27 



may cause pronounced but local relief within the nearshore zone 

 (Schupp, 1953 )„ The shifting of bottom material in the form 

 of longshore troughs and bars is generally confined to water 

 less than about 15 to 20 feet deep (Shepard, 1950). 



With the exception of the surf and nearshore zone, most 

 of the shelf is devoid of any significant relief. Small current 

 and wave formed ripples , marks, and burrows caused by bottom 

 dwelling animals may form mounds, depressions or undulations 

 a few feet high. However, these features like the bars and 

 troughs generally are not permanent. 



A large number of shallow-water fathograms to 300 feet 

 were taken by the VELERO IV over the central part of the Santa 

 Monica shelf. It was observed that much of the shelf had 

 virtually no relief while other parts had significant changes 

 in elevation. Generally, the surface of any continental shelf 

 has small features consisting of mounds, ridges, depressions, 

 or undulations called Miic ro- relief . Features about three feet 

 high are usually the smallest that can be determined by an 

 echo-sounder due to the motion of the ship caused by sea and 

 swell. Ripple marks and mounds formed by organisms are, there= 

 fore, too small to be recorded, A special study of the shallow- 

 water fathograms collected in Santa Monica Bay was made in 

 order to find the extent, type, and distribution of micro- 

 relief on the shelf (Staff, Allan Hancock Foundation, 1956). 

 Figure 7 shows the tracklines along which the fathograms were 

 obtained and studied, and Figure 8 shows the different types 

 and distribution of micro-relief which were then plotted along 

 the trackline. After all fathograms were examined and plotted, 



