Along the Southern California coast "slugs" of poorly sorted sediment are injected periodically into the littoral zone. This river-borne sediment is subjected to transport and sorting in the nearshore area by currents generated by progressive surface waves, tides, and winds. The wave-induced currents dominate and are powerful enough to move boulders in the breaker zone and sand in depths greater than 150 feet. Net transport of water along the bottom is shoreward and results in net transport of sand to the beach. Waves not parallel with the shore transport sand along the beach. Silt and clay tend to be kept in suspension by wave-induced surge and form a dense turbid layer at the bottom. The turbid layer is believed to flow slowly down the sloping sea floor into deeper wate . Sand is known to be removed from the nearshore where it is trapped in the heads of submarine canyons. Thus separation of sand from silt and clay results. (Vernon)

The shoreward margin of the California continental borderland, termed a basin slope because it does not descend to the abyssal sea floor like true continental slopes, appears to be primarily a depositional feature, silt being the predominant sediment. A profile from the beach seaward usually shows a narrow rocky shelf thinly covered with sand or sandy silt, and in some places with rock cropping out at the shelf edge. The basin slope itself continues as a bedrock surface which becomes increasingly buried under an encroaching prism of sediment building up from the basin floor. Sea gullies found locally on the upper parts of the slope, but not on the shelf, are attributed to subaerial erosion during periods of exposure when lowered sea-level

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and local tectonic movements combined to permit stream erosion. Bedrock outcrops have been seen from research submersibles. These outcrops are in the walls of the gullies as deep as 400 meters below sea-level. Only a thin film of silt covers the gullies which show a very youthful V-shape cross section. Their identity also is lost at depth beneath the encroaching prism. The gullies do not extend inshore close enough to trap sand, but may serve as channels for funneling silt to the basin in concentrated turbid flow. Likewise considerable transport may result from sheet flow of turbid layers down the slope itself. (Buffington)

Coarse-grained sediments moving parallel with the beach are intercepted by the nearshore heads of submarine canyons. Diverted seaward, coarse sand is known to be mixed with large amounts of plant material. The fill in these canyons accumulates rapidly and is marginally stable. Periodic failure causes mass movement of sediment down the canyons. The dominant present-day processes of sediment movement are (1) slow gravity creep of the entire sedimentary fill of the canyon, (2) progressive slumps and slides of parts of the fill, (3) sand flows and falls in areas where the bottom slopes exceed 30°, and (4) traction resulting from strong bottom currents of various origins. This mass movement of sediment causes both downward and headward erosion of the canyon walls. Box cores from basin fed by nearshore canyons suggest that the coarser and cleaner sand occupies the seaward side of the basin and fine-grained clay the nearshore side. Approximately 70 per cent of the sand contains current-derived structures, indicating reworking after deposition. (Dill)

The California continental borderland contains 17 major basins 20-80 miles long, with depths to more than 1,600 fathoms and distances to sediment source 0-110 nautical miles. Numerous gullies and several large canyons are associated with the basins. Earlier investigations led to conclusions that the coarser silt- and sand-size sediments of the inner basins were largely turbidity-current deposits, whereas the finer sediments were hemipelagic. A recent study, based on the interpretation of more than 1,700 miles of continuous reflection profiles, has produced new data on the structure of both the regional rocks and the basin sediments. The principal objective has been to differentiate between sequences of folded and faulted pre-orogenic rocks which form the present topographic basins, an post-orogenic sedimentary fill of these basins. Sections accepted as post-orogenic fill are divided into turbidity-current and hemipelagic deposits. Post-orogenic sediment thickness is also determined areally to define geographic distribution and relation to topographic features. It is concluded that deposition by turbidity currents has been of paramount importance, virtually to the exclusion of hemipelagic deposition. Because relatively fine sediments constitute the bulk of the basin fill, this conclusion is incompatible with earlier theories which classify only the relatively coarse-grained layers as turbidites. It is postulated (1) that the fine turbidites are of a very low-density, low-velocity variety with a shelf-depth origin, (2) that these layers travel diagonally from the shore ine under control of downslope gravity flow, orbital wave motion, and local tidal currents, (3) that these turbid layers commonly intercept canyon and gully systems where they then flow long distances down distributary systems to basin floors and basin-slope aprons where gradual dissipation occurs, and (4) that they do not erode or carry shallow-water forms into the deep basins, but are deposited slowly and gently enough to be incorporated with the benthonic forms. (Moore)

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