General Geology

Sandstone Cliff

The Cannon River Wilderness Area occupies a valley that was created during the last Ice Age (more than 12,000 years ago). Huge amounts of glacial melt water during the waning stages of the last Ice Age incised the Cannon River through the Wilderness Area.

Bedrock exposed in the valley includes limestone, shale, and sandstone originally laid down during the Ordovician Period (roughly 443-485 million years ago). The river is now cutting through the top of the Shakopee Dolostone (part of the Prairie du Chien Group). If you walk from the bottom to the top of the bluffs on either side of the valley you will hike through successive layers of Ordovician bedrock including the St. Peter Sandstone, the Glenwood Shale, and the Platteville Formation at the top of the hill.  Here is a brief field guide describing the local bedrock.  While you’re unlikely to find fossils in the CRWA park, this guide to southern Minnesota fossils has a more user-friendly diagram of the bedrock stratigraphy on the first page.  There can be a layer of glacial till on top of the Platteville Limestone on the high ridges in the park as shown in Fig. 1 in the Fossils guide, although the Galena and Decorah bedrock units are absent.  The glacial till is much younger than any of the bedrock units and was deposited onto whatever bedrock units were exposed at the surface at the time.

Springs at the base of sandstone contribute to the marshy land in the valley bottom. Current geological thinking is that the spring water originates as artesian flow from the underlying and very porous Prairie du Chien Group, although in the past geologists attributed it to water percolating down through the St Peter Sandstone.

Large, active gullies have formed in the park because of changes in agricultural practices in the fields above the park, affecting the drainage of water from upland areas. The erosion peaked during the late 19th and early 20th centuries; in more recent decades, erosion control practices have helped reduce soil erosion.