12 HAYDEN: GEOLOGY OF THE PROVINCES OF TSANG AND 0. 
encircled the central hill-mass and extended to some height up the 
mountain sides. 
The broad valley running out from Kasang Sampa— a dam partly 
natural and partly artificial— at the north-west corner of the lake, was 
evidently the original outlet whence the drainage escaped down the 
valley of the Rong Chu ; this, however, was gradually closed and the 
Rong no longer rises in Yamdrok Tso, but in a small lake at about 
6 miles further to the west-northwest. Between this and Yamdrok is 
another small lake, whence the water flows back through Kasang Sampa. 
90 
•5 90 
s6 
29 
^,9(5 
li Scale, 1 Inch =4 Miles so 
3d 1 
Map showing reversal of drainage at head of Rong Chu, 
a«-c, line of water-parting. 
( '33 ) 
