A. The granite needles of Harney Peak in the Black Hills of South Dakota (after Darton).
B. Castellated erosion chimneys in El Cobra Cañon, New Mexico. (Photograph by E. C. Case.)
Fig. 230.—Billowy surface of the salt crust on the central sink in the Lop Desert of central Asia (after Ellsworth Huntington).
The zone of vegetation, as already stated, lies near the foot of the alluvial bench, so that here are found the oases about which have clustered the cities of the desert from the earliest records of antiquity until now. Just without the line of oases is the wall of dunes held back from further advance only by the vegetation which in turn is dependent upon the rains in the neighboring mountains. With every diminution in the water supply, the dunes advance and encroach upon the oases ([plate 7 B]); while with every considerable increase in this supply of moisture the alluvial bench advances over the dunes and acquires a strip of their territory. Thus with varying fortunes a war is continually waged between the withering river and the flying sand, and the alternations of climate are later recorded in the dovetailing together of the eolian and alluvial deposits at their common junction ([Fig. 231]).
Fig. 231.—Schematic diagram to show the zones of deposition in their order from the margin to the center of a desert.
