Ill 



lower part of the sea floor by currents, turbulence, and 

 gravity. The settling velocities for very fine sand (0.1 mm) 

 at 20OC is 0,778 cm/sec. and for clay 0,000892 cm/sec. Since 

 the settling velocities for particles of these sizes are low, 

 they could be carried far before settling. The distance of 

 transport would depend on the height above the bottom and the 

 velocity of the transporting current. 



Turbulent motion may be caused by waves or currents. In 

 Santa Monica Bay convection currents resulting from the distri- 

 bution of density are too small to produce any significant 

 turbulence near the bottom. Revelle and Shepard (1939) believe 

 that the chief transporting forces must be tidal and non- 

 permanent eddying "slope" currents resulting from wind action. 

 Although these currents produce no net transport of water, 

 they are rapid enough even at considerable depths to generate 

 pronounced turbulence. Turbulence thus produced tends to be 

 strongest over banks and other obstructions. It is likely 

 that such turbulence exists on the outer shelf in Santa Monica 

 Bay. Current measurements and the analyses of slope currents 

 at depths of 100 to 180 feet show that velocities of as much 

 as 0o2 knot may occur on occasion. 



The only data known concerning threshold velocities is 

 the work by Hjulstrom (1939) and Inman (1949), The minimum 

 threshold velocity necessary to erode sediments already 

 deposited on the bottom finer than silt (l/l6 mm) is about 

 35 cm/sec. (0.7 knot). It has been noted by several sedi- 

 mentologists that particle sizes of about 0,18 ram (fine sand) 

 require minimvim tlireshold velocities, and that the threshold 



