Canada's geography: a shield of bare rock ringed by younger land

How the Canadian Shield shapes the whole country

The Canadian Shield is the core of the continent. Its crystalline rocks formed in several phases of mountain building between 4 billion and 1 billion years ago, and for the last billion years it has sat still while plate movements built mountains around its edges. Erosion wore it flat. What remains covers close to 5 million square kilometres, almost all of Quebec and Ontario, much of Manitoba and northeastern Saskatchewan, and large parts of the Northwest Territories and Nunavut.

The Canadian Encyclopedia describes its shape as a soup plate: a raised rim on the south, east and northeast, and a basin of sedimentary rock in the middle whose southern fringe underlies the Hudson Bay Lowlands. About half the Shield is upland at 200 to 500 metres, with local relief of only 50 to 60 metres. Toward the east the ground rises from 300 metres near the coasts to 900 metres in central Labrador and Quebec, and on Baffin Island and in northern Labrador it stands at 800 to 1,500 metres, cut by glacial troughs into a fjord coast. The ice sheets that spread out from two centres east and west of Hudson Bay, from 75,000 to 6,000 years ago, did the rest. They stripped the soil, left a thin cover of till and pocked the rock with irregular shallow lake basins. Eskers mark the beds of rivers that once ran under the ice, and moraines show where the retreating front paused.

The result cannot carry intensive agriculture. There are subsistence plots and small dairy farms in the river valleys and around the lakes of the south, and little else. The Shield carries boreal forest of spruce and pine, and it carries metals: iron, nickel, zinc, copper, gold and uranium, worked from mining towns such as Sudbury. Its lakes and falling rivers feed the hydroelectric dams of northern Quebec and Ontario. The six younger regions lie around it in two rings. The outer and older ring holds the Western Cordillera, the Arctic and the Appalachians; the inner and younger one holds the Interior Plains, the Hudson Bay Lowlands and the St. Lawrence Lowlands.

The shares add up to more than a hundred because the Arctic overlaps the Shield, the Cordillera and the Plains in the far north.

Where the western mountains come from

The Western Cordillera is a belt about 800 kilometres wide that runs from southern British Columbia north through the Yukon to the Beaufort Sea. It holds three mountain systems. The eastern one, the Rockies, is sedimentary rock that has been tilted, faulted and folded. The interior system is a mix of folded sediments, lava and metamorphic rock cut into plateaus. The western system is the Coast Mountains, a mass of granite and schist formed about 100 million years ago above an old subduction zone, with a seaward face of fjords and islands and very large icefields along its spine.

The Canadian Rockies run 1,200 kilometres from the American border to the Liard River basin and carry the continental divide. Water on one side goes to the Pacific; on the other it goes to the Arctic Ocean or Hudson Bay. The Canadian Encyclopedia dates the main phase of their formation to the Jurassic, around 200 million years ago, when North America was pushed west into a series of microcontinents as Pangaea broke up. Mount Robson in British Columbia stands at 3,954 metres. Mount Logan, at 5,959 metres, is not in the Rockies at all: it rises in the Saint Elias Mountains of the southwestern Yukon, near the Alaska border.

The Cordillera is still active. Canada has frequent earthquakes and potentially active volcanoes at Mount Meager, Mount Garibaldi, Mount Cayley and the Mount Edziza complex, and some of the youngest lava flows in the interior are only a few hundred years old. During the last glaciation only the highest summits south of 60° N stood above the Cordilleran ice sheet, as nunataks. Rain and snow off the Pacific fall heavily on the Coast Mountains and keep glaciers there at low elevations. Inland the snowfall drops, the timberline and snowline rise to the east, and in the dry valleys of the southern interior the lowest vegetation zone is grassland. The Athabasca Glacier of the Columbia Icefield has retreated more than 1.5 kilometres since 1890, and in the Canadian Rockies the rate of glacial melt has increased eightfold since 2011.

How the prairies step up toward the Rockies

The Interior Plains fill the space between the Shield and the mountains, 1.8 million square kilometres of soft shale, siltstone and sandstone laid down by ancient seas and by rivers running east off the young mountains. The glaciations of the last 2.6 million years spread till, lake clays and moraines over all of it.

The southern plains climb west in three steps. The Manitoba Plain, below the Manitoba Escarpment, lies under 400 metres and is the flattest, floored with the silts and clays of former glacial lakes. Above the escarpment the Saskatchewan Plain runs from 460 to 790 metres. Beyond the Missouri Coteau the Alberta plains rise to about 1,100 metres, and the Southern Alberta Uplands reach toward 1,650 metres before the foothills. In the dry south, soft rock has been cut into badlands.

Vegetation changes with the rain. The semiarid south is grassland, the native prairie. Where it is slightly cooler and wetter, to the north and east, the grass gives way to aspen parkland, then to mixed forest, and finally to the coniferous boreal forest that runs north to the Mackenzie valley. The prairies are where Canada grows its wheat, oats, barley and flax, and cattle, sheep and hogs are raised across them. Alberta and Saskatchewan are the two provinces with no coastline. Every other province and territory touches one of the three oceans.

Why most Canadians live in the southern lowlands

The St. Lawrence Lowlands cover 180,000 square kilometres, 1.8% of Canada's land, and they hold Toronto, Montreal and Quebec City. They lie between the Shield to the north and the Appalachians to the east: limestone plains in southern Ontario, bounded by the Niagara Escarpment, and in southern Quebec a floor of marine clays and sands left by the Champlain Sea, which flooded the lowland about 13,000 years ago as the ice withdrew. The seven Monteregian Hills, Mount Royal among them, are the exposed roots of volcanoes that formed about 120 million years ago as the Atlantic opened.

