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.