RIPPLE-MARK AND CURRENT-MARK 281 



and in each pool among the boulders the whole 

 body of water can be seen to be heaving. On 

 the little beaches in backwaters the progressive 

 waves become steep-faced, with straight, broad 

 fronts, and can be seen charging obliquely upon 

 the shore in rapid succession. In this position the 

 sand-ripples are almost entirely due to the pro- 

 gressive waves. Thus in a backwater below Spey 

 Bridge, near Grantown, in Scotland, when the river 

 was swollen during a rapid thaw, I observed pro- 

 gressive waves coming in upon the shore, thirty - 

 four to the minute, making sand-ripples. These 

 were parallel to the shore, with a steeper front on 

 the shore side, but I could see the sand-grains 

 drifting with the current along the ridges, parallel 

 to the shore. This existence of the two modes of 

 motion in the sand, one general, parallel to the 

 ridges, the other differential, at right angles to 

 them, is a good illustration of the fact that sand- 

 ripples, though passive, are truly waves. 



Except near the shallowing shores the progres- 

 sive water-waves in rapid rivers travel in the 

 direction of the river and not athwart it. They 

 are not very noticeable, but can often be detected 

 by half-closing the eyes so as to blur the outline 

 of stationary bodies such as standing waves, a 

 device which brings moving objects into relatively 



