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| 0742 |
Southern Tibet : vol.7 |
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plateau-land would only be a question of time. A bed of deposits in the region of
Ye being some 500 m. in thickness would reach even as high as La-rok, and a rather
broad plain would expand here between the Transhimalaya and Himalaya.
It is an interesting fact that along the uppermost courses of all the three great
rivers, Indus, Satlej and Tsangpo-Brahmaputra, there is a very evident tendency to
plateau-formation. From Shamsang and down to Tradum there is a series of flat
plains in the Tsangpo valley, and such is also the case along the left Indus branch
below Gartok and a long way down the joined Indus. This formation of plains
between the parallel ranges has only been possible during an arid period. During
the same arid period the enormous deposits in the originally narrow valley of the
upper Satlej were also formed. Below these plateau-shaped regions all three rivers
cut their beds deeply down into wild narrow valleys. This took place as soon as
the volume of water grew to a great quantity and the slope became steep. The
uppermost Satlej, however, shows in the plateau-shaped portion of its valley a character
quite different from either the uppermost Indus or the uppermost Tsangpo. For it has
cut down its bed through the enormous deposits of the arid time, and the tributaries
have had to follow at the same speed of mighty erosion. The upper Indus and
Tsangpo, on the other hand, have not been able to cut down their beds through the
deposits in their valleys. Both these rivers flow in very shallow beds. The cause
seems to be twofold. The region were the Satlej cuts through the old deposits is
much more exposed to the monsoon rains than the similar region of the two other
rivers, and the Satlej therefore here is several times mightier than the uppermost
Indus and Tsangpo. In the case of the Satlej the slope is also much steeper, which
immensely increases the erosive power. From the confluence of the Singi-kamba and
Gartang (4254 m.) and down to Dungkang (4186 m.), the Indus valley has a fall of
only 68 m. in 92 km., or as 1:1353. From Shamsang (4697 m.) to Camp CLXXXVIII
(4583 m.) the Tsangpo falls 114 m. in 96 km., or as 1:842. But the Satlej from
Chunglung-gompa (4239 m.) to Totling (3700 m.) falls no less than 539 m. in 90 km.,
or as 1:167. Its slope is therefore eight times as steep as that of the uppermost
Indus, and five times as steep as that of the uppermost Tsangpo. The erosive power
of the Satlej is therefore enormous, if compared with that of the two other rivers.
Comparing finally the valleys falling to the north from the great water-parting
of the Transhimalaya, with those falling to the south, we find a striking difference
in their morphology, to which I have already directed the reader's attention in the
latter half of Vol. III when describing my eight crossings of the Transhimalaya.
Here it will be sufficient to state the fact that the valleys going to the lake basins
in the great latitudinal depression have a much slower fall than the southern ones,
and that the steepness of the latter increases from west to east, which is quite
natural on account of the gradual fall of the Tsangpo. The following table shows
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28
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38
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49
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60
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70
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81
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92
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105
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117
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128
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138
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150
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161
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177
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190
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202
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214
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225
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237
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251
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263
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277
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291
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302
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315
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329
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342
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352
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363
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375
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386
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397
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407
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420
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432
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444
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457
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467
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478
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488
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499
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510
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520
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530
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541
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552
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563
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573
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583
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593
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605
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615
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625
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635
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646
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656
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666
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681
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693
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704
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714
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726
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737
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740
741
742
743
744
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747
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758
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773
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788
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801
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813
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833
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848
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864
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876
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888
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