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basin and/or the more eastern Great Lakes region during a pre-Illinois Episode glaciation (Willman and Frye 1970). Various glacial deposits from both episodes have been reported by MacClintock (1929), McKay (1979), and Phillips (2004) in this region. Glacial ice did not reach the study area during the Wisconsin Episode; however, glacial meltwater streams from the upper Mississippi River drainage basin deposited outwash throughout the middle Mississippi River valley. This outwash was the source of the loess deposits (windblown silt) that blanket uplands in southwestern Illinois. During the Illinois and pre-Illinois Episodes, outwash was regionally deposited in the ancestral valleys of Silver Creek (Phillips 2004) and the Kaskaskia River (Grimley 2008), both of which drained to the south and southwest. In response to Mississippi River aggradation, large slackwater lakes formed in many tributary valleys, including glacial Lake Kaskaskia (late Wisconsin Episode) in the present-day Kaskaskia River valley (Shaw 1921, Willman and Frye 1970) and similar slackwater lakes that formed during earlier glaciations. During interglacial (Yarmouth and Sangamon Episodes) as well as postglacial periods, the Kaskaskia River and its tributaries were incised in response to periods of downcutting of the Mississippi River (Curry and Grimley 2006). Thus, the Kaskaskia River valley has experienced a succession of cut-and-fill sequences over approximately the last 500,000 years. The lower Kaskaskia River valley is also the site of numerous archeological sites (Conrad 1966).
Methods
Surficial Map
The surficial geology map is based in part upon soil parent material data (Wallace 1978, Natural Resources Conservation Service 1999), supplemented by data from outcrop studies, stratigraphic test holes obtained for this STATEMAP project, engineering borings from the Illinois Department of Transportation (IDOT) and the St. Clair County Highway Department, coal test borings, and water-well records. Map contacts were also adjusted according to the surface topography, geomorphology, and observed landform-sediment associations.
Localities of important data used for the surficial geology map, cross sections, or landform-sediment associations are shown on the map. All outcrops and stratigraphic test holes are shown on the surficial map, as well as key engineering and water-well borings with confirmed locations. Coal and oil and gas type borings are shown only where utilized for cross sections. The locations of many water wells and coal borings were verified by plat books, permit maps, and/or field confirmations (for water wells only). Many data in this quadrangle are not shown due to poor descriptions of surficial materials or unconfirmed locations. Further information on all data shown, as well as other data, is available from the ISGS Geological Records Unit. Data can be identified based on their labeled county number (5-digit portion of the 12-digit API number).
Cross Sections
The cross sections portray unconsolidated deposits as would be seen in a vertical slice through the earth down to bedrock (vertically exaggerated 20 times). The lines of cross section are indicated on the surficial map. Data used for subsurface unit contacts (in approximate order of quality) are from studied outcrops, stratigraphic test holes, engineering boring records, water-well records, coal test borings, and oil-well records. Units less than 5 feet in maximum thickness are not shown on the cross sections. Dashed contacts are used to indicate where data are less reliable or are not present. The full extent of wells that penetrate deeply into bedrock is not shown.
Bedrock Topography Map
Maps of bedrock topography (fig. 2) and drift thickness (fig. 3) are based on data from which a reliable bedrock elevation could be determined (fig. 2). Data within about a mile of the map were also utilized (not shown). A total of 339 data locations were used, including 2 outcrops, 8 stratigraphic tests, 22 engineering borings, 42 water-well borings, 131 coal borings, 44 oil and gas test borings, and 90 other borings. The bedrock surface was modeled utilizing a “Topo to Raster” program in ArcMap 9.2 (ESRI). This program incorporated a combination of three information types: (1) the 339 data points coded with bedrock top elevations, (2) digitized contour lines coded with bedrock top elevations (from outcrops and soil survey observations), and (3) digitized “streams” (ArcMap term) that forced the bedrock surface model to conform to a typical stream drainage network, guided by geological insights.
Drift Thickness Map
A drift thickness map (fig. 3) was created by subtracting the bedrock topography digital elevation model (DEM) from a land surface DEM, both with a 30-m cell size. Areas shown to have a drift thickness less than zero were reevaluated and modified through use of additional “streams,” artificial points, or reinterpretations of original data. Multiple iterations of this process were repeated until reasonable bedrock surface and drift thickness maps were obtained, reflecting our visualized model of bedrock surface and land surface topographic relationships. For the final bedrock topography map, any bedrock elevations higher than the surface DEM were replaced with the value of the surface DEM, so that the final calculated drift thickness map does not have values less than zero.
Surficial Deposits
The surficial deposits can be divided into four landform-sediment associations: (1) bedrock-controlled uplands in the central portion of quadrangle, with a thin cover of glacial and windblown (loess) sediment; (2) isolated upland ridges and knolls, sporadically distributed, containing ice-contact sediment and capped with loess; (3) broad terraces and tributary valleys (primarily Doza and Mud Creek valleys),
