Southern Tibet : vol.7 / 737 ページ（カラー画像）

0737	Southern Tibet : vol.7
南チベット : vol.7

The first lake in this series is Dangra-yum-tso with an altitude of only 4646 m.
But well-developed beach-lines were clearly visible along the sides of the Targo
valley and in its tributary valleys. The highest I observed was 90 m. above the
present lake, provided the altitude of 4646 m. is correct. The Shuru-tso, 4725 m.
high, was surrounded by old beach-lines some 30 m. above the present lake. The water-
parting between the two lakes is at 4763 m. The Dangra-yum-tso would thus have to
rise 117 m. or slightly more to be in connection with the Shuru-tso, as it no doubt
was during the wet period. At that epoch the joined lake may easily have been 200 or
300 m. higher than now, an assumption that does not seem too audacious if we
remember that I measured old beach-lines at 133 m. above the present Lakor-tso.¹

The joined lake being thus filled by the Targo-tsangpo, the overflowing water
had to escape somewhere, probably to the east, to Ngangtse-tso, which is now
situated at an altitude of 4694 m., and to Marchar-tso which is at about the same
height, and which at a very recent period, perhaps only a century ago, formed one
lake with Ngangtse-tso. The water may thence have escaped somewhere in the
region Jhiakta where NAIN SING has an altitude of 4651 m., to Chikut-tso (4502+?),
Kyaring-tso (4502, probably too low), Mokieu-tso, Bul-tso, Ring-tso, Tengri-nor,
4630 m., Bum-tso, 4580 m., Bul-tso, 4430 m., and finally to Nak-chu, 4445 m., the
uppermost course of the Salwen.

The source branch of this old river must have been the Targo-tsangpo which,
from Chang-la-Pod-la to its mouth in the Dangra-yum-tso, is 135 km. in length.
From the lake to the Nak-chu we have to add 600 km., so that the old river would
have been some 735 km. in length. Tagrak-tsangpo, 110 km. in length, was its
first Transhimalayan tributary. As to the others, discovered by Nain-Sing, Bara-
tsangpo and Dumphu- or Ota-tsangpo (affluents of Kyaring-tso), and Thama-tsangpo
(affluent of Mokieu-tso), they are known only where the Pundit crossed them, but it
seems very probable that they have their sources on the Pabla Range, the same
range from which the Targo-tsangpo and Tagrak-tsangpo take their origin. On
Nain Sing's map their direction is nearly straight from south to north. The Targo
and Tagrak, on the other hand, flow from S. E. to N. W. just as the Soma, the
Buptsang and the Sumdang. One therefore feels tempted also to join the Tagrak
and the Targo with the western river system. But the altitudes as they are now,
and the configuration and arrangement of the mountains around the Dangra-yum-
tso, make it more probable that the water-parting range between the two old
rivers has been situated on the western side of the last mentioned lake.

The next great latitudinal valley to the south is that which follows the southern
base of the Transhimalaya and separates that system from the water-parting range

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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
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