SNOW- WAVES AND SNOW-RIPPLES 137 



The four groups of erosion -waves referred to 

 above have an average proportion of length to 

 height of 1 8" 22 5 ; i.e., their steepness is equal to 

 that of waves in loose sand but less than that of 

 the large ripples produced when the wind cuts down 

 into slightly coherent sand. 



The erosion-waves in compact snow are, as I 

 stated when describing my observations in Scot- 

 land, an initial form which is finally replaced 

 by longitudinal ridges having a similar profile. 

 During the intermediate stage it sometimes happens 

 that the depressions and not the elevations of the 

 eroded surface are the most noticeable forms. 

 Sometimes these depressions are pits, roughly ellip- 

 tical in plan, and of which the profile has one 

 long and gentle and one short and steep slope. 

 They have therefore a resemblance to the pits which 

 I described in the loose sand at Helwan, but in 

 the erosion form the steep cliff faces the wind. 

 I have not been able to make out from the accounts 

 of travellers whether the sides of the fuljes, or 

 pits, in the sand of the Arabian Nefud are in- 

 variably composed of loose sand or whether it is 

 sometimes " set " and compacted. In either case 

 the profile due to wind action would have the two 

 characteristic slopes, one long and gentle, one short 

 and steep, but whereas pits left among accumula- 



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