202 EXPLORATIONS IN TURKESTAN.
The number of terraces ranges from none to nine, but neither extreme represents the true state of affairs. Where terraces are absent it is either because the slope of the streams is so gentle that there is no erosion or because the slope is so steep and the country rock so resistant that the streams have as yet been able to cut only narrow gorges. Where the number is over five the material is usually unconsolidated gravel, and some of the terraces are usually small and seem to be mere stages of larger ones. Ordinarily there are from three to five terraces. The number of valleys for which the writer has a record is forty-three, and the number of terraces in these valleys is shown in Table IV. The number of valleys with only one or two terraces was really larger than appears from the table ; for after the widespread distribution of the terraces had been noticed, valleys where only one or two occurred were not recorded.
Valleys with— Valleys with—
One terrace. 3 Six terraces 4
Two terraces 3 Seven terraces 3
Three terraces. r2 Eight terraces o
Four terraces. 8 Nine terraces r
Five terraces 9
The terraces are sometimes cut in gravel (fig. 140), and sometimes in rock (fig. 141), but in the latter case there is always a cover of gravel lying over the rock. It may happen in a single valley that the upstream portions of the terraces are almost wholly cut in rock, while the downstream portions are entirely in gravel, as, for instance, along the Kuzzil Su, at the southeast end of Issik Kul. In valleys such as those of the Ispairan Su, flowing to Marghilan, and of the Ak Bura, flowing to Osh, it often happens that the terraces are cut for a certain distance in soft strata, or in gravel that fills a basin where soft strata have been excavated, although farther down the stream flows through a narrow canyon in hard strata, without a trace of terracing; but when the hard strata end and soft ones begin once more, the terraces are resumed as . though they had never been interrupted.
Another and perhaps the most characteristic feature of the terraces is the persistence with which the different members of a series preserve the saine relative
height and width. In terraces due merely to the swinging of the stream from side 1
to side as it cuts steadily downward, one terrace is here or there cut off either at the upper or lower end by another terrace of later date, and a pattern of cusps and bays is thus formed along the valley side. In such cases a single terrace can only be traced a short distance, and the number of terraces is continually changing. In the mountain valleys of Central Turkestan, on the other hand, although it sometimes happens that one terrace truncates another and thus forms a cusp, this is far from being a prevalent condition ; each level is, as a rule, distinct and does not interfere with its neighbors. Several terraces often run for many miles side by side without interfering with one another, each one preserving an almost uniform width with remarkable persistency. As a rule, too, the uppermost terrace possesses not only the greatest width, but the greatest height. Such a regular diminution in size can mean only that the cause of the terracing was of steadily decreasing efficiency.