Bedrock topography of northeast 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 foot (0.3 m) to more than 400 ft (18 m), respectively. The configuration of the bedrock surface is the result of a complex system of ancient drainage courses when were developed during a long period of preglacial erosion and during shorter, but more intense, periods of interglacial erosion.