind erosion and deposition is displayed on the Digital Elevation 330 + Qp 340 Qnl St. Cloud to about 1,080 feet (329 meters) at the northwest corner Mayhew Qct 340 Qp Model (Fig. 2). 340 Qct 330 St. Francis 15 A Sauk River Stony of the county. Most contacts with other map units (except eolian
Qci 330 330 Qco Rapids Qns 25 sand) are scarps; however, the contact with adjacent outwash is 330 320 SUMMARY OF GLACIAL HISTORY 340 330 commonly gradational. The original surface of the terrace has been 320 Qco T. 36 N. 320 MINDENQct ST. GEORGE GLENDORADO Qcs Qci T. 36 N. The surficial geology of Benton County, like most of Minnesota, is dominated altered in places by wind action, and by ice-block melt-out. 330 Qco Qcs 310 Qp Qp by unconsolidated sediments laid down by glacial ice and meltwater towards the LOCATION DIAGRAM 310 Qct 23 Qcs Qwh Sand overlying reddish sandy till—Sand and gravel at the Richfield 320 320 end of the "Ice Age," during the Wisconsin Episode (Johnson and others, 1997). Qwl Creek terrace level less than 10 feet (3 meters) thick over till of the 330 10 Qno 330 Qcd 330 Qcd Qcd These sediments bury eroded remnants left by multiple earlier glacial advances 310 95 330 95 Cromwell Formation. Boulder lags are common at the contact. 320 330 Qcs that once covered the county (Plate 4, Quaternary Stratigraphy). During each of 330 Qno Qci Qf Qco these advances, glacial ice from the Laurentide ice sheet to the north entered the New Ulm formation (Meyer and Patterson, 1999)—Glacial, fluvial, and 320 Qcs Qnw 320 320 Qno 320 county from different directions, reflected in the diverse deposits left behind. The lacustrine sediment of Riding Mountain provenance (Table 1) 320 310 Qcs Qnw Qco Qno provenance of an ice advance (Fig. 3, Table 1) is the unique area of bedrock to the deposited by ice and meltwater of the last glacial advance into 300 Qct 310 Qct 310 320 A St. Cloud 320 north that the ice passed over and incorporated, and then deposited to the south. the county, that of the Grantsburg sublobe of the Des Moines lobe D Donovan Qci Qp D' 31 320 Qnd Lake 320 River (Fig. 3). Included in these map units are some low-lying areas 36 31 Qco Qns At the onset of the most recent glaciation, the Michigan Subepisode of the STEARNS 320 36 36 Qwr 320 Qp 320 310 31 Qcd 36 where New Ulm formation sediment may be overlain by 3 feet (1 COUNTY Qcs Qco Wisconsin Episode, ice carrying debris of Rainy and Superior provenance covered Qno 320 meter) or more of sandy to clayey, organic-bearing colluvium, or 320 Qci 320 Qp MILLE LACS COUNTY the county (Fig. 3; Meyer and Knaeble, 1996). Following the northwestward retreat SHERBURNE COUNTY R. 30 W. of the Wadena lobe (the ice carrying Rainy provenance debris), the Superior lobe by thin peat. R. 29 W. R. 28 W. SHERBURNE COUNTY 94° W. advanced during the Emerald phase (Johnson, 2000) to cover the entire county. Qnl Lake clay and silt—Laminated to thick bedded clay to silt; generally GIS compilation by R.S. Lively Following the Emerald phase, the Superior lobe melted back out of the county, greater than 5 feet (1.5 meters) thick. Covered by patches of SCALE 1:100 000 Edited by Lori Robinson 1 0 1 2 3 4 5 MILES and then readvanced during the St. Croix phase (Wright, 1972) to once again silty, fine-grained sand. Thin beds of silty, fine-grained sand completely cover the county. Its termination to the west and south was marked to gravelly sand occur at boundaries and at or near the base in MAP SYMBOLS 1 0 1 2 3 4 5 6 7 8 KILOMETERS by the prominent line of hills called the St. Croix moraine (Meyer and Knaeble, places. The unit was deposited in deeper, quiet water of glacial geologic contact—Approximately located. 