AEOLIAN SAND-RIPPLES 89 



accumulation of haze in the room attested the fact 

 that the finer particles were being thrown into 

 suspension. Watching the process at work, I was 

 quickly convinced that the rippling action was due 

 to the concurrence of two modes of motion. In 

 the lee of the large grains eddies were formed which 

 caused the small grains there to be thrown into 

 suspension. The large grains, when apart, rolled 

 freely owing to their projection above the surface, 

 but when gathered in groups were less readily 

 moved owing to mutual shelter and interlocking, 

 so that they tended to mass themselves together. 

 The more they massed the stronger was the eddy 

 on their lee and the more rapid the erosion of the 

 fine sand there. Thus the barriers of gravel 

 became the crests of ripples, which had finer sand 

 in the troughs. The relative mobility of heavy 

 and large and of small and light particles under 

 the action of wind depends upon its direction, the 

 mobility of the larger particles being almost 

 entirely due to the component of the wind which 

 is parallel to the surface, whereas that of the fine 

 particles is largely due to the upward component 

 of the wind's motion. 



At Hayle, on the north coast of Cornwall, the 

 sand of the beach is mainly composed of frag- 

 ments of shell. When I visited the place one 



