268 TIT BASIN OF EASTERN PERSIA AND SISTAN.
which the younger 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 happens 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 saine 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 hypothesis 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, looking east. In the Middle Foreground the Fields are Terraced for
Cultivation. I n 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 formation 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 athing as an unconsolidated terrace should be preserved from one cycle to the next.
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