This feature dataset shows continuous lines representing the lowermost elevations/locations of (buried) bedrock valleys in the state of Illinois. Additionally, names of buried valleys are annotated. Scale is 1:500,000. Several major bedrock valleys are found across the state. The greatest control for the present bedrock surface configuration was probably the preglacial surface configuration. 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. Please refer to the metadata for the parent coverage, Buried Bedrock Surface of Illinois (GISDB_BEDGEO.IL_Bedrock_Topography_1994_Ln) for more information.
