52 PRINCIPLES OF STRATIGRAPHY 



Wind Erosion and Translocation. 



Walther (102, 104) has repeatedly insisted upon the importance 

 of distinguishing the twofold process of denudation performed by 

 the wind, namely, corrosion, or the abrasive process, and deflation, 

 or the process of denuding by removal of mineral particles loosened 

 by weathering. The latter is by far the more effective process, and 

 • applies not only to the immediate products of weathering, but to 

 all the loose material of the earth's surface, fine enough to be sub- 

 ject to wind transportation. 



CoRRASiON. Wind corrasion is accomplished by means of the 

 material carried by the wind. Sand grains are the commonest and 

 most effective agent in this respect, though blown snow crystals have 

 a similar though less violent effect. Wind-corraded rocks usually 

 have a smooth and highly polished surface, which, however, is fre- 

 quently irregular, with the harder minerals or bands projecting, or 

 with cavities due to corrasion of the soft parts. Hard limestones 

 often have their otherwise obscure bedding emphasized by the wear- 

 ing of the softer layers ; while hard fossils or concretions will ap- 

 pear in relief by the filing away of the surrounding softer rock. In 

 the Libyan desert the blocks of Tertiary Operculina limestone are 

 corraded by the wind in svich a manner that the somewhat harder 

 Foraminifera are filed out in relief, each resting on a pyramid or a 

 needle of limestone, the latter often 2 cm. long, and sometimes 

 giving the rock the aspect of a pincushion stuck full of long pins. 

 ( Walther-106: ^/^.) On the Libyan limestone plateau, where the 

 surface is formed by an exceptionally hard siliceous lower Eocenic 

 limestone bed, wind-carved furrows have been discovered, which 

 extend in a N.N.W. — S.S.E. direction and vary in depth up to a 

 meter. These were carved by the sand-laden winds which for thou- 

 sands of years swept across this surface, with little or no variation 

 of direction. (Walther-106 : .//j, and Martonne-62, pi. XXXIII, 

 b.) Similar furrows, a foot or more wide, but shallow, occur in 

 the surface of the late Siluric Anderdon limestone of western On- 

 tario, and probably represent wind erosion under desert conditions 

 during Lower Devonic time. They are covered with Middle De- 

 vonic sands, both siliceous and calcareous, about 12 feet thick, ap- 

 parently free from fossils, and probably representing the somewhat 

 reworked calcareous sands drifted across the old limestone plateau. 

 These lower beds, chiefly brown dolomitic rocks, fill the fissures 

 above referred to. (Grabau and Sherzer-36 : ^7.) The yardangs 

 of Central Asia (47) are groove-like hollows, sculptured out of ar- 



