20250530 14565800 1.0 TRUE TRUE Driftless_Areas This is a polygon feature class containing driftless areas in Illinois. These areas are analogous to areas of the state where the bedrock surface is not buried. Data are originally from the Buried Bedrock Surface map (1:500,000) by Herzog et al (1994) and the related feature class Bedrock_Topography_1994_Ln. There are no attribute data associated with the spatial data in this data set. The nominal 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 for more information. geology bedrock driftless drift glacial surface surficial Illinois Driftless areas in Illinois. These areas are analogous to areas of the state where the bedrock surface is not buried by glacial drift. Data originally from the Buried Bedrock Surface map by Herzog et al. (1994). There are no user defined attribute items. ISGS, 1994 (revised 1998). 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