Fig. 211.—Granite blocks in the Sierra de los Dolores of Texas, rent into several fragments by the dash of rain (after Walther).
The natural sand blast.—Because of the saucer-like shape, the vast expanse, and the absence of wind breaks, the potency of wind as a geological agent is in desert areas not easily overestimated. While most of its work is accomplished with the aid of tools, it has been proven that even without this help, considerable work is done through the friction of the wind alone, particularly when moving as powerful eddies in cracks and crannies. This wear of the wind, unaided by cutting tools, is known as deflation.
The greater work of the wind is, however, accomplished with the aid of larger or smaller rock particles, the sand and dust, with which it is so generally charged above the deserts. Unprotected by any mat of vegetation the materials of the desert surface are easily lifted and are constantly migrating with the wind. The finest dust is raised high into the air, and is carried beyond the marginal barriers, but none of the sand or coarser materials ever passes beyond the borders.
Fig. 212.—“Mushroom rock” from a desert in Wyoming (after Fairbanks).
The efficiency of this sand as a cutting tool when carried by the wind is directly proportioned to the size of the grain, since with larger fragments a heavier blow is struck when carried at any given velocity. These more effective grains are, however, not lifted far above the ground, but advance with a squirming or hopping motion, much as do the larger pebbles upon the bottom of a river at the time of a spring freshet. To quote Professor Walther: “Whoever has had the opportunity to travel over a surface of dune sand when a strong wind is blowing has found it easy to convince himself of the grinding action of the wind. At such times the ground becomes alive, everywhere the sand is creeping over the surface with snake-like squirmings, and the eye quickly tires of these writhing movements of the currents of sand and cannot long endure the scene.”
Fig. 213.—Windkanten shaped by the desert sand blast (after Chamberlin and Salisbury).
A direct consequence of this restriction of the more effective cutting tools to the layer of air just above the ground, is the strong tendency to cut away all projecting masses near their bases. The “mushroom rocks”, which are so characteristic of desert landscapes, have been shaped in this manner ([Fig. 212]). Another product of the desert sand blast is the so-called Windkante (wind-edge) or Dreikante (three-edge), a pebble which is usually shaped in the form of a pyramid ([Fig. 213]).
Whenever a rock face, open to direct attack by the drifting sand, is constituted of parts which have different hardness, the blast of sand pecks away at the softer places and leaves the harder ones in relief. Thus is produced the well-known “stone lattice” of the desert ([Fig. 214]). Particularly upon the neck of the great Sphinx have the flying sand grains, by removing the softer layers, brought the sedimentary structures of the sandstone into strong relief.