vast horseshoe-shaped geologic or physiographic region, composed mainly of granitic and metamorphic Precambrian rocks, the deeply eroded roots of ancient mountains, located primarily in central and eastern Canada and also including small parts of the northern U.S. The oldest part of the North American crustal plate, its rocks contain some of the earliest forms of life on earth—fossils of algae and bacteria over 2 billion years old. The shield extends in a great semicircle around Hudson Bay, ranging from the Arctic coast north of Great Bear Lake in the Northwest Territories to northern Québec Province and Labrador. It covers about 4.4 million sq km (about 1.7 million sq mi) and occupies almost one-half of Canada’s total area. In the U.S. it includes the Superior Upland of northern Minnesota, northern Wisconsin, and northern Michigan and the Adirondack Mountains of northeastern New York.

During the most recent of the ICE AGES, (q.v.), beginning about 2 million years ago, continental ice sheets covered the region, stripping away soil, depositing glacial drift, and creating many lake basins and riverbeds. Present-day elevations in the shield are mostly between 300 and 600 m (984 and 1969 ft), except in limited areas, such as the Québec-Labrador Peninsula, where they rise about 1070 m (about 3510 ft). The shield has a poorly integrated drainage system.

The climate is arctic in the northern shield, where the mean January temperature is about –32° C (–25° F), the growing season is less than 40 days a year, and lakes are free of ice for only about three months annually. Permanently frozen subsoil is continuous northwest of Hudson Bay and is sporadic east of it. The more temperate southern shield has forests of pine and northern hardwoods. Such vegetation gives way northward to open tundra carpeted with sedges, grasses, lichens, and dwarf shrubs. Muskegs, swamps, and lakes are found throughout the shield area.

The northern Canadian Shield is sparsely populated, mainly by Inuit (Eskimo) and Indians, but the southern shield has numerous urban areas, which have developed principally as centers for exploiting natural resources. The southern shield is Canada’s main source of metallic minerals, hydroelectricity, and pulpwood. Minerals recovered include iron, nickel, copper, zinc, uranium, gold, silver, platinum, and molybdenum. Many of the mines are located south of James Bay.