52 



Physiography 



derlain by thick sediments and nearly sur- 

 rounded by ridges and hills known as the 

 San Diego Trough. In all probability this is 

 a basin that has become filled to overflowing 

 with sediments. Several other filled basins 

 may be present (between Oceanside and 

 Santa Catalina Island, 20 miles west of Des- 

 canso Point, and 30 miles west of San Nico- 

 las Island), but the evidence for them is less 

 definite. Four basins on land represent a 

 further stage of filling by sediment because 

 they have become filled to above sea level. 



Altogether, there are eighteen known ba- 

 sins within the area of Chart I. These can 

 be grouped into parallel belts progressively 

 farther seaward as follows: (1) Ventura, San 

 Fernando, Los Angeles, San Gabriel; (2) 

 Santa Barbara, Santa Monica, San Pedro, 

 and San Diego; (3) Santa Cruz, Santa Cata- 

 Una, San Clemente; (4) San Nicolas, East 

 Cortes, No Name; (5) Tanner, West Cortes, 

 Velero; and (6) Long. In general, the basins 

 along each belt become deeper to the south- 

 east. This arrangement of basins is followed 

 by Table 5, in which the dimensions are from 

 physiographic, but not necessarily structural, 

 data. Areas of the basins at their sill depths 

 range from 118 to 1027 square miles. The 

 volumes range from 10 to 231 cubic miles 

 and total 913 cubic miles (3790 cu km), but 

 this is only about 5 per cent of the total vol- 

 ume of water atop the continental border- 

 land. 



Examination of Chart I shows that several 

 of the outer basins are compound, that is, 

 they enclose several areas much deeper than 

 their surroundings on the basin floors: San 

 Clemente, East Cortes, West Cortes, and 

 Long. The basins having the flattest floors 

 are among those closest to shore: Santa 

 Monica, San Pedro, San Diego, Santa Cata- 

 lina (Fig. 50). Minor irregularities charac- 

 terize the basins at intermediate distance: 

 Santa Cruz, San Nicolas, San Clemente, and 

 Tanner. The flattish submarine basin floors 

 beyond the foot of the basin slopes have a 

 total area of about 5120 square miles, about 

 60 per cent of the total area of deep flat por- 

 tions of Chart I (Tables 1, 5) and about 78 

 per cent of the total area of the basins at 

 their sill depths. For the nearshore basins 



the area of flat floor exceeds the area at sill 

 depth, whereas for offshore basins the area 

 of flat floor averages about half that at the 

 sill depths. Thus, the basins present a grad- 

 ual shallowing (both absolute and with re- 

 spect to their sills), a broadening, and a 

 smoothing of irregularities from far off'shore 

 to nearshore (Fig. 51). The trend is con- 

 tinued by the basins on land, except for the 

 eff'ects of stream erosion along their inner 

 margins. This trend of characteristics of 

 basin floors strongly indicates progressively 

 thicker fills of sediments in basins closer to 

 shore, a deduction which is also supported 

 by data on the rate of deposition of sedi- 

 ments and by hmited seismic measurements. 

 Most of the basins have rectangular out- 

 lines, although some (Santa Cruz, San Nico- 

 las) have one or more curving sides and 

 several (San Clemente, West Cortes, Tanner) 

 are irregular with sides consisting of several 

 segments, each more or less straight. Long 

 dimensions of afl except Santa Barbara Basin 

 trend northwest-southeast, paralleling the 

 structural trend of the peninsular ranges in 

 southern California. The long axis of Santa 

 Barbara Basin is east-west, paralleling the 

 structural trend of the Transverse Ranges 

 which are represented on its north side by 

 the Santa Ynez Mountains and on its south 

 side by the San Miguel-Anacapa chain of 

 islands. Trends of the land basins show the 

 same division, and in fact there is some justi- 

 fication for considering the Ventura Basin as 

 a landward extension of the still submerged 

 Santa Barbara Basin (Rand, 1951). These 

 relationships show that the submarine basins 

 are closely related in present form and prob- 

 able origin to the land basins, and that both 

 are controlled by regional structure. 



Filling by Sediments 



The chief source of detrital sediments is 

 the mainland from which sediments are 

 contributed by streams, waves, and wind. 

 Streams, carrying the bulk of contribution, 

 obtain most of their load from the areas of 

 steepest slope and highest rainfall. Thus in 

 southern California it might be expected that 

 most of the sediments reaching the ocean 



