THE TSOLLA-RING-TSO AND THE LUMA-RING-TSO. 209
One thing at once arrested attention, namely that these terraces exist solely and alone on the north-west slope of the pass, whereas on its eastern side they are entirely absent. In this circumstance I see a fresh proof of the correctness of the theory which I have already advanced, that the arrangement of these Tibetan lacustrine terraces proves generally, that even at the time when the climate was moister than it is now, and when the lakes were therefore bigger, the prevailing wind blew, as it blows to-day, from the west. The slope on the west side of the pass has in especial been in a high degree exposed to wind and waves beating upon it from the west, or rather from the west-north-west and north-west, for the shape of the valley forces the wind to make a local deviation. It was in that same direction that the old Tsolla-ring-tso stretched, and the waves which beat upon the eastern shore of the lake, along the lines of the four or five terraces, came from a long way off, namely from its distant western side, and consequently had plenty of time in which to grow to a considerable height. Thus, even though the period during which the lake maintained itself at the one level or the other may have been fairly short, the powerful and energetic action of the waves was nevertheless sufficient to accomplish a work which under other conditions would have required a much longer period.
On the eastern slope that stretches down from the threshold pass to the Perutse-tso no strand-terraces have been able to develop, and that for three reasons. The first and most important is, that this slope was sheltered from the everlasting westerly wind, the effects of which upon the western shore of the lake were practically neutralized. In the next place this slope is much flatter than the western slope, which reaches down to a deeper depression. Finally, at the period in question the Perutse-tso was considerably smaller than the Tsolla-ring-tso. The circumstance that the Perutse-tso remained for a relatively longer period at each of its respective levels ought to have operated in the opposite direction; but beach-lines are nevertheless entirely absent on the eastern slope of the threshold. A thorough examination of the whole depression would without doubt reveal the fact, that the best developed strand-terraces are to be sought for at the east end, upon which the waves, gathering way in their passage across the entire lake, beat with great force.
While the eastern slope down from the threshold is practically quite free from eroded watercourses, and in any case possesses no main drainage-channel, there does exist one in the north-western part of the latitudinal valley. It does not however come down off the pass, but is formed gradually out of the transverse glens of the southern range, the brooks of which unite one after the other to form a main watercourse. This stream flows close along the foot of the southern mountains, and finally expands as it approaches Lake Harschu, but disappears before it quite reaches it. From the northern mountains this main stream does not receive a single contributory. Thus the temporary torrents which arise in the northern glens never reach down as far as the principal glen.
In travelling through a latitudinal valley like this, — absolutely dry except for the two lakes and the Ombo-tsangpo — I was astonished to find no accumulations of sand anywhere; we did not detect so much as a rudimentary tendency to the formation of dunes. And yet one would suppose that the conditions favourable to
Iletlin, 7durnt ' in Central Asia. It". 27
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