Dill 



It thus appears that the sea cliffs were cut during the Pleistocene 

 lowering of sea level. That such is the case is important because it 

 permits the speculation that similar sea cliffs exist throughout the 

 world and at similar depths, in those areas known to be relatively 

 stable and unaffected by tectonic movements of the continental margin. 



Nature of the Sediment-Cov ered Slope 



Most of the continental slope off San Diego is covered with a 

 deposit of fine-grainedj greenish, clayey-silt. The sediment is 

 cohesive and has shear- strength values indicating that it has built up 

 slowly, probably as a prograding slope deposit over a considerable 

 period of time (Moore, 1961). The benthonic Foraminifera associated 

 with the clay are deep-water forms, characteristic of the depths in 

 which they are found. 



The slope below Dive 175 has been investigated by Hamilton (1963) 

 in the bathyscaphe Trieste I and with cores. He reported that the bot- 

 tom was composed of a sandy silt at a depth of 188 fathoms (344 m). 

 He states that the bottom, when viewed from the window of the bathy- 

 scaphe, is relatively smooth and slopes gently toward the west. A 

 10. 5- foot long piston core taken at a depth of 187 fathoms (342 m) 

 cored 50 cm of clayey silt overlying 250 cm of sandy silt. This was 

 underlain by 10 cm of silt with abundant broken shell debris. 



Bottom Currents on the Slope 



Bottom currents were measured during the DEEPSTAR dives 

 that reached velocities up to 0. 4 knot. In most instances they were 

 flowing diagonally down slope. However, in one instance in the vicin- 

 ity of a sea cliff, currents were observed to be flowing up slope. 

 This might have been a counter current set up by the prevailing down- 

 slope current spilling over the upper lip of the small sea cliff. Simi- 

 lar counter currents have been observed in submarine canyons where 

 there are abrupt dropoffs in the axial slope. 



Another important observation was that scour depressions are 

 frequent around large pieces of man-made junk that had been dropped 

 on the sea floor. In one instance a bathroom sink approximately 2 

 feet in diameter had developed a scour depression of over 3 feet in 

 diameter. It had settled at least 1 foot into the sedimient and rested 

 on a lag deposit of broken shells. The constrictions of flow stream- 

 lines created by this foreign object caused the erosion (or lack of 

 accumulation) of the otherwise stable fine-grained sediments that are 

 prograding the slope. This indicates that the sediment forming and 

 building up the continental slope are at equilibrium with the prevail- 

 ing bottom currents. Scour can therefore be expected around arti- 

 facts placed on the sediment- covered slopes; an important considera- 

 tion when planning the placement of bottomed equipment if instru- 

 ments must remain upright or stable. 



112 



