6 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL, 27 



Shepard and Beard (1938) reported that the axial slope of Cali- 

 fornia submarine canyons is steepest at the head — 14.5°, moderate at 

 the middle — 5.5°, and gentlest at the seaward end — 4.0°. The new pro- 

 files were made too far from the shallows at the heads of the canyons 

 to cross the steepest part of the canyon axes, but axial slopes which they 

 did encounter in the shelf portions usually exceeded 5°. All except three 

 canyons (Coronado, Santa Catalina, and Tanner) have longitudinal 

 profiles that are concave upward. As shown by Figure 16, there is only 

 a slight correlation between steepness of canyon walls and of canyon axes. 



Heights of canyon walls in the shelf portion range upward to 480 

 meters and average about 170 meters. In five canyons (Hueneme, Santa 

 Monica, Redondo, Newport, and Santa Cruz) the greatest wall heights 

 occur at the outer part of the shelves ; in all the others, the greatest 

 heights are slightly farther seaward, near the top of the basin slopes. 



Basin-slope Portion: — Basin slopes in the region average about 8°. 

 The portion of some of the canyons traversing the basin slope is longer 

 than that across the shelf, but for other canyons the reverse is true. All 

 except Newport, San Clemente, and Tanner canyons have broadly 

 curved courses down the basin slopes. For four canyons the curvature 

 is to the right and for six to the left ; this curvature appears to be the 

 result of differential erosion along structural irregularities in the basin 

 slopes. 



Just as for the shelf portions, the intersections of the canyon walls 

 with the basin slopes are not usually abrupt, but the basin slopes bend 

 gradually inward toward the canyons. Indicated steepnesses of the can- 

 yon walls range up to 40°, averaging slightly less than for the shelf por- 

 tion. In both portions the opposite walls exhibit considerable asymmetry, 

 with one-third of all pairs of profiles having one wall more than twice 

 as steep as the opposite wall. Heights of the walls range up to 500 

 meters and average 170 meters for 79 measurements, the same as the 

 average for the shelf portions of the canyons. The heights of both walls 

 are about equal, except where the canyon lies at the foot of a basin slope. 



The echograms present a minimum width of the canyon floors be- 

 cause of reflections from the canyon walls, as discussed also by Northrop 

 (1953) for Hudson Canyon. Often a faint echo from a horizontal sur- 

 face can be detected through the traces produced by echoes from the 

 walls. This faint echo, the presence of flat bottoms on some echograms, 

 the collection of several samples from about the same wire depth on a 

 profile across a canyon, plus the observations of divers in shallow water 

 indicate that the canyons in both shelf and slope portions may have flat 



