Bedrock topography of north-central Iowa

The bedrock in Iowa (Hershey, 1969) is generally overlain by deposits of glacial drift and alluvium, which range in thickness from less than 1 ft to more than 400 ft, and from less than 1 ft to about 60 ft respectively. The configuration of the bedrock surface 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 mroe intense, periods of interglacial erosion.