42 



Physiography 



5 6 



MILES 



Figure 43. Profiles of slope oflf Palos Verdes Hills. Profiles A to F extend southwesterly downslope at approximately 

 1-mile spacing. Note the presence on each profile of the following zones: mainland shelf, smooth upper slope, un- 

 dulating lower slope, and basin floor. Profiles G to N extend parallel to the slope at approximately one-third-mile 

 spacing. The shallowest profile, of the mainland shelf, is very flat; those of the lower slope have broad gentle undu- 

 lations. Dashed lines show possible connections of sea gullies. The broad sag in the middle left is a major indenta- 

 tion, atop which the small irregularities are superimposed. 



summarizing most of his information in a 

 mongraph (Parts III and IV of Shepard and 

 Emery, 1941). His later papers have con- 

 centrated on changes at the heads of the 

 canyons, on results of diving operations, 

 and on hypotheses of origin. Most of the 

 more recent work was done on La JoUa and 

 Scripps Canyons. Thirteen canyons in 

 southern California have been assigned 

 names (Chart I). At least 19 additional 

 canyons are present, but most of these are 

 small in comparison with nearby named 

 canyons (like those between Hueneme and 

 Mugu Canyons and those near Newport 

 Canyon) or are poorly known (like those 

 northeast of San Nicolas Island). Of this 

 total of 32 submarine canyons, 20 border 

 the mainland, 10 border islands, and 2 are 

 off submarine banks. There is no question 

 but what others are present off islands and 

 banks where the density of soundings is less 

 than in areas near the mainland but where 



existing soundings show some irregularities 

 of the slopes. In fact, on the basis of these 

 irregularities Ma (1952) drew courses for 

 120 submarine canyons in the area of Chart I. 

 Doubtlessly, more canyons exist even near 

 the mainland but are buried beneath thick 

 sediments which locally mantle the slopes. 

 Finally, there is probably a size gradation of 

 canyons down to the scale of the smaller sea 

 gullies discussed in the previous section. 



Description of the physical characteristics 

 of the submarine canyons is simplified by 

 considering them in relation to the three 

 environments that they cross, shelf, slope, 

 and basin or trough floor. Far more is 

 known of the canyons in the shelf environ- 

 ment than elsewhere, owing to ease of 

 extrapolation of geological data from land, 

 to certainty of positions, and to shallowness 

 of water there. 



Eight of the submarine canyons (including 

 nearly all the named ones off" the mainland) 



