Shaded Relief Map of the Bedrock Surface of Iowa Most areas in Iowa are covered by multiple layers of soft, easily-eroded sediments consisting of sand, silt, gravel or clay, ranging in thickness from a few feet to several hundred feet. Underneath these deposits is the bedrock surface. This image map shows what this surface would look like if the overlying materials were stripped off. It reveals a landscape that is similar to the present topographic surface in some areas, while other areas are quite different with large buried river valleys that do not show in the present surface at all. One such valley is in western Iowa, which stretches from northwest Missouri to the Minnesota border. Other buried valleys are in Scott, Muscatine and Lee Counties representing former channels of the Mississippi River.