From the Laurentian Plateau and the Appalachian Highland on the east to the Rocky Mountains on the west, and from the Arctic Ocean in the north to the Gulf of Mexico in the south, there stretch the great Continental Plains of which the Gulf Coastal Plain forms the southern part. The character of this continental interior varies considerably throughout, and several important subdivisions must be recognised. The sub-Arctic Plain, around the shores of the Arctic Ocean and along the lower course of the Mackenzie River, corresponds generally in configuration and altitude with the Gulf Plain, and on the south gradually merges into the Great Plateaus. In the east of the Continental Basin the altitudes are low and the plains are either flat or gently rolling, while in the west they rise by degrees to a height of 5,000 feet along the foot of the Rocky Mountains. As these two regions—the Prairie Plains and the Great Plateaus— gradually merge into one another, it is difficult to draw exactly the dividing line between them ; but it may be taken roughly as follow ing the Missouri Coteau in Canada, and in the United States as separating eastern and western North and South Dakota, eastern and central Nebraska and Kansas, and eastern and western Texas. To the east of this line, however, there are two regions which cannot properly be included within the Prairie Plains, one being the Ozark uplift, and the other the Lake Plains, a region lying to the south of the Great Lakes and formerly covered by them.
The Western Cordillera is the last great physical region of North America ; its structure is very complicated, and only its salient features can be mentioned here. In the north, bordered by mountain ranges, there is an undulating plateau country which forms the basin of the Yukon. Further south, besides the Rocky Mountains proper there are, in the east, the Selkirks and the Gold Range, between the last of which and the coastal mountains— the Northern Cascades—lies the interior plateau of British Columbia.
South of the international boundary, the term Rocky Mountains is generally applied to the wide stretch of country extending from the western margin of the Great Plateaus in the east to the Wahsatch Mountains in the west. Over this tract there are many ranges running north and south, with great intervening valleys known as parks. To the north-west of it lies the Columbia Plateau, which has been built up by volcanic outpourings filling the depressions between the Rocky Mountain system and the Southern Cascades ; to the south of it there is the Colorado Plateau, an elevated region composed of horizontal rock in which deep canons have been cut by the Colorado and its tributaries. Between the Wahsatch Mountains and the Colorado Plateau in the east and the Sierra Nevada in the west, lies the Great Basin, a region of inland drainage with a general elevation of 5,000 feet, and with many short ranges running across it from north to south. The general characteristics of this region are preserved in the country to the south of the Colorado Plateau, though there the drainage is to the sea, and in the Mexican Plateau, where many of the basins are again closed. To the west of the Southern Cascades and the Sierra Nevada lies a series of coastal ranges, with the Puget Sound valley and the California valley intervening.
Mexico is the southern continuation of the Cordilleran system. Two great ranges, the Sierra Madre Oriental and the Sierra Madre Occidental, whose precise relationship with the Rocky Mountain system has not yet been definitely determined, border the country on the east and west respectively, while the intervening region is a plateau which rises from a height of about 4,000 feet on the United States boundary to over 8,000 feet in the south. This plateau has been built up in part by the debris from the surrounding mountains, and in part by discharges from the volcanoes which form so prominent a feature in the topography of the country.