ORIGIN OF EOLIC DEPOSITS 699 



plify this essential feature, although Johnson^ ^ regards them as merely 

 preventing erosion from rainfall. I have already pointed out the im- 

 portance of plant growth in the formation of the loess derived from wind- 

 blown silts along the Missouri Eiver.^^ Shimek^*^ notes the same phe- 

 nomenon in connection with the Iowa loess, Huntington^^ observes that 

 in the Kwen-Lun Mountains, on the borders of Turkestan, "wherever 

 there is sufficient vegetation . . . the dust is held in place and 

 heavy deposits of loess are in the process of accumulation." 



In the desert itself, where deflative processes greatly predominate, cul- 

 tivated areas soon become higher than the surrounding barren grounds. 

 This is due partly to the retention of the wind-blown dusts, partly to the 

 silts brought in by the muddy irrigation waters, and partly to the re- 

 moval of the soils through deflation of the adjacent lands devoid of, or 

 only scantily provided with, plant growth. These features are well dis- 

 played at Socorro, Las Cruces, and other old Mexican towns in the 

 southwestern part of the United States. In the same region like phe- 

 nomena are presented by many of the old Indian villages and the sites 

 of the ancient Aztec communities. In the Lybian desert, especially in 

 the oasis of Kharga, BeadnelP^ reports that the gardens have been raised 

 many feet in perhaps as many centuries by the constant lodgment of 

 wind-blown materials. 



CHARACTERISTICS OF EOLIC DEPOSITS 



Critical criteria for the recognition by lithologic means of wind-blown 

 deposits have never been satisfactorily formulated. The finer and the 

 coarser materials should be considered separately. In deserts, which are 

 essentially areas of degradation, sands prevail, but they are ephemeral 

 in character and seldom constitute prominent deposits except when 

 blown directly into the sea. Accumulating mainly outside of the limits 

 of desert areas in the semi-arid and moister regions, extensive eolic de- 

 posits are characteristically loamy. In the determination of eolic forma- 

 tions this distinction requires fullest attention. 



Typical wind-formed deposits possess all the physical characters of the 

 familiar loess. The most striking feature is the peculiar loamy nature 

 already mentioned. Neither strictly clay nor strictly sand, the size of 

 grain is intermediate. The homogeneous texture, the remarkable capac- 



^"^ Twenty-second Ann. Kept. TJ. S. Geological Survey, 1002, pt. iv, p. G37. 

 ^American .Tour. Sci. (4), vol. vi, 1898, p. 299. 

 50 Proc. Iowa Acad. Sci., vol. Iv, 1898, p. 68. 

 8«Bull. Geol. Soc. America, vol. 18, 1907, p. 359. 

 «An Egyptian Oasis, 1909, p. 78. 



