The Precambrian Peninsula

The western third of Michigan's Upper Peninsula, including the Keweenaw Peninsula, does not lie within the Michigan Basin. Rather, from a geophysical point of view, it is part of the Wisconsin Dome, which also includes much of northern Wisconsin and northeast Minnesota; in this region, which blends with the Canadian Shield, Precambrian rocks outcrop at the surface since overlying, younger sediments were either scraped away by the Pleistocene glaciers or eroded away in the distant past due to the general uplift of these ancient basement rocks.

We decided to give our legs a rest today and took a road trip to Copper Harbor, stopping at various lighthouses and beaches along the way. The highlight of the trip was the view from Brockway Mountain, just outside Copper Harbor; from this summit, one enjoys a panorama of Lake Superior and the northern Keweenaw Peninsula, including the shipping lanes, the rocky shoreline, inland lakes and colorful, forested ridges. Wherever we stopped, outcrops of Precambrian rock, volcanic or continental, dominated the scene; along portions of the coast, hogbacks and shoals of ancient basalt created a fascinating and scenic topography.

As I discussed yesterday, the Porcupine Mountains are volcanic in origin, as are some other ridges and uplifts across the Wisconsin Dome. Other ranges, such as the Huron Mountains east of Keweenaw Bay, have eroded from localized Precambrian domes (similar to the formation of the Black Hills and Adirondacks). Of course, the highlands across the Wisconsin Dome are among the oldest in North America (1-3 billion years old) and, over time, have eroded into hills and relatively low ridges. Just knowing you are walking on or amidst rock that pre-dates terrestrial life is an exhilarating experience.