69 



also have coarse fractions which have been identified as 

 relic material. These are likely old beach ridges or near- 

 shore deposits associated with a rising sea level. The 

 intervening band of fine grained material which has a coarse 

 fraction composed of fine quartz-feldspar sand is evidently 

 sediment being deposited at the present time. This sediment 

 has either filled in a low area behind an older deposit 

 farther out on the shelf or has been deposited across the 

 variable topography of an older series of deposits formed 

 by an encroaching sea to create the smooth topography of 

 the shelf (Fig, 9). The inner shelf appears to be a 

 depositional apron, apparently formed by filling in of 

 irregularities and making a smooth plain since the last 

 lowering of sea level. 



Fine material is kept from being deposited in the near- 

 shore zone by the action of waves. The patch of fine-grained 

 material near the Hyperion outfall, reported at this location 

 by divers, is probably due to the deposition of sludge from 

 the outfalls, which is relatively rapid so that all of the 

 fine material cannot be removed. 



Normal deposition of fine-and medixim-grained marine 

 sediments is taking place on the shelf off the Malibu coast 

 and the median diameters decrease outward as a result. A 

 clue to the areas where marine sedimentation is taking place 

 at present is offered by a graph of mediaji diameters related 

 to depth (Fig, 22). In the depth interval from 125 to 175 

 feet, which is the depth range of the band of fine sediment 

 on the central shelf, and also the depth range in which 



