SECT. 2] SUBMARINE CANYONS 481 



straight-walled valleys in tectonically active areas following structural trends 

 rarely normal to the slope, containing basin depressions, and virtually lacking 

 tributaries ; (3) trough-shaped valleys found off deltas, lacking tributaries but 

 differing from the second type in liaving virtually continuous outward-sloping 

 floors ; (4) short gullies found on the foreset sloj)e off a few deltas ; (5) the 

 relatively shallow, flat-floored valleys with levees wliich extend beyond true 

 submarine canyons ; and (6) relatively straight, shallow valleys extending down 

 submarine escarpments and having few tributaries. 



The advantage of separating these types before considering their origin can 

 be seen when we read such statements as "submarine canyons could not have 

 been cut by subaerial erosion because they extend out to the deep ocean floor". 

 One has but to ask what type of submarine valley is found extending to the 

 ocean floor, before considering the merits of such a statement. 



3. Canyon Descriptions 



Not very many submarine canyons have been sufficiently surveyed for them 

 to be described in any detail. The best known are along the California and 

 Lower California coasts. Several of these have been sounded with P.D.R. 

 recordings from their heads down to where they disappear on the outer fans. 

 Furthermore, aqualungers have explored the canyon heads and brought back 

 many pictures along with descriptions. 



The most explored canyon is located off La Jolla, California, and has its two 

 main branches, La Jolla and Scripps, coming into the coast on either side of 

 Scripps Institution (Fig. 1). The numerous dives into the head of Scripps 

 Canyon have revealed a series of narrow, rock-walled gorges with some over- 

 hanging precipices (Figs. 2 and 3). The inner ends of the canyons have what 

 might be termed sand chutes which extend in almost to the shore. These chutes 

 fill rapidly with sand carried to them along the shore by wave action and 

 currents. Kelp and sea-grass accumulate below the zone of active sand deposi- 

 tion. Periodically they discharge much of their sand and kelp load and at such 

 times the canyon floors deepen by amounts of as much as 5 fm (about 9 m) 

 (Chamberlain, 1960). Two of the deej)enings have been connected with earth- 

 quakes and two others with periods of large waves. Other deepenings have 

 taken place without any apparent triggering effect. Chamberlain found that 

 the sea-plant debris becomes comj)ressed and this increases the instability of 

 the inner sand deposits. According to Dill (1962), after the sand chutes have 

 been evacuated, polished rock walls and the floors of these small tributaries 

 are exposed, suggesting considerable wear from moving currents of sand. 

 Traced outward, Scripps Canyon continues as a precipitous, rock-walled gorge 

 as far as its juncture with La Jolla Canyon, a mile from the canyon head and 

 at a depth of 150 fm (274 m). 



La Jolla Canyon winds out to sea with relatively gentle walls except at the 

 head where small cliffs are found in what appears to be Pleistocene or Holocene 

 alluvium. Farther out the walls are of soft rock, but beyond axis depths of 



