Near Fort Snelling, ten thousand years ago, melt water from the Wisconsin glacier was discharged through the Mississippi River and plunged over a ledge of Platteville limestone into a gorge cut chiefly in the white St. Peter sandstone. The undercutting action in the soft sandstone caused the limestone ledge to break off with a vertical face, thus maintaining the falls while causing them to retreat upstream. When the falls in the main channel passed the upper end of the island where the Soldiers Home now stands, the entire flow in the river was diverted to the main gorge and the falls in the west channel were abandoned. This unique and unusual geologic feature, an abandoned waterfall, is located at the north end of the former west channel. The cataract in the Mississippi has migrated to St. Anthony Falls and Minnehaha has retreated from the abandoned channel to its present location, where the undercutting action responsible for the migration is apparent.
