198 



Sediments 



Figure 1 74. Growth of a sand beach and development of 

 a long-Hved berm at the north side of the shoreline bulge 

 developed inshore of breakwater at Santa Monica Har- 

 bor. These profiles were measured by the Los Angeles 

 City Engineer (Venice Office) during the years between 

 construction of the breakwater in 1933-1934 and its first 

 major dredging in 1949-1950. 



that there is Uttle or no difference in grain 

 size between the cusp and its adjacent 

 embayment. 



Recognition of ancient beach deposits 

 during the past has depended almost ex- 

 clusively on only two criteria of vastly dif- 

 ferent size scale. One of these is the sedi- 

 ment texture, and the other is the general 

 shape of the deposit. The minor features of 

 intermediate size described earlier can also 

 serve as reasonably certain criteria of beach 

 environment when and if ancient represen- 

 tatives are found and adequately studied. 

 Moreover, some of the features are diagnos- 

 tic of a particular part of a beach. For ex- 

 ample, above high tide on a beach we find 

 berms, washover fans, wind ripples, and the 

 uppermost irregular swash marks. Near 

 midtide of the foreshore are cusps, ordinary 

 swash marks, sand holes, sand domes, 

 cavernous sand, and erosional ripples. Just 

 above the low-tide level are rhomboid ripples 

 and rills, and below low tide are oscillation 

 ripples. The manner of preservation of the 

 smaller of these features is not well under- 

 stood, but the uniformity of laminae over 

 distances of at least several meters indicates 

 that deposition of sand by waves can occur 

 without great erosion of the sediment sur- 

 face. Preservation of dehcate features such 

 as swash marks is probably enhanced by 

 the development of a rigid crust by growth 

 of salt crystals in the interstices. In any 

 event swash marks are known to have been 



preserved in Paleozoic sandstones of eastern 

 United States (Fairchild, 1901: Clarke, 

 1918). Partial impregnation of recent beach 

 deposits in such a way that laminae can be 

 separated would be of interest in showing 

 just how and what features may be preserved. 



Mainland Shelf 



Approximately 5000, samples have been 

 taken on the 1890 square miles (4900 sq km) 

 of shelf bordering the mainland coast. 

 Nearly 1700 of them were collected by the 

 University of California Division of War 

 Research in the San Diego region during 

 World War II in a program of measuring 

 the effects of bottom sediments on under- 

 water acoustics (Russell, 1950; Emery, 

 Butcher, Gould, and Shepard, 1952). About 

 1 500 samples in a dense grid between Santa 

 Barbara and Gaviota, by-products of oil 

 company explorations, were studied by 

 Thompson (1957). Most of the rest were 

 taken by the University of Southern Cali- 

 fornia along the entire shelf between Point 

 Conception and Mexico, but with a concen- 

 tration in Santa Monica Bay. This sam- 

 pling was part of two large studies of sewage 

 pollution, one for Hyperion Engineers (Terry, 

 Keesling, and Uchupi. 1956) and the other 

 for the California State Water Pollution Con- 

 trol Board (Stevenson and Terry, 1957). The 

 densest and most systematic sampling is in 

 the four widest places of the shelf: off San 

 Diego, San Pedro, Santa Monica, and Santa 

 Barbara. These areas comprise about two- 

 thirds of the total area of mainland shelf 



Many methods can be used for describing 

 the sediments in an area and have been 

 used during the past. For depicting areas 

 having many samples the ideal that is usu- 

 ally sought is a map on which a single com- 

 pletely descriptive parameter can be con- 

 toured or patterned. One obviously 

 important parameter is grain size, usually 

 expressed as median diameter of each of 

 many mapped and analyzed samples. Such 

 an isopleth map for the San Diego region (Fig. 

 175) reveals a complex pattern of coarse-to- 

 fine median diameters, but it fails to show 

 whether all areas of equal median diameter 



