206 



EARTH FEATURES AND THEIR MEANING 



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 joint- 

 ing, forms of a very high degree of ornamentation are developed. 

 Some of the most remarkable forms are due to the protection af- 

 forded to the sun-exposed surfaces by the shell of desert varnish. 

 In the shaded portions of projecting masses there is no such pro- 

 tection, 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 some- 

 times 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). 



Worn by its impact upon neighboring sand grains while in trans- 

 port, 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 sur- 

 face which sea sand 

 acquires by attrition, 

 this shaping process is 

 much the more effi- 

 cient, since in the 

 water the beach sand 

 is buoyed up and is 

 more effectively cush- 

 ioned against its neigh- 

 boring 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. 

 The dust carried out of the desert. When, standing upon the 

 mountain wall that surrounds a desert, the traveler gazes out to 



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). 



