MILITARY SIGNIFICANCE OF DEEPLY SUBMERGED 

 SEA CLIFFS AND ROCKY TERRACES ON THE CONTINENTAL SLOPE 



Robert F. Dill 

 U. S. Navy Electronics Laboratory 



ABSTRACT 



A series of narrow, step-like, rock terraces and low sea cliffs 

 (between depths of 325 and 1170 feet) have been observed on three 

 DEEPSTAR dive-traverses up the continental slope off San Diego, 

 California. The terraces are cut in bedrock and covered with coarse 

 shelly sand of shallow water origin, and large rounded boulders. The 

 flat terraces are less than 100-feet wide and the maximum observed 

 relief of the adjacent cliffs is less than 6 feet. The best-developed 

 cliff and terrace is between 600 and 700 feet (water depth), and 

 extends at least 10 miles along the slope off San Diego. Although 

 large when viewed from the window of a subnnersible, these features 

 are usually overlooked on most echo-sounder surveys because of the 

 lack of definition within the wide angle of the sound cone and the high 

 speeds of the survey traverses. Restudy of existing echo-sounder 

 and acoustic reflection profiles, especially those made with narrow 

 beams, may show that these features are important world-wide top- 

 ographic expressions. If they are related to still- stands of lowered 

 sea-level during the Pleistocene, these terraces should be found in 

 many areas at nearly the same depth, permitting correlation over 

 great distances. 



Acoustic reflections fromi the rocky cliffs and associated coarse 

 sand and cobbles should be greatly different than those from the adja- 

 cent slopes which' are usually covered with a thick mantle of fine- 

 grained sediment. This difference must be considered in any type of 

 ASW or lost-instrument search and recovery employing acoustic 

 sound sources. Also, large numbers of fish were over these rock 

 areas and may be important sources of false echos on search sonars. 

 In addition, these terraces could afford a firm, fairly level founda- 

 tion for vehicles, weapons, and equipment placed on the sea floor. 



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