206 



Sediments 



essentially at the same positions, the average 

 percentage of sand (coarser than 62 microns) 

 in the earlier survey was 78 and in the later 

 one 53. As pointed out by Terry, Keesling, 

 and Uchupi, this decrease in grain size has 

 probably been accompanied or caused by a 

 decrease in the rate of contribution of sedi- 

 ments to the bay, owing to both natural and 

 artificial factors. The chief natural factor is 

 that of diversion of the mouth of the Los 

 Angeles River from Santa Monica Bay (via 

 Ballona Creek) to San Pedro Bay. This 

 diversion began in 1825 and became perma- 

 nent in 1884 (Troxell and others, 1942), thus 

 diverting away from Santa Monica Bay large 

 amounts of coarse flood-borne sediments. 

 Artificial changes tending to reduce the 

 amount of sediment contributed to Santa 

 Monica Bay include construction of water 

 supply reservoirs and flood debris basins in 

 stream valleys, urban paving, installation of 

 storm sewers and lining of river channels, 

 building of highways and sea walls at the 

 base of sea cliffs, and trapping of sediment 

 behind breakwaters and groins. It is believed 

 certain that the supply of sediment to the 

 ocean has been reduced more by these fac- 

 tors than it has been increased by the minor 

 amount of cultivation on the watershed. 

 However, accelerated erosion in areas of 

 brush fires locally causes such an enormous 

 increase in the rate of erosion that reservoirs 

 and debris basins fail to retain the materials 

 and prevent them from reaching the shore. 

 In spite of the abundant evidence of move- 

 ment of present-day detrital sediments on 

 the mainland shelf, extensive areas remain 

 uncovered by them. Some of these areas 

 rise little if any above the general level of the 

 shelf but remain uncovered, perhaps because 

 wave turbulence is great enough to prevent 

 the deposition of fine-grained detrital mate- 

 rial but not great enough to remove coarser 

 sediment of other origin. Most important 

 in terms of area is relict sediment, ancient 

 detrital sediment that is believed to have 

 been deposited mostly during Late Pleisto- 

 cene or Early Recent times when sea level 

 was still far below its present position. Most 

 abundant by far of these relict sediments are 

 red sands which occur in patches between 



at least San Francisco and Todos Santos Bay 

 of Baja California. Most of the 550 samples 

 and chart notations of red sand occur off San 

 Diego, San Pedro, and Santa Monica where 

 its mean depth is 36 meters, 23 meters, and 13 

 meters, respectively. This northward shoal- 

 ing parallels that of the warping of shallow 

 terraces on the shelf; also, the red sands oc- 

 cur on both the first and second submarine 

 terraces and on the bordering slopes of each 

 terrace, but nowhere are they as deep as the 

 third terrace nor deeper than 60 meters. The 

 red sand has an average median diameter of 

 480 microns, and thus it is considerably 

 coarser than the present-day detrital sedi- 

 ments (Keller, 1957). The average sorting 

 coefficient is 1.33, about the same as that of 

 the present-day detrital sediments. Round- 

 ing is good, with values mostly between 0.4 

 and 0.6; sphericity also is high, 0.80 to 0.85. 

 Calcium carbonate averages 3.2 per cent and 

 heavy minerals about 3.1 per cent. The ratio 

 of quartz to feldspar is 1.6: 1 at Santa Mon- 

 ica Bay, 1.7: 1 at San Pedro Bay, and 2.9: 1 

 off San Diego and thus is similar to the ratios 

 in the later detrital sediments. The red color 

 is due to a thin stain of ferric oxide. Some 

 of the samples from San Pedro Bay were 

 found to contain a Pleistocene Timms Point 

 foraminiferal fauna (Moore, 1954a). In ad- 

 dition, the sediment is lithologically closely 

 similar to the Pleistocene semiconsolidated 

 dunes that border Santa Monica Bay (Mer- 

 riam, 1949), and it may also be related to a 

 red sand that was reported from borings in 

 the alluviated Tia Juana River valley. The 

 fossil content, lithological similarities, iron 

 stain, coarse grain size, good sorting, shape, 

 and patterns of distribution indicate that the 

 sand is an ancient deposit now being gradu- 

 ally buried beneath prograding detrital sedi- 

 ments. Clearly this coarse material cannot 

 now be in the process of being carried over 

 and past the finer present-day detrital sedi- 

 ment to its areas of abundance. Another 

 relict sediment possibly related to the red 

 sand is a rock fragment sand at Santa Mon- 

 ica Bay and other areas. As described by 

 Terry, KeesHng, and Uchupi (1956), this 

 sand also is coarse-grained and weathered 

 but was derived partly from local sea floor 



