204 
EXPLORATIONS IN TURKESTAN, 
each of the mountain masses, large or small, should be intermittently elevated or 
that each of the basins should be intermittently depressed in such a way that all the 
streams should be intermittently accelerated in their work of erosion. This pro- 
cess involves an alternation of movement and rest from four to six times in each 
separate drainage area, and at each alternation the amount of uplift and the length 
of the period of rest nuist have decreased. All this seems improlxible, whatever 
may be thought of its possibility. ^ 
It is, however, not only the erosion of the terraces that has to be accounted 
for; in most cases each terrace in\ol\es an epoch of deposition preceding the epoch 
of erosion. The gravel deposits in which the terraces are carved occur not onlj- at 
the mouth of every vallc)- where it opens on the plain, but also along the course of 
Fig. HI. — Terraces and Meanders of another Kuzzil Su near Chadir Kul. on the Tian Shan Plateau, at an 
elevation of I 1,000 feet. These terraces are cut partly in gravel and partly in red Tertiary limestone. 
many streams almost to their heads. Sometimes the gravel lies on rock-cut terraces 
(fig. 142), where it might have been formed during periods of rest when no uplift was 
in progress. In other cases, however, the terraces along large portions of the stream 
course are cut in gravel only, and the rock bottom of the valley is now no deeper than 
when the first gravel deposits were begun (fig. 143). Where this is tnie the net result of 
whatever crustal movements have taken place has been that they have balanced one 
another in such a way as to bring the region back to essentially the same position 
that it first occupied. There must have been depression to cause the aggradation 
of the valleys by gravel deposits, and this must have been followed by periodic and 
decreasing uplifts of which the sum was equal to the total previous depression ; 
