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0200 Explorations in Turkestan 1903 : vol.1
Explorations in Turkestan 1903 : vol.1 / Page 200 (Color Image)

OCR Text



I 70



In the preceding sections the various rock formations and the Tertiary peneplain have been spoken of as though each of them represented a definite space of time rather than a stage of development. That the latter is the truer view is well shown by the phenomena along the borders of the Kashgar basin. South of the village of Artush, about I2 miles northeast of Kashgar, the Quaternary plain is broken by a ridge of interstratified silt and gravel which runs nearly east and west and rises 200 or 30o feet. It is an anticline so recently uplifted that its original form remains almost unchanged, although the material is soft and unlithified. The dip is gentle, about 6° on the north side and less on the south. The greater part of the strata which compose the anticline consists of buff silt, which is like loess in color and texture, although some portions at least are more clayey. It is apparently the same material as that which is now being deposited in the playas of the surrounding plain. Interstratified with the silt are layers of stream gravel, showing old channels with cross-bedding and lateral unconformities. The top of the ridge is covered with gravel from 5 to 20 feet thick. West of Kashgar the silt continues, although with less of the character of loess. The valley followed by the caravan road to Osh leaves the plain through some low hills which seem to be a continuation of those already described near Artush. They consist of the same silts and gravels, which dip greatly toward the plain and are well exposed in the steep sides of the valley. At the top lie several hundred feet of gravel, then come numerous alternations of gravel and silt, with a gradual increase in the thickness of the finer material, and at last very thick yellow silts. The upper part of the latter are full of lenticular stream-channels, which grow broader and less numerous lower down. Although all the strata are very soft, they correspond in position to the formations which have been described above as the brown conglomerate and brown sandstone of the upper Tertiary, and by the rules of ordinary stratigraphy would be reckoned as of the same age. That they are younger is shown by their less degree of consolidation and by the fact that they can hardly be distinguished from the strata now in process of formation. Their folds, too, are younger than those of the sandstones, for although they rise above the level of the old peneplain, they are not beveled by it. Apparently, the Kashgar basin has long been growing smaller by a process of continuous folding along the edges, and as it has grown smaller the locus of deposition of the gravels which accumulate along its edges has gradually been pushed inward.

To make this more concrete, let us take the cross-section at Sugun Karaul, west of Shor Kul (fig. 124). In the Tian Shan plateau, 10 or 15 miles from the edge of the Quaternary plain of the Kashgar basin, the conglomerate at the top of the Tertiary is highly folded and very hard, but as the same stratum is traced southward and westward it becomes softer and less folded, until finally it seems to run into the soft gravel of very recent date which has been described in the preceding paragraph. Moreover, the old resistant conglomerate of the Tian Shan region has been smoothly baseleveled since its severe folding was completed, while the gentle folds of the soft young gravel have only been dissected by narrow valleys which have not yet produced a maximum of relief. This seems to mean that somewhere in Tertiary time