112 



velocity increases for sediments both finer and coarser than 

 0.18 mm (Inmanj, 1949). It takes a current velocity between 

 0.5 cm/seCo (0.01 knot), and 20 cm/sec, (0.4 knot) to keep 

 sediment finer than silt in suspension, but if the velocity 

 falls below about 0,5 cm/sec. the material will be deposited, 

 Shepard (1948, p. 63) made 14 to 21-hour current measurements 

 in Santa Monica Bay with a tripod resting directly on the 

 bottom. Three current meters were superimposed on the tripod 

 at 21, 51, and 126 cm above the bottom. He found few measure- 

 ments over 10 cm/sec. and the maximum was 15.9 cm/sec, 84 cm 

 above the bottom. One series showed extreme variability and 

 the other series in the same general locality showed less 

 variability but equally low maxima. Bottom current measure- 

 ments by the Hancock Foundation over most of the shelf 

 generally were between 0.1 and 0.4 knots. Consequently, if 

 Hjulstrom^s values are correct, then fine sediments should 

 be deposited. Since the fine material is not continuously 

 deposited, some mechanism must operate to keep it in sus- 

 pension and carry it beyond the shelf. Inraan (personal 

 communication) has theorized on the possibility of "beat" 

 effects resulting from the impinging of waves on the adjacent 

 shores as a possible mechanism. 



Shepard (1941) has discussed some of the possible 

 explanations of nondeposition on the sea floor off southern 

 California and covers the following possibilities ^ (1) effect 

 of waves, (2) tsunamis (tidal waves), (3) tidal currents, 

 (4) non^tidal currents, and (5) mudflows or submarine land- 

 slides. 



