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.
The Buried Bedrock Surface of Illinois will be useful in fields such as groundwater resource planning, engineering geology and earthquake hazard mapping. Data shown on this map were compiled from geologic data from drill holes, samples, surface observations and geophysical data from downhole logging and surface studies.
The data are not appropriate as a geodetic, legal or engineering base. The data set was not and is not intended as a substitute for surveyed locations, such as can be determined by a registered Public Land Surveyor. Although useful in a GIS as a reference base layer for maps, the data set has no legal basis in the definition of boundaries or property lines.
Links to these are provided in the Cross References section.
Individuals or entities may make fair use of copyrighted ISGS material, such as reproducing a single figure or table, or using a brief text quotation, without obtaining formal permission, but in all cases the Illinois State Geological Survey must be credited as the source of the material. To reproduce ISGS information beyond the fair use standard, permission must be obtained from the ISGS Information Office, 615 East Peabody Drive, Champaign, Illinois 61820, 217-333-4747, firstname.lastname@example.org. License fees and a license agreement may be required, depending on the proposed usage.
Map information is to be used at a scientifically and cartographically appropriate scale, that is, at a scale no greater than indicated on the map or as described in the documentation of the map or map data. Map information is not appropriate for, and is not to be used as, a geodetic, legal, or engineering base. Map information has no legal basis in the definition of boundaries or property lines and is not intended as a substitute for surveyed locations such as can be determined by a registered Public Land Surveyor.
The data do not replace the need for detailed site-specific studies.
The features were imported into the ISGS SDE enterprise database following steps outlined in ISGS procedure GISDB_0009.
The correction was done with SQL directly against the business table, rather than use SDE versioned editing. The SQL statement used was:
update GISDB.IL_Bedrock_Topography_1994_Ln set contour = 550 where objectid = 938;
CONTOUR The elevation above msl in Lambert feet. Values range from 200 to 1000 with a contour interval of 50 feet. A value of zero (0) is used to indicate the state boundary or areas where bedrock is at or near the surface.
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Illinois Natural Resources Geospatial Data ClearinghouseGenerated by mp version 2.8.25 on Thu Apr 02 09:53:58 2009