PHYSICAL FEATURES. 
A complete reversal of the drainage has thus taken place and the line 
of water-parting between the Rong Chu and Yamdrok Tso lies 
between the villages of SaUiang and Teshung. 
Numerous small lakes of comparatively recent origin lie among 
the glaciers and old moraines of Sikkim and 
Small glacial lakes. 
Tibet : examples of these are Tso Lhamo, Bam 
Tso and Gyamtsonang in the Lachen basin^ numerous small lakes 
among the old moraines on the western flanks of the Lingshi range 
and a small tarn at the end of the great glacier opposite Dzara, the 
Chinese post-house to the east of the Karo La. 
The origin of the Tibetan lakes was formerly ascribed to the 
Origin of Tibetan lakes, damming up of rivers by the debris brought 
down by side streams, but some years ago 
an ingenious suggestion was made by Mr, R. D. Oldham that the true 
explanation might lie in local uplift of the river-bed ; hence if the rate 
of elevation exceeded that of erosion of the river, the stream 
would be unable to keep its channel open and a lake would conse- 
quently be formed. The valley between Yamdrok Tso and Teshung 
(see wood-cut, p. 12) is broad and open, and the amount of debris 
brought in from the lateral valleys even when they carried glaciers 
would have been too insignificant to have arrested the flow of the 
important river that must once have passed through the Rong valley. 
If, however, we assume that by a slight local uplift the bed of the river 
was raised between Yasik and Teshung, the deposition of a compara- 
tively small amount of debris might have been sufficient to result in 
the formation of a lake. The contrast between the deep and preci- 
pitous gorge of the Rong some way below Teshung and the open 
valley above is a further indication of local elevation having enhanced 
the erosive power of the river below and diminished it above the line 
of uplift. 
Turning now to Kala Tso we find no evidence of uplift such as is 
furnished by Yamdrok and the valley of the Rong Chu. The eastern 
margin of the lake lies many miles from the valley of the Nyang Chu, 
the intervening area is now covered by fine silt such as is found 
( m ) 
