Toward the end of the great ice ages about 10,000 years ago, a glacier that had pushed along the trough of Lake Superior retreated toward the northeast and near Moose Lake crossed the divide between the Mississippi River and Lake Superior. When the ice lobe had shrunk so that it lay wholly within the rim of the lake basin, Glacial Lake Nemadji formed around the southwest margin of the ice. Its earliest outlet was at the western end, when the lake stood 523 feet above the present level of Lake Superior and nearly reached the elevation of the state hospital in the distance. During centuries of drainage from here through the Moose River to the Mississippi, this channel was eroded downward to the present level. When lower outlets for the Lake Superior basin opened, the Moose River Valley was abandoned as an outlet, and this part of the ancient watercourse became the basin of Moosehead Lake.