When the last of the Pleistocene Glaciers plowed through New England, it molded the landscape, sculpting the mountains and eroding broad valleys. One of the latter, stretching between the Adirondacks of New York and the Green Mountains of Vermont, would become the basin for Lake Champlain.
After peaking in its coverage, some 20,000 years ago, the Wisconsin Glacier began to melt back to the north, opening the newly formed basins. Some of these drained to the south or east, while others, including the Champlain Basin, drained northward. Blocked by the retreating glacier, the north flowing rivers gave rise to valley lakes, filled by mountain streams and by meltwater from the glacier itself. By 15,000 years ago, Lake Vermont, more extensive than today's remnant, spread across the Champlain Basin. Then, as the ice sheet retreated further into Canada, the rising ocean spilled through the St. Lawrence Valley and southward into the basin of Lake Vermont. Turning brackish, the latter became the Champlain Sea, an arm of the North Atlantic, and, for a thousand years or more, harbored a wide variety of marine life; fossils of these sea creatures are found across the basin today.
Finally, as New England rebounded from the weight of the glacier, the Sea slowly drained back toward the Gulf of St. Lawrence and freshwater reclaimed the basin, giving rise to Lake Champlain. Sustained by snowmelt and rainwater from the flanking mountains, the Lake drains northward via the Richelieu and St. Lawrence Rivers, flowing through a glacial valley that once admitted the sea.
