Great Lakes Origin by Diastrophic Processes

A brief outline of a theory on the formation of the Great Lakes of North America.


Lake Superior is eroded in granite and other rocks of Precambrian basement, and the rock basins of the other Great Lakes are eroded into Paleozoic sedimentary rocks such as shale and limestone. In these pages the origin of lake basins is attributed to the erosional effects of currents when the region was submerged, as it was raised to its present elevation, and a disintegration process that accompanied the erosion. These were natural processes associated with catastrophic events. Large scale crustal movements generated powerful currents that swept across the Great Lakes region, providing an environment for very effective erosion.

The generation of the currents powerful enough to excavate the basins of the Great Lakes was due in part to diastrophic processes associated with crustal subsidence and uplift, and the effects of the earth’s rotation. It is suggested the differential crustal movements may have formed the basement complex of the Michigan Basin, and other basement depressions and domes in areas near the Great Lakes.

The erosion of the basins of lakes Erie and Ontario was likely associated with a  southwesterly flow, caused by uplift in the Canadian Shield north of the region. Because of the principle of conservation of angular momentum, the mass of displaced water flowing south would tend to drag behind earth's rotational motion. The current would be deflected to the southwest, and would cause streamlining of sediments at the earth's surface and erosion of lake basins. The excavation of the Great Lake basins, as well as other lakes in the region, was probably was not connected with former ice sheets or glaciers.

The excavation of the lakes was accomplished by powerful currents generated by differential crustal movements. Flood waters were displaced by uplift and spilled over surrounding lower regions, and the resulting fast currents eroded deep basins in the softer sediments. The process was enhanced by a disintegration effect that accompanied the rapid removal of overburden.

Previous Work

The problem of explaining the origin of the Great Lakes was summarized by Sir J.W. Dawson in his book Acadian Geology:
Our American lake-basins are cut out deeply into the softer strata. Running water on the land would not have done this, for it could have no outlet; nor could this result be effected by breakers. Glaciers could not have effected it; for even if the climatal conditions for these were admitted, there is no height of land to give them momentum. But if we suppose the land submerged so that the Arctic current, flowing from the north-east, should pour over the Laurentian rocks on the north side of Lake Superior and Lake Huron, it would necessarily cut out of the softer Silurian strata just such basins, drifting their materials to the south-west. At the same time, the lower strata of the current would be powerfully determined through the strait between the Adirondac and Laurentide Hills, and, flowing over the ridge of hard rock which connects them at the Thousand Islands, would cut out the long basin of Lake Ontario, heaping up at the same time, in the lee of the Lawrentian ridge, the great mass of boulder clay which intervenes between Lake Ontario and Georgian Bay. Lake Erie may have been cut by the flow of the upper layers of water over the Middle Silurian escarpment; and Lake Michigan, though less closely connected with the direction of the current, is, like the others, due to the action of a continuous eroding force on rocks of unequal hardness.
Dawson thought the "Arctic current" mentioned in the paragraph quoted above was responsible for distributing the drift.

J.W. Spencer also thought that ice action would be ineffective in deepening the basins of the Great Lakes. He thought the Great Lakes basins were formed mainly by the erosion due to pre-glacial rivers, followed by blocking of the drainage by elevation of the land to the northeast. He dismissed the suggestion that glaciers had eroded the Great Lake basins. He wrote [Spencer, 1891, p. 93]:

At the present time in the investigation this subject can be quickly dismissed. The question whether glaciers can erode great lake-basins is hardly pertinant, for nowhere about the lakes is the glaciation parallel to the shores or vertical escarpments which are associated with the lakes.
Spencer believed crustal warping was a major factor in the formation of the lake basins. The Great Lakes region, Spencer said, was 2,000 feet higher than now, during the last ice age. This explained the erosion of deep canyons in the continental shelves by rivers. [Coleman 1941, p. 32]  H.L Fairchild argued that glacial erosion could not account for the rock basins of the Great Lakes, claiming it would be physically impossible for ice to do so.

Many geologists, unwilling to admit the possibility of former catastrophic events, remain uncertain about the question of the origin of the lakes; some argue the erosive power of glaciers was effective in forming the lakes (White, 1972), while others suppose the Pleistocene ice sheets must have modified already existing rock basins eroded by rivers (Gravenor, 1975). Chapman and Putnam offered only a single sentence on the origin of the Great Lakes in their book The Physiography of Southern Ontario. They wrote (Chapman and Putnam, p. 13):

The basins of Lakes Ontario, Huron, and Superior extend to below sea level; as such scooping-out could not have been the work of a river, it is attributed to glaciers.
Chapman and Putnam  apparently dismiss the thoughful research of Sir J.W. Dawson, who remained skeptical of the glacial theory of Louis Agassiz all his life.


© 1999 by Douglas E. Cox
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