SECT. 3] 



BEACH AND NEAKSHORE PROCESSES 



509 



wliicli they enter the sea-bed zone, is straightforward in theory but difficult in 

 practice. In common with the very similar problem of j)redicting the ground 

 destination of fall-out from a nuclear explosion it requires a detailed knowledge 

 of the speed and direction of the sea currents in every fluid layer. 



The fall velocity of a quartz-density cobble 10 cm in diameter which might 

 be rafted out to sea by an iceberg and loosed into deep water is about 150 cm/sec 

 or 5 ft/sec. It would fall to a depth of 10,000 ft in less than an hour and would 

 drift but little during its fall. On the other hand, since the fall velocity of fine 

 grains decreases as the square of their size, very small particles may drift 

 immense distances. The fall velocity of a solid quartz particle 10 [x in diameter 



FALL VELOCITY IN CM/SEC 

 Fig. 1. Fall velocities in water at 20°C of quartz -density grains of average natural shape. 



is 10~2 cm/sec, approximately; and the particle would take 10^ h to reach the 

 10,000-ft depth. 



A graph giving the fall velocities in water at 20°C of natural quartz grains of 

 average shape is shown in Fig. 1. 



3. Transport of Sediment over the Sea Bed 



The transport of sediment over a gravity bed by the combined action of fluid 

 flow and of a gravity component parallel to the bed surface is a phenomenon 

 which occurs in nature under many different conditions. It appears in such 

 varied guises that, until recently, no common characteristics have been looked 



