The cold days of Canada
For the last two million years or so, large parts of the northern hemisphere, all of Antarctica and many mountain chains on other continents have been covered and recovered by ice sheets. The surface of most of Canada has been shaped by the advancing and retreating of these glaciers. Most of Great Lakes’ basins of and other Canadian lakes both large and small were carved out by the moving ice. Soil and rocks were scooped off the Canadian Shield and deposited over southern parts such as Toronto.
Many causes have been suggested for the Pleistocene Glaciation and others earlier in our planet’s history including: changes in solar flux; changes in the shape of Earth’s orbit & the tilt its axis; the massing of the continents and deep oceanic currents. Probably it was a combination of several such influences.
North America’s continental ice sheet apparently started in Labrador or Ungava then spread south in the east and west in the north. Other ice sheets grew from the coalescing of mountain glaciers in the west and in the Arctic. We may be closer to new continental ice sheets than we think. The picture below was taken in July 1995 along the south coast of Labrador. There are many such ice fields in that area. What would it take for these small ice fields to join and grow into a glacier covering Labrador and Quebec? Whenever a combination of cold climate and sufficient precipitation in the form of snow occurs, as it has several times in the past few million years and in the more distant past, continental glaciers form and grow.
The Ice age in Toronto
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.
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, 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.
(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.)
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. Most of the landforms here are of glacial origin.
Places like where these coordinates have taken you right now (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.
In order to log this Earthcache you must answer the following questions at the posted coordinates and tell me them in an email or GC Message:
1. Check out the hill, to the east: this hill is known for sledding down in the winter (when the pond is frozen over) What geological features caused the land to be in this form and why?
2. Head to the lookout waypoint for Grenadier pond: Looking across the lake what evidence of glaciation is there?
3. OPTIONAL: At the same waypoint is a great photo oppurtunity spot, you can take a photo here and post it to your log, just avoid posting pictures including the west border of the lake because that could spoil Question 2.
4. Measure the elevation of the water at the pond, how do you think the elevation of the water in this area has changed in the past 1000 years?
The bridge waypoint is out of the way and is not necessary to visit to log as a find, but I found it interesting how grenadier pond acts like a river and continues underground north of high park. If you choose you can come here and see where the underground river starts too!
(An old map showing the river in the park before being covered up, you can see north of grenadier pond how it continues.)
I hope you have fun with this Earthcache and learning about the Ice Age's affect on our lake, city, and pond!