1996; Knaeble and Meyer, 2007). Early during its recession from the St. Croix Lake Grantsburg. contour interval 10 METERS general flow direction of braided streams—Arrow points downstream in the direction moraine, the Superior lobe paused briefly at the St. Stephen ice margin (Meyer Qns Lake sand—Very fine- to fine-grained sand and silty sand, with minor glacial meltwater last flowed. and Knaeble, 1996) at the southwest corner of Benton County (Fig. 4), but then interbeds of silt and medium-grained sand. The unit generally coarsens upwards, but locally, coarse-grained, gravelly sand occurs Stream-cut scarp—Hachures point downslope; dashed where discontinuous or obscure; apparently retreated rapidly across most of the county, perhaps slowing again in along boundaries and at or near the base. Many of the coarser- marks the flanks of a former fluvial channel. Boundaries of outwash and terrace the northeastern portion (Fig. 5), as reflected by various ice-contact land forms. grained clasts are reworked from underlying and adjacent Cromwell units and alluvium are commonly at scarps, so are not shown by a scarp symbol. The Pierz drumlin field, formed beneath the Superior lobe and extending across Formation deposits. Where patchy, the unit is commonly less than Where paired, scarps bound stream-scoured areas. Till surfaces on the hachured most of the county (Wright, 1972; Mooers, 1990), was exposed at this time. As 10 feet (3 meters) thick. The unit was deposited in shallower, side of scarps are fluvially scoured and mantled in places by sand and gravel too the Superior lobe pulled back from the St. Croix moraine, its meltwater became ponded in low terrain north of the Twin Cities, forming glacial Lake Lind (Johnson more active water of glacial Lake Grantsburg. Deposits adjacent Superior-lobe ice thin and patchy to map separately. and others, 1999). The lake may have extended up the Mississippi River valley to to unit were likely laid down, at least in part, in slackwater of Maximum extent of glacial Lake Grantsburg—About 1,120 feet (341 meters) above mean Qno western Benton County (Meyer, 1998). meltwater streams that deposited the outwash following drainage sea level (Meyer, 2008). The lake was bounded to the southeast by the Grantsburg of glacial Lake Grantsburg (Fig. 7). The upper few feet (1 meter) sublobe (Fig. 6). The last glacial advance into the county, the Grantsburg sublobe, was an offshoot of the Des Moines lobe (Fig. 3), a tongue of ice that flowed down the Minnesota have been reworked in places by wind. + Esker—A sinuous ridge of predominantly sand and gravel, interpreted to have been + + + River valley and then south to Des Moines, Iowa. The northeast flank of the Des Qno Outwash—Sand, gravelly sand, and gravel. Deposited by meltwater St. Croix moraine + + deposited in an ice-walled tunnel of a glacial meltwater stream flowing at the base Moines lobe overrode the St. Croix moraine and continued northeast as far as issuing from the ice margin. Includes clasts of Superior provenance of Superior-lobe ice. Points in the inferred flow direction. Grantsburg, Wisconsin. The Grantsburg sublobe blocked south-flowing streams eroded from the Cromwell Formation. The upper few feet (1 meter) Drumlin—A streamlined hill or ridge, typically of glacial till, with the arrow showing in the Mississippi River valley and other valleys to the east, forming glacial Lake have been reworked in places by wind. Commonly bounded by the inferred direction of ice movement. The length of the arrow is approximately Benton County Grantsburg (Fig. 6; Cooper, 1935; Johnson and Hemstad, 1998). Meltwater streams scarps. equivalent to the drumlin length. Drumlins are partially masked in places by from the west and north began to fill the lake with coarse-grained sediment, and Twin Cities member (Meyer and Patterson, 1999)—Complexly supraglacial sediment or eolian sand. Qnd finer-grained sediment accumulated in the deeper basins within the