The leading geographical features of the North American continental basin are its generally low elevation, the mildness of its topographic details, and, with two exceptions, the absence within its borders of elevations having a mountainous structure. In general the rocks beneath the surface are horizontally stratified marine sediments. The stream-cut valleys are shallow and usually broad, except in the bordering plateaus and foot-hills on the east and west sides, where the streams frequently flow several hundred feet below the surface of the broad, flat-topped interstream spaces. The drainage of the continental basin serves as a convenient basis for subdividing it into three separate portions. These are the Gulf slope, which discharges its surplus waters into the Gulf of Mexico and is drained principally by the Mississippi; the St. Lawrence slope, occupied in part by the Great Lakes and drained by the St. Lawrence River; and the arctic slope, down which the Mackenzie, Nelson, and other rivers flow to the Arctic Ocean or to Hudson Bay. At no place are the Pacific mountains broken by cross-drainage, so as to allow the continental basin to send a tribute to the Pacific Ocean.
The vast extent of the Continental basin, embracing, as it does, some three-fourths of the entire area of North America, makes it necessary, even in a general review of the large geographical features of the continent, to recognise smaller subdivisions than the three great drainage slopes referred to above. For this purpose we select the more or less well-defined plains and plateaus into which the region is naturally subdivided. The portion of the Continental basin embraced within the boundaries of the United States has been shown by J. W. Powell to consist of the following physiographic regions, namely, the Gulf plains; the Prairie plains; the Lake plains, including the region draining to the Great Lakes; and the Great plateaus or Great plains, as they are more generally termed, adjacent to the eastern border of the Pacific mountains. Several of these divisions need to be extended and still others recognised in order to include the entire region under review. The portion of the Continental basin to the north of the United States-Canadian boundary has been only partially explored, and the subdivisions of it suggested below are to be considered as provisional.