IIIFカラー高解像度
国立情報学研究所 - ディジタル・シルクロード・プロジェクト
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| IIIFカラー高解像度 |
| Scientific Results of a Journey in Central Asia, 1899-1902 : vol.4 | |
| 1899-1902年の中央アジア旅行における科学的成果 : vol.4 |
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引用情報
THE OMAN-TSO, THE DADAP-TSO, AND THE PERUTSE-TSO. 203
races. In other words, the slope down from the foot of the western mountains is much more gentle than the slope along the line which we levelled. Accordingly the contour-lines have been more easily modelled in the latter quarter, while they stretch a good long way westwards in order to keep to the foot of the mountains. And with this the position which the lake occupies with regard to the adjacent country is in full agreement; that is to say, when the lake was 39 m. higher than it is now, it was elongated in the same direction as its latitudinal valley runs, namely from east-south-east to west-north-west. There is presumably nothing to prevent one Tibetan lake from drying up more quickly than another. If the desiccation is occasioned, as in the present instance, by a diminishing precipitation, then the several lakes ought on the whole to dry up at approximately the same rate, and their subsidence ought to proceed at pretty much the same rate in them all. But there is yet another factor that has to be taken into account, namely the different positions which the lakes occupy with regard to their environment and in relation to the water-bearing strata. A very extensive and shallow salt-lake will of course dry up faster
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Fig. 116. Horizontal scale = r : r o,000. Vertical scale = r : 2,000.
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than a small and relatively deep lake; and in the case of the former the desiccation may proceed so fast that terraces have hardly been able to come into existence on its flat shelving shores. Accordingly, in the case of the Lakor-tso, it is the highest terraces that are the most distinctly developed, and they originated at a time when the neighbouring mountains plunged straight down into the lake. The smaller the lake grew the nearer did it approach in shape to the saucer type, and the more indistinct did its terraces become. Beside the Perutse-tso the terraces are only visible where the slope of the shores is more pronounced, to speak solely of those parts of them that I visited. If, as we have seen, the Lakor-tso once lay 133 m. higher than it does now, we may certainly assume that the fifth terrace beside the Perutsetso, which only reaches 39 m. above the existing level of the lake, in no sense corresponds to the altitude of 133 m. which was simultaneously reached by the Lakor-tso; but at that time the Perutse-tso also rose far above the level of its present fifth terrace. A more detailed and more extensive examination all round the lake would undoubtedly reveal the existence of several older terraces, even though they be in a fragmentary condition. That we were unable to detect any above the fifth terrace may have been purely an accident, and due more particularly to the erosion of the wind acting in the very open valley. At the present time the lake has reached such a languishing condition that its remaining days are not many; the exposure of the bottom of the lake in the middle is a proof of this.
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