lake. The Duelm intermixed yellowish-brown to gray and reddish-brown to reddish- Broad, irregular trough—Hachures point downslope; identified by alignment of depressions channel (Meyer and Hobbs, 1993) was cut apparently when glacial Lake Grantsburg's gray, sandy clay loam- to sandy loam-textured, unsorted sediment and lakes. Likely mark collapsed and filled channels. Many of these troughs are level had dropped substantially, isolating water bodies to the west (Fig. 7). As the (diamicton), pebbly, with cobbles and boulders. This mixture of interpreted to reflect valleys cut by meltwater flowing beneath Superior-lobe ice Grantsburg sublobe retreated, glacial Lake Grantsburg deposits were exposed or both Riding Mountain and Superior provenance sediment formed that were partially buried by subsequent glacial events. Drainage channels beneath buried by fluvial sediment as the lake level lowered, and then the lake completely by the erosion and incorporation of Cromwell Formation sediment Figure 4. The Superior lobe paused in its retreat from the St. Croix Superior-lobe ice locally eroded deeply into the substrate, in places exploiting drained down the St. Croix River valley. A large outwash plain formed south of by the overriding ice of the Grantsburg sublobe. Small lenses moraine at the St. Stephen ice margin, before rapidly retreating across pre-existing bedrock valleys. These tunnel valleys in places were reoccupied by Benton County (Cooper, 1935; Meyer and Knaeble, 1996; Meyer, 1998). of stratified sediment are common in many areas. Cromwell Benton County. proglacial meltwater of the receding Superior lobe. Most of the southwesterly The Des Moines lobe and Grantsburg sublobe eventually retreated beyond the Formation deposits are commonly within 10 feet (3 meters) of the trending valleys in the county likely originated as subglacial valleys. St. Croix moraine, and the Mississippi River, still greatly swollen by meltwater, surface, and locally are at or very near the surface. Commonly glacial striation measurement—Arrow shows direction of ice flow (Upham, 1888; Jirsa, became restricted in a narrow channel from Sartell to St. Cloud as it began to cut water-washed and overlain in places by a few feet (1 meter) of M.A., unpub. data). through hard glacial till of the Cromwell Formation. The river, probably running fluvial or eolian sand in the vicinity of units Qno and Qwr. Figure 1. The map units from the 1:100,000 surficial geology map are combined into three simplified units in Line of geologic cross section illustrated on Plate 4, Quaternary Stratigraphy. in multiple channels, continued to meander widely to the north and south of this A A' Qnw Outwash over Cromwell Formation deposits—Sand, gravelly this figure: green is less permeable diamicton (glacial till) and bedded silt and clay; yellow is more permeable, narrow channel, where it was down-cutting softer sand and silt. Eventually the sand, and cobbly gravel less than 20 feet (6 meters) thick over bedded sand to gravel; and purple is bedrock. Water bodies are white. Superior-lobe ice river became restricted to its present channel, leaving a broad terrace north of Cromwell Formation till, sand, or gravel. Includes areas where Sartell and south of St. Cloud (unit ; Meyer and others, 2001). REFERENCES Qwr Cromwell Formation deposits are at or near the surface. HUDSON EPISODE EVENTS Cromwell Formation (Wright and others, 1970)—Glacial and fluvial Cooper, W.S., 1935, The history of the upper Mississippi River in late Wisconsin and postglacial time: Minnesota sediment of Superior provenance (Table 1), deposited by the Geological Survey Bulletin 26, 116 p. While the channels of the Mississippi River and its tributaries were being Superior lobe and its meltwater. Included in these map units are Ellingson, J.B., 2002, Aggregate