127 



redistribution of sediments makes it difficult to determine 

 how fast sediments accvunulate at any^^rticular point. Sedi- 

 ment traps have been constructed by Emery and placed on the 

 sea floor off southern California. Apparently, most of the 

 sediment collected in the traps is the result of the movement 

 of sediment along the bottomo A number of investigations in 

 various parts of the world have shown that in relatively 

 shallow water, where the investigations were made, there is 

 an almost continuous change in the texture of the sediments. 

 Thus, the median diameter, or per cent sand, silt, and clay 

 vary greatly over relatively short periods of time. It can 

 only be assvuned that sediment movement along the bottom is 

 zero at any particular time when the calculated rate of sedi- 

 mentation is made; or in other words, the sediments are in a 

 "steady state condition". 



Assuming that 80% of the sediments are deposited within 

 8 miles of shore or over an area of 200 square miles, and the 

 estimated volume of sediments is 478,000 to 650,000, then the 

 rate of sedimentation would be between 0.02 and 0.03 inches 

 per year. Thus, even if the calculations are off by factors 

 of 2 or 3, the overall rate of sedimentation is slow at^ the 

 present time . 



While it is not possible to quantitatively estimate the 

 rate of sedimentation over a long period of geologic time, a 

 few conclusions can be reached on the relative rate of sedi- 

 mentation and to changes in the depositional history of the 

 bay. 



