SECT. 2] TOPOGRAPHY OF THE DEEP-SEA FLOOR 245 



The structural benches which are present on the continental slope are 

 somewhat more clearly defined in the walls of the submarine canyons. 



e. Marginal plateaus 



A marginal plateau is a shelf-like feature having a greater depth than the 

 continental shelf and separated from the continental shelf by a low continental 

 slope. The surfaces of the marginal plateaus off Pernambuco Province, Brazil, 

 and west of Angola are irregular in profile. In contrast, the Blake Plateau is 

 superficially similar to the continental shelf in slope and in the frequency and 

 magnitude of relief features. On closer examination, however, it is found to be 

 much rougher than the continental shelf and is dominated by relief which 

 appears to be primarily tectonic in character. 



The Blake Plateau is a well-developed representative of this morphologic 

 type. In the area of the Blake Plateau, the shelf break lies parallel to the coast 

 about 60 miles offshore (Figs. 3, 13). The continental sloj)e extends at a gradient 

 of 1 : 40 only to depths of 300 to 400 fm, where the lower gradients, i.e. about 

 1 : 1000, of the Blake Plateau are found. The main Blake Plateau is 170 miles 

 wide (east-west) and extends from the Grand Bahama Island to.3«^N. The 

 Blake Escarpment forms a precipitous drop to abyssal depths along the eastern 

 edge of the plateau. The top of the Blake Escarpment lies at about 550 fm and 

 its base at 2600 fm. The escarpment is typically formed by two or three distinct 

 slope segments. The outer part of the Blake Plateau is typically smooth, with 

 very low and rolling topography. The inner part, however, lying at the base of 

 the continental slope, is broken by many small, steep -sided valleys 1 to 3 miles 

 wide, and from 5 to 20 fm deep. The trend of this topography appears to 

 parallel the continental margin and one trough has been traced for a con- 

 siderable distance by Hersey. 



The outer edge of the Blake Plateau breaks off abruptly at about 550 fm. 

 Here gradients increase to 1 : 30. This segment continues with a few minor 

 breaks to a depth of 1200-1500 fm, where a narrow bench occurs. Below this 

 bench the escarpment drops so steeply that only a few side echoes are recorded 

 on the echogram. The gradient here exceeds 1 : 2 in several profiles. At 2400 fm 

 there is another narrow bench in the southern part of the area, but toward the 

 north the abyssal plain of the Blake-Bahama basin lies directly at the foot of 

 the steepest segment. A peculiar fact is that along many east-west cross- 

 sections, the deepest point in the basin lies directly at the foot of the escarp- 

 ment. A similar deepening, adjacent to the Campeche and West Florida 

 escarpments in the Gulf of Mexico, has also been found. 



Marginal plateaus and marginal escarpments are found off Recife, Brazil, 

 Rio de Janeiro, Brazil, Angola and the Congo (Heezen et al., 1959). A very 

 similar feature extends for several hundred miles eastward from the Falkland 

 Islands. 



Much of the great Melanesian subcontinent resembles in depth and in topo- 

 graphy the smaller marginal plateaus mentioned above. The area off Southern 



