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Buried Bedrock Surface: Illinois, 2004

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Herzog, B.L.. B.J. Stiff, C.A. Chenoweth, K.L. Warner, J.B. Sievering, C. Avery and Illinois State Geological Survey
The Buried Bedrock Surface of Illinois map represents the first revision of the bedrock topography map since the second edition was published by the Illinois State Geological Survey (Horberg, 1957). The map is useful for the regional and statewide studies such as groundwater resource planning and geologic mapping. The data included on this map were compiled by many downhole logging and surface studies. This information was combined to revise the bedrock topography map for the entire state. Since some of the most extensive and productive aquifers in the state are located in thick glacial sediments of major buried bedrock valleys, special attention was given to data for known bedrock valleys. The revised map was published by the ISGS (Herzog et al, 1994) and much of the following text appears on the published map. The bedrock topography is shown as 50-foot (15-meter) contours of bedrock elevation above mean sea level (msl). Bedrock elevations range from less then 250 feet (76 meters) in southern Illinois to more than a 1000 feet (300 meters) in the northwestern part of the state. Several major bedrock valleys are found across the state. The greatest control for the present bedrock surface configuration was probably the preglacial surface configuration. The interpretations of bedrock topography have been modified since the 1950's. Horberg contoured his map with the philosophy that the bedrock surface map represented the preglacial topography modified by glacial erosion, which formed U-shaped valleys. The current map, showing narrow valley channels and bedrock knobs in wide channels, indicates complex preglacial and glacial erosion primarily from running water. The major valleys were formed before the minor valleys that appear as tributaries. Streams that formed during successive glaciations probably eroded to bedrock and produced the small tributary valleys to the main channels. The bedrock surface was less likely to be eroded as sediments accumulated during each successive glaciation.
University of Illinois Urbana-Champaign
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Illinois, United States
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