This small lowland is where the country's people are concentrated. Statistics Canada figures cited in the Wikipedia survey of the country put 72% of Canadians within 150 kilometres of the United States border and more than 60% along the Great Lakes and the St. Lawrence between Windsor and Quebec City. About 79.7% live in urban areas.

The Appalachian Region to the east is old, eroded mountain country, 360,000 square kilometres folded between 480 and 280 million years ago into a Z-shaped belt that runs from the Vermont border through the Gaspé Peninsula and New Brunswick to the western spine of Newfoundland. Mont Jacques-Cartier on the Gaspé reaches 1,268 metres. Its soils are mostly sandy and poor, and the arable pockets lie on raised marine terraces along the coast and in valleys such as the Annapolis, where Nova Scotia grows its fruit.

The ground is still moving after the ice. On the Atlantic coast the sea is rising against the land by up to 30 centimetres a century. On the shores of Hudson Bay, where the crust is rebounding faster, relative sea level is falling by about 90 centimetres a century, and storm waves have left scores of parallel gravel beach ridges across the Hudson Bay Lowlands, a plain of muskeg, peat and ponds that covers 320,000 square kilometres.

What permafrost and ice do to the north

The Canadian Arctic covers 2.1 million square kilometres, 21% of the land, and its defining conditions are continuous permafrost and treeless tundra. The Arctic Archipelago alone holds 94 major islands larger than 130 square kilometres and tens of thousands of smaller ones, 1.4 million square kilometres in all. Since 14 June 2022 Canada has also had a land border with Greenland, on Tartupaluk, known in English as Hans Island.

Permafrost, ground that stays at or below 0 °C for at least two years, underlies 40 to 50% of Canada. It is more than 1,000 metres thick at high elevations on Baffin and Ellesmere islands and 60 to 90 metres thick at the southern limit of the continuous zone. The coldest permafrost in the country, on Ellesmere, sits at about minus 15 °C. The ground builds its own landforms: tundra polygons of ice-filled cracks up to 30 metres apart, and pingos, ice-cored mounds up to 100 metres high, near Tuktoyaktuk and elsewhere.

Building on it takes care. Heated houses stand on piles with air beneath them so the ground does not thaw, roads and runways need an insulating layer, and water and sewage pipes run in insulated boxes above ground called utilidors. Agriculture is close to impossible, which is one reason food costs more in the north. The islands of the Arctic carry about 130,000 square kilometres of glaciers and ice caps, most of them on Ellesmere, Axel Heiberg, Devon and Baffin. The glaciers that are monitored have thinned by 8 to 12 metres since the early 1960s. The permafrost boundary has moved before: its southern edge in the Mackenzie valley shifted north with a 4 °C winter warming after the mid-nineteenth century and south again with a 2 °C cooling after 1940, and it is moving north once more.

Where Canada's rivers drain

Canada has more than two million lakes, 563 of them larger than 100 square kilometres, and water covers 8.62% of its total area. Five drainage basins take the runoff.

The Atlantic watershed drains the Atlantic provinces, most of settled Quebec and much of southern Ontario, and its trunk is the St. Lawrence, 3,058 kilometres long, which carries the outflow of the Great Lakes to the Gulf of St. Lawrence. The Saguenay, the Manicouagan and the Ottawa join it.

The Hudson Bay watershed drains more than a third of the country: Manitoba, northern Ontario and Quebec, most of Saskatchewan and southern Alberta. Its rivers, the Nelson and the two Saskatchewans among them, are dammed for hydroelectricity, especially in Manitoba, northern Ontario and Quebec. Wollaston Lake sits exactly on the divide between this basin and the Arctic one and drains into both.

The Arctic watershed is gathered almost entirely by the Mackenzie, which with its tributaries runs more than 4,200 kilometres from Great Slave Lake to the Beaufort Sea. Great Bear Lake and Great Slave Lake both belong to it. Apart from the Mackenzie itself, the Arctic basin has been little used for hydroelectricity.

West of the continental divide the Fraser, the Columbia and the Yukon carry water to the Pacific and irrigate the Okanagan and Kootenay valleys. One small corner goes the other way: the southernmost strip of Alberta drains through the Milk River into the Missouri and eventually the Gulf of Mexico.

How the climate changes from coast to interior

Distance from the sea decides most of it. Victoria on the Pacific has a mild, wet winter, with about 130 millimetres of rain a month in an average winter; Winnipeg, in the middle of the continent, is colder, and across the interior and the prairie provinces daily winter averages sit near minus 15 °C and can drop below minus 40 °C. Snow covers the ground in the interior for close to six months of the year, and in parts of the far north it never fully melts. Precipitation ranges from about 100 millimetres a year in the High Arctic to more than 1,500 millimetres on the west coast, because the prevailing winds come off the Pacific and drop their moisture on the mountains. The prairies are dry: Edmonton receives 447 millimetres a year against a potential evapotranspiration of 555, and summer deficits of about 120 millimetres are common. Halifax, on the Atlantic, has rain spread through the year and 773 millimetres of runoff.

The north-south spread is widest in winter. Alert, at the northern tip of Ellesmere Island, has a mean January maximum of minus 28.6 °C; Windsor, in southern Ontario, has minus 0.4 °C. In December southern Canada gets about eight hours of daylight and the far north gets none. Heating need follows: southern Ontario counts about 4,000 heating degree days a year, the high Arctic islands about 12,000.

Canada's average annual temperature over land has risen by 1.7 °C since 1948, with regional changes from 1.1 to 2.3 °C and the fastest warming in the north and on the prairies.