OF SAND FOBMATIONS ON MARINE COASTS. 



19 



water. When the wind blows upon the sand, a winnowing process takes 

 place, the finer particles being carried farther away than the coarser 

 material, which then produces the ridges of heavier grains mentioned 

 above. This uniformity of pattern connected with the fact that the 

 wave length increases with the time during which the force of the 

 wind is acting upon the sand led Cornish * ) to advance the following 

 law for rippling by wind: The rippling takes place when the eddy 

 in the lee of the larger grains is of sufficient strength to lift the smaller. 



The systematically corrugated surface of loose sand can only be 

 produced by a wind that is not too strong for the larger grains to remain 

 on the ground. If the breeze is too strong no rippling whatever takes 

 place as all the particles of sand then will be transported. If on the 

 other hand the wind is too weak to make an eddy, the sand moves slowly, 

 but does not form ripples. 



The height of the waves and their distances from each other is larger 

 the larger the grains are. The movement of the waves is of different 

 rapidity and depends on the force of wind and the size of the grains. 

 It is naturally more rapid when the wind is stronger and the sand fine. 

 Following results were obtained by the author from a number of meas- 

 urements of amplitude or height of ridge and wave-length of sand 

 ripples made by wind on the coast of West Australia. All measurements 

 are given in millimeters. 



These measurements serve to support the results obtained by Cornish 

 and the law advanced by him, that amplitude and wave-length increase 



280. 



1 ) On the formation of sand dunes. Geographical Journal, vol. IX : 



