BARKHANS OF THE KARA—KUM. 57
The right-handed shifting and bluff-cutting of the Amu, as a result of which the admirably irrigated and fertile oasis of Charjui is on its left side, has been taken as an illustration of the deflective force of the earth's rotation (cf. Walther, 61, 14); but it is still questionable whether this small force is sufficient to produce the results that are ascribed to it. The doubt on this point has greatly increased in my mind by reason of the measured deflection of the Mississippi, as determined recently by one of my students from the maps published by the Mississippi River Commission, being greater on the east (left) than on the west (right) in a period of twelve years. In any case, the bluff-cutting by the Amu must be determined by some other cause
Fig. 33.— Sand Dunes south of Charjui, looking northwest._
besides the earth's rotation, inasmuch as it involves not merely a right-handed shifting, but a degrading action at the same time ; and degradation by such a river implies some alteration in individual régime, such as climatic change or crustal movement would produce.
The railroad crosses a tract of typical crescentic dunes (barkhans) before reaching the oasis of Charjui on the Amu. Some areas had scattered bushes among the sands; others were essentially without vegetation, and there the dunes were at their best (fig. 33). They were under the influence of northerly winds, for their longer slopes were to the north, and their steep scarps and lateral horns were to the south. According to Russian observers, the form of the barkhans has a seasonal variation, following the change in the prevailing winds. Besides the seasonal variation of dune form, there appears to be a secular extension of the dune-covered area, in some places averaging 20 feet a year (Walther, 190o, I19).