Southern Tibet : vol.7 / 699 ページ（白黒高解像度画像）

0699	Southern Tibet : vol.7
南チベット : vol.7

had its gathering-grounds and firn-basin to the west, and itself travelled east through
the valley; and it is in consequence of this that the bottom is so smooth and level
as we actually find it to be . . . . There, however, we are confronted with the
difficulty of explaining the possibility of movement on the part of the ice-stream
along what was practically level ground, or even somewhat rising ground.»¹

However, having studied the problem of the Tibetan lakes more thoroughly
and compared the different theories concerning them, I now feel inclined to abandon
the theory of their glacial origin completely. For even if this theory seems to suit
the formation of the Panggong-tso very well, it is not sufficient to explain the for-
mation of the hundreds of lakes spread over the Tibetan plateau-land. Further I do
not think that Drew's theory of talus fans in the mouths of tributaries damming up
main valleys can simply be dismissed, for in some cases such fans may have played
a considerable part. The Panggong valley, not only the part of it that now is
covered with water, but also its eastern continuation from Tso-nyak, has all the
characteristics of a fluviatile formation and the features of having been eroded by a
considerable river. My soundings in the Panggong lakes, undertaken from the ice
in the freshwater lakes, and from a boat in the western, salt-water lake, proved that
the depth, as a rule, increases from east to west until it reaches its maximum with
47.50 m. in the western part of the Panggong-tso. Until a detailed measurement
of the height of the threshold between the Indus system and the lake has been
carried out, the real height of the threshold above the Panggong-tso remains un-
certain. Strachey says the water-divide lies 100 feet above the lake, Huntington
gives 90 feet, though I do not know by what means these figures have been ar-
rived at. On December 22nd, 1901, I got an altitude of 4327 m., and on August 22nd,
1906, one of 4331 m., both by boiling-point thermometer and three aneroids. The
altitude of the lake being 4317 m. according to my observations, the height of the
threshold would be only 12 m. above the lake. As the maximum depth of the western
part of the lake is 47.50 m., a talus fan of a thickness of 60 m. where it is lowest
would be sufficient to dam up the valley and make the formation of a lake both
natural and necessary. The progress of this process has obviously been gradual and
periodical. As a rule the erosive power of the out-flowing river has been active at
the same rate as the increase of the talus fan. During comparatively dry periods
the erosive power has diminished, but the increase of the talus fan has continued,
though at a much slower rate. At periods the lake has been cut off completely.
During the next moist period the lake has again risen to the lowest part of the
talus threshold then existing. Such periods of rise and fall of the surface of a lake
on account of alternating moist and dry periods may still be observed in the Manasarovar

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