RIPPLE-MARK AND CURRENT-MARK 261 



by the swell of the sea in pools off-shore or where 

 the waves are not near the breaking-point. Both 

 are steeper than the sand -ripples formed by a 

 simple current. I measured a series of eighty-six 

 consecutive ripple -marks formed by waves on the 

 flat shore of the Mawdach estuary in North Wales, 

 and found that they had an average length of 1-99 

 inches and height of 0'36 inch, so that the length 

 was only 5- 5 3 times the height. The measure- 

 ments were made after the water had receded, and, 

 as the ridges had not preserved the sharp crests 

 which they have when under water, I presume that 

 the height had been originally somewhat greater. 

 The ridges extended in broad, straight lines, and 

 the wave-length had an appearance of almost per- 

 fect regularity. The measurements gave 10 per 

 cent, of the mean wave-length as the average 

 difference between the length of succeeding waves. 



At Felpham, near Bognor, on the Sussex coast, 

 I found almost symmetrical ripple-marks left dry 

 on a plank. Seven ridges had an average height 

 of 0534 inch and an average length of 3*24 

 inches, so that the length was 6" 07 times as great 

 as the height. 



The late Professor Osborne Reynolds in his 

 working models of estuaries used " tides " with 

 periods of from thirty to sixty seconds, which is 



