Dill 



INTRODUCTION 



The U. S. Navy, although capable of operating throughout all the 

 world's oceans, conducts many of its operations over the relatively 

 shallow waters of the continental shelves and slopes immediately adja- 

 cent to strategic land areas. The increasing reliance on sophisticated 

 acoustic equipment for search in both ASW and Mine Hunting opera- 

 tions along with a requirement to retrieve lost ordnance and valuable 

 equipment from the sea floor, makes a detailed knowledge of the 

 bottom in these operational areas necessary. In most instances fleet 

 personnel assigned to the foregoing tasks must rely on published 

 hydro graphic charts if they want to know the nature of the bottom over 

 which they are working. The deeply submerged rock terraces and 

 associated sea cliffs discussed in this paper, although capable of 

 greatly affecting sonar search capability, do not appear on these 

 charts. Even more critical is the fact that they do not show on most 

 echo-sounder profiles across the continental slope when made with 

 conventional echo- sounding equipment. (Fig. 1). 



The existence of rocky areas in regions presumably covered 

 with fine-grained sediment have been long known and exploited by 

 fishermen. Abrupt changes in the bottom slopes recorded on echo- 

 sounder profiles have led marine geologists (Emery and Terry, 1956; 

 Emery, 1960; Terry, 1965; Goreau and Burke, 1966) and many others 

 to propose the existence of deep terraces throughout the world. How- 

 ever, the extent that these terraces were correlatable, their size, 

 and the nature of the sediments associated with them were unknown 

 until world-wide surveys could be conducted with sub-bottom acous- 

 tic profilers (Moore, 1957; 1960; Buffington and Moore, 1963; Moore 

 and Curray, 1963; Curray and Moore, 1964; Garrison and McMasters, 

 1966) and direct observations made from deep submersibles (Busby, 

 1965, Shepard and Dill, 1966). The profilers permitted a rapid sur- 

 vey of the relationship between the bedrock forming the continental 

 margin and its cover of recent sediment. The submersibles permit- 

 ted a visual and photographic record of bottom roughness, provided 

 bottom samples, and allowed accurate measurements of slopes and 

 micro-relief in the vicinity of the terraces and sea cliffs. For the 

 first time the in situ factors controlling bottom reflectivity and prob- 

 lems associated with operating acoustic equipment in these areas 

 could be made, and the areas could be assessed as foundations for 

 vehicles, weapons, and equipment. 



Terraces and Sea Cliffs off Southern California 



The direct observation of deeply submerged terraces and sea 

 cliffs were made during three DEEPSTAR dive-traverses up the 

 continental slope approximately 6 miles off San Diego, California 

 (Fig. 2). The traverses were made between depths of 325 and 1200 

 feet. Within this interval, 7 cobble- covered terraces and 2 small 

 sea cliffs were found. The terraces are cut in bedrock and covered 

 with coarse, shelly sand of shallow-water origin. Large rounded 



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