Fig. 75.—Bad-land topography. North of Scott’s Bluff, Neb. (Darton, U. S. Geol. Surv.)
Bad-land topography.—To a type of topography developed in early maturity in certain high regions where the rock is but slightly, though unequally, resistant, a special name is sometimes given. Such regions are termed bad lands. Some idea of bad-land topography is gained from Figs. [75] to [78]. Bad-land topography is found in various localities in the West, but especially in western Nebraska and Wyoming, and the western parts of the Dakotas. The formations here are often beds of sandstone or shale, alternating with unindurated beds of clay. Climatic factors are also concerned in the development of bad-land topography. A semi-arid climate, where the precipitation is much concentrated, seems to be most favorable for its development. The bad-land topography is most striking in early maturity.
Fig. 76.—Toadstool Park, Sioux Co., Neb. The peculiar topography is the result of erosion working on jointed rocks of unequal hardness in an arid region of considerable elevation where rainfall is unequally distributed. (Darton, U. S. Geol. Surv.)
Special forms of valleys; canyons.—Various conditions influence the size and shape of valleys, especially in the early stage of their development. If the altitude of the land be great, the gradient of the streams at this stage will be high. A high gradient means a swift stream, and a swift stream erodes chiefly at its bottom. High altitudes therefore favor the development of deep valleys. Such valleys will be narrow if the conditions which determine widening are absent or unfavorable. Since slope wash is one of the main factors in the widening of valleys, an arid climate favors the development of narrow valleys, if there be sufficient water to maintain a vigorous stream. Narrowness and steepness of slopes will also be favored if the valley is cut in rock which is capable of standing with steep faces. Thus a stream may develop a narrow valley in indurated rock where it would not do so in loose gravel, and, other things being equal, it will develop a narrower valley in rock which is horizontally bedded than in rock the beds of which are inclined. Aridity, high altitude, and the proper sort of rock structure therefore favor the development of canyons, and many of the young valleys in the western part of the United States where these conditions prevail, belong to this class.
Fig. 77.—Detail of bad-land topography. Head of Indian Draw, Washington Co., S. D. Protoceras sandstone on Oreodon clay. (Darton, U. S. Geol. Surv.)
Fig. 78.—Detail of bad-land topography. Southwest foot of Mesa Verde, Colo. (Matthes, U. S. Geol. Surv.)