Deglacial History and Paleoenvironments of Northeastern Illinois

11 glacial drift units and three bedrock units (Dey et al., 2007b, c, d) as well as an aquifer sensitivitymap (Dey et al., 2007e). The 14 layers were digitally stitched together to form a 1:100,000scale three-dimensional model of the geology of Kane County (Abert et al., 2007; Figure 5). TheIllinois State Water Survey recently adapted this model for groundwater flow by converting it to a3-D array of square grid cells about 200 m across with a minimum thickness, employed in areasof absence, of 0.3 m. The groundwater flow model of the glacial drift and uppermost bedrockconsists of 18 layers and approximately 1.5 million grid cells. The groundwater flow model wascalibrated using stream flow measurements and by numerous water level measurements fromwells open to the various aquifer units, including a “synoptic” measurement of water levels fromover 1000 wells during a 60-day period (Locke and Meyer, 2007). Ultimately, the model will permitplanning officials in Kane County to evaluate water availability under various scenarios ofclimate change to help them plan for future growth.ObjectivesOur discussions will focus on the deglacial history of northeastern Illinois based on identificationof key landforms and their constituent sediment and fossils. We will examine the glacial landscapeof northeastern Illinois and discuss the physical characteristics of the sediment units thatwe have identified and mapped. The succession and unit characteristics are the basis for ourstratigraphic framework; our mapping is influenced further by interpretation of the environment ofdeposition of the units. We will also highlight the geomorphology the area by referring often to ashaded relief maps based on either 10-m DEM data (McGarry, 2000), LiDAR data (Lake CountyDepartment of Information and Technology, 2004), or 30-m DEM data (Luman et al., 2003).Physical SettingNortheastern Illinois is located in a region of varied geomorphologic character (Figure 2). Severalnamed moraines that formed during the last glaciation occur in upland areas. We will be travelingacross the Burlington, Marengo, Woodstock, and Ransom moraines, and the Bloomingtonand Marseilles Morainic Systems (Figure 1). The hummocky or ridge-like topography of themoraines contrasts with intervening tracts of lakebeds and the valleys of waterways. Most rivervalleys were formed by meltwater erosion, in some cases, clearly by the catastrophic release ofmeltwater.Previous InvestigationsDuring the late 19th century, the Geological Survey of Illinois reported on many aspects of northeasternIllinois geology (e.g., Bannister, 1870, 1882). At that time water was being obtained frommajor waterways such as the Fox and Kishwaukee Rivers and from relatively shallow sand andgravel alluvial aquifers. Building stone was mined from dolomite that cropped out along majorrivers (Conover, 1884). Ancient lake bed sediments supplied clay for manufacture of brick anddecorative terra cotta; peat was mined for fuel and soil conditioning.Between World Wars I and II, information gained from the drilling of water wells had become amajor source of geologic information (Udden, 1914) as well as test borings for determination ofdolomite resources (Thwaites, 1923, 1927; Palmer, 1933). Evidence was emerging for the distributionof buried bedrock valleys, filled and covered by more recent glacial sediments, and thecharacter of the deposits within those valleys (Leighton, 1925; Horberg, 1953).In the late 1950’s, geologists at the ISGS began to recognize that glacial deposits are traceablein the subsurface and their relationships could be classified based on stratigraphic principles,like rocks of Paleozoic age. The physical characteristics of glacial deposits such as particle-sizedistribution, clay mineralogy, and clast lithology began to be routinely analyzed on core sub-7