484 



SHEPARD 



[chap. 20 



300 fm (550 m), the rock apparently ceases and muddy walls with some sand 

 are found. At dej^ths of 350 fm (G40 m) the walls appear to ])e related to ridges 

 \\hich can be termed natural levees. These levees traced still farther out have 

 become definitely a i)art of the outer fan with a shallow channel in the mid- 

 portion. All along the canyon and channel, sand occurs largely as layers 

 between mud. The channel gets less and less deeply incised until it disappears 



32*35'- 



32° 30" 



32° 25' 



32° 20' 



32° 1 5' 



32°IO'h 



117° 35' 



117*30' 



II7°25' 



117*15' 



CORONADO SUBMARINE CANYON 



32°35f 



32°3tf 



32°25' 



32°20' 



32° 15' 



117 35 



117 30' 



1 1 7° 25' 



1 17° 20' 



_L 



32° 10 



M7°I5' 



Fig. 4. Coronado Submarine Canyon, and outer channel. Note the high levee (or dissected 

 delta) on the outer curve below the inner gorge. Loma Sea Valley contains a divide 

 draining north and south. (From Scripps Institution P.D.R. surveys.) 



and the fan merges w itli the flat, but gently sloping, floor of San Diego Trough, 

 where the dei)th is 600 fm (1100 m). 



Coronado Canyon heads 8 miles off the Tia Juana River near the United 

 States and Mexican border. The inner canyon has a rock gorge crossing what 

 appears to be a hogback ridge (Fig. 4). Beyond the ridge, instead of directly 

 entering San Diego Trough, the valley turns to the left and extends as a narrow 



