AEOLIAN SAND -RIPPLES 87 



only attains a greater wave-length than that of 

 ripples formed in quite loose sand, but a greater 

 steepness also. Thus on the nearly flat top of a 

 sandhill near Lake Timsah, in Egypt, I measured 

 such ripples with a wave-length of 7 feet 2 inches 

 and a height of 6 inches, being a ratio of length 

 to height of 14* 3. This kind of rippling is quickly 

 produced in positions where the supply of drifting 

 sand is small and the wind is swirling upwards. 

 The lower photograph of Plate XL shows this 

 erosion rippling in such a position to leeward of 

 a sand-dune at Abu Racan, near Ismailia. The 

 crest of the ripple is more nearly in the middle 

 than is the case with the ripples in quite loose 

 sand, and this form suggests that these erosion 

 ripples were almost or quite stationary. 



The fact that there is a sorting of sand-grains 

 during rippling, the coarser grains accumulating 

 at the crest, affords a clue to the problem of the 

 simultaneous formation of two orders of undula- 

 tion, the ripple and the wave, upon the sandbank 

 at H el wan and elsewhere. The theory of the 

 formation of the wave has already been given. 

 Its rippled surface is due to the circumstance that 

 the sand-grains are of various sizes. I have been 

 at considerable pains to investigate the precise 

 manner in which the wind acts upon particles of 



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