Slopes 



duced by the internal waves or deep seiches 

 within the basins; these were revealed by 

 periodic short-term temperature changes in 

 Santa Catalina Basin (Fig. 1 14). 



Slopes 



Only a few studies of sediments on slopes 

 have been made anywhere in the world, 

 owing possibly to the small area of slopes 

 as compared with those of shelves and deep 

 floors. The general absence of data has 

 given rise to speculation that fine-grained 

 sediments should not be present because 

 their angle of repose ought to be smaller 

 than the steepness of some slopes. During 

 1929, however, Kuenen (1943, pp. 34, 35) 

 found sediment at least a meter thick on 

 slopes of 15° in the East Indies. Observa- 

 tions from the bathyscaphes Trieste (Piccard 

 and Dietz, 1957) and F. N. R. S. ///(Tregou- 

 boff, 1958) in the Mediterranean Sea during 

 1956-1957 also revealed many instances of 

 sediment blanketing slopes as steep as 70° 

 according to R. S. Dietz (personal com- 

 munication). 



Off" southern California some sediment 

 occurs on all slopes, but it is patchy on many 

 slopes that are distant from the mainland, 

 so that rock is usually exposed or only thinly 

 mantled (Fig. 41). One nearshore slope 

 which was studied in some detail by Emery 

 and Terry (1956) is covered by sediment at 

 least 6 meters thick. This sediment has an 

 average median diameter of about 22 microns 

 and is homogeneous. Distinct layers were 

 penetrated by the corer only at the very top 

 of the slope and beyond its base on the 

 basin floor. The sediment has the usual 

 high water content of freshly deposited 

 mud, even though it lies on a slope of 9 to 

 18°, and exceptionally of 30°. Measure- 

 ments showed the water content to be about 

 15 per cent higher than the Atterberg Uquid 

 limit (Fig. 186). Thus, the mud must be 

 classed as a liquid rather than as a plastic 

 or rigid material. Evidently, the water con- 

 tent tells only part of the story, and other 

 properties permit the sediment to remain on 

 the slope. Perhaps chief of these properties 

 is shear strength produced by thixotropy, a 



215 



gel-like stiff"ening of the sediment. Resis- 

 tance of clayey sediment to being cut by a 

 wire was found by Emery and Rittenberg 

 (1952) to decrease to less than 1 per cent of 

 its normal value after being shaken. When 

 the sediment is allowed to remain undis- 

 turbed for a few minutes after being shaken, 

 its strength returns. Similar results were 

 obtained by Moore (1956) using more elab- 

 orate equipment to measure shear strength. 

 When sediments are deposited grain by grain 

 on a slope, there clearly is ample time for 

 them to become stiffened. However, if they 

 start to slide or are shaken by an earth- 

 quake, their shear strength should largely 

 disappear, permitting them to flow easily 

 and to produce the elongate sea gullies 

 described in the section on physiography 

 (Fig. 43). 



The sediments of still other basin slopes 

 have been investigated briefly in connection 

 with other problems. In most instances the 

 main objective was the study of sediments 

 on shelves and bank tops, but complete 

 sampling included the tops of adjacent 

 slopes (Figs. 175, 181, and 185). The more 

 than 100 samples available from slopes 

 show them to be intermediate in properties 

 between those of the adjoining shaUow areas 

 and basin floors. Because of the large num- 

 ber of samples from the tops of basin slopes, 

 the sediment parameters given by Table 12 

 are weighted somewhat too heavily toward 

 sediments like those on the adjoining shelves 

 and bank tops. 



Fewer samples are available from the con- 

 tinental slope. Because the continental 

 slope is farther from the source of detrital 

 sediments than most basin slopes, its sedi- 

 ments might be expected to be diff'erent 

 from those of the basin slopes. Sampling 

 attempts indicate that almost certainly the 

 percentage area of exposed rock is greater 

 than on basin slopes; however, only two 

 areas have been studied in any detail. One, 

 near Point Conception, was investigated by 

 Dill (1952) with a series of eleven short 

 cores. Most were on the gently sloping and 

 irregular upper part of the slope where the 

 sediments are coarse with abundant glauco- 

 nite, phosphorite, and rock fragments. Since 



