52 WAVES OF SAND AND SNOW 



the barchan or medafio form in close proximity 

 to each other. Structurally there is little differ- 

 ence between the elevated plain of loose sand 

 closely pitted by fuLjes and the low plain of bare 

 ground closely studded by barchan-shaped sand- 

 hills, but in the one case the eye takes note of the 

 shape of the depressions, in the other of the form 

 of the hillocks. 



In both cases the profiles are essentially wave 

 profiles, differing but little from those of the 

 regular ridges formed in the earlier stages of wind 

 action on a deep deposit of loose sand. In the 

 regular ridges the crests of the waves extend in- 

 definitely in breadth, so that there are only two 

 dimensions to be studied, just as is the case in 

 waves of the sea. When, however, the loose sand 

 no longer completely covers the ground, we have 

 to study the plan as well as the profile of the 

 moving mounds of sand. Are we to regard the 

 mound of moving sand, called a barchan, as a 

 wave? When barchans occur in regular series one 

 behind the other, each dependent in form, position, 

 and movement upon the shelter afforded by that to 

 windward, I think they should certainly be regarded 

 as a group of sand-waves. The fact that they 

 have a definite lateral boundary imparts a complete- 

 ness to their study which is novel in the subject 



