SUN AND WIND IN LANDS OF INFREQUENT RAINS 205 



FIG. 212. "Mushroom rock" from a 

 desert in Wyoming (after Fairbanks) . 



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 oppor- 

 tunity 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." 



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 Wind- 

 kante (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 sedi- 

 mentary structures of the sandstone into strong relief. 



FIG. 213. Windkanten shaped by the 

 desert sand blast (after Chamberlin and 

 Salsbury). 



