115 



California. It must be noted, however, that no detailed 

 studied have been made on the effect of tidal currents in 

 relation to nondeposition off this part of the coast. 



A few bottom current measurements were made up to 24 

 hours duration in various parts of Santa Monica Bay, but no 

 currents were ever recorded which could be directly related 

 to tidal action. It must be concluded, therefore, that tidal 

 currents cannot be the direct cause of the low sedimentation 

 rate on the outer shelf, 



Non -tidal Currents 



Non-tidal currents include internal waves and large 

 moving eddies with vertical axes. Shepard (1941), utilizing 

 Hjulstrom*s calculations, reports that in all of the localities 

 investigated off southern California, sufficient velocities 

 were found which would transport fine material, and in some 

 gravel could be rolled across the bottom. There were con- 

 siderable lapses in current velocities in some nondepositional 

 areas which would allow deposition of all sediments except 

 silt and clay. To put the material that had been deposited 

 back into suspension would take high threshold velocities, 

 but the measmred velocities (36 cm/sec) would take sand 

 (.05 - 2.0 mm) back into suspension. Thus, the measurements 

 by Shepard and his associates are sufficient to explain the 

 by-passing of sediments over the nondepositional surfaces, if 

 Hjulstrom'^ data applies to the ocean. Shepard (1941, p. 1882- 

 83) questions the importance of these currents because (1) the 

 currents are much weaker than those which are observed in land 

 stream where deposition is gradually building up the stream 



