26 THE FORMING OF THE OASIS OF ANAU. 
appreciated, for we find hills of pure culture growth 80 feet high which have been 
occupied till within a thousand years. The reason for this preference may have 
been partly the strategic advantage offered, but it is doubtless also to be sought 
in the very marked relief, found at even a slight elevation, from the burning summer 
atmosphere of the plain, as stated by Mr. Huntington in his report on the distri- 
bution of kurgans on the oasis of Merv. 
Let us now consider the agencies that have been active in these processes of 
cutting-down and rebuilding. They form one of the most interesting illustrations 
of the law of compensation in the grand cyclical action of forces that have modeled 
the relief of the surface of our planet. 
A great mountain range, the Kopet Dagh, several hundred miles long, forms 
the sharply defined southern edge of the desert plains of Western Central Asia. 
It rises everywhere abruptly from this plain to a height of from 5,000 to 10,000 
feet; and its height is sufficiently great to cause it to receive abundant precipita- 
tion and a heavy covering of winter snows. Within this mountain system the 
trunk valleys, after following a longitudinal course, turn sharply and, after cutting 
through the border range and piedmont hills, discharge their waters cnto the 
MOUNTAINS 


PLAINS 





Rising 
AS 
Dislocations SX 
Sinking 
Fig. 15.—ideal Sectioa of a Delta-oasis oa an Uptilting Piedmont. (By R. W. Pumpelly.) 
plains. The mountain masses, lacking the protection of a heavy forest growth, 
are subjected to rapid disintegration and decay, and the resulting detritus is carried 
by the torrential rivers down to the plains. 
In a coastal region these waters would flow onward to the ccean, and the silts 
they had brought from the mountains would ultimately complete the same course, 
to be deposited at the mouth of the river, to form there a submarine delta. But 
in an arid ‘“‘central’’ region, such as is Turkestan, the conditions are different. The 
precipitation is confined to the mountains, and on leaving these the rivers enter 
a desert region of rapid evaporation, where there is no compensating rainfall. The 
valley ends at the mouth of the mountain gorge. Thus all the coarse and fine 
materials brought by the torrential rivers from far and near in the mountains, as 
well as those carried down by smaller streams from the declivities bordering the 
lowland, and from the piedmont hills, are deposited within a zone along the edge 
of the plain at the base of the Kopet range. The rock-mass of the mountains is, 
therefore, being continually removed and loaded onto this long zone. 
Now, two connected phenomena are observed to result from this process. 
On the one hand, the zone of deposition is continually and proportionately sinking 
