38 



Physiography 



Figure 38. Depth distribution 

 off southern CaHfornia of the 

 outer or lower edge of the 

 terrace which marks the 

 shelf-break (depth at which 

 slope becomes markedly 

 steeper than at any point 

 closer to shore). The con- 

 tour interval is 25 feet. 

 Depths are based on average 

 shelf-break shown by groups 

 of detailed sounding profiles. 

 Italicized numbers on the 

 land side of the shoreline 

 show depth in feet below sea 

 level of alluvial fill in mouths 

 of valleys. From Emery 

 (1958a, Fig. 13). 



Basin and Trough Slopes 



The slopes that border the shelves and 

 bank tops and lead down into the adjoining 

 basins or troughs are commonly long and 

 straight. Several are more than 50 miles 

 long with curvatures or departures of less 

 than 5 miles from being straight lines. 

 Forming the largest irregularities of the 

 basin and trough slopes are sharp and more 

 or less right-angled offsets. Examples can 

 be seen on Chart I opposite La Jolla, off 

 Newport Beach, west of Santa Catalina 

 Island, south of San Nicolas Island, and 

 south and east of Sixtymile Bank. These 

 offsets have much the aspect of cross faults. 

 Topographic evidence suggesting fault origin 

 of the slopes is provided by the straightness, 

 height, and steepness of the slopes, the oc- 

 casional presence of "sagponds" and offsets, 

 and the general parallelism of the slopes to 

 major known faults on the adjoining land. 



Heights of the slopes range up to more 

 than a mile above their bases. In general, 

 the steep slopes of basins near the mainland 

 have their bases near or even above the sill 



depth of the basin, but the slopes of basins 

 farther from shore extend as much as 3000 

 feet below the sills; this, of course, is in re- 

 sponse to the greater degree of filling of the 

 basins closer to shore. The steepest por- 

 tions with angles between 10 and 22° are 

 commonly nearer the top than the bottom 

 of the slopes. As shown by Figure 39, the 

 mean of 239 measurements of basin and 

 trough slopes of Chart I is 8°, considerably 

 less than the steepness of the continental 

 slope and of the sides of mountains located 

 within the area of Chart I. Five slopes are 

 steeper than usual (mostly 15 to 20°). These 

 are the Coronado Escarpment off San Diego, 

 the slopes that border the two long sides of 

 Thirtymile and Fortymile Banks, and the 

 ones that border the two long sides of Santa 

 Catalina Basin. The base of each of these 

 five slopes also makes a more abrupt angle 

 with the basin or trough floors than does 

 that of most other slopes of the region 

 (Chart I). These facts suggest that the five 

 slopes may have undergone more recent 

 major faulting than the other slopes. 



Detailed profiles of slopes (Fig. 40) reveal 



