CHAPTER II I , 



AEOLIAN SAND-RIPPLES 



All aeolian sand-waves have a rippled surface — Description of 

 the formation of aeolian sand-ripples in level deposits of 

 loose sand blown from the beach — Measurements of height 

 and length — The ripples grow without change of shape — 

 Ratio of length to height the same as in the larger sand- 

 waves — Rate of advance — Winnowing of fine from coarse 

 sand-grains observed during growth of aeolian ripples — 

 "Erosion-ripples" formed where the level of the sand is 

 being lowered — The formation of a protecting cover of 

 " lag gravel " on the crests — Experiments by means of an 

 artificial blast — No rippling of assorted sand-grains of 

 uniform large size, although travelling freely — Immediate 

 rippling when small sand-grains mixed with them — Ex- 

 planation offered, that eddy in lee of larger grains tosses 

 away the smaller, hence the existence of ripples upon the 

 larger waves — Ripples in shell-sand showing lines of 

 notches in the wave-fronts where the troughs are deeper. 



The little sand-dunes at Helw^an were covered w^ith 

 small ripples of a form very much like that of the 

 dunes themselves — i.e., with a gentle slope to wind- 

 ward and a steep lee face. The great desert dunes 



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