OF SAND FORMATIONS ON MARINE COASTS. 17 



We have here also to consider the peculiar results of the wave-action 

 of both water and wind, which are generally known by the name of 

 ripple marks, a term introduced by Lyell, 



The phenomena of rippling have in recent times had a careful ob- 

 server in Cornish, and the following statements are principally based 

 on his studies of this problem, although the laws and facts presented by 

 him have been subjected to a detailed investigation in the field by the 

 present writer, and I am able in a few instances to bring forward some 

 evidence in support of Cornish's theories. 



The same factor which causes the wave formation of water has a 

 similar influence on the sand. The resulting wave-forms or ripples 

 consist of alternate ridges and furrows made by the wash of the waters 

 over a sand flat or beach, or over the bottom within soundings. They 

 may also be made by the action of wind on a surface of sand. When 

 the ripples are formed through the action of water we can distinguish 

 between 



1. wave formed ripples, 



2. current made ripple marks, and 



3. tidal sand ripples. 



The parallel formations of wind made sand waves are 



4. eolian sand ripples, and 



5. dunes. 



Comparing waves of water with those of a more solid medium, such 

 as sand, we find that, while in the case of water two kinds of waves, 

 oscillatory and wind driven, can be recognized, wave formation in sand 

 is always connected with onward movement of the particles. In oscil- 

 latory waves the water particles on the crest are moving forward, but 

 those in the trough backward with the same velocity and consequently 

 the water body as such does not move in either direction. It is cus- 

 tomary to express this motion by saying that the particles move in a 

 circular orbit. When the waves are wind driven the forward velocity 

 is greater than the backward, and a bodily movement of the water in 

 the direction of the acting force is the result. The curve described by 

 the water particles is still closed, having a trochoidal form. In the case 

 of drifting sand the particles from the crest of the wave move in curves, 

 which are open. 



Wave-formed sand ripples have an unsymmetrical form, always facing 

 with the waves. Current made ripple marks are similarly unsymmetrical 

 in form, the sheltered side being steeper, and the front facing the cur- 



