A Connecticut Department of Energy & Environmental Protection and University of Connecticut Partnership
Geological > Geology of Long Island Sound
Figure 6:  21,500 - 17,500 Years Ago
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A View of Connecticut and the Moraines on Long Island as the Last Glacier Was Melting Out of Glacial Lake Connecticut, About 17,500 Radiocarbon Years Ago

It took about 1,500 years for the Wisconsinan glacier to melt back from its position along the north shore of Long Island to the position depicted here. As this happened, melt water from the glacier formed glacial Lake Connecticut, which was dammed by the Orient Point-Fishers Island moraine. A similar lake, dammed by the terminal moraine, existed in Block Island Sound.

The lakes remained fresh and the continental shelf to the south remained exposed because the sea had not yet risen sufficiently to encroach on the area. The melting glacier supplied a tremendous volume of fine-grained sediment to glacial Lake Connecticut. Glacial lake clay deposits, up to 150 meters thick, had almost completely filled in the Long Island Sound basin by the time the glacier had melted out of the lake. As the Connecticut coast emerged from under the glacier, streams reoccupied glacially modified bedrock valleys. These streams delivered more sediment to Lake Connecticut as they built deltas at their mouths. The net result was that the last glacier did more to fill in the Long Island Sound basin than it did to carve it out.

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