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0131 Explorations in Turkestan 1903 : vol.1
Explorations in Turkestan 1903 : vol.1 / Page 131 (Grayscale High Resolution Image)

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doi: 10.20676/00000177
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of cross-bedded sands and gravels, with occasional silty layers, sloping toward the lake. Here and there low hills rise over the plain. The hills are chiefly made of silt, but contain also certain layers of rough gravel with broken shells of land snails. The body of the plain is about 200 feet above the lake. During the formation of the plain, the local baselevel at the eastern end of the lake may have been relatively higher than now, but whether the strata of the plain are fluviatile or lacustrine does not clearly appear. Semenof noted the marginal conglomerates in 1856, and inferred from them that " the lake in former times occupied a far more extensive basin " (1869, 331). The same explorer states that the mountains on the north of the lake are named from the Kirghiz word " kungei," meaning "toward noon," and those on the south from " terskei," meaning «toward midnight," thus referring to the opposite aspects of the two piedmont slopes (1858, 359). Severtzof noted in his journey of 1867 that the Ak-su, entering Issik Kul at the southeastern corner, had cut its valley through zoo feet of sands and conglomerates, and inferred from this that the lake was once higher than now (1875, 21). Capus (1892, 56) and Schwarz (1900, 581) probably base their statements that Issik Kul once stood 6o meters higher than now on Severtzof s observations. All estimates of the former higher stand of Issik Kul based on the distribution of sands and conglomerates seem untrustworthy, because such deposits are more likely of fluviatile than of lacustrine origin.

The piedmont slopes and the eastern plain are not now in their original condition. They are more or less dissected by open valleys and branching gullies. The valleys are not distinct near the western end of the lake. They are from 5o to 70 feet deep where we crossed many of them on the northern piedmont slope a few miles back from the inidlake shore, but they decrease to less and less depth toward the present shoreline. The eastern plain is well dissected by branching terraced valleys with open straths. Even at the shoreline the valleys at the middle, and still more at the eastern end of the lake, are eroded distinctly beneath the piedmont and the eastern plains ; and, as will be more fully stated below, the lake waters now invade the valley mouths, the invasion being of increasing measure eastward. It is inferred from this that the sloping plains were not graded with reference to the present level of the lake, but with reference to a lake surface that descended gently eastward with respect to the present lake surface.

The valleys emphasize this conclusion. It has just been mentioned that they increase in depth as one passes from west to east, along the north side of the lake. They were not noticed at the west end. They became serious obstacles in road building near the middle of the lake, and at its eastern end the road winds about on the plain to avoid them ; hence it is probable that the cause of the valley erosion should be associated with a tilting of the lake basin, whereby the eastern end was raised more than the western, after the piedmont slopes and the eastern plain had been formed. Climatic change is also to be considered as a cause of the valley erosion, because the depth of the valleys below the piedmont slopes increases toward the mountains. This indicates a change in the règime of the streams, such as a change of climate commonly produces, and such as is commonly associated with