Herzog, B.L., B.J. Stiff, C.A. Chenoweth, K.L. Warner, J.B. Sievering, and C. Avery
Topography of the bedrock surface in Illinois. Bedrock topography is shown as 50-foot (15-meter) contours of bedrock elevation above mean sea level (msl). Several major bedrock valleys are found across the state. Attributes include contour values. ISGS, 1994.