Bedrock topography of south-central Iowa

The bedrock surface in Iowa is covered nearly everywhere by unconsolidated deposits of glacial drift and alluvium which range in thickness from less than 1 foot to more than 400 feet, and from less than 1 foot to about 60 feet, respectively. The bedrock surface is the result of a complex system of ancient drainage courses, which were developed during the long period of preglacial erosion and during shorter, but more intense, periods of interglacial erosion.