64 



Distribution of Median Diamet ers ,. 



An isopleth map of median diameters of the bottom sedi- 

 ments in Santa Monica Bay is shown in Figure 20 o In con- 

 structing this maps, information from samples obtained with an 

 underway sampler was not used, as the finer portions of 

 material collected with this device are partially washed away 

 during the sampling process » Samples of sediment collected 

 while dredging for rocks were not used for the same reasono 

 Results of the analyses of cores were also not used for this 

 mapo Elimination of the foregoing types of samples left 364 

 samples available fox tlie preparation of this charts 



There are important limitations in the interpretation of 

 the contoured information in Figure 20 o The first is that 

 the contour internal is geometric, rather than arithmetic, 

 using boundaries of Wootworth grade sises as the contour values 

 (2, 1, "I, i, etc ram) J anid, secondly, there is a decrease of 

 the number of samples with increasing water depth. In Figure 

 21 is a cumulative curve of the percentage of samples with 

 depth which indicates the sparsity of samples at the greater 

 depths o Seventy one pe-r cent of the samples were obtained 

 from the shelf at depths less than 300 feet and the remaining 

 29 per cent was from depths greater than 300 feet. Thus, the 

 contours of the median diameters are based on successively 

 decreasing amounts of information with increasing depth. 

 Median Diameters of Shelf Sediments 



Offshore on the outer shelf, median diameters are relatively 

 high and have an irregular distribution. This portion of the 

 shelf contains extensive rock and gravel (Fig, 13) which are also 



