84 WAVES OF SAND AND SNOW 



ment. As long as sand is being so plentifully 

 supplied that the deposit accumulates, or does not 

 become depleted, the growth of the ripples is slow, 

 they never attain a very large size, and the sand 

 upon their crests does not become coarse. But 

 when the supply of sand from the windward is 

 scanty or fails altogether the ripples grow to larger 

 dimensions and the sand-grains upon the crests are 

 coarser. This coarse sand, which becomes of the 

 dimensions of small gravel, ultimately collects in 

 a sheet, which covers and completely protects the 

 upper part of the ripples. The completeness of 

 this protection depends upon the circumstance that 

 individual particles of coarsest sand or very fine 

 gravel lie wholly above the general surface and, 

 being thus exposed to the horizontal pressure of 

 the wind, roll freely, whereas when they are 

 collected in a sheet the wind glides ineffectively 

 over their surface. The rate of their subsidence, 

 or fall, in air is, moreover, such that it would 

 need a very strong eddy to suspend them. Thus 

 when this sheet of coarse sand, or " lag gravel," 

 has been formed it protects the sand below it and 

 the ripples grow by deepening of their intervening 

 troughs. These are dug out in sand which has 

 become somewhat compact owing to superincum- 

 bent pressure. This kind of erosion -ripple not 



