Bedrock Topography of Iowa

A map of bedrock topography portrays the hills and valleys that are present on the bedrock surface, beneath the sands, gravels, glacial till, and other unconsolidated materials in Iowa. Geological Survey Bureau geologists completed a series of county bedrock topography maps for about 90% of the State of Iowa in 1991 as a part of the production of a map of Groundwater Vulnerability. The area not yet completed is in the northeast corner of Iowa, an area of many bedrock exposures where unconsolidated cover is thin.

The map shows that the highest area of bedrock (over 1400 feet above sea level) lies in northwest Iowa (in Lyon County) where the Precambrian-age Sioux Quartzite, a dense, hard, pink rock is exposed at Gitchie Manitou State Preserve and in a farm field about 2 miles east of the park. The lowest area (less than 300 feet) is within an abandoned and buried channel of the ancestral Mississippi River that cuts across Lee County in southeast Iowa.

The Iowa bedrock has been sculpted by numerous river channels that cut into rock both before and between separate advances of continental ice sheets into the state beginning about 2.5 million years ago. One of these ancient river valleys, called the Fremont Channel, is one of the most dramatic features on the Iowa bedrock surface. The Fremont Channel is a north-south trending valley in western Iowa, that begins in Dickinson and Emmit counties and crosses into Missouri through Fremont County, and may be considered the trace of an "ancestral Missouri River" that was present over 500,000 years ago. The Fremont Channel is incised into bedrock more deeply than the modern Missouri River.

For more information contact Ray Anderson e-mail: Raymond.Anderson@dnr.iowa.gov at (319)335-1575.