Unconsolidated deposits, mainly glacial drift, overlie the bedrock surface in most of Illinois. Glacial drift ranges from less than a few feet to approximately 600 feet in thickness. The thickest drift occurs in major preglacial valleys cut into the bedrock and filled with glacial sediments. Regionally thick drift occurs in the N.E. portions of the state. The thinnest drift, less than 25 feet thick and intersected by numerous bedrock outcrops, occurs widely in southern and western portions of the State. Drift, as shown on this map, may include any of the following: unconsolidated deposits, glacial drift, Wisconsinan deposits, Illinoian deposits, till, glaciofluvial deposits, glaciolacustrine deposit, wind-blown deposits (loess).
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.
In April 2004 edition 2.0 of the data set was transitioned to storage in an Oracle RDBMS using the ArcSDE geodatabase model. The result (this version) is designated Edition 20040423.
Edition 1.0 of this data set was replaced edition 2.0 in June, 1998. The data were reviewed as described in the logical consistency report and process steps for May, 1998. Contour values, which were not included in edition 1.0, were included in version 2.0.
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.
Some changes were made, as follows:
Arcs were coded with contour values. This was done with an AML program that assigned contour values to arcs based on the drift thickness values for the adjacent polygons. This processing was done in late 1996, but the updates were not posted to the general-use data set until June of 1998.
The existing set of tics was replaced by the set of tics from the coverage RE-STATE1. These were derived from 7.5 minute quadrangle corners (data set QUAD75) and give a good distribution of tics immediately outside the state boundary.
COVER-ID values were recalculated to be unique.
The features were imported into the ISGS SDE enterprise database following steps outlined in ISGS procedure GISDB_0009.
Polygon Attributes: THICK - thickness of drift 1 - Less than 25 feet 2 - 25 to 50 feet 3 - 50 to 100 feet 4 - 100 to 200 feet 5 - 200 to 300 feet 6 - 300 to 400 feet 7 - 400 to 500 feet 8 - Greater than 500 feet Thick. 9 - No Drift (Driftless Regions)
RANGE - range of thicknesses in feet for that polygon
Arc Attributes: CONTOUR - The contour value for the arc.
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Illinois Natural Resources Geospatial Data ClearinghouseGenerated by mp version 2.8.25 on Thu Apr 02 09:54:03 2009