RIPPLE-MARK AND CURRENT MARK 279 



it, and presently will be uncovered as the ridge 

 passes on up-stream. The up-stream sand-ripples 

 occur, as I have said, in groups, and each member 

 of the group moves up-stream, but the first member 

 — viz., that which is exposed to the undeflected 

 current — is being flattened out all the time and 

 soon disappears, number two succeeding it as the 

 head of the group. This in like manner continues 

 to travel up-stream, but its life is short, for the 

 current planes off the top, making the ridge flatter 

 until it vanishes. 



I have already put forward an explanation of 

 the origin of these sand-ridges in a current of this 

 velocity. I proceed to offer an explanation of their 

 up-stream motion. 



The shallowness of the stream causes the surface 

 of the water to rise at once in standing waves, and 

 there is no eddy on the side of the ridges, a sort 

 of stream-line motion being substituted. Thus the 

 weather face of the ridges is exposed to a shower 

 of the subsiding particles from the turbid liquid 

 denser than that which falls upon the lee side. This 

 is one-half of the process of up-stream motion. 

 The other half is, I think, due to the circumstance 

 that below the sandy water is a stratum of watery 

 sand, which is just visible and appears to be 

 dragged along. The firmly bedded sand, with only 



