MAP DETAIL from
"Map Showing the Thickness and Character of Quaternary Sediments in the Glaciated United States East of the Rocky Mountains"
U.S. Geological Survey Miscellaneous Investigations Series Map I-1970
1993-1998
by David R. Soller
The Mississippi River was a major drainage for glacial meltwaters, and in most places the river channel is filled with glacial sediment more than 100' thick. In some places, however, the Mississippi River now flows directly on the bedrock -- there, glaciers filled the original river channel with sediment and forced the river to cut a new channel, around the obstruction. These images show this situation, SW of Des Moines, between Keokuk and Fort Madison; the glaciers advanced from the northwest, causing the Mississippi River to migrate to the east, cutting a new channel into thin glacial sediments and bedrock.
