Geology
The Great Lakes are in a constant state of change. Water levels rise and fall; cliffs and dunes are eroded by wind and sea. The Lakes have existed for about 10,000 years in a variety of forms. Formation of the Lakes was caused by the slow and deep gouging of the earth by the Wisconsin Glaciers. About 12,000 years ago, temperatures began to warm globally, and the great sheets of ice started to recede. This left behind large bowl-like land depressions that filled with the melt waters. They also began to fill from nearby large rivers. It was only about 2,000 years ago, however, that the Lakes took on the configuration that we see today.
Walking along the shores of Lake Erie, one can find evidence of the past in the forms of rock carried and deposited by the glaciers, and layers of strata exposed by the cutting and gouging action of the glaciers: Kelleys Island glacial grooves, for example, or Devonian shale along the Rocky River. The glaciers also left behind the beginning of some of the most fertile soils in the nation.