SECT. 2] 



SUBMARINE CANYONS 



485 



trough for 1 1 miles along the base of the ridge leading down from the Mexican 

 Coronados Islands. On the outside of the trough a great mass of sediment rises 

 100 fm (183 m) or more above the floor. This may well be a large natural levee 

 although the alternative explanation given previously (Emery et al., 1952) was 

 that it was a delta which had undergone extensive slumping. Beyond, the 

 channel crosses the lower end of San Diego Trough and shows definite levees on 

 both sides. Coming to a rock ridge, Boundary Bank, the channel deepens 

 considerably as its gradient increases in starting the descent toward San 

 Clemente Trough. However, it terminates part way down the descent. 



Fig. 5. Photograph by Conrad Limbaugh of sandfall observed in a tributary of San Lucas 

 Canyon. Height of fall estimated as about 5 ft. The gently flowing sand above the fall 

 was found in the lower end of a funnel-like narrow valley at a depth of about 15 to 

 25 fm. 



San Lucas Canyon is located off the tip of Lower California. It heads in a 

 small bay of a relatively large fan-filled valley coming down from the high 

 mountains of the southern end of the peninsula. The main canyon head extends 

 into the end of a pier at the small fishing village of San Lucas, as can be seen 

 through the clear water from a plane or from the adjacent ridge. Along the tip 

 end of the peninsula there are several tributaries, one being located off an 

 intermittent tombolo which is occasionally topped by large waves from the 

 open Pacific. At such times quantities of sand are carried across the tombolo 

 and down into one of the tributaries. At the lower end of a funnel-shaped valley 



