Throughout practically the whole of the region occupied by the subarctic forest, between Hudson Bay and the foot-hills of the Rocky Mountains, the land is low and the
valleys monotonous. Many lakes are present, several of them of large size, and the rivers are remarkable for their lengths, low gradients, and large volumes.
The subdued topography of the region here considered, and the presence of vast numbers of lakes and swamps, is due in general to the influence of the ice-sheets which formerly covered it. In a minor way the presence of the innumerable small lakes and swamps is owing to the obstructions formed by growing vegetation, the damming of streams by driftwood, the work of beavers, and possibly the influence of subsoil ice.
To the north of the Subarctic Forest plain, as already described, occur the desolate tracts known in Canada as the Barren Grounds, which form a part or merge into the tundras bordering the Arctic Ocean.
The Great Plateaus.—The boundary between the prairie plains of the central portion of the interior Continental basin and the Great plateaus (Great plains) bordering them on the west is usually indefinite. The prairies pass into the more elevated and drier plateaus by insensible gradations. The plateaus rise gradually from east to west, and along their western margin, adjacent to the east base of the Pacific mountains, attain a general elevation of from 5,000 to 6,000 feet. Over vast areas these monotonous plateaus, with their even sky-lines, are higher above the sea than the crests of the Appalachians, and along their western margin in many localities even surpass in elevation the most prominent peaks in the eastern portion of the United States. Accompanying this increase in elevation from east to west there is a decrease in precipitation, and in consequence a marked change in the vegetation. The plateaus, like the prairies, are treeless in their most characteristic portions, but the larger rivers winding across them are margined in many instances by giant cottonwoods.
The mental picture that a traveller over the broad plateaus retains in after-years is of a vast treeless level tract of country, boundless as the ocean, which is bright green and decked with lowly flowers in the early spring, but becomes
yellowish brown as the heat and dryness of summer increase and the grasses lose their freshness. Various portions of the plateaus, however, have their own individuality and present characteristics which make them conspicuously different from other portions of the same great series of steppes. At the south, in the region of the Rio Grande and of the Pecos and Canadian Rivers, the plateau is dissected by stream-cut valleys 1,000 feet or more deep, and from one to two score miles across, which divide it into a number of individual table-lands. The plateau margins for many miles on each side of the larger river-valleys have been carved by a complex system of secondary and usually ephemeral streams into a great variety of rock forms with deep trenches between. These conspicuously sculptured areas constitute what are commonly termed Bad Lands. In certain regions also the surfaces of the plateaus, more especially in Nebraska and South Dakota, are broadly undulating or reveal a seemingly endless succession of ridges and hills separated by shallow depressions, due to the presence of large tracts of drifting sand. In spite of these several variations, however, the leading characteristics of by far the larger portion of the plateau country are the generally level grass-covered surfaces extending away in all directions far beyond the reach of vision. On the rolling prairie one can frequently see the undulating surface about him for a distance of 15 or 20 miles, but the curvature of the earth usually draws still narrower limits to the region within the view of the plainsman. In riding over the plains the scene changes but little from day to day and from week to week. Monotony is the one word that best describes the lives of those whose lot is cast on these broad featureless surfaces. In journeying westward across the plateau over any one of the transcontinental railways a moment of excitement occurs when the even line of the western horizon is broken by the summit of a cloud-like mountain-peak. "Land ho!" is no more thrilling to voyagers on the ocean than the shouts which first made known the presence of a mountain-peak to the bands of immigrants who slowly voyaged across this sea of grass with their picturesque "prairie
schooners" previous to the building of the railroads which now bind together the East and the West.
The Great plateaus begin indefinitely to the south of the Rio Grande, broaden in the United States to a general width of about 400 miles, and extend far northward into Canada. Their northern limit has not as yet been determined, but is to be looked for near the head waters of the Mackenzie. The length of the plateau country is in the neighbourhood of 2,000 or 2,500 miles, and its average width about 300 miles. An estimate of the area with a generally plane surface and an elevation of from 1,000 to 6,000 feet above the sea places it at about 700,000 square miles.
The eastern portion of the Great plateaus includes western Texas, Oklahoma, the central and western portions of Kansas and Nebraska, the western half of South Dakota, western North Dakota, western Assiniboia, and thence extend northward so as to include portions of Saskatchewan, Alberta, and Athabasca. On the west the plateau region includes the eastern portions of New Mexico and Colorado, extends far into Wyoming, and embraces central and eastern Montana, and thence reaches northward to Mackenzie.