Great Lakes Origin by Diastrophic Processes

Denudation of the Shield

The scale of the Great Lakes implies very effective agents of erosion. Whatever the process, it must also have eroded the rock basins of other lakes around the perimeter of the Canadian Shield, including Great Bear, Great Slave, and Athabasca lakes, all hundreds of meters deep and cut into bedrock. How could ice have accomplished it? There has been a long controversy about the question. Or, could rivers have done so? But Lake Superior extends to well below sea level. Rivers seem inadequate too. The answer, of course, is catastrophic currents.

Currents that formed the lake basins were fast, and sustained. This implies powerful uplifts of large areas of the earth's crust, that spilled enormous volumes of water over the area now occupied by the Great Lakes.

There is evidence the Canadian Shield has been exhumed; its once extensive cover of sediments has been swept away (White, 1972). Remnants of the former sediment cover remain as outliers in different parts of the Shield. Outliers occur at New Liskeard, Ontario, and scattered locations in Quebec. Eroded remnants of a more extensive Paleozoic sedimentary cover occur in Hudson Bay, Ungava Bay, in the Foxe Basin west of Baffin Island, and in Cumberland Sound.

Map showing the Canadian Shield Highlands occur in the northeastern section of the Shield in Canada, in Ellesmere Island, the northeast coasts of Baffin Island, and the coast of Labrador. The coast is indented with many deep fjords, and rises steeply from the sea to uplands of 2,000 meters elevation, with glaciers in the northern region. The southern boundary of the Shield in Quebec along the St. Lawrence estuary is a prominent escarpment of tectonic origin, 500 to 1,000 meters high. In contrast, most of the western and southwestern border of the Shield is low, and dips gently beneath the Paleozoic sediments, forming the basement beneath them over most of the continent.

Currents generated by uplift almost completely eroded away the sediments from the exposed parts of the Canadian Shield, and eroded the lake basins along the western edge. Uplift alone would not cause the removal of the sediments from the Shield, but uplift when it was submerged could generate powerful currents, which would explain the erosion of the sedimentary cover. The bulk of the sediment eroded from the Great Lakes area must have been carried far to the south, and perhaps was redeposited in the thick coastal sediments of the southern U.S. or in the sediments of the continental shelves.

Reference

White, W.A., 1972. Deep erosion by continental ice sheets: Geol. Soc. Amer.Bull., v. 83, p. 1037-1056.

Links

Physiographic Division - Information about the Canadian Shield.