resources, Benton County, Minnesota: Minnesota Department of Natural established, organic debris (unit Qp) began to accumulate in ice-block melt-out some low-lying areas where Cromwell Formation sediment may be St. Croix moraine Resources, Division of Lands and Minerals Report 305, 4 pls., scale 1:100,000. depressions, drumlin swales, and abandoned drainageways across the county. Little Rock Lake formed at this time, and as the Mississippi River continued to down- overlain by 3 feet (1 meter) or more of sandy to clayey, organic- Johnson, M.D., 2000, Pleistocene geology of Polk County, Wisconsin: Wisconsin Geological and Natural cut its channel, the lake level dropped, exposing nearshore sediments (unit ). A bearing colluvium, or by thin peat. Where mapped within the History Survey Bulletin 92, 70 p., 1 pl., sclae 1:100,000. Ql study of sediments cored from Lake Ann in bordering Sherburne County (Keen and basin of glacial Lake Grantsburg, Cromwell Formation deposits Benton County Johnson, M.D., Addis, K.L., Ferber, L.R., Hemstad, C.B., Meyer, G.N., and Komai, L.T., 1999, Glacial Lake Shane, 1990) indicated that eolian sand dunes in western Benton County (unit ) are commonly reworked at the top and are mantled in places Lind, Wisconsin and Minnesota: Geological Society of America Bulletin, v. 111, no. 9, p. 1371-1386. Qe may not have formed until an extended dry period, beginning about 8,000 years too patchy to map by generally less than 5 feet (1.5 meters) of Johnson, M.D., and Hemstad, C., 1998, Glacial Lake Grantsburg: A short-lived lake recording the advance and ago (based on radiocarbon dating). However, evidence elsewhere in Minnesota younger deposits. Beyond the bounds of glacial Lake Grantsburg, retreat of the Grantsburg sublobe, in Patterson, C.J., and Wright, H.E., Jr., eds., Contributions to Quaternary implies some eolian activity began earlier (Marlow and others, 2005), possibly the Cromwell Formation is commonly mantled by up to 3 feet (1 studies in Minnesota: Minnesota Geological Survey Report of Investigations 49, p. 49-60. as early as the retreat of the Superior lobe from the county, when vegetation was meter) of windblown silt (loess), especially in the northeast portion Johnson, W.H., Hansel, S.K., Bettis, E.A., III, Karrow, P.F., Larson, G.J., Lowell, T.V., and Schneider, A.F., apparently sparse and sand was at the surface over much of the area. of the county (Natural Resources Conservation Service, 2009). 1997, Late Quaternary temporal and event classifications, Great Lakes region, North America: Quaternary Qco Outwash—Sand, gravelly sand, and gravel. Cobbly in places, Figure 5. Retreatal position of the Superior lobe just prior to its final Research, v. 47, p. 1-12. ACKNOWLEDGEMENTS especially near till and ice-contact deposits. Laid down by exit from Benton County. Keen, K.L., and Shane, L.C.K., 1990, A continuous record of Holocene eolian activity and vegetation change meltwater issuing from the ice margin; generally bounded by Robert Kozel and Gerald Hovde of the Benton County Highway Department at Lake Ann, east-central Minnesota: Geological Society of America Bulletin, v. 102, p. 1642-1657. scarps. Where patchy it is commonly less than 10 feet (3 meters) arranged permission for drilling in county road right-of-ways and gave access to thick. Knaeble, A.R., and Meyer, G.N., 2007, Quaternary stratigraphy, pl. 4 of Setterholm, D.R., project manager, county bridge boring records. Thanks are extended to Gary Johnson-Cheeseman, Outwash, sandy—Fine- to medium-grained sand, generally coarsening Geologic atlas of Todd County, Minnesota: Minnesota Geological Survey County Atlas C-18, pt. A, 6 pls., Gordon Jurek, Ronald Larson, Michael Schneider, and Allan Wollak, landowners Qcf downwards to sand to gravelly sand. The unit may have been scale 1:100,000. who