14 HAYDEN : GEOLOGY OF THE PROVINCES OF TSANG AND 0. 
around the present shore and all evidence points to the fact that the 
lake once extended to the river valley at Mangtsa. From this point 
a broad glaciated valley runs up to the snow-clad peaks on the Bhutan 
frontier, and the extensive moraines with which it is now filled show 
that a large glacier once flowed from these peaks to the valley of the 
Nyang Chu ; this glacier, as it retreated, closed with its moraine the 
tributary valley of Kala Tso and thus produced a lake, the waters of 
which stood at a height of about 30 feet above their present level for a 
period long enough to enable the lake to leave a clearly defined terrace 
to mark its old shore-line. Once desiccation set in, it appears to 
have continued steadily and, so far as can be ascertained from local 
evidence, to be still in progress at the present day. This process is 
usually attributed to the rise of the main range of the Himalaya and 
the consequent arrest of the moisture-laden winds of the Indian mon- 
soon, and should it be possible to prove by a series of observations 
that desiccation is still in progress, this would strengthen the prevalent 
belief that the Himalayan mountain-system has not yet reached 
maturity. It would of course be necessary for such observations to 
extend over a long period of years, but under existing circumstances, 
there should be no difficulty in establishing gauges both on Hram Tso 
and Kala Tso. Similar observations might be made on the Rupshu 
lakes, of which Tso Moriri would probably be the most suitable, and 
it might even be feasible to extend them to Manasarowar and 
Rakas Tal. 
4. Glaciation. 
In Central Tibet, as in all parts of the Himalaya, there is ample 
evidence to show tliat glaciation was formerly very much more exten- 
sive than at the present day. The old moraines between the Yaru 
plain and the head-waters of the Lachen have already been referred 
to. Further to the west, vast moraines extend from the northern 
slopes of Chomoyumo and from the Naku La far into the Kampa plain, 
whilst broad spreads of talus and moraine cover the northern slopes of 
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