Abyssal Sea Floor 



61 



the gap in the continental slope west of San 

 Miguel Island, and it may have caused the 

 gap. Sounding profiles across the Murray 

 Fracture Zone show it to be irregular with 

 several asymmetrical ridges, some of which 

 rise as much as 6800 feet above their sur- 

 roundings. The width of the zone is 30 to 

 60 miles, about the same as that of the 

 Transverse Ranges. 



Also present on the abyssal sea floor are 

 numerous seamounts. One of these, San 

 Juan Seamount, just at the border of Chart 

 I, was surveyed in detail in 1938 (Shepard 

 and Emery, 1941, p. 3). It has the appear- 

 ance of a volcano, even to the possible pres- 

 ence of several craters at the top. A dredg- 

 ing also recovered a few chips of basalt. 



Other smaller seamounts occur near the 

 continental slope both to the south and 

 north. Still other seamounts which are pres- 

 ent farther seaward have been sounded and 

 dredged (Carsola and Dietz, 1952), and ba- 

 salt was recovered from two of them. Many 

 rounded cobbles from the flattish top of one 

 suggest that its top was planed off" during a 

 lower relative stand of the sea. 



In summary, the topography of the abyssal 

 sea floor is of diastrophic origin (volcanic, 

 faulting, and folding) except where sediments 

 have blanketed it. Erosion is considered vir- 

 tually nonexistent. Thus, this topography is 

 more closely allied in origin to that of the 

 moon than to that of the continents (Dietz, 

 1958). 



