┌─────────────────────────────────────────────────────────────────────┐ │ PLATE 8. │ │ │ │ [Illustration: _A._ The granite needles of Harney Peak in the Black │ │ Hills of South Dakota (after Darton).] │ │ │ │ [Illustration: _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).
[Illustration: FIG. 231.—Schematic diagram to show the zones of deposition in their order from the margin to the center of a desert.]
FIG. 232.—Mounds upon the site of the buried city of Nippur (after the cast by Muret).]
In addition to the smaller periodic alternations of pluvial and interpluvial climate—the “pulse of Asia”—the record of the Asiatic deserts indicates a progressive desiccation of the entire region, which has now given the victory to the dune. The ancient history of the cities of the plains supplies the records of many that have been buried in the dunes. To-day, where once were prosperous cities, nothing is to be seen at the surface but a group of mounds (Fig. 232). Exhumed after much painstaking labor, the walls and palaces of these ancient cities have once more been brought to the light of day, and much has thus been learned of the civilization of these early times (Fig. 233). Quite recently the mounds which cover between one and two hundred buried villages have been found upon the borders of the Tarim basin of central Asia, where they were lost to history when they were overwhelmed in the early centuries of the Christian Era.