280 WAVES OF SAND AND SNOW 



stagnant water in its interspaces, is lower down and 

 cannot be seen.' Now, the superincumbent water, 

 being in standing waves, flows almost exactly- 

 parallel to the surface of the sand-ripples, so that 

 the watery sand must be dragged up the weather 

 slope more slowly than it slides down the lee slope, 

 and there is no eddy to bring it back on the lee 

 side. Therefore, both on account of the sand which 

 travels in suspension and on account of that which 

 travels superficially, the ripples grow on the 

 weather side and lose on the lee side, consequently 

 advancing up-stream. 



I do not think that up-stream sand-waves 

 would be formed in deep water, for unless the 

 water surface were in waves there would be an 

 eddy on the lee side of the sand-ridges which 

 would ensure a down-stream motion. 



In mountain torrents I have often seen progres- 

 sive waves assisting in the formation of the sand- 

 ripples called " current-mark." The broken water 

 of a waterfall descends intermittently, each lump 

 of descending water sending out a travelling wave,2 



^ I examined a sample of firmly bedded sand taken from the 

 bed of a stream where the velocity was only 0*54 foot per 

 second, and found the volume of the interstices to be f that of 

 the real bulk of the sand-grains — z.e., y^ of their apparent bulk. 



"^ See " Waves of the Sea and other Water Waves," by 

 Vaughan Cornish, p. 346, 