allowed rotary-sonic drilling on their property, and to all gravel pit operators deposited, at least in part, in deltaic and lacustrine environments Marlow, L.M., Larson, P.C., and Mooers, H.D., 2005, Field trip 3, Day 2—Deposits along the central and eastern and landowners who gave permission to examine exposures on their property. (possibly the northwest end of glacial Lake Lind). The surface of parts of the St. Louis sublobe, in Robinson, L., ed., Field trip guidebook for selected geology in Minnesota the deposits has been pitted both by ice-block melt-out and wind and Wisconsin: Minnesota Geological Survey Guidebook 21, p. 51-72. erosion. Meyer, G.N., 1998, Glacial lakes of the Stacy basin, east-central Minnesota and northwest Wisconsin, in Patterson, DESCRIPTION OF MAP UNITS Ice-contact stratified deposits—Sand, gravelly sand, and cobbly C.J., and Wright, H.E., Jr., eds., Contributions to the Quaternary of Minnesota: Minnesota Geological Qci Benton County QUATERNARY gravel; deposited by meltwater flowing at or behind the ice margin. Survey Report of Investigations 49, p. 35-48. Figure 2. Physical relief of the land surface in Benton County. Elevation is shown by color: red (higher Sediment can be quite variable and is typically faulted and folded. ———2008, Surficial geology of the Mora 30 x 60 minute quadrangle, central Minnesota: Minnesota Geological surface elevation) grading to blue (lower surface elevation). A false sun illumination at an elevation of 30° Commonly includes interbeds of, and in places is capped by, sandy Survey Miscellaneous Map M-180, scale 1:100,000. Qe Eolian sand—Very fine- to medium-grained sand; more than 3 feet (1 from the northwest (315°) provides contrast (gray shadowing) to accent details of landforms. The map was to silty diamicton (mudflow sediment) and silt (lake sediment). Des Moines-lobe ice meter) thick; windblown into low-lying dunes. Meyer, G.N., and Hobbs, H.C., 1993, Quaternary geologic map of Sherburne County, Minnesota: Minnesota created using the U.S. Geological Survey's Digital Elevation Model with a 30-meter grid. St. Croix moraine Some deposits contain boulders. Most deposits were laid down Glacial Lake Grantsburg Alluvial fan deposit—Slopewash sediment consisting of loamy Geological Survey Miscellaneous Map M-77, scale 1:100,000. Qf by meltwater in coalescing fans at the ice margin, but some were fine-grained sand to gravelly sand. Contains minor amounts of Meyer, G.N., and Knaeble, A.R., 1996, Quaternary geology, in Meyer, G.N., and Swanson, L., eds., Text deposited beneath or surrounded by ice. After melting of the ice, disseminated organic debris. Deposited at the base of steep slopes supplement to the geologic atlas, Stearns County, Minnesota: Minnesota Geological Survey County Atlas these deposits generally stand as positive features. and at the mouths of deep gullies. C-10, pt. C, p. 16-39. Qct Supraglacial till—Chiefly sandy loam-textured, unsorted sediment Ql lacustrine deposit—Beach and nearshore sediment. Consists of sand Winnipeg Meyer, G.N., Knaeble, A.R., and Ellingson, J.B., 2001, Surficial geology of the St. Cloud 30 x 60 minute quadrangle, (diamicton), with pebbles, cobbles, and boulders; sandy silt to Provenance to silty, very fine-grained sand with common shells; organic-rich Riding Mountain central Minnesota: Minnesota Geological Survey Miscellaneous Map M-115, scale 1:100,000. cobbly gravel lenses are commonly present, particularly in the Provenance layers in places. Unit is mapped only along Little Rock Lake; other Grantsburg-sublobe ice vicinity of units and . The unit is generally more than Meyer, G.N., and Patterson, C.J., 1999, Surficial geology of the Anoka 30 x 60 minute quadrangle, Minnesota: deposits along the edges of lakes and marshes are too narrow to be Qci Qcs Minnesota Geological Survey Miscellaneous Map M-97, scale 1:100,000. 10 feet (3 meters) thick over more dense and massive subglacial Rainy Figure 6. Maximum extent of the Grantsburg sublobe and glacial shown. Provenance Minnesota Department of Transportation, 2009, Office of materials and road research (borings): St. Paul, Minn., till. Beyond the bounds of glacial Lake Grantsburg, the unit is Lake Grantsburg in Benton County. qa Floodplain alluvium—Sediment of modern rivers. Unit is typically
