Great Lakes Origin by Diastrophic Processes

Escarpments a Barrier to Ice

Cliffs at Lion's HeadThe Niagara Escarpment, shown at Lion's Head, along the eastern shore of the Bruce Peninsula in Ontario in the photo at right, is a line of resistant cliffs composed of dolomites and limestones of the Niagara Formation. It forms a cuesta around the Michigan Basin.

Near the escarpment, directional indicators such as drumlins and aligned valleys show the former current flowed towards the major sedimentary basins and away from the Canadian Shield, and flow was from the lower side of the escarpment to the higher. South of Lake Ontario, the escarpment runs east-west and has a similar relation to the Allegheny Basin. Drumlins in New York are also aligned normal to the trend of the escarpment. These facts indicate no former ice sheet could  have been involved in its formation. The escarpment would have presented a barrier to the motion of former ice sheets, as the proposed ice flow would have to be up and over the line of steep cliffs.

In Lake Huron, another submerged escarpment stretches from Clark Point, Ontario, to North Point, Michigan. It slopes away steeply to below sea level on the north. This escarpment also would present a barrier to the flow of any hypothetical former ice sheet eroding the basin of Lake Huron.

The escarpments in the Great Lakes region indicate that large scale, catastrophic currents were a necessary cause for the excavation of the lakes, when the region was submerged, and the crust was actively warping and generating currents. The trends of drumlin fields suggest the cause of the currents was the differential uplift that caused the dip in the sedimentary rocks, and formed the sedimentary basins and uplifted the shield areas. The excavation of the lakes was aided by the in situ disintegration of bedrock surfaces, forming drift sand and gravel, as the overburden was being removed by the currents.

Flowerpot Island

On Flowerpot Island, north of the Bruce Peninsula, there are two unstable stacks or pillars of fractured rock, standing on the eastern shore, a location which would have received maximum wear and tear from the supposed movement of a hypothetical ice sheet flowing over the region. Presumably, the ice would have to sweep up and over the escarpment from the depths of Georgian Bay, leaving these rock pinnacles intact! The presence of these stacks seems to discredit the idea of former ice sheets in the region of Georgian Bay.
The larger Flowerpot

A.P. Coleman noted evidence within the Great Lakes discrediting idea that ice sheets could have scoured the region. He wrote [Coleman 1941, p. 123]:

In 1901, I rowed a clumsy fishing boat seven miles out from the north shore of Lake Superior to the Slate Islands and found no drift on them, only deeply weathered gneiss, above which stood walls and pinnacles of dike rocks; and in my notes there is a suggestion that the islands must have been nunataks. A. L. Parsons, who visited them later, got the same impression, and mentions also deep weathering of rocks a hundred miles south-east in the Michipicoten mining district.

On the north shore of Lake Huron, too, Collins has noted dikes which stand up as walls 10 feet above weathered quartzite; and on Bruce Peninsula between Lake Huron and Georgian Bay the tableland rising 400 to 1,000 feet above the water shows only Precambnan boulders and a greatly weathered surface of Niagara limestone, while at low levels there is fresh boulder clay with beautifully polished and striated limestone pebbles, indicating that the Wisconsin ice lobe was thin and failed to sweep over the highland.


A.P. Coleman,  1941.  The Last Million Years, U. of Toronto Press.
© 1999 by Douglas E. Cox
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