42 



Rock and Gravel 



Hand lens identifications of rock types are made on all 

 gravel and bedrock samples from the sea floor. During the 

 identification of each sample, the roundness and sphericity, 

 nature of fracturing if present, and the presence or absence 

 of borings and encrusting organisms on the individual pieces 

 of rock are noted as an aid in determining whether they 

 represent a fragment of bedrock or rock in place. 



BOTTOM MATERIALS 



Unconsolidated Sediments 



The distribution of the unconsolidated sediments of Santa 

 Monica Bay is based on the examination of 364 samples, and to 

 a minor extent on the notations of the Uo S, Coast and Geodetic 

 Survey. The sediments are classified using a three component 

 system based upon gravel, sand, and silt percentages, as 

 shown in Figure 12. Because of the low volume of clay in the 

 shelf sediments, it is possible to include that fraction with 

 the silt and still retain the three component system. The 

 boundaries for the per cent gravel, sand, and silt fractions 

 in Figure 12 have been modified so that minor conponents 

 could not aggregate more than 20% of the sample. A sajnple 

 containing 857o sand, 9% gravel, and 6% silt is classified as 

 a sand . If it contains 76% sand, 17% gravel, and 97© silt it 

 is a gravelly sand . A sample containing 70% silt, 15% gravel, 

 and 157o sand is a sandy gravelly silt . Sands were further 

 subdivided on the basis of color into two sediment sub-types. 



