RIPPLE-MARK AND CURRENT-MARK 277 



however, difficult to be certain of the exact level 

 of the surface of the sand, for below the turbid 

 water the superficial layers of sand are gliding 

 along. The superposed water-wave is, unlike the 

 standing waves, usually formed in natural streams 

 in that its front extends in a straight line quite 

 across the stream at right angles to the current. 

 It evidently owes its existence to the sand-wave, 

 and not vice versa. The reason why these sand- 

 waves suddenly arise when the velocity of the water 

 is sufficient to throw large quantities of sand into 

 suspension is, I think, revealed by a discovery made 

 by Dr. J. C. Owens. ' He found that sand-grains 

 settle more slowly in sandy water than in clear 

 water. Thus wherever erosion of a sandy bottom 

 becomes so rapid as to make the water thick the 

 rate of settlement is thereby diminished. Now, 

 the level of a sandy bottom affected by the action 

 of a current is only maintained constant when the 

 amount of erosion and deposition are equal. It 

 appears, therefore, that the velocity at which large 

 quantities of sand are thrown into suspension is 

 a critical velocity at which any minute depression 

 due to erosion will be immediately deepened. If 

 the sand, or some part of it, be heavy enough to 



' Dr. J. C. Owens on " Experiments on the Settlement of 

 Solids in Water," Geographical journal, January, 1911. 



