16 HISTORY OF THE OCEANS 



commonly situated only a few hundred miles off shore from a 

 continent and enclosing a marginal sea (e.g., the West Indian 

 Island Arc and the Caribbean Sea). The mountain range may 

 sometimes be found at the margin of the continent, (e.g., the 

 Andes) or far out in the ocean (e.g., the Marianas), but in all cases 

 the mountains consist largely of andesitic rocks. There is a parallel 

 deep-sea trench, on the side toward the ocean, and there is seismic 

 activity which usually includes foci at greater than normal depths. 

 The deeper foci are systematically displaced toward the continent 

 side of the arc. There are also significant gravity anomalies over 

 the trenches. 



Since the importance of turbidity currents in long-range trans- 

 portation of deep-sea sediments has been recognized (Heezen and 

 Ewing, 1952), it has been clear that deep-sea trenches were likely 

 to become rapidly filled with sediments (Ewing and Heezen, 1955). 

 This was to be expected provided rivers carrying large volumes of 

 sediment were so located that no topographic barriers intervened 

 to deny access to the turbidity currents. The presence of such 

 filled trenches has been demonstrated by seismic refraction and 

 by gravity measurements. For example, the Puerto Rico Trench 

 is a structural feature which curves southward around the Lesser 

 Antilles and continues through Barbados (Officer et al., 1957). But 

 the trench has disappeared as a topographic feature through being 

 filled with sediments to a thickness of about 8 km, evidently from 

 the Orinoco and the rivers of the Guianas. As additional examples 

 of filled trenches, we may cite that just north of the islands Aruba, 

 Curac^ao, and Bonaire, (Hennion and Ewing, in preparation) and 

 one in the Scotia Arc, from Staten Island and Burdwood Bank to 

 the vicinity of South Georgia. Here also the sedimentary filling is 

 about 8 km thick (Ewing and Ewing, 1959b). By way of contrast, 

 the South Sandwich Trench is an excellent example, like many of 

 those in the Pacific, of a trench which is not being rapidly filled 

 owing to the lack of access of turbidity currents. 



Marginal Seas 



The marginal seas, such as the Caribbean and Scotia seas, have 

 sometimes been described as oceanic regions which are partially 



