Southern Tibet : vol.7 / 697 ページ（カラー画像）

0697	Southern Tibet : vol.7
南チベット : vol.7

0697

Southern Tibet : vol.7

南チベット : vol.7

while the existence of an exit would depend on the rapidity of the movement, the
supply of water, and the nature of the climate.» He feels persuaded that there
have been irregular movements of the beds of the streams and rivers within a very
recent period, and in the existence of these movements he finds an evidence that
the disturbance which caused the elevation of the Himalayas is still in progress.
However, he does not deny that lakes under certain circumstances may be directly
formed by talus dams. If a portion of a river valley were elevated more rapidly
than the rate of erosion of the river, »the barrier so raised would react up stream
and cause the formation of a sloping surface of river gravels. If then for some
distance the configuration of the river valley was such that but little débris was shed
into it, and below this region the amount of debris suddenly increased, it is quite
conceivable that the rock barrier lower down might prevent this being carried away
as fast as it was shed, and so a talus dam formed across the valley.» Only the
various degrees of desiccation of the different lakes is difficult to reconcile with the
talus theory. Some lakes have entirely disappeared while others, as the Ling-shi-
tang Lake, have dwindled to one-tenth of their original size. Again, others, as the
Panggong-tso, have half of their original area, and Tso Morari has contracted only to
four-fifths of its former extent. Oldham therefore concludes that these great variations in
the degree of desiccation would hardly be possible if the lakes had been formed at one
period. If we, on the other hand, accept his theory of the differential earth movements,
the varying degrees of desiccation in different lakes would be quite natural.

Returning now to the great well-defined valley occupied by the chain of lakes
to which the Selling-tso in the east and the Panggong-tso in the west belong, we
will, in a few words, consider the different opinions regarding the formation of the
last mentioned lake.

According to Drew the Panggong-tso was formed by the damming up of
the main valley by the talus-fan of a southern tributary at the place where the low
threshold is situated between the lake and the Shayok drainage area.

Oldham, on the other hand, says that the formation of the lake is entirely
due to differential movements of the surface, »which raised a portion of the original
river bed at a more rapid rate than the stream was able to erode, and dammed
back the drainage to produce the present lake», in the same way as he imagines
the formation of Tso-moriri in Rupshu.

Ellsworth Huntington who visited the Panggong-tso, May 1 to 6, 1905, as a
member of the Barrett Expedition to Central Asia, expresses the opinion that the lake
basin, with a length of 105 miles and an average width of only 1.8 miles where
covered with water, is due to glacial erosion.¹

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国立情報学研究所 - ディジタル・シルクロード・プロジェクト 『東洋文庫所蔵』貴重書デジタルアーカイブ

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国立情報学研究所 - ディジタル・シルクロード・プロジェクト
『東洋文庫所蔵』貴重書デジタルアーカイブ