SPECIAL FEATURES RESULTING FROM SPECIAL CONDITIONS OF EROSION.
Many striking topographic and scenic features result from rain and river erosion. Some of them depend primarily on the conditions of erosion, such as climate, altitude, etc., while others depend largely on the structure and resistance of the rock. Between these two classes there is no sharp line of demarkation. Illustrations of two types, dependent largely but by no means wholly on conditions independent of the rock, are cited at this point. Others will be mentioned in other connections.
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.)
Fig. 79.—Grand Canyon of the Colorado. (Peabody.)
Fig. 80.—Grand Canyon of the Colorado. (Peabody.)
Fig. 81.—Diagram showing the relations of depth and width of a valley, the width of which is eight times the depth.
While all canyons are valleys, most valleys are not canyons. The distinction between a canyon and a valley which is not a canyon is not sharp. The canyon depends for its distinctive character on the relation of depth, width, and angle of slope to one another; but any definition of the depth, width, and angle of slope necessary to constitute a valley a canyon is arbitrary.[31] In popular usage the rule seems to be that if a valley is sufficiently deep, narrow, and steep-sided to be distinctly striking, it is called a canyon in regions where that term is in use. Whether a valley is deep, narrow, and steep-sided enough to be striking clearly depends on the observer. The Colorado Canyon (Figs. [79] and [80]) is the greatest canyon known, but it is rarely more than a mile deep, and where its depth approaches this figure it is often eight, ten, or even twelve miles wide from rim to rim. Its width at bottom is little more than the width of the stream; that is, a few hundred feet. Its cross-profile throughout much of its course is therefore not in keeping with the conventional idea of a canyon. With a depth of one mile and a width of eight, the slope, if uniform, would have an angle of less than 15°. Such a valley is represented in [Fig. 81]. As a matter of fact the slopes of a canyon are not commonly uniform. The slopes represented in [Fig. 82] correspond more nearly than those of [Fig. 81], to the actual slopes of the Colorado Canyon. The inequalities of slope are occasioned by the inequalities of hardness. It is perhaps needless to say that to an observer on the rim of the canyon the slopes seem several times as steep as those shown in the diagrams.
Like all valleys which are narrow relative to their depth, the Colorado Canyon, great as it is, is a young valley; for it represents but a small part of the work which the stream must do to bring its drainage basin to base-level.
While aridity and altitude are conditions which favor the development of canyons, as shown by the fact that most canyons are high and dry regions, they are not indispensable. Niagara River has a canyon below its falls ([Pl. IX]), and the surrounding region is neither high nor arid. The narrow part of the valley has been developed by the recession of the falls, and is so young that side erosion has not yet widened the valley or lowered its angle of slope to such an extent as to destroy its canyon character. This canyon is often called a gorge, a term frequently applied to small valleys of the canyon type.
Fig. 82.—Cross-section of the Colorado Canyon. (After Gilbert and Brigham.)
Fig. 83.—Detail of erosion in the Grand Canyon. The inequalities of slope are the result of unequal hardness. The vertical planes which give the architectural effect are the result of joints. (Holmes.)
[Plate X] shows portions of the canyons of the Yellowstone and the Colorado rivers respectively. In the first the contour interval is 100 feet, and in the second, 250 feet. The horizontal scale is ¹⁄₁₂₅₀₀₀ (about 2 miles to the inch) in the first, and ¹⁄₂₅₀₀₀₀ in the second. These scales should be borne in mind in interpreting the map.
Falls, rapids, narrows, and other peculiar features, due primarily to inequalities in the hardness of the rock affected by erosion, will be considered later.
Fig. 84.—A surface illustrating the struggle for existence among gullies. Most of the smaller gullies shown on the slope can have but little growth before being absorbed by their larger neighbors. A type of erosion surface common in the Bad Lands. Scott’s Bluff, Neb. (Darton, U. S. Geol. Surv.)