The portion of the Great plateaus in Canada, like the similar region to the south, is covered with bunch-grass, which dries as it stands and forms highly nutritious self-cured hay. Formerly this region was the winter feeding-ground for vast herds of bison. The winters, although cold, are not characterized by a heavy snowfall, and even in midwinter the warm dry chinook winds, as they are termed, similar to the foehn winds of Switzerland, frequently cause the snow to disappear and leave the brown plateau surfaces free for grazing. Now that the bison has disappeared, this immense region is favourable in many ways for stock-raising, but, unlike the lower prairies to the east with their rich black soil and long hot summers, is not suitable for agriculture. The main difficulties in the way of successful farming lie in the dryness of the summers, and the scarcity of water available for irrigation. The rivers flow in valleys several hundred feet below the general plateau surfaces, and hence cannot be made available for irrigating the uplands without too great an expense. In the bottoms of the valleys, however, adjacent to the stream, limited areas are now under cultivation, and it is to be expected that the wheat-fields of the prairie region will be gradually extended into the valley to the westward, and perhaps even to the eastern margin of the plateaus. A greater extension of the wheat-belt to the north and west than is now thought practicable
has been predicted, but what the ultimate limit will be cannot be told.
The Black Hills of Dakota.—As stated in the brief account already given of the Prairie plains, their monotony of surface and of geological structure is broken by a single area of disturbance termed the Ozark uplift. Similarly, the vast generally level expanse of the Great plateaus is broken by a single rudely circular region of elevation, the Black Hills of Dakota, which has been sculptured by atmospheric agencies and given a diversified topography, in striking contrast with the even monotony of the country surrounding it.
This protuberance on the surface of the Great plateaus is situated in the southwest portion of South Dakota, and embraces also a part of Wyoming, and about 140 miles east of the nearest range—the Big Horn Mountains—of the Rocky Mountain chain. It rises from the surrounding plateau to a height on an average of about 2,000 to 2,500 feet; the highest summit, Harney Peak, is 3,000 feet above the plain, and 7,216 feet above the sea. The uplift is elliptical in ground plan, with a northwest and southeast axis measuring about 120 miles, and a transverse diameter of 40 to 50 miles. Its area is in the neighbourhood of 6,000 square miles.
While the generally level plateau surface about the Black Hills is treeless, except for the scattered groves of wide-spreading cottonwoods along the immediate banks of the larger streams, the central and higher portions of the elevation itself is clothed with an open but abundant forest, consisting principally of pines. The evergreen forests give to the hills a nearly black colour when seen from a distance, and have gained for them the name they bear.
The rocks which have been forced upward so as to form the Black Hills dome were previously like those in the surrounding plain, quite horizontal, and had a vertical thickness of at least 5,000 feet. The uplift, if uneroded, would rise from the surrounding plain as a flat-topped dome about 6,600 feet high, as is suggested by the highest dotted line in the following diagram. In reality such a dome never
existed, for the reason that its growth was slow, and perhaps is not completed even at the present day, and as soon as the rocks began to rise, the rain, wind, streams, etc., commenced their task of destruction. The higher the rocks were elevated the more powerfully those agencies acted. The top of the dome was soon broken and its internal structure revealed.
Fig. 18.—Ideal east-and-west section through the Black Hills. The vertical scale is about six times the horizontal. The dotted lines indicate the portion of the uplift removed by erosion. After Henry Newton.
1. Archean slates and schists.
2. Granite.
3. Potsdam, sandstone, resting unconformably on 1 and 2.
4. Carboniferous, mostly limestone.
5. Red Beds (Trias), sandstone with included limestone.
6. Jura, shales.
7. Cretaceous, shales.
8. White River Tertiary, shales, resting unconformably on 7.