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]).
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).
Worn by its impact upon neighboring sand grains while in transport, but much more as it is thrown against the ground or hard rock surfaces, the wind-driven or eolian sand is at last worn into smoothly rounded granules which approach the form of a sphere. Compared to the surface which sea sand acquires by attrition, this shaping process is much the more efficient, since in the water the beach sand is buoyed up and is more effectively cushioned against its neighboring grains. The grains of beach sand when examined under a microscope are found to be much more irregular in form and usually display the original fracture surfaces only in part abraded.
Fig. 216.—Cliffs in loess 200 feet in height which exhibit the characteristic vertical jointing (after von Richtofen).
The dust carried out of the desert.—When, standing upon the mountain wall that surrounds a desert, the traveler gazes out to windward over the great depression, his field of view is generally obscured by the yellow haze of the dust clouds moving across the margins. Upon the mountain flanks and extending far outside the borders, this cloud of dust settles as a shrouding mantle of impalpable yellow powder, which is known as loess. These deposits are continually deepening, and have sometimes accumulated until they are hundreds or even thousands of feet in thickness. Before reaching its final resting place the dust of this deposit may have settled many times, and has certainly been in part redistributed by the streams near the desert margin. In it are the ingredients which are necessary for the nourishment of plants, and it constitutes the most important of natural soils. Continually fed by new deposits from the desert, and refertilized from below by a natural process so soon as the upper layers become impoverished, it requires no artificial fertilization. Without artificial aids the loess of northern China has been tilled for thousands of years without any signs of exhaustion.