Southern Tibet : vol.7 / 700 ページ（カラー画像）

0700	Southern Tibet : vol.7
南チベット : vol.7

0700

Southern Tibet : vol.7

南チベット : vol.7

and its channel to the Rakas-tal, though the latter is not dammed by any talus
fans from tributaries.

Comparing different lakes in Tibet with one another, I have arrived at the
conviction that the problem of the formation of the lakes is much more complicated
than it would seem to be if we accepted as a general rule only one of the three
theories mentioned above. It cannot be said that Drew alone is right and Oldham
and Huntington wrong, nor that Oldham or Huntington were right to the exclusion
of the two others. As far as Panggong-tso is concerned, I think that all three
theories may be applied at different periods of the history of the lake and its
valley. Very likely the Panggong valley may have been filled with glacier ice during
the glacial period. But during ages before the glacial period, even from the period
when the Himalayan and Transhimalayan fold systems began to be formed, and until
the present day, differential movements of the surface have been going on. The ice
has dissappeared and the climate has become drier. But even during the post-glacial
time, the precipitation has been incomparably more abundant than nowadays, and
great rivers have been flowing through the long latitudinal valleys of the Tibetan
high-lands, valleys which still exist, although the rivers have dwindled and disappeared
because of the periodically proceeding desiccation. Finally the continual differential
movements of the surface have gradually divided every latitudinal valley into a series
of separated self-contained basins, usually with a salt lake in the centre.

At a period when the latitudinal valleys were divided in this way, the talus
fans of Drew may easily have played a certain part in the formation of some of the
peripheric lakes of Tibet. And thus all the three theories may be said to be correct.
The differential movements belong to the whole history of the building up of the
southern mountains, the glacier erosion to the ice age and the talus fans to the post-
glacial epoch with a growing dryness of the atmosphere.

Burrard and Hayden may be said to have added a fourth theory to the three
of Drew, Oldham and Huntington. They say¹: »The further suggestion, now made
by us, that the damming of the main valley may have taken place owing to its
conversion into a tributary valley, may be regarded as a modification of Mr. Drew's
hypothesis, and if we add to this the damming of tributary valleys by moraines of
glaciers occupying the main valley we shall probably have included all the causes
at work to form the more important lakes of Tibet. But we are not disposed to
think that any single theory can be of universal application: thus Kala Tso may be
regarded as a type of the first hypothesis (with its corollaries), Yamdrok Tso of the
second and, according to Mr. Huntington, Panggong is a type of the third.»

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