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0198 Explorations in Turkestan 1903 : vol.1
Explorations in Turkestan 1903 : vol.1 / Page 198 (Color Image)

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doi: 10.20676/00000177
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and its residual mountains, and in order that the weak Tertiary strata should still exist in the warped and uplifted plains, its deformation must have been compara-

tively recent.

Numerous other examples of this kind might be cited, but one will suffice. At Gulcha, about 3o miles southeast of Osh, on the border between the Fergana basin on the north and the Alai Mountains on the south, the Gulcha River flows in a valley 2,000 feet deep. Between this valley and the next there is an upland which in a general view appears to slope smoothly and gently to the north, although it is somewhat notched here and there. The surface of the upland truncates inclined strata which vary in hardness from the resistant oyster-bearing limestone to the soft, shaly sandstones of the Tertiary. It is still well preserved, in spite of the fact that there is in some places a descent of 2,000 feet in 3 miles. The sloping upland plain must have been formed as a peneplain, and must have been given its present inclination at a somewhat recent date. In the western part of the Tian Shan range, where the plateau character is less marked, and in the main range of the Alai Mountains, the old peneplain is shown chiefly in the level crests of the ridges. Even in the lofty Pamir there are certain ranges where the snowy peaks are smoothly truncated, as though by the old peneplain, in spite of the fact that they are from Is,000 to 20,000 feet high. The fragments of old surfaces are indeed so numerous that it seems safe to conclude that much of the country was once reduced to a peneplain, and the rest of it at least to the stage of late maturity. The extent of this degraded region was fully ioo,000 square miles—that is, at least 400 miles east and west and 250 miles north and south, and probably much more. Although the age of the peneplain is not closely fixed by the evidence of fossils, it may be referred to the end of the Tertiary, because its erosion was completed after practically the whole Tertiary series of the region had been laid down and warped. For the present we shall consider that the erosion of the peneplain marks the close of the Tertiary era and that the Quaternary is introduced by the succeeding changes of elevation.