Randy McDonald (rfmcdpei) wrote,

[URBAN NOTE] "Prehistoric Toronto: Glacial Lake Iroquois"

Torontoist's Daniel Sellers has a neat post examining the history of Glacial Lake Iroquois, Lake Ontario's Ice Age predecessor that--so long as the ice dam held--had a shoreline well to the north of Toronto's existing waterfront.

For 100 years, Casa Loma has sat just a short, steep ridge away from the intersection of Spadina and Davenport roads, and gazed out over downtown Toronto. But, at one time, the view looking south from what is now the castle grounds would have been all water as far as the eye could see; in the very late Pleistocene epoch, Davenport was the beach above vast Lake Iroquois.

“I think a lot of people go up and down that ridge every day and don’t even give much thought to it,” says Rob MacDonald of heritage conservation consultants Archaeological Services Inc. “But that was [a] shoreline 12,500 years ago.”

Some 8,000 years before that, a massive glacier called the Laurentide Ice Sheet covered most of Canada, and had managed to extend as far as present-day Ohio at the tail end of its slow creep south. Then it began to melt and retreat, and, eventually, huge basins in the Great Lakes region, formed by previous glaciers and further gouged out under the movement of the Laurentide, would fill with meltwater.

An early version of Lake Erie appeared. Further west, the lakes we now call Michigan and Huron made up the bulk of glacial Lake Algonquin. Lake Iroquois formed in the basin of today’s Lake Ontario; it was bounded by ice to the northeast and drained through New York state’s Mohawk River.

The glacier continued to recede.

By about 12,000 years ago, the ice over the St. Lawrence River had disappeared. Lake Iroquois, finding this new, lower outlet, drained quickly and dramatically to as much as 85 metres below present-day water levels; an ancient shoreline now found at the bottom of Lake Ontario is evidence of this.

This smaller lake would not last long, either. After millennia beneath a heavy glacier more than a kilometre thick, Toronto was going through a process called isostatic rebound.
Tags: history, lake ontario, toronto, urban note
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