50 



San Pedro, Catalina, Santa Cruz, and Tan- 

 ner, for example) follow the trend of faults 

 known on land, they are not necessarily fault 

 valleys any more than is the Grand Canyon 

 of the Colorado whose course is partly con- 

 trolled by preferred erosion along fault 

 zones. Many other submarine canyons are 

 transverse to general fault trends or are in 

 areas of only slight faulting. Two other 

 hypotheses, no longer requiring much discus- 

 sion, are those of submarine spring sapping 

 (Johnson, 1938-1939) and erosion by tsuna- 

 mis (Bucher, 1940). The former is opposed 

 by the continuous outward profiles, the rarity 

 of canyons off limestone coasts, and the rar- 

 ity of limestone and favorable structure in 

 areas of abundant canyons, such as southern 

 California. The latter, tsunami origin, is 

 opposed by the abundance of canyons in 

 nonseismic areas, in the lee of islands such 

 as those off San Nicolas Island, and by de- 

 tails of their topography. Landslides, in the 

 form of mudflows, have been proposed as 

 an origin for the sea gullies of southern Cali- 

 fornia (Buffington, 1951; Emery and Terry, 

 1956) and have been measured at the heads 

 of La JoUa, Scripps, Newport, Redondo, and 

 possibly Mugu Canyons as well as in others 

 outside of southern California (Shepard, 

 1949, 1951a); however, it is difficult to con- 

 ceive of them as the chief origin of entire 

 submarine canyons owing to the continuous 

 outward profile of the canyons and to their 

 elongate plan, numerous tributaries, and 

 varied kinds of wall rocks. 



Davis (1934), recognizing the erosional 

 character of the canyons, attempted to ex- 

 plain them through a seaward flow of bot- 

 tom water created by the piling up of surface 

 water against the coast by strong onshore 

 winds. However, the water is so well strati- 

 fied by density that return flow should occur 

 at intermediate depths — at the top of the 

 thermocline — just as it is known to do in 

 fresh-water lakes (Mortimer, 1951). 



Finally, there is the turbidity current hy- 

 pothesis, proposed by Daly (1936) and ex- 

 tended by Kuenen in many papers beginning 

 in 1937. According to this theory, during 

 times of glacially lowered sea-level waves 

 were able to attack accumulations of muddy 



Physiography 



sediment on the outer edge of the continental 

 shelf, presumably forming a dense suspen- 

 sion which could flow along the bottom be- 

 neath clear surface water. In addition, sedi- 

 ment that accumulates from httoral drift at 

 the heads of some canyons is known to sHde 

 periodically, and on sliding it may also be 

 converted into a turbidity current. It is 

 believed that such a heavy turbidity current 

 may easily flow down a canyon and across 

 the sea floor beyond its mouth. The pres- 

 ence of littoral sand was noted along the axis 

 of Redondo Canyon by Ritter (1902) and in 

 later studies of other workers. Extension of 

 the turbidity currents beyond the mouths of 

 canyons is believed by Ewing and associates 

 (1953), Dietz (1953), Menard (1955), and 

 others to have formed the deep-sea channels 

 by eroding the channels and by building up 

 the adjacent levees. Evidence for erosion of 

 the canyons themselves by turbidity currents 

 is not very convincing as yet, being based 

 only on Kuenen's (1951) laboratory experi- 

 ments and on the breaking of submarine 

 cables by the currents (Heezen and Ewing, 

 1952). 



In summary, it seems probable that at 

 least the upper (less than 400-foot depth) 

 part of the canyons was cut by subaerial 

 erosion during times of glacially lowered sea 

 level. The heads of many of the canyons, 

 however, have been modified by deposition 

 of postglacial sediments, accounting for the 

 steep gradients at shallow depths discussed 

 by Woodford (1951). The deeper portions 

 of many canyons cannot be older than Mio- 

 cene, the age of the wall rocks, yet it is not 

 possible to make a clear-cut choice of a sin- 

 gle agent of erosion. Perhaps several agents 

 are responsible as long ago advocated by 

 Smith (1902) and Ritter (1902) and recently 

 reiterated by Shepard (1952) and Kuenen 

 (1953a). 



Basins and Troughs 



Description 



A basin in the usage of marine geology is 

 a more or less equidimensional closed de- 

 pression deeper than its surroundings. The 



