133 



Other changes appear to have taken place since 1934-38 

 in the bottom sediment distribution. The most outstanding is 

 the lacK of evidence of a rocky area reported by Shepard ajid 

 MacDonald to be one to two miles offshore between Playa del 

 Rey and Manhattan Beach. Shepard and MacDonald stated that 

 part of their data was based upon reports by fishermen and 

 also bottom notations by the U. S. Coast and Geodetic Survey. 

 It is possible that this region was delineated largely on the 

 basis of scattered notations of hard bottom that actually may 

 have been gravel, since it has been found that gravel is wide- 

 spread nearshore. At that time extensive patches of coarse 

 gravel perhaps were exposed at the surface and these were 

 taken to indicate rock. Nevertheless, there is no indication 

 of rock bottom in this region so far as could be determined 

 during the present survey, and either no rock exists here, or 

 it has been covered by a considerable thickness of sediment 

 since the earlier report was made. The submarine topography 

 gives no clue, for the micro-relief in this region is very 

 smooth. 



Submarine Landslides and Slumping 



While the vertical changes in lithology over most of the 

 shelf are explained by variations in the quantity and quality 

 of sediments deposited and also in part to oceanographic 

 conditions (waves, currents, etc), the frequent textural 

 changes in cores taken in submarine canyons are due largely to 

 an additional factor - submarine landslides or slumping. 



