Page:Popular Science Monthly Volume 53.djvu/24

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deeper submarine valleys was thus still left unexplained, as if they were either unimportant phenomena or as if they had been formed by causes not in operation at the present time.

Character of Land Valleys.—Ordinarily speaking, valleys are produced by the chemical and physical action of the rains, rills, and rivers denuding-the surfaces of the land, while the character of the rocks gives rise to modified PSM V53 D024 Cross section representing a broad valley.pngFig. 1.—Cross-section representing a broad valley, a b r b a; with base level lowered, the valley (b c r c b) was produced. Another rise of the district causes the stream to make a new cañon (d) too recently for the valley to become mature. The terraces at b b and c c show the positions of former base levels. features. Streams flowing through old valleys are usually insignificant, if compared with the size of the valleys. When a stream commences its work, it gradually deepens its channel until it is reduced to the base level of erosion—that is to say, when the slope of the floor of the valley rises so gently from the sea level, or from some other barrier, that it does not permit any further deepening of the river channel. In the first stage, the stream only excavates the narrow gorge or cañon. Upon the base level being reached, the rains and rills widen the valley, while the work of the river itself is chiefly that of carrying away the mud washed off the neighboring lands by the rains, or of undermining an occasional bank. The process continuing, the valley may become dozens of miles in width. With the subsequent rise in the land, a lower base level is formed, and accordingly the process is repeated from the canon-making stage to that of mature valleys, as illustrated in Fig. 1.

With the land rising at intermittent epochs, the valley becomes a series of steps, as illustrated in Fig. 2. Each of the steps is being cut slowly backward, but, owing to the character of the rocks, the

PSM V53 D024 Longitudonal section of a valley.png

Fig. 2.—Longitudinal section of a valley dissecting a plateau (represented by broken shading along a line a g x), showing three base levels, with the front of the platforms (b c and d e) characterized by waterfalls.

lower platforms may be worn away faster than the upper, so that in time the steps will disappear as the valley becomes mature, when its whole floor is as low as it is possible for the stream to deepen it.

If upon the completion of a portion of a valley the country subsides, the lower reaches may become submerged, as shown in