! !
! meltwater channels. Generally mapped only where unit is greater ! Benton County was initially covered by ice carrying ! ! Till, sand and gravel—Sandy till capped by, and/or interbedded Des Moines lobe ! Qcs St. Croix! phase ! than 4 feet (1.3 meters) thick. Includes fine-grained organic matter ! Des Moines-lobe ice ! debris of both Rainy (Wadena lobe) and Superior ! ! with, sand and gravel. Locally, consists of patchy till over thick ! ! St. Croix moraine ! Grantsburg ! Table 1. Physical characteristics of glacial deposits in the Benton County region. laid down in ponded water, and marl (calcareous clay) at depth in ! sublobe provenance. The approximate interlobate boundary Duelm channel ! ! deposits of sand and gravel. May include small bodies of thick ! ! Glacial Lake Grantsburg ! places. Also includes narrow deposits of alluvium along streams, ! and easternmost extent of the Wadena lobe is shown ! ! silt. Exhibits gradational contacts with units and . Glacial ! ! Qct Qci ! PROVENANCE RIDING MOUNTAIN WINNIPEG RAINY SUPERIOR ! narrow beach deposits, and small bodies of open water. Where by the dashed line running through western Benton and fluvial sediment are too intricately associated to distinguish TILL TEXTURE Sandy loam to Loam to silt loam Loam to Loam to crossed by roads, the unit has been buried under artificial fill; the County. When the Wadena lobe waned, the Superior at map scale. Pre-Wisconsin lobe flowed west, completely covering the county sandy clay loam to clay loam sandy loam loamy sand organic sediment is commonly removed prior to filling in areas deposits where major structures are built. (the line of maximum extent is dotted where covered TILL COLOR PALEOPROTEROZOIC AND ARCHEAN by the Des Moines lobe). After the retreat of the Oxidized Light olive-brown Light olive-brown Yellow-brown Brown to red-brown West Campus formation (Meyer and Patterson, 1999)—Fluvial sand Grantsburg-sublobe ice < Undifferentiated Paleoproterozoic and Archean rocks—Bedrock at or Superior lobe, the flank of the Grantsburg sublobe Unoxidized Gray to dark gray Gray to dark gray Gray to brown-gray Gray to red-gray and gravelly sand of mixed provenance. Coarsens to cobbly A near the land surface; consists of intrusive, gneissic, and schistose of the Des Moines lobe moved into southernmost Figure 7. Isolated water bodies connected by the Duelm channel to PEBBLE TYPE gravel in places. Unit was laid down during early, higher stages rocks. For details, see Plate 2, Bedrock Geology. Benton County. a reduced glacial Lake Grantsburg. Carbonate Common Common to abundant Uncommon Rare to uncommon Gray-green rock Uncommon to common Uncommon to common Uncommon to common common to abundant Red felsite Rare to uncommon Absent to uncommon Rare to uncommon Uncommon to common Every reasonable effort has been made to ensure the accuracy of the factual data on which this map Gray shale Uncommon to common Absent to rare Absent Absent 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 Minnesota Geological Survey in St. Paul. In addition, effort ©2010 by the Regents of the University of Minnesota 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 The University of Minnesota is an equal opportunity educator guide engineering-scale decisions without site-specific verification. and employer GEOLOGIC ATLAS OF BENTON COUNTY, MINNESOTA