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Great Lakes Region: Roles of Weathering, Erosion & Glaciation

Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

The Great Lakes and their surrounding region were shaped by the most recent ice sheet advances. This lesson is going to examine how glaciers can alter the landscape and the resulting features you see.

America's Great Glaciation

If you had to pick one word to describe the landscape of the Great Lakes region, you'd probably pick 'flat'. You'll find lots of lakes in that area of the U.S., but very few large hills. The area's distinctive flat topography is due to ice sheets that covered the midwestern United States from approximately 85,000 to 16,000 years ago.

These ice sheets -- also known as glaciers -- basically acted like nature's bulldozers. As ice sheets flowed over the earth, they pushed the dirt and underlying rock in front of and to the sides of the ice. This action, coupled with the drainage under and around the glaciers, produced some of the characteristic features we see in the Great Lakes Region.

Glaciers are bodies of ice that show internal deformation and flow under the force of gravity. Currently, glaciers are either constrained to mountain ranges or the limited continental glaciers as seen in Greenland and Antarctica. During periods called ice ages, the continental glaciers grow to cover larger portions of the earth, starting from the poles. The most recent one of these periods that affected the U.S. was the Wisconsin Glaciation from approximately 85,000 to 16,000 years ago.

One lobe of the continental glacier on Greenland
Greenland glacier

Glacial Advance and Erosion

But how, exactly, did glaciers shape the Great Lakes region? Well, much of it was due to glacial advance and erosion. When a glacier flows across a landscape, initially the tremendous weight and force of the ice will push the underlying dirt and rock out of the way. This bulldozer reaction creates moraines, which are mounds of dirt and rock that are formed in front of and on the sides of the glacier as it flows. In fact, moraines are one of the main ways that geologists track glacial advances because they mark the farthest location that a glacier traveled.

Terminal moraine at the end of a glacier
Moraine at the end of a glacier

Otherwise, the glacier creates a flat landscape during advance. When the glacier begins to retreat, meltwater reworks the relocated soil to create interesting landscapes beneath and adjacent to the glacier. These glacial deposits are some of the only topography that is found in this region.

Glacial Retreat and Deposition

Beneath melting glaciers, streams carry soil and rock outward in tunnel features. Once the glacier has totally melted, these streams leave behind a hill of coarse gravel called an esker.

Long sinuous esker left behind from an ancient glacier
Grass covered esker

There are two types of lakes that can form as glaciers retreat: kettle lakes and moraine-dammed lakes. Kettle lakes form when chunks of ice are left behind after a glacier has melted away, and they eventually also melt to form steep-sided lakes. Moraine-dammed lakes form when meltwater is trapped behind the moraine left behind at the end of a retreating glacier.

The Great Lakes actually began as moraine-dammed lakes. The area was already a depression, but the glaciers further scoured out these future lakes. As the ice retreated, the meltwater filled in the depressions, which were further supported by the moraines holding in the water, much like dams.

Moraine-dammed lake trapped by the moraine in the lower left corner of the photo
Glacially dammed lake

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