Jay Cooke State Park - Geology

Geology

The geologic history of the Lower St. Louis River can be reconstructed from the rocks and sediments exposed in the river bed and along the shoreline. The present river channel was shaped primarily by the glaciers of the Pleistocene epoch, which began approximately 2 million years ago. As glaciers advanced and retreated, receding for the last time around 10,000 years ago, the melting ice and flowing meltwater left behind complex patterns of sediment, including moraines, drumlins, beach sands, and lake-bottom clays. In the places that the river has cut through the glacial sediments, the underlying Middle Precambrian Thomson Formation is exposed. Evidence that the river was once a raging torrent is evidenced by potholes eroded into the Thomson Formation tens of meters above the present river level. Continued erosion along the river valley has exposed tilted black slates and metasediments that now make up a series of gorges, falls, and rapids. Slate beds were formed from original layers of mud and sand, which were deposited in a sea that occupied this area 1.9 billion years ago. These thick deposits compacted into shale (from mud) and greywacke (from sand) and then heat, pressure, and earth movements converted the deeply buried shale into slate. Later, about 1.1 billion years ago, molten rock was forced into fractures in the beds and when they cooled, these intrusions formed volcanic dikes which can be seen along the river bed today. Some dikes are visible just north of the bridge near Thomson.

The bedrock over which the St. Louis River flows is part of the Canadian Shield, the stable ancient core of the North American continent. On the Wisconsin side of the river, bedrock is buried beneath thick layers of red clay, silt, and sand– remnants of a time over 11,000 years ago when the area was covered by glacial Lake Duluth, which formed as meltwater was trapped in front of the ice of the retreating Superior Lobe of the Laurentide Ice Sheet. About 12,000 years ago Glacial Lake Duluth reached about 165 m above its present height--there are wave-sculpted cliffs along Skyline Drive in Duluth.