268 
THE BASIN OF EASTERN PERSIA AND SISTAN. 
which tlie youn<(cr terraces are cut (fig. i6i). This is a good illustration of the way 
in which older terraces disappear, and explains why, in regions of gravel deposition, 
it frequently hapi)ens that only one terrace exists where we should expect to find more. 
The upper terrace at Bajistan consists of fine silt with a cover of gravel from 
I to 3 feet thick. It is the same phenomenon of gravel lying with a slight 
unconformity on fine silt, which is so noticeable throughout the whole of Central 
Asia from Kashgar to Sistan. On the tectonic hj'pothesis it can only be explained 
by supposing that the times of the deposition of the valley fillings lasted so long 
that the processes of erosion and weathering reduced the slopes of the mountains to 
a well-graded condition, which allowed them to furnish the streams with nothing 
but finely comminuted detritus. Times of uplift then ensued and at first caused 
Fig. 161. — The Town of Bajistan, loolcing east. In the Middle Foreground the Fields are Terraced (or 
Cultivation. In the immediate foreground lies a fluviatile terrace cut in silt and gravel. 
rapid erosion and a flooding of the valleys with gravel. Soon, however, the 
accumulations upon the graded slopes were all washed away, and the streams relied 
merely upon the products of contemporary weathering, which naturally furnished 
a much lighter load than the sudden carrying away of the accumulated product of 
many years' weathering. When the streams were thus more lightly loaded, they at 
once began to deepen their channels and form the terraces. Such an explanation 
is quite possible in the case of a single terrace, but it fails entirely when we come 
to two or more. If the interval between the fonnation of two successive terraces 
was so long as to allow the mountain slopes to be reduced from an ungraded to a 
graded condition, it is inconceivable that so slight a thing as an unconsolidated terrace 
should be preserved from one cycle to the next. 
