64 



Lithology 



SEA FLOOR OFF SOUTHERN CALIFORNIA 



STATUTE MILES 



CONTOUR INTERVAL — 1500 FEET 



300 FOCfT CONTOUR ADDED 



ROCK BOTTOM 



. KNOWN 

 ^PROBABLE 



Figure 60. Distribution of rock bottom. Samples containing rock judged to be in place are shown by dots or by large 

 black spots where many rock samples are available. Areas believed to be underlain by rock, patchily covered by sedi- 

 ments, are indicated by cross-hatching. 



supplementary indicators of rock bottom, 

 however, adds to the completeness of the 

 map, showing that in an area of about 750 

 square miles (about 3 per cent of the region) 

 heavy dredges and corers are more likely 

 than not to encounter rock, whereas in the 

 rest of the region the same equipment is 

 more likely than not to encounter only un- 

 consolidated sediment. In general, rock 

 bottom is likely to be found in the following 

 kinds of areas: 



1. Adajacent to rocky shores. 



2. On irregular hills which rise above the 

 level of the mainland or island shelves or 

 above the level of large flat bank tops. 



3. At the outer edge of the mainland or 

 island shelves or of large bank tops. 



4. On the walls of submarine canyons. 



5. Atop small sharp banks. 



6. Atop even small deep hills which rise 

 above the floors of basins or of the abyssal 

 sea. 



7. On the upper parts of steep basin 

 slopes or of the continental slope. 



Kinds of Rocks 



Rocks judged to be in place include aU 

 three major groups: sedimentary, igneous, 

 and metamorphic. Sedimentary rocks (Fig. 



