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.” [Illustration: 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. [Illustration: FIG. 214.—The “stone lattice” of the desert, the work of the natural sand blast (after Walther).] When guided both by planes of sedimentation and planes of jointing, forms of a very high degree of ornamentation are developed. Some of the most remarkable forms are due to the protection afforded to the sun-exposed surfaces by the shell of desert varnish. In the shaded portions of projecting masses there is no such protection, and here the sand blast insinuates itself into every crack and cranny. In this it is aided by shadow weathering due to the differential strains set up at the border of the expanded sun-heated surface. As a result, projecting rock masses are sometimes etched away beneath and give the effect of a squatting animal. These forms, due to shadow erosion, have also been likened to projecting faucets. (Fig. 215). [Illustration: FIG. 215.—Projecting rock carved by the drifting sand into the form of a couchant animal as a result of shadow weathering and erosion. Cut in granite on the north Indian Desert (after Walther).]