Prehistoric Toronto: Glacial Lake Iroquois
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Prehistoric Toronto: Glacial Lake Iroquois

As the last Ice Age glacier retreated from above Toronto, its meltwater, for a time, overwhelmed the city.

Mammoths, mastodons, and giant beavers! Prehistoric Toronto looks back—wayyyyyy back—and explores the terrain that is now Toronto, as it developed through the ages.

The blue border shows the outline of the lakes today. The colour shapes with the wavy lines show the glacial lakes as they were 12,000 years ago.

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.

“Isostatic rebound is kind of like memory foam,” MacDonald explains. “If you push down on a bed that has a memory foam top on it, and take your hand away, you can still see the impression of your hand, but it’ll gradually come back up. So, the tectonic plates of the Earth are similar to that in some respect, in that if there’s a great weight added to them they will be depressed and then, gradually, over time, rise back up.”

This rebound from beneath the glacier—which is still slowly ongoing today—did not happen evenly. With land rising faster near early Lake Ontario’s northeastern outlet than it did in the south and the west, drainage along the St. Lawrence slowed. Water in the lake began to rise once more, reaching its current level some 4,500 years ago.

Of course, Laurentide ice did more than just temporarily create a giant lake. Much of Toronto’s present geography has been formed by the city’s history of repeated glaciation.

“If you strip up all the glacial sediment and just looked at the bare bedrock in southern Ontario, you’d see a landscape that’s similar, but in some ways different than what you see on the surface,” MacDonald says. “Most of the landforms [here] are of glacial origin.”

The Scarborough Bluffs, for instance, are an erosional feature representing a section of Lake Iroquois shoreline. High Park’s Grenadier Pond—and other lagoons and estuaries at the mouths of Toronto’s rivers—formed when isostatic rebound caused lake tributaries to back up. And sediments being pulled laterally across the north shore of the lake by a current were deposited near the mouth of the Don River, forming a long, sandy spit that would eventually be the Toronto Islands.

Amid all of this geographical formation, life in Toronto forged ahead. Spruce, birch, poplar, and alder trees populated mixed post-glacial forests. Humans had already crossed over the Bering land bridge from Siberia to Alaska, and would soon be in Toronto.

Just before they arrived, an extinct deer about which we know almost nothing was making its living in this city’s west end. And then one springtime day, that animal lay down and died. Some 11,300 years later, after an unsuspecting TTC worker discovered a fragment of its fossilized remains, it would come to be known as Torontoceros.