About 15,000 years ago, lobes of the last Pleistocene glacier gouged the basins of the Great Lakes. Some 10,000 years ago, the melting ice sheet filled the basins and created both Lake Michigan and Lake Erie. Further melting filled the other basins to create the world's largest system of freshwater lakes.
Lake Michigan, the second largest Great Lake, is the only one entirely within the United States. Approximately 118 miles wide and 307 miles long, Lake Michigan has more than 1,600 miles of shoreline. Averaging 279 feet in depth, the lake reaches 925 feet at its deepest point. The lake's northern tier is in the colder, less developed upper Great Lakes region, while its more temperate southern basin contains the Milwaukee and Chicago metropolitan areas.