68 



The gravels of the central shelf projection are relic 

 and were deposited during a lower stand of sea level. However, 

 the finer sediments occtirring on the outer shelf are probably 

 a recent sediment cover. The coarse fractions of this finer 

 covering sediment contain authigenic minerals, such as phos- 

 phorite, and abundant shell fragments possibly indicating 

 an environment of non- or slow deposition. Often this coarse 

 fraction material is in the size range of granules and pebbles 

 (2 to 64 mm in diameter). There must be sedimentation of 

 fine-grained material here as in other portions of the bay, 

 or at least over a portion of this area, but it may be slow 

 enough to allow the formation of phosphorite and to favor the 

 abuiKiant growth of shelled animals. It is also possible that 

 the phosphorite and shells are relics of a prior environment 

 and are now being covered or reworked. Then too, the topo- 

 graphy of this area consists of small highs and depressions 

 of low relief (Fig. 8)„ Gravel and rock may be exposed at 

 the highs with sediment accumulating in the lows. The high 

 areas may be the loci of formation of phosphorite and the 

 places of growth of shelled animals which are subsequently 

 swept away and added to sediment accumulating in depressions. 



The coarse material at the head of Santa Monica Canyon 

 and on the shelf behind the central projection has been 

 identified from its coarse fractions as being mainly relic 

 sand with a low silt and clay content. This sediment is 

 probably an old nearshore deposit in which some silt and 

 clay accumulated with the sand. The tongues of coarse 

 material nearshore in the vicinity of Venice and Hyperion 



