This medial region of the continent is bordered on the east for some 2,000 miles by the Atlantic mountains, and on the west throughout its entire extent by the Pacific mountains. It is open to the sea at both the north and the south, and extends in one continuous series of plains and plateaus from the Gulf of Mexico to the Arctic Ocean. The southern portion of this interior basin or trough has already been briefly described in discussing the characteristics of the Gulf plains. The northern portion has also been considered in describing the tundra region adjacent to the Arctic Ocean.
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.
The Lake plains include in Canada the country to the north of the Great Lakes, which drains to them, but excepting the flat lands bordering Lakes Erie and Ontario and once covered by their waters, the region referred to is rather a roughened plateau than a plain. From a geological point of view the hilly country composed of crystalline rocks to the north of Lakes Superior and Huron and included within their hydrographic basins partakes more of the character of the Laurentian Highlands than it does of the features of the portion of the Lake plains situated in the United States.
The Prairie plains also extend far to the north of the international boundary, and on their northern border merge with the forest-covered plains in central Manitoba and the northern portion of Saskatchewan, which are drained by northward-flowing rivers. These plains in the far north differ from the Prairie plains in the fact that they are forested and acquire greater diversity from the presence of innumerable lakes, several of which are of large size. For convenience we may designate this vast and but little known northern region as the Subarctic Forest plains. Still farther north, where the forest dies away, lie the Barren Grounds, which merge on their northern border with the frozen morasses or tundra of the arctic coastal plain.
To acquire just conceptions of the topographic and other characteristics of the several regions of mild relief which make up the Continental basin is a difficult task, as each one is of great extent and possesses many peculiarities of its own, and besides, in two separate regions, each embracing many hundreds of square miles, movements in the earth's crust have occurred of such a nature as to elevate the rocks and give them the general structure commonly found in mountain ranges. Reference is here made to the Ozark uplift in the southwestern portion of the Prairie plains and the Black Hills of Dakota which rise from the Great plateaus.
The Ozark Uplift.—There is an area embracing about 75,000 square miles in southern Missouri, northern Arkansas, and the eastern border of the Indian Territory, in which the rocks have been upraised above the surrounding Gulf and Prairie plains. The uplift, if we imagine it uneroded, would have the general form of an inverted canoe; that is, it would form an elongated ridge, broad and dome-like in the central portion and dying away on all sides into the great surrounding region of undisturbed and essentially horizontal rocks. The major axis of the uplift, although exhibiting a double curvature, has a general northeast and southwest trend. It is about 500 miles long, and in the widest part is approximately 200 miles broad. What the
height of the dome would be had the rocks composing it not yielded to the destructive influences of the air or been removed by streams cannot be readily estimated, since the movements of the earth's crust which upraised it occurred at several widely separated intervals with intervening periods of decay and erosion, and downward movements have also been experienced which submerged the region and permitted the deposit of sheets of sediment over it. If the results of the upbuilding agencies had not in a large measure been counteracted in these several ways, the dome to-day would have a height of several thousand feet. In the present condition the deeply eroded dome presents the net result of elevation over subsidence and erosion. The dome-like form is lost, and in its place is a complex series of ridges and valleys. The higher summits now remaining, situated principally in the Iron Mountain district in northeastern Missouri, rise from 1,400 to 1,800 feet above the neighbouring plains, and from 1,800 to 2,100 feet above the sea.