Trail Geology
Linking people with nature by footpath along Lake Superior's North Shore.
Minnesota's shore lands of Lake Superior - the North Shore is a land of rugged, forested hills, sweeping vistas of blue, green, autumn red and gold, and winter white, rocky headlands and crashing waves, cozy valleys and surging waterfalls. The dramatic beautiful landscape that we see today is a consequence of a geological history that goes back more than a billion years into late Precambrian time.
Regional geologic studies have shown that what is now the upper Great Lakes area had undergone several major periods of volcanism, intense deformation of the Earth's crust, sedimentation and mountain-building. By about 1200 million years ago, erosion had eventually reduced the area to a low, rolling plain. There were no Great lakes.
'Then about 1100 million years ago the center of North America began to split apart as slow upwellings in the Earth's stiff-plastic mantle (beneath the crust) began to melt, and huge volumes of molten rock (magma) leaked up to the surface along fissures in the crust.
As the crust was pulled apart, stretched and thinned, and magma erupted onto the surface from the mantle beneath, the center of the rift gradually subsided, leaving the rock layers tilted on the flanks towards the rift axis. Erosion during the last billion years has etched out these tilted layers to form the Sawtooth Mountains in Cook County. These are a series of long ridges with a relatively gentle southeast slope toward Lake Superior and a steep northwest slope, each one sculpted from a single huge lava flow.
The last chapter in the saga of the North Shore's landscape is the Great Ice Age. Several times during the last two million years (most recently only about 14,000 years ago) great continental glaciers, up to one or two miles thick, built up and oozed southward from Canada.
The great ice streams were mainly eroding the underlying rock, some of which had become deeply weathered. Moving southwestward, the Superior Lake of the ice sheet carried debris (including volcanic rocks, agates and sandstone) from the North Shore area as far as the Twin Cities, the Minnesota River Valley, and even to Iowa. The ice found the sedimentary rocks in the middle of the old Mid-continent rift System to be relatively easy to erode, and it excavated what was to be the Lake Superior basin well below sea level. As the glacier melted back about 11,000 years ago, it uncovered this great scooped-out depression which of course filled with water. Early stages (such as Glacial Lake Duluth) were several hundred feet higher than the present Lake, because the ice was still blocking the outlet. Look for rounded beach stones along the Trail, high above the current Lake level. About 5000 years ago, Lake Superior as we know it today was well established. Since glaciation the forests have covered the land, the North Shore rivers have been eroding their gorges, and waves have been making beaches and eating away at the shore cliffs and bluffs.
As you hike the trail, remember this geologic history that has shaped the landscape. look for evidence of volcanic activity, the squeezed in intrusions, glacial erosion and deposition, abandoned beaches far above the present Lake level and on-going geologic processes. Enjoy the Geologic Dimension.
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