Southern Tibet : vol.4 / Page 480 (Color Image)

0480	Southern Tibet : vol.4

finally reaches the foot of the hills at the left side of the valley and finds nothing
except sand. After a while we found some snow buried under drift sand.
In the valley of Nagrong, we had thus witnessed a phenomenon that is very
rare in Tibet, viz. the formation of sand dunes. The highest of them, only two or
three feet, were situated near the base of the eastern mountains. I have already
pointed out the surprising fact that sand dunes are extremely rare on the Tibetan
plateau-land, though the conditions for their formation nearly everywhere seem to be
existing, first of all the sand which continually is formed by destruction of the moun-
tains, and then the regular wind with which I, during two winters, had become so
intimately acquainted. To this comes the dryness of the climate and the want
of vegetation, the whole country being not far from a perfect desert. And still one
may wander for months through the Chang-tang plains and broad open valleys
without seeing a single dune. This means, of course, that one or several conditions
for the formation of dunes are not fulfilled. But on the eastern side of the lower
part of the valley of Nagrong, there are none of the conditions missing, and the
result is the belt of dunes we have crossed. There must, therefore, exist some im-
portant difference between the Nagrong valley and other valleys in Tibet. One
great difference is that this valley runs N. N. E.—S. S. W. whilst most of the others
run N. W.—S. E., W. N. W.—E. S. E. or W.—E. This is not sufficient for ex-
plaining the existence of dunes as I have seen other meridional valleys, for instance
between Targo-gangri and Shuru-tso, where no dunes have accumulated. Perhaps
the relative height of the hills and ridges on both sides of the Nagrong valley plays
an important part. Perhaps also the absolute altitude and the rarefaction of the air
have something to do with the problem. Thus, for instance, there is to the east
and S. E. of Kum-köl, south of the Kalta-alaghan Mountains in northern Tibet a
sand desert with considerable dunes, along which I travelled in 1900, but there the
absolute altitude is only 3,900 m. In the Tsangpo valley in 1907 I also passed
through regions of sand dunes, viz. east of Karu at a height of about 4,000 m.
and at Dongbo, Camp CLXXXIX, at a height of not quite 4,600 m. as described
in Vol. II, p. 294 and 323. In both these instances the valleys containing the sand
dunes are running from west to east and are large latitudinal valleys. As we have
seen many latitudinal valleys in Northern and Central Tibet which have no dunes
at all, and as the Nagrong valley, which is rather meridional has dunes whilst other
meridional valleys have none, the direction of the valleys seems to play a subordinate
part. The valley of the Tsangpo should, perhaps, not be directly compared with
the valleys of the Chang-tang, as its sands are brought down by the river and not
by the wind. But in the other valleys mentioned, the same conditions are prevailing.
Now as regards the Nagrong valley, we have found that its absolute altitude was
lower than the whole country we had travelled through ever since December 17th,

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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