Southern Tibet : vol.4 / Page 482 (Color Image)

0482	Southern Tibet : vol.4

mountains afford a lee side sufficient for the small dunes to remain at their place.
It may be that the height and shape of the dunes are changed during other
seasons when the winds are less strong or coming from other directions. But in the
winter the surplus of sand will again be carried away and the dunes return to their
normal size. In the latitudinal valley I travelled through in 1896 and in the one of
1906, no dunes had been formed because all conditions were not existent, and the
existence of sand and wind alone, is not sufficient for the formation of dunes. In
how far and why the absolute altitude plays a part, I cannot tell, but, as I have
mentioned above, it is noteworthy that the sand belts are only to be found at a
comparatively moderate height, and that there are no dunes at all on the highest
regions of the Tibetan plateau-land.

There is another interesting problem to be considered in connection with this
question. The destruction of the mountains goes on irresistibly. The peaks, ridges
and ranges, therefore, become lower and lower, a fact that is by no means influenced
by the slowness of the procedure. The hills west of the Nagrong dunes will, one
day, become so low that there is not lee side enough for the dunes, and the par-
ticles of drift sand will no more find a station along the base of the eastern hills.
It is probable that this state of things has already entered in several Tibetan valleys
where, at an earlier period, dunes have existed. But, on the other hand, it is not
likely that there have been extensive sand deserts in the great latitudinal valleys in
post-glacial time. The greater precipitation, which now is going towards desiccation,
would not have allowed the formation of dunes. The dryness of the air is an important
factor in all sand deserts. As now, during the present epoch, the air is becoming
more and more dry, is it then likely that the Tibetan highlands are at the beginning
of a period more adapted to the formation of sand deserts? This is not probable
as long as the winter storms sweep across the country with their enormous force.
In those parts of the Lop Desert and the desert of Seistan in Persia where the
regular wind is at its strongest, there are no sand dunes. But in other parts in the
neighbourhood, where the wind is more moderate, dunes of enormous height are
formed, as to the west of the lower Tarim River. Here, therefore, it is obvious
that if the wind surpasses a certain degree of velocity, no dunes may be formed,
even if all other conditions are satisfied. This is the case in Tibet: the wind is too
strong. Dunes that are formed at seasons are again swept away by the storms.

On March 11th, our route is 8,6 km. south, a little to the east. The ground
falls 97 m. or at a rate of 1:88. The minimum temperature was at —16,6°. In the
morning the wind was moderate, but at 10,40 o'clock a. m., the usual storm began with
full strength and continued the whole day. In the evening, again, the wind was not
strong. Before the storm began, the landscape became visible and its principal features
could be inserted on the map.

National Institute of Informatics - Digital Silk Road Project
Digital Archive of Toyo Bunko Rare Books

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National Institute of Informatics - Digital Silk Road Project Digital Archive of Toyo Bunko Rare Books

Citation Information

OCR Text

National Institute of Informatics - Digital Silk Road Project
Digital Archive of Toyo Bunko Rare Books