Bedrock topography of central Iowa

The bedrock in Iowa (Hershey, 1969) generally is ovelain by deposits of glacial drift and alluvium. The drift, comprised of glacial till and glacial outwash, varies in thickness from less than 1 foot to more than 400 feet; the alluvium in central Iowa varies in thickness from less than 1 foot to about 60 feet. The configuration of the bedrock suface is the result of a complex system of ancient drainage courses which were developed during a long period of preglacial erosion and during shorter, but more intense, periods of interglacial erosion. This map, for a 10 county area in central Iowa, is the seventh of a series of 9 reports that will provide statewide coverage of the bedrock surface of Iowa.