76 WAVES OF SAND AND SNOW 



of quartz sand, which I have described, were like- 

 wise covered with ripples. In fact, all aeolian 

 sand-waves, whether they be the small dunes 

 elementaires of French writers such as those at 

 Helwan, or the great dunes of the desert, have 

 their otherwise smooth surface corded with a 

 delicate and regular pattern of ripples. These 

 ripples have a length of some inches from crest 

 to crest, relatively great lateral extension or 

 breadth, and level crests. They face everywhere 

 the direction in which the air flows over the surface 

 of the dune, and show precisely how the shape of 

 the dune deflects the wind. They are formed on 

 all parts of the dune where the air flows parallel 

 to a surface of non -coherent sand. The slipping 

 clifl", which forms sometimes the whole, sometimes 

 only a part, of the lee side, is unrippled. There 

 are also on the greater dunes positions where the 

 wind, cutting down to lower layers of sand which 

 have become somewhat coherent owing to super- 

 incumbent pressure, makes another variety of minor 

 wave or ripple, of greater wave length but less 

 breadth. These, which I call erosion ripples, I 

 shall deal with presently, confining myself now to 

 ripples in loose sand, which are the most regular 

 and the prettiest of the patterns modelled in sand 

 by the wind. 



