into the porous, thirsty gravel. Most of the smaller streams thus disappear entirely, and many of the larger ones do likewise except during the flood season. The effect is most marked where the break between the plateaus and the basin floor takes the form of a fault, for there the steepness of the slope of the valleys cut in the escarpment causes the streams to be very swift and to bring down a great amount of coarse detritus. The sudden change of grade at the base of the mountains necessitates the deposition of much material, with the result that huge slopes of piedmont gravel are formed. Between Keriya and Cherchen they sometimes reach a width of forty or fifty miles. Only a large stream can succeed in crossing such an expanse of parched, naked gravel. It is evident that the major part of the rather abundant supply of water from the mountains must disappear in the piedmont gravel zone long before it can reach a point where it might support much vegetation, or be utilized by man. The water is not wholly lost, however, for it cannot sink straight downward. The gravel contains frequent lenses of somewhat clayey, less pervious material, along the slightly sloping surface of which the water is obliged to flow away from the mountains. Moreover, the few wells which have been dug show that the lower layers of the gravel are saturated with water, and hence whatever more seeps down from above must tend to flow along the surface of the previous ground water, the level of which necessarily slopes away from the mountains. The net result is that the vanished streams spread out into an underground sheet, which slowly, but steadily, flows toward the centre of the basin. As it gets away from the rather
IIIF Color HighRes
