RIPPLE-MARK AND CURRENT-MARK 285 



co-operation by progressive waves. Eight ripples 

 had an average length of 6156 inches and height 

 of o-6i inch, so that the length was iot times 

 the height. They were sharp-crested, and there- 

 fore of a very different appearance from the sand- 

 ripples of the same length formed in the creeping 

 current of the canalized Bournemouth brook, which 

 had almost flat summits, and were only about half 

 the height. On the sandy road on the top of the 

 East Cliff at Bournemouth I found current-mark 



Fig. 29. — Current-mark on Road after Thunderstorm. 



left after a thunderstorm. Where a pool had 

 formed the ripples had a wave-length of 3 inches, 

 and the ratio of length to height was 12-41; 

 but where the water had run off instead of escaping 

 by subsidence the ripples were left flatter, the wave- 

 length being 1680 times the height. Fig. 29 

 shows four consecutive ridges. 



In many clear rivers which have a sandy bottom 

 sand-ripples are a noticeable feature, ranged in 

 long series, sharp-crested, with gentle slope on 

 the weather side and steep on the lee. I may 



