Submarine Canyons 



45 



is larger in cross section than near the head, 

 but not in all instances is it larger than the 

 canyon near the outer edge of the shelf. 

 The large cross section is evidently a result 

 of the deep cutting permitted the agents of 

 erosion by the initial steepness of the basin 

 and trough slopes. It has its counterpart on 

 land in the large cross sections of stream 

 valleys where they cross the emerged sea 

 cliffs as compared to their small size at the 

 back edge of the emerged flat terraces. 

 Axial profiles of the canyons generally pre- 

 sent a fairly smooth decrease of gradient 

 from the head to the mouth, where the base 

 level is the basin or trough floor — corre- 

 sponding to the basin floor base level of 

 valleys in closed basins on land Hke Death 

 Valley. 



Crowell (1952) commented on the possible 



significance of nickpoints in this portion of 

 the canyon axis, but two factors make the 

 existence of nickpoints exceedingly uncer- 

 tain. One factor is the difficulty of recogniz- 

 ing in the field the deepest individual sound- 

 ing on successive transverse sounding pro- 

 files. This difficulty is particularly great 

 where canyons are deep and narrow and 

 where audiovisual echo sounders are used 

 (contours of Chart I were based entirely on 

 such spot soundings made before the advent 

 of automatically recording sounding instru- 

 ments). The second factor is the uncertainty 

 in positions. Errors due to both factors in- 

 crease with depth and distance from shore, 

 and the wonder is not that occasional un- 

 substantiated nickpoints remain but that 

 more depressions and bars are not errone- 

 ously shown by the charts. This does not 



Figure 45. Sketch of head of Scripps Canyon. Based on many observations and photographs by diver. Nearly ver- 

 tical walls consist of Eocene conglomerate and sandstone. Sediment on the floor has a steep slope and chokes tribu- 

 taries. Kelp and other organic debris have accumulated in the sediment. Drawn by T. E. Mahnker. Jr. Courtesy of 

 F. P. Shepard. 



