River Warren Falls - Geologic History

Geologic History

The area now occupied by the Twin Cities generally consisted of a 155 foot (47 m) thick layer of St. Peter Sandstone, under a 16 foot (5 m) thick layer of shale, under a 35 foot (11 m) thick layer of Platteville limestone. These layers were the result of an Ordovician Period sea which covered east-central Minnesota 500 million years ago. The hard limestone cap was formed from fossilized shell fish. About 20,000 years ago, the area was covered by the Superior Lobe of the Laurentide ice sheet, which left the St. Croix moraine on the Twin Cities as it receded. Later the Grantsburg Sublobe of the Des Moines Lobe also covered the area. Under these vast layers of ice, tunnel valleys were formed, cutting through the limestone layer with tremendous force, to release ice meltwater and glacier effluence. The result was a series of troughs in the limestone, which were filled by glacial till and outwash deposit as the glaciers receded. Sometimes the sediment would be mixed with huge chunks of ice, which would leave voids in the soil, or kettles. These kettles created basins for the Twin Cities Lakes, such as Harriet and Calhoun. Connecting the city lakes in several north-south arteries are gorges cut through the bedrock, but filled with sand and sediment.