1882.] 



On the Formation of Ripplemark. 



3 



attention to the fact that waves, if they affect the bottom at all, do so 

 by setting up alternate currents :* and that, though at great depths 

 the action is very small, theoretically it has no limit. 



Having observed that ripplemarks are commonly better preserved 

 in pools between tidemarks than on those parts of the tidal strand 

 left dry at low tide, and that the bottoms of these pools must be in 

 some measure protected from the continuous currents that are com- 

 monly supposed to produce the ripplemarks, it seemed to me probable 

 that they were produced by the alternate action of waves described 

 to me by Lord Rayleigh. 



One fine and almost calm clay in the summer of 1881, being at 

 Broadsands in Torbay, and seeing that the strand was covered with 

 ripplemarks, I proceeded to watch carefully the action of the water 

 with a view of ascertaining, if possible, the process of their formation. 

 Floating in my boat a few yards from the shore in about 18 inches of 

 water, 1 narrowly scanned the effect of the very gentle swell that was 

 breaking on the beach. I observed that a small shell lying in one of 

 the furrows instead of being steadily washed shorewards by the in- 

 coming waves, was washed backwards and forwards from one furrow 

 to another; sometimes it would stop on the intervening ridge, and so 

 for the moment help to build it up ; at others it would fall over into 

 the furrow towards which for the moment it was being propelled, but 

 in no case did it show any tendency to travel continuously in any 

 particular direction along the bottom. 



On a subsequent occasion, having to land on the beach at Paignton, 

 and seeing the ripplemarks well developed, I again carefully watched 

 them seawards. At a point where the bottom was too indistinct for 

 me to observe its condition, I could distinguish fragments of sea- 

 weed gently moving backwards and forwards in the direction of the 

 beach, and at right angles to the ripplemark where last visible. This 

 observation was unexpected, as it proved a gentle swinging motion of 

 the water in the vicinity of the shore, when the surface motion was 

 so slight as not to interfere with my landing on a flat open beach 

 from a very small boat. 



On the 19th October, 1881, there was a strong south-easterly gale 

 in Torbay, and the waves rolled on to the Meadfoot Sands at the rate 

 of 1\ per minute. The distance between the southern point of an 

 outlying islet known as the Shag Rock .and the rocks at the western 

 end of Meadfoot Sands (two points in line with the direction of pro- 

 pagation of the waves), 275 yards by the chart, was covered by 

 exactly three waves, so that each one must have measured 275 feet 

 from crest to crest. In midbay this length was probably exceeded. 

 As from the manifest turbidity of the water, the bottom was unqu.es- 



* " Trans. Tev. Assoc.," vol. x, p. 191 (1878). 



B 2 



