National Institute of Informatics - Digital Silk Road Project
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Explorations in Turkestan 1903 : vol.1 |
Terraces and Meanders of another Kuzzil Su near Chadir Kul, on the Tian Shan Plateau, at an elevation of 11, 000 feet. These terraces are cut partly in gravel and partly in red Tertiary limestone. |
EXPLORATIONS IN TURKESTAN.
each of the mountain masses, large or small, should be intermittently elevated or that each of the basins should be intermittently depressed in such a way that all the streams should be intermittently accelerated in their work of erosion. This process involves an alternation of movement and rest from four to six tunes in each separate drainage area, and at each alternation the amount of uplift and the length of the period of rest must have decreased. All this seems improbable, whatever
may be thought of its possibility.
It is, however, not only the erosion of the terraces that has to be accounted for; in most cases each terrace involves an epoch of deposition preceding the epoch of erosion. The gravel deposits in which the terraces are carved occur not only at the mouth of every valley where it opens on the plain, but also along the course of
Fig. 141.—Terraces and Meanders of another Kuzzil Su near Chadir Kul, on the Tian Shan Plateau, at an elevation of 11,000 feet. These terraces are cut partly in gravel and partly in red Tertiary limestone.
many streams almost to their heads. Sometimes the gravel lies on rock-cut terraces (fig. 142), where it might have been formed during periods of rest when no uplift was in progress. In other cases, however, the terraces along large portions of the stream course are cut in gravel only, and the rock bottom of the valley is now no deeper than when the first gravel deposits were begun (fig. 143). Where this is true the net result of whatever crustal movements have taken place has been that they have balanced one another in such a way as to bring the region back to essentially the saine position that it first occupied. There must have been depression to cause the aggradation of the valleys by gravel deposits, and this must have been followed by periodic and decreasing uplifts of which the sum was equal to the total previous depression;
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