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
bop OROGRAPHY OF CENTRAL TIBET.
exhibited in the region of western Tibet that Deasy and others have explored, but then the maps of those same travellers show that the divergences extend also to the associated mountain-ranges. Seeing that these last are so pressed together, it is not surprising that the lakes — the shapes of which are determined by the shapes of the depressions in the intervening valleys — should themselves exhibit great diversity of outline. In the suothernmost of the great valleys instead of a chain of lakes, we find rivers, the Indus and the Tsangpo. The precipitation is there so abundant that the water which drops to the earth must seek an outlet for itself; but up on the high plateau the evaporation is more than sufficient to get rid of the superfluous water.
Having regard to their shape, we may discriminate between
flat lakes,
mountain lakes,
a type intermediate between the two,
annular lakes.
By a »flat» lake I understand one that occupies the bottom of a very shallow
self-contained drainage-basin in a latitudinal valley. We have seen that the bottoms
of the latitudinal valleys, in consequence of the advanced filling up, are as nearly as possible level, or at any rate the angle of slope on both sides from the foot of the mountains down to the lowest part of the valley is extremely slight. And when a lake forms in a valley such as that it must consequently be exceedingly shallow. An example of a lake of this kind is the big salt lake of the summer excursion of
1900, which, although covering a fairly large area, was only a couple of meters
deep. It is characteristic of these flat lakes, that they possess exceedingly flat shores, which frequently form extensive plains perfectly level to look at and narrower on north and south than on east and west. In consequence of this their shore-lines are very regular and even ; bays, capes, and islands are exceptional. The water is very shallow, and almost always salt; and where it is not so, the
existence of some subterranean outlet -is to be suspected. These flat lakes
may be
I ) elongated,
2) round,
or 3) of a more irregular shape. But the elongated type, with the long axis
stretching east and west, is however the rule.
By mountain lakes I understand those that possess steep shores and lie for
the most part squeezed in between mountains. One such lake is the Tschargut-tso. The characteristics of this type of lake are, that the shore-line is very irregular and indented, and deeply penetrating bays, projecting headlands, as well as islands, are usually found in them. By reason of their confined position between the mountains, their basins are generally deep. The water is fresh, because these lakes possess for the most part an emissary. Very often they are in fact »through» or passage lakes strung along the course of some river, as, for instance, the Tschargut-tso, the Tso-ngombo, and probably several of Nain Singh's lakes. The Panggong-tso, on the other hand, contains salt water, notwithstanding that it lies compressed between the mountains; but then it is the terminal reservoir of a self-contained drainage-basin.
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