The bedrock in southwest Iowa is covered by layers of loose materials like glacial drift, alluvium, and loess. Loess, a type of wind-blown sediment, is thickest near the Missouri River, reaching over 200ft in some spots. The total thickness of these loose sediments varies from less than 1 ft to more than 450 ft. The shape of the bedrock underneath is shaped by ancient rivers that formed during periods of intense erosion between ice ages.