172 WAVES OF SAND AND SNOW 



Isolated mounds of loose snow or sand travelling, 

 before the wind assume a crescentic form' with 

 cusps, or horns, pointing to leeward — the form to 

 which the name " barchan " is given. What, then, 

 is the connection between the shape of a barchan, 



Fig. 25 



Hollow etc cut by wind, from left , around a sfconcfrOlTJ a. 

 photo0ra.ph taken in Scotland February 1900, 



or crescentic moupd, of particles drifting before the 

 wind, and that of an eddy space? When a patch 

 of loose sand or snow happens to form during drift- 

 ing upon hard ground it tends to grow, because 

 other particles driven by the wind have their pro- 

 gress retarded when they pass over the soft and 

 porous surface. The low mOunds are at first 

 roughly elliptical in plan, with their longer axis 

 parallel to the direction of the wind, and are highest 

 near the middle ; but eddy action soon occurs. 

 Its first effect upon the plan of the mound is to 

 truncate the lee end, which becomes a nearly 

 straight line transverse to the wind (see ante, 



