Gravel Beaches 



183 



Figure 1 56. Typical cumulative 

 curves of total sediment from 

 various environments. Sedi- 

 ments subject to strong win- 

 nowing, wide lines: A, beach 

 gravel, 3 km north of Enci- 

 nitas; B, sheh sand, shelf off 

 Anacapa Island; C, foraminif- 

 eral sand, Santa Rosa-Cortes 

 Ridge; D, red relict sand, shelf 

 off San Diego; E. dune sand. 

 El Segundo; F, beach sand, 6 km 

 west of Santa Monica; G, detri- 

 tal sand, Santa Monica Bay. 



Sediments carried in suspen- 

 sion, narrow lines: H, wave- 

 borne sediment, Huntington 

 Beach; /, wind-borne sediment 

 carried by Santa Ana, El Se- 

 gundo; J, river-borne sedi- 

 ment, Los Angeles River. 



Sediment deposited in quiet water, medium lines: K, sediment from north slope of San Pedro Basin; L, sediment 

 from floor of Santa Barbara Basin; M, marsh silt. Upper Newport Bay; A', Red clay, 80 km west of Northeast Bank. 



128 64 32 



1 .5 .25 .125 .062 031 



MEDIAN DIAMETER-MM 



016 .008 .004 .002 .001 



ganic content closely correspond to boun- 

 daries of the various floral communities with 

 average values for communities from above 

 high tide to below low tide as follows: Dis- 

 tichlidetum, 29.8 per cent; Monanthocloe- 

 tum, 28.0; Suaedetum, 24.1; Salicornietum, 

 14.7; Spartinetum, 8.4; and Zosteretum, 0.5. 

 Because of the large productivity and burial 

 of organic material, the interstitial water 

 loses its dissolved oxygen and is character- 

 ized by a negative Eh (oxidation-reduction 

 potential) and the presence of hydrogen sul- 

 fide and iron monosulfides that contribute 

 toward the typical black color of the marsh 

 sediment. Organic acids released to the in- 

 terstitial water give it a low pH (hydrogen 

 ion potential) that decreases from about 8.0 

 in the tidal channels through 6.9 in the 

 Spartinetum and 6.7 in the Salicornietum to 

 5.7 in the Distichhdetum. Such />H's are 

 sufficiently low to dissolve shells buried in 

 the mud. As a result the average content of 

 calcium carbonate in the marsh surface is 

 only 0.6 per cent, as compared with about 

 80 per cent in the sands of the tidal channels. 

 Recognition of ancient marsh deposits 

 must be made mostly on the basis of sedi- 

 mentary characteristics owing to the general 

 absence of fossils. Chief among the petro- 

 graphic characteristics are fine grain size, 



high organic content, dark color, and pres- 

 ence of marcasite. In addition, thin layers 

 of muds deposited by floods or special high 

 tides produce the well-known platy laminae 

 of Paleozoic black shales, many of which 

 have been considered to be former marsh 

 deposits. Ripple marks and mud cracks are 

 sometimes, but not invariably, present. 

 Tracks and trails should be common in an- 

 cient deposits as they are in the modern 

 marshes. 



Gravel Beaches 



Beaches consisting mostly of gravel fringe 

 about 5 per cent of the coast, occurring at 

 the base of many of the cHffed shores of the 

 mainland and islands. They are most com- 

 mon at the sides of projecting points rather 

 than at the very ends of the points where the 

 exposure to waves is so great that gravels 

 cannot remain. They are also especially 

 likely to occur where the chff's consist of con- 

 glomerate, mostly Eocene and Pleistocene, 

 or where streams contribute large quantities 

 of gravels (Fig. 157). These multiple-cycle 

 gravels are believed to form about 75 per 

 cent of the gravel beaches in the region. 

 Most of the boulders, cobbles, and pebbles 

 consist of dense igneous rock, but chert, 



