4" Midcontinent Rift Section - Ontario, Canada

The Midcontinent Rift was a major geologic event that occurred approximately 1.1 billion years ago in what is now the Lake Superior region. During this time the Midcontinent Rift System, also known as the Keweenawan Rift, was splitting east and west North America apart from each other. The rift spanned from Oklahoma/Kansas up through Iowa and Minnesota, arching over into Ontario and back down through Michigan, possibly into Ohio. It's believed that compressive forces from the Grenville Orogeny played a role in the rift's failure, leaving behind faults and zones of fracturing in the host rock, along with deposited gabbro, basalt, and granite intrusive rocks.

During the Pleistocene Epoch, glaciers carved out the rift valleys which were later filled in with water, resulting in Lake Superior's existence and unique shape.