The Great Plains rise in elevation from under 100 feet at the lower Mississippi River to just over a mile high (1 mile = 5,280 feet) at the foot of the Rocky Mountains. It is composed of layer on layer of sedimentary rock washed out from the Rocky Mountains to the west. Rivers such as the Platte, Missouri, and Red parallel one another as they flow from west to east down this gentle grade. Much of the Great Plains was a swampy area during the Carboniferous Period. Large portions were still under water prior to the uplift of the Rocky Mountains some 70 million years ago. This history has resulted today in large deposits of coal, oil, and natural gas from Texas through Colorado, Wyoming, and Montana, and into Alberta in Canada.
While most of the Great Plains is flat and undistinguished, it does contain some distinct physical anomalies worthy of mention. In Nebraska, a belt of sand hills has been formed by wind and the outwash of the last continental glaciers. In South Dakota, however, water erosion has created a vast badlands landscape of deeply cut and barren soils. The one major mountain system that interrupts the evenness of the Great Plains is the Black Hills of South Dakota, which rise to over 7,000 feet. Here old crystalline igneous rock breaks throughout the surface in a dome-like swelling believed to be an outlier of the Rocky Mountain system. The oldest and largest gold mine in the US, as well as the famous Mount Rushmore, are both located in the Black Hills.
Great Plains Climate and Vegetation
The Great Plains have a considerably drier and more unpredictable climate than do the Midwest and Coastal Plains. Prairie grasses are the natural vegetation of this area. For this reason, early settlers from northern Europe referred to it as the "Great American Desert." Soils that formed under the prairie grasses, however, were actually richer than those of the more forested Midwest, making the region the primary wheat-growing area in the US and Canada. The Great Plains become more narrow as they extend into Canada and gradually disappear in northern Alberta. North of Edmonton (Alberta), the prairie grasslands give way to a belt of mixed coniferous (pine) deciduous trees. Eventually, these become spruce and aspen forests which dominate the cold northern latitudes all the way to Alaska, both on the Canadian Shield to the east and the Rockies to the west.
The Canadian Shield (Laurentian Plateau)
Underlying more than 1 million square miles of the Interior Lowlands of Canada is the Canadian Shield. While the Canadian Shield is almost entirely in Canada, geologically important extensions of it lie in northern Michigan, Minnesota, and Wisconsin. In New York, the Adirondack Mountains are an extension of the Shield. The Canadian Shield contains some of the oldest exposed rock on the surface of the earth. It is primarily composed of hard, metamorphic rock created 4 billion years ago in the Precambrian Period. The metamorphic process has created zones rich in metallic mineral deposits. Successive periods of glacial cover have removed most of the sedimentary material that may have once covered the Shield, exposing rich deposits of metallic ores. Canadian Shield iron ores from Minnesota and Canada were one of the main resources allowing for the development of the US and Canadian manufacturing core area around the Great Lakes. Hudson Bay, the largest bay in North America, is situated at the center of the Canadian Shield. During the great ice ages, vast sheets of snow extended out of the Hudson Bay area, covering much of lowland Canada and the northern US.
The Canadian Shield dips from between 1,000 and 2,000 feet in elevation in the south and west to sea level at Hudson Bay. A string of lakes and other water bodies surrounds the edge of the Shield to the south and west. The Great Lakes, Lake Winnipeg, Great Slave Lake, and Great Bear Lake are the largest lakes in North America. They, along with the St. Lawrence River and Lake Champlain Lowland (New York and Vermont), and the Mackenzie River in the far north demarcate the boundary between the Canadian Shield and surrounding uplands. The St. Lawrence and Mackenzie Rivers are two of the largest in North America. Unlike the rivers that flow parallel across the Great Plains, those located on the Canadian Shield form a zigzag pattern interspersed with numerous lakes. This pattern reflects the underlying hard rock and is typical of landscapes created by continental glaciation. Most of the Shield is vegetated with skinny spruce and aspen forests, with the exception of the far north, which has treeless tundra vegetation. The tundra line also marks the southern limits of continuous permafrost soils.
The southern two-thirds of the Canadian Shield experiences mild summers (50 to 70 degrees F) and cold (below freezing) winters. The region north of Hudson Bay, however, remains at freezing and below throughout most of the year, with very short summers when high temperatures reach into the 40s. (Only interior Greenland averages below freezing all year round.) The major air mass influencing this region is the dry and cold continental polar system. This air mass is weaker in the summer, but comes on strong in the winter months. Annual precipitation is under 10 inches for the entire Canadian Shield, except for the areas to the east and south of the Great Lakes.