Champlain Sea

[1][2] The inlet once included lands in what are now the Canadian provinces of Quebec and Ontario, as well as parts of the American states of New York and Vermont.

At the end of the last glacial period, while the rock was still depressed, the Saint Lawrence and Ottawa River valleys, as well as modern Lake Champlain, at that time Lake Vermont, were below sea level and flooded with rising worldwide sea levels, once the ice no longer prevented the ocean from flowing into the region.

[14] The Sea also left ancient raised shorelines in the former coastal regions, and the Leda clay deposits in areas of deeper water.

[15] The northern shore of the lake was in southern Quebec where outcrops of the Canadian Shield form Eardley Escarpment.

It is part of the Mattawa fault at the southeastern edge of the Ottawa-Bonnechere Graben, in Eastern Ontario and the Outaouais region of Quebec, more commonly known as the Ottawa Valley.