SURVEYING THE DESERT. 247
but somewhat deeper than usual. Some of these pits might just as readily belong to the one class as to the other; it was impossible to tell which. Even in the case of the unmistakable river-beds it was strange how they dwindled away in both directions until they disappeared. Seeing that the parts which we crossed over all lay in a south-west to north-east direction, parallel to the lowest Tarim and the Karakoschun, and to the general relief of the desert, it is conceivable, that only those parts of their courses are preserved which run parallel to the prevailing wind, while the other parts have been obliterated. A photograph of one of these river-beds is shown on plate 29. In its case there cannot exist any doubt as to the origin of the channel. It is not however always so easy to interpret straightaway the traces which erosion has left behind it on the surface. If, for example, the whole of this region were at one time a lake, it is difficult to account for the presence of the round pit-like hollows we passed during this day's march. Are they simply deeper spots in the larger lake, or are they survivals of other lakes which were formed at some later epoch in the basin of the older lake? Certain things are at any rate clear, namely that throughout long periods the water, river sediment, drift-,,and, and dust have been everywhere levelling down this great basin, that the hollows which descend one or two meters below the general level have been formed by water, that the directions and distribution of the flowing water became gradually more undecided in proportion as the basin was filled and levelled up, and that in consequence of this the tendency of both rivers and lakes to shift their positions has been accentuated.
I have already said, that the absence of the wind-gullies in the schor indicate that this type of desert offers more effective resistance to the wind's erosive energy than does the clay desert. I have also assumed, that the southern part of the Lop-nor dried up sooner than the northern, just as is the case at the present time with the Kara-koschun. Thus the southern parts of the floor of the Lop-nor would be exposed to the wind for a longer period than its northern parts, and the effects of the wind in the former would be more distinguishable than in the latter. But in reality the case is exactly the reverse of this, the cause being the induration of the schor formation. However that may be, the appearance of these secondary depressions and watercourses seem to indicate, that even in the schor desert the wind does possess a certain abrasive power, although it is, it is true, incomparably less effective than in the clay desert. In the round pit-like depressions considerable portions of the margins are wanting; either this shows where the lakes shallowed or else they have crumbled away under the attacks of the wind. With regard to the surviving south-west to north-east portions of the watercourses, it is probable that the wind, instead of obliterating them, has deepened them still further. As with the missing parts of the pit-like hollows, so here, we may assume that the absent portions were either especially exposed to the levelling power of the wind or else the channels themselves were very shallow. Anyway it is . to the wind that the defective condition of . these watercourses must in the main be ascribed. The absence of wind-grooving and of jardangs must not therefore be regarded as proving that the wind is altogether powerless in the schor desert; it is not indeed powerless even in the mountains, with their immeasurably greater hardness. Perhaps too the greater sculp-
Hedin, Journey in Central Asia. H. 32
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