The presence of raised beaches around Lake Minto, northern Quebec, and the washing of till in the upper part of the Leaf River valley are attributed to the existence of a glacial lake. It drained westward into the Tyrrell Sea and was dammed in the east by glacial ice that occupied the main part of the Leaf River valley. The disappearance of this ice prior to 5200 years BP led to rapid drainage of the glacial lake into an arm of the D'Iberville Sea. Postglacial isostatic rebound has subsequently led to warping of the highest water plane associated with this glacial lake, as well as those of the postglacial seas.