According to geological theory the continents are masses of lighter rock floating upon the heavier deep seated rocks of the earth's crust which underlie the floors of the ocean basins. Basically the land mass or continent of North America has a three-part structure. The northeastern portion consists of hard old rocks, once strongly folded and altered, former mountain ranges long since completely worn down, now constituting a stable land mass or Shield. The mid-portion of the continent consists of vast plains underlain by later beds of rock which have departed little from their original horizontal position because they rest upon the ancient Shield. The western margin of the continent is a zone of high and rugged young mountains which were raised up in comparatively recent geological periods and have not yet been worn down. Apparently this is a belt of weaker rocks which has been crumpled between the Shield and the resistant rocks of the floor of the Pacific basin. The eastern margin of the continent also has a mountain belt but it is older and considerably worn down. Canada, the northern half of the continent, has nearly all of the exposed area of the ancient Shield, hence the interior lowlands are considerably less than those of the United States. Canada also has a very irregular coastline and many large islands. Much of the lower marginal area of the continental mass is flooded and the higher parts protrude while great bays invade the heart of the country. The drowned portion of the mass is known as the continental shelf. Map of Canada showing the natural physiographic regions into which the country may be divided.
1. The Appalachian Region, comprising the Maritime Provinces, the island of Newfoundland and the hilly portion of Quebec south of the St. Lawrence River.
2. The Canadian Shield, a vast, rough area of very old Precambrian rocks surrounding Hudson Bay and comprising the northeastern one-third of the country.
3. The Interior Lowlands, underlain by almost undisturbed Paleozoic rocks. These small northern extensions of the great central plains of the United States are found along the St. Lawrence, the Great Lakes and in southern Manitoba.
4. The Great Plains, a vast area extending from western Manitoba to the Rocky Mountains and northward into the Mackenzie Valley. It is underlain by gently dipping beds of Mesozoic and Tertiary age. This is the northern half of a physiographic province which extends south to the Rio Grande.
5. The Cordilleran Region, extending from Western Alberta to the Pacific Ocean. It may be divided into the Rocky Mountains, the Interior Plateaus and the Goast and Insular Ranges.
6. The Hudson Bay Lowland, an area of flat-lying paleozoic rocks much like those of the interior lowlands farther south.
7. The Arctic Archipelago, a composite area of many large islands of Northern Canada, resting upon the continental shelf which borders the Arctic Ocean. The ten largest islands range from 10,000 to 200,000 square miles in area.
Only a small portion of the country, practically all located in the Cordilleran region, is above 4,000 feet in elevation. More than 70 named peaks exceed 11,000 feet in elevation. The highest peaks are in the St. Elias Mountains in the southwestern corner of Yukon where Mount Logan reaches 19,850 feet above sea level. Mount Robson (12,972 feet) is the highest peak in the Rocky Mountains and Mount Waddington (13,260 feet) is the highest of the coast mountains. In eastern Canada, the Torngats of Labrador rise to about 5,500 feet while the summit of Mount Jacques Cartier, the highest peak in the Shickshocks of the Gaspé Peninsula, is 4,160 feet above sea level.
The Great Plains are, for the most part, above or very close to the 2,000 foot level while the Canadian Shield, with the exception of some parts of Quebec and Labrador, is less than 2,000 feet in elevation. Baffin Island, Devon Island and Ellesmere Island also have considerable areas above 2,000 feet in elevation. These high areas carry extensive ice fields.
The Hudson Bay Lowland and adjoining portions of the Canadian Shield appear as an enormous basin surrounded by an upland rim. A somewhat similar but much smaller basin contains the Gulf of St. Lawrence. A third great depression is found in the northwest, between the Shield and the Rocky Mountains. Unlike the other two, it is not low enough to be invaded by the sea but it does contain several very large lakes. These major depressions, sometimes called geosynclines, exert a controlling influence upon the drainage pattern of the country.