IIIF Color HighRes
National Institute of Informatics - Digital Silk Road Project
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| IIIF Color HighRes |
| Scientific Results of a Journey in Central Asia, 1899-1902 : vol.1 |
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| Eroded dunes. | |
| Eroded dunes. |
Citation Information
THE STRUGGLE BETWEEN THE RIVER AND THE SAND. 173
What actually occurs is shown in fig. 143, C; that is to say, both the disintegrating forces act turn and turn about. After a storm or storms have carried away the layer of sand (a), the river penetrates the dune-foot to (c), whereupon the section of sand (6) slips and glides down into the river, leaving behind it a fresh sharp edge between the steep slope and the flat dome of the dune-summit. After that comes a fresh storm, which sweeps away the layer (a'); then the river eats its way into the base of the sand as far as (c'), and the section (6') in its turn plunges into the stream, which carries it away, either adding it to its existing sand-banks or forming new ones out of it. Meanwhile the loop increases in breadth precisely at the spot where these sand-slides have taken place. Finally the contours assume the shape shown by the stippling. All the rest of the mass of sand has been swept away, a part blown into the interior of the desert, but by far the greater part used by the river as an instrument for modifying the relief of the country through which it flows. When the river shifts its channel, the sand-banks it has deposited become gradually exposed, and serve as the foundation for river-dunes. These are, however, rare in the Tarim, and when they do occur they are so small that they may be disregarded in comparison with the immense masses and vast extent of the desert dunes. Where they do come into existence, they very soon get covered over with vegetation.
~i~%//~
~'' - ///~//~
' - ///~///~/
> b, ,b„-.-~,,,~~~j~~~~~
~ ' -'' ~%%~~%%%~~//
)--
-_~%%/
- --'c„ c;,;
Fig. 143 C.
The order of the occurrences is, however, different in different parts of the same loop; a very illustrative example is afforded by the sharp bend to the south just below Jallang-dschajir. All round the outer periphery of the loop the sand descends into the river with the same degree of steepness, and the effect of the river's action upon it is uniformly the same throughout. But on the eastern side the sand would go down into the river even though the latter produced no effect upon it, for the wind drives it over and down upon the leeward side. On the west side, however, the wind conveys no sand into the river. On the east side the two agents work together towards the same end; the wind seeking to move the dune to the position marked (a) in fig. 144 and the river endeavouring to deposit it at (b). Yet the
Fig. 144.
C
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