Ice Age Canada: How Glaciers Sculpted the Land

Toronto under a kilometre of ice

About 20,000 years ago, almost all of Canada was buried under ice. Where Toronto, Winnipeg and Montréal stand today, the ice was more than a kilometre thick. The ice sheets slowly melted, and most of southern Canada was free of ice by about 10,000 years ago, very recently on the geologic time scale.

The ice is mostly gone, but its fingerprints are everywhere: the Great Lakes, the thousands of lakes of the Shield, the rolling hills north of Toronto, the flat farmland of the Red River Valley, and the dramatic fjords of British Columbia and Newfoundland. If you understand glaciers, you understand a huge part of Canada's landscape.

Two kinds of glaciers

A glacier forms where more snow falls in winter than melts in summer, year after year. The snow piles up, gets squeezed into ice, and becomes so heavy that it starts to flow, very slowly.

• Continental glaciers (ice sheets) spread out over huge areas like pancake batter. During the last ice age, the Laurentide Ice Sheet covered most of Canada east of the Rockies, and the Cordilleran Ice Sheet covered most of British Columbia. Today, ice sheets cover Greenland and Antarctica. (Interestingly, much of the Yukon stayed ice-free because it was too dry for enough snow to pile up.)
• Alpine (valley) glaciers form in the mountains and flow down valleys. Canada still has thousands of them in the Cordillera and the Arctic, such as the Athabasca Glacier at the Columbia Icefield in Alberta.

Over the last 2.6 million years or so, ice sheets have advanced and retreated many times. Each advance scraped and reshaped the land.

Glaciers as bulldozers: erosion

As ice moves, rocks frozen into its base act like sandpaper.

• Abrasion: rocks in the ice grind the bedrock, leaving scratches called striations that show the direction the ice moved.
• Plucking: ice freezes onto loose bedrock and pulls chunks away.

Results in Canada:

• On the Canadian Shield, the ice scraped away most of the soil and gouged out countless hollows in the bedrock, which filled with water. That is why the Shield has thin soils, bare rock and countless lakes.
• The Great Lakes basins were deepened by ice flowing along old river valleys and lowlands, and they filled with meltwater as the ice retreated.
• In the mountains, valley glaciers turned V-shaped river valleys into wide U-shaped valleys and carved bowl-shaped cirques, sharp ridges (arêtes) and pointed peaks (horns).
• Along the coasts of British Columbia, Newfoundland and Labrador and the Arctic, glaciers carved deep valleys that the sea later flooded, forming fjords. Western Brook Pond in Gros Morne National Park is a fjord that was later cut off from the sea.

Glaciers as dump trucks: deposition

Everything the ice picked up had to go somewhere when it melted.

• Till is the unsorted mix (clay to boulders) dropped straight from the ice. It covers much of southern Ontario and the Prairies.
• Moraines are ridges of till. An end moraine forms at the front of a glacier where it stayed in one place for a long time. The Oak Ridges Moraine north of Toronto stretches about 160 km and formed between lobes of melting ice. Its sand and gravel store groundwater that many communities rely on.
• Drumlins are smooth, oval hills shaped by moving ice. They often occur in groups called drumlin fields, such as the one around Peterborough, Ontario.
• Eskers are long, snaking ridges of sand and gravel left by meltwater rivers that flowed in tunnels in the ice. They are common on the Shield and in the North, and they are important sources of gravel and travel routes for caribou and people.
• Kettle lakes form when a block of ice is buried in sediment and melts, leaving a hollow.
• Erratics are boulders carried far from home. Shield rocks are found on farm fields across southern Ontario.
• Outwash plains are flat areas of sorted sand and gravel spread out by meltwater streams in front of the ice.

Giant lakes of meltwater

As the ice sheets melted, huge glacial lakes formed in front of them. Glacial Lake Agassiz covered much of what is now Manitoba and parts of Saskatchewan, Ontario and the northern United States. Fine clay settled on its floor, leaving the very flat, fertile land of the Red River Valley around Winnipeg. In Toronto, the steep slope that Casa Loma sits on is the old shoreline of Glacial Lake Iroquois, a larger ancestor of Lake Ontario.

The ice still matters: isostatic rebound

Ice is heavy. The ice sheets pressed the Earth's crust down. When the ice melted, the land began slowly springing back up, like a foam mattress after you stand up. This is isostatic rebound. Around Hudson Bay, where the ice was thickest, the land is still rising, about a centimetre a year in places. Old beaches now sit high above today's shoreline as raised beaches.

This is a great example of the concept of interrelationships: an event that ended thousands of years ago still shapes coastlines, soils, lakes and even where we find gravel for roads today.