Basin and Trough Slopes 



39 



them to be more irregular than can be shown 

 by the contours of Chart I. Irregularities 

 consist partly of uneven rocky bottom, as in 

 the profile of the east side of Santa Cruz 

 Basin (Fig. 41), partly of incomplete blanket- 

 ing of sediment, as in the profile off^ Santa 

 Catalina Island, and partly of sediment be- 

 lieved to have slid downslope to accumulate 

 at the foot, as off" the Palos Verdes Hills. 

 Some step-like irregularities of the slopes 

 may result from parallel splinter faults. 

 Near the top of the slopes there commonly 

 is seen a slight concavity that may represent 

 the form taken by a terrace cut into a steep 

 slope. At still greater depths of about 1800 

 feet the slopes commonly contain a slight 

 nick. The frequency of its occurrence and 

 the uniformity of its depth suggests an 

 origin as a narrow terrace produced by 

 eustatic lowering of sea level. However, 

 such a lowering far exceeds the generally 

 accepted maximum for the Pleistocene 

 Epoch. For the present, the nick is con- 

 sidered an unsolved problem. 



Close examination of Chart I shows that 

 the slopes bordering the mainland shelf are 

 commonly fringed at their bases by an 

 apron that generally is lacking for slopes 

 bordering islands and banks. Where sam- 



pled, this slope consists of sediment. A 

 sequence can be set up of slopes that de- 

 scend into small linear depressions similar 

 to sagponds on land, through ones that 

 have a small apron at their base to a larger 

 apron, and finally to gentle slopes that ap- 

 pear to be aprons from top to bottom (Fig. 

 40). This sequence is exactly parallel to 

 that for continental slopes proposed by Dietz 

 (1952) as stages from initial, to early youth, 

 late youth, maturity, and old age. In gen- 

 eral, the slopes of greater geomorphic age 

 border the mainland shelf; thus, the age 

 corresponds closely to the availability of 

 sediments for building up aprons. In ad- 

 dition to forming aprons, sediments may 

 mantle the top of the slope, building it out- 

 ward in areas where sediments are contributed 

 to the ocean most rapidly. Detailed studies 

 by Emery and Terry (1956) showed the 

 presence of sediment at least 18 feet thick 

 on a slope of 18° off" the Palos Verdes Hills 

 (Figs. 41 and 42). From many of the off- 

 shore slopes rock has been dredged, suggest- 

 ing less mantling by sediment on slopes 

 farther from the mainland source of 

 sediment. 



Irregularities are not restricted to features 

 which parallel the slope. Others have been 



100 



UJ 50 



O 



< 



I- 



z 



LJ 



u 

 cr 

 u 



CL 



STEEPNESS 



15 20 



OF SLOPE— DEGREES 



25 



Figure 39. Cumulative curves of measurements of steepness of canyon axes, basin and trough slopes, land mountains, 

 continental slope, and submarine volcanoes of Chart I. The number of measurements for each kind of feature is indi- 

 cated in parentheses. Each measurement is that of the steepest 1500-foot vertical span of profiles spaced about 10 

 miles apart, except for submarine canyons for which profiles are closer. 



