38 



A gravity core sample is representative of all depths 

 penetrated;, but the in situ depth of a given layer may be as 

 much as twice its actual depth in the coreo The difference 

 between recovered core length and the depth of penetration is 

 produced by the thinning of sediments at the cutting edge of 

 the core barrels Thus, the depths of penetration given for 

 gravity core samples in this report may be up to two times the 

 length of the core recovered., However, since most of the 

 sediment was of sand, or non-uniform sediment, the core length 

 was usually almost equal to the depth of penetration. Since 

 it is virtually impossible to calculate the depth of penetration 

 because of the heterogeneity of most shelf sediment, the outside 

 of the core barrel was greased to determine the depth of pene- 

 trationo 



Figure 11 shows the location and apparatus used to collect 

 the bottom samples in Santa Monica Bay» 



Laboratory Studies 



Mechanical Analysis 



In a typical sediment analysis, twenty^five to fifty grams 

 of sediment are washed through a 250 mesh screen with ,061 mm 

 square openings co separate silt and clay (grain diameter 

 less than „062 mm) from sand and gravel (grain diameters 

 greater than .062 mm). The coarse fraction is dried and 

 weighed. Gravel (grain diameters greater than 2 mm) is 

 separated from the coarse fraction by screening, and weighed 

 to determine its proportion in the entire sediment. Grain- 

 size distribution of the sand portion of the coarse fraction 



