31 



crest of the ripple and then downward into the depression of the 

 next ripple. See figure 17. Simultaneously an eddy is formed in the 

 depression and is loaded with small sand particles picked up from the 

 depression. The hydrodynamic reaction between the rotating eddy 

 and the sand surface ejects the eddy together with its sand particles 

 upward some distance. With the trough of the wave following, the 

 movement of water particles near the sand ripple surface is reversed. 

 This moving mass of water carries with it toward the sea the sus],ended 

 sand particles of the previous eddy. In this reversed motion the 

 major portion of the suspended particles is deposited on the surface 

 of the sand ripple from which it initially derived and a minor amount 

 is taken over the next ripple on the seaward side. The cumulative 

 effect of this cyclic process is that a ripple moves gradually by small 

 steps toward the shore. In the process the larger particles of sand of 

 the environment are taken to the shore and the lighter particles, 

 through suspension, are taken to sea. Thus the sand ripple, rather 

 than its motion, becomes the sorting agent. 



eddy formed 



CREST PASSING 



eddy disperses 



A< 



TROUGH PASSING 



MECHANISM OF MOVEMENT OF SAND OVER SAND RIPPLES 



Figure 17. 



