The Great Lakes
1. Pre Glacial Plate Tectonics and Volcanic Activity with formation of the Mid Continent Rift and the St. Lawrence Rift forming the basins for Lake Superior and the lower Great Lakes and St. Lawrence River.
2. Before glaciation, Lake Huron was a depression through which flowed the now-buried Laurentian and Huronian Rivers; the lake bed was criss-crossed by a network of tributaries to these ancient waterways, with old channels still evident on bathymetric maps.
3. The basins that contain the Great Lakes are the product of repeated scour and erosion of weak bedrock by continental glaciers that advanced into the Great Lakes watershed beginning as early as 2.4 Ma. Most of the scouring, however,
occurred after about 0.78 Ma when episodic glaciation of North America was more extensive.
4. Stratigraphic evidence from outside the watershed indicates that glacier ice extended over all or part of the watershed at least six times since 0.78 Ma. extending as far south as Kentucky.
5. The last glaciation of the Great Lakes watershed is well documented by glacial sediments, recessional moraines, and buried organic deposits. The eastern part of the watershed was first glaciated between 65 to 79 ka and that the ice margin oscillated there until about 25 ka.
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