Hiking the Parks
The bedrock exposed along Lake Superior’s North Shore has a geologic history that goes back some 1.1 billion years. During the dramatic volcanic activity of that time, molten lava poured through great fissures that developed in the Earth’s crust. One particular flow complex, the Devil’s Kettle rhyolite flow, visible along the Brule River, is thought to be as much as 770 feet thick. As these flows accumulated, the land along the rift zone sank to form a great basin, into which huge volumes of sediment were deposited after volcanic activity ended. A long period of erosion followed. The local Sawtooth Mountains of the Grand Marais area are the remnants of these great, tilted lava flows. Much more recently, glaciers took their toll on the area as massive ice sheets gouged out the Lake Superior basin, mainly from the post-volcanic sediments, and scoured the bedrock surface. In Cook County, where the park is located, the glacial action eroded more earth and bedrock than it deposited.