PHYSICAL FEATURES. ig 
boundary of the Yaru plain, and runs far to the westward, merging 
into the northern range of the Central Himdlaya. 
At the north-east corner of the Yaru plain the line of water-parting 
suddenly bends round to the south and runs back to Pauhunri, thence 
east through the Tang La to Chomolhari and continuing thence to 
the north-east lies on the Lingshi range — the dividing range between 
Northern Bhutan and Tibet. 
Thus the Tang La constitutes the crest of the watershed 
between the basin of the Nyang Chu and that of the Ammo Chu. The 
latter river rises in the glaciers of Pauhunri and Chomolhari, one 
branch draining the Phari plain and the other the Khongbu valley ; 
these unite at Bakcham and flowing through the narrow valley of 
Chumbi debouch on the plains of Bengal as the main branch of the 
Torsa. 
The largest, in fact the only important, river flowing to the north to 
join the Tsangpo in this part of Tibet, is the 
Nyang Chu. . ... 
Nyang Chu. Its various branches rise in the 
glaciers of Chomolhari and the Lingshi range, in the hills to the west 
of Kala Tso and in the group of peaks in the neighbourhood of the 
Karo La. 
There is no evident connection between this river and the streams 
falling into Hram Tso and Kala TsO; but near Mangtsa, at about lO 
miles to the east of Kala Tso, a stream of considerable volume issues 
from under the gravel terraces of the plain, and there can be little 
doubt that its waters are derived from Kala Tso by underground 
percolation. 
The two main branches of the Nyang Chu unite at the southern 
end of ihe Gyantse plain. One of these flows north across the strike 
of the folded and crushed sedimentary beds at Dzamtrang, cutting 
out a deep and narrow gorge through a mass of granite and other 
intrusive rocks ; the other branch is formed by the union of the Nyiru 
Chu, deriving its waters from the snowy peaks to the north of Bhutan 
—the Lingshi range — and the Ralung Chu coming from the glaciers 
of Nojinkangsang and surrounding peaks. From Gyantse the river 
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