The Ontario landmass traces its origins to the last ice age, which ended some 10,000 years ago. The province’s topography is mainly a result of the effects of glacial action. When the glaciers retreated at the end of the ice age, they carved out valleys that became lakes, and left soil deposits that became hills and ridges. Ontario can be divided into three major natural regions: the Canadian Shield (also known as the Laurentian Plateau), which cuts a wide swath across the center of the province; the Hudson Bay Lowlands to the north; and the Great Lakes and St. Lawrence Lowlands, which form the southernmost region.

The Canadian Shield is part of a huge area of water, forest, and Precambrian igneous rock—mostly granite—that occupies 50 percent of Canada’s total land area. It stretches in a great crescent from the Labrador coast, through Québec and Ontario, into the Prairie provinces of Alberta, Manitoba, and Saskatchewan, and then northwards to the Arctic Ocean. An extension of the shield called the Frontenac Axis penetrates the lowlands of southern Ontario to the St. Lawrence River, where it forms the rocky Thousand Islands.

The Canadian Shield makes up approximately half the land area of Ontario. Its poor soils and rough terrain have discouraged farming and extensive settlement, although several areas of clay deposits—the beds of ancient glacial lakes—are suitable for raising crops and for grazing. The region is a rich mineral storehouse and produces, among other metals, much of Canada’s copper, lead, nickel, zinc, and uranium. The Canadian Shield’s forests yield a variety of trees, particularly those used for making pulp and paper. In addition, the region’s lakes, rivers, and wooded hills attract vacationers year-round. Ontario’s highest point is in the Canadian Shield near Lake Timiskaming: Ishpatina Ridge (693 m/2,274 ft).

The Hudson Bay Lowlands in the north sit atop sedimentary rocks, mainly limestone, and dip gently northward from the Canadian Shield toward the shore of Hudson Bay. The lowlands are flat, poorly drained, and characterized by areas of swampy bogs called muskeg. In the far north of the region is a small belt of permafrost, land that is continually frozen. The rocks of the Hudson Bay Lowlands are the remains of the great beds that once covered the Canadian Shield. The beds of the lowlands remained because they were less subject to erosion than those at higher elevations. Sizeable areas of trees, commonly black spruce and larch (also called tamarack), give way to isolated stands and then stunted growths further northwards.

The third physical region of Ontario is the Great Lakes and St. Lawrence Lowlands, home to most of the province’s population, agriculture, and industry. Sedimentary deposits underlie the soils of this region. The Frontenac Axis divides the region into two parts: the St. Lawrence Lowlands, which are located southeast of the city of Ottawa, and the Great Lakes Lowlands, which lie to the west. Scientists believe that a considerable area in the lower Ottawa Valley, on the province’s eastern border, was once covered by the Champlain Sea, an arm of water reaching in from the Atlantic Ocean. Clay beds of marine origin were deposited in this region. Also, sand plains formed as deltas where the Ottawa River entered the Champlain Sea. These regions are now agricultural areas.

The most notable topographical feature of the Great Lakes Lowlands is the Niagara Escarpment, a high ridge of limestone cliffs that runs about 400 km (250 mi), north from Niagara Falls through the Bruce Peninsula to the Manitoulin Islands in Lake Huron. Niagara Falls and the gorge below it are the result of the Niagara River, which empties from Lake Erie into Lake Ontario, cutting slowly through the ridge. The escarpment cradles much rich fruit-growing land near the cities of St. Catherines and Niagara Falls. Otherwise the topography of the Great Lakes Lowlands is a rolling plain once thickly carpeted with forests and now given over to agricultural, domestic, and industrial use. Glacial deposits called moraines occur in several belts that run roughly parallel to Lakes Huron and Ontario. Along the shorelines of these lakes are clay lands and beach deposits formed when even larger glacial lakes existed. The rich soils and fundamentally level lands, coupled with easy access from water routes, attracted early agricultural settlement in the area. The region now contains many of Ontario’s largest cities, including the capital, Toronto.