44 



Physiography 



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Figure 44. Contact of sloping canyon fill with vertical north wall of Sumner Branch of Scripps Canyon. Note pholad 

 borings in wall and eel grass in fill. Depth 100 feet. Courtesy of F. P. Shepard. 



break would be progressively shallower as 

 shore is approached, paralleling a profile of 

 the shelf itself. If, on the other hand, the 

 canyon were older than the date of cutting 

 of the shelf, we would expect the waves 

 entering an embayment of the lowered 

 shoreline to have cut the shelf back along 

 the sides of the canyon. The depth of the 

 canyon shelf-break, thus, would be nearly 

 uniform along the canyon. In other words, 

 if the canyon were older than the shelf, the 

 shelf could have been cut not only from its 

 seaward side but also along re-entrants into 

 it. Clearly, the only canyons that can pro- 

 vide such data are those that cross most of 

 the shelf width, such as Redondo, Hueneme, 

 and perhaps Coronado, Scripps, and Mugu. 

 Only one special survey has so far been 

 made in an effort to settle this question. 



This one, at Redondo Canyon, showed a 

 uniform depth of canyon shelf-break except 

 at the head (Fig. 46); therefore, we might 

 conclude that only the head of the canyon 

 postdates the time of cutting of the shelf. 

 Much more work should be done along this 

 line in order to find the depth of the canyon 

 shelf-break elsewhere and to learn the rela- 

 tion of the canyon shelf-break to the several 

 terraces on the shelf. 



The second portion of the submarine 

 canyons is that which crosses the basin or 

 trough slopes. North of Point Conception 

 in California and elsewhere in the world this 

 portion would correspond to the continental 

 slope. If any canyons cross the true conti- 

 nental slope of southern California south of 

 San Miguel Island, they have not yet been 

 discovered. The slope part of the canyons 



