STEWART KNOWITALL: Austin asks, "how was Michigan's Upper Peninsula formed?"
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Austin, your question is a great one! As you probably know, a peninsula is a long piece of land that is surrounded by water on all sides except for one, where it is connected to the mainland. Michigan’s Upper Peninsula exists because of the Great Lakes that border it on three sides: Lake Superior to the north, Lake Michigan to the south, and Lake Huron to the east.
So the answer to your question is also about how and why these lakes formed where they did. About 80,000 years ago a huge mass of glacial ice called the Laurentide Ice Sheet began to form and spread southward from the Hudson Bay region in Canada, which is north of Michigan. We can know from different glacial sediments and landforms that as the ice moved into the Great Lakes region it traveled as several different lobes. These lobes of ice moved along stream valleys that already existed around the Upper and Lower Peninsulas of Michigan. As they moved, the thick, heavy ice lobes dug deeply into the rocks and carved the original valleys into long, deep basins. As the ice lobes continued to move south they also covered the Upper and Lower Peninsulas but these areas were highlands, so the ice was thinner, and they are made of harder bedrock than in the valleys. For these reasons the glacier could not erode the two peninsulas as deeply as the basins.
By about 20,000 years ago the ice sheet had reached south into central Ohio, Indiana, and Illinois. After that it began to melt back to the north and eventually uncovered the Great Lakes region. The deep basins carved by the ice lobes filled with water to become the Great Lakes and the harder rock highlands were left standing above the lakes as the Upper and Lower Peninsulas of Michigan. We are very fortunate to live in such a beautiful and special place of rock shaped by ice.
Austin, I hope this has helped answer your question about how the Upper Peninsula formed. As to why the peninsulas are made of harder bedrock than the lake basins, that is another story for another time! For anybody who wants to learn more about the geology and geography of Michigan, an excellent resource is Michigan Geography and Geology, 2009, edited by Randall Schaetzl, Joe Darden, and Danita Brandt.
Professor of Geology at Ferris State University
He teaches non-geology majors about the Earth system through courses in Physical Geology, Historical Geology and Earth Systems Science. All of his classes include a day trip to Sleeping Bear Dunes National Lakeshore, and he has led students on two-week summer field trips to explore the geology of Michigan, the Canyon Country of southern Utah, and the national parks of the Rocky Mountain region.