Geology
The lakes originated as a series of northward-flowing streams. Around two million years ago the first of many continental glaciers of the Laurentide Ice Sheet moved southward from the Hudson Bay area, initiating the Pleistocene glaciation. These glaciers widened, deepened and accentuated the existing river valleys. The glacial debris, possibly terminal moraine, left behind by the receding ice, acted as dams allowing lakes to form. Despite the deep erosion of the valleys, the surrounding uplands show little evidence of glaciation, suggesting that the ice was thin, or at least unable to cause much erosion at these higher altitudes. The deep cutting by the ice left some tributaries hanging high above the lakes—both Seneca and Cayuga have tributaries hanging as much as 120 m above the valley floors.
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