Answer:
About 20,000 years ago, the climate warmed and the ice sheet retreated. Water from the melting glacier filled the basins , forming the Great Lakes. Approximately 3,000 years ago, the Great Lakes reached their present shapes and sizes. How They Were Made. Thousands of years ago, the melting mile-thick glaciers of the Wisconsin Ice Age left the North American continent a magnificent gift: five fantastic freshwater seas collectively known today as the Great Lakes — Lake Superior, Lake Huron, Lake Michigan, Lake Erie and Lake Ontario.
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