SECT. 2] TOPOGKAPHY OF THE DEEP-SEA FLOOR 277 



Azores Plateau is characterized by a series of ridges trending NW-SE at 

 approximately right angles to the trends in the Mid-Atlantic Ridge whose 

 spacing is entirely different from the ridges forming the Mid- Atlantic Ridge. 

 Instead of being close together with only a narrow, steep-walled valley separat- 

 ing two ridges, the ridges on the Azores Plateau are separated by broad valleys. 

 There is a strong possibility that other mid-oceanic plateaus exist, particularly 

 in the vicinity of St. Paul Island in the Indian Ocean ; however, so little of the 

 mid-oceanic ridge has been explored, even in a reconnaissance fashion, that 

 it is impossible at this time to make an accurate prediction. 



3. Summary of Submarine Topographic Forms 



The ocean floor consists of three major morphological divisions: (1) con- 

 tinental margin, (2) ocean-basin floor, and (3) mid-oceanic ridge. The continental 

 margin is formed by three categories of provinces, representing (1) the sub- 

 merged continental platform, (2) the steep edge of the continental block, and 

 (3) the raised or depressed edge of the ocean floor. The topograpliic detail of the 

 continental margin is predominantly smooth except for the submarine canyons 

 and minor irregularities of the upper continental rise. A close correspondence 

 of topography and distribution of recent sediments is apparent. For example, 

 deep-sea sands are found in the canyons and on the fans of the lower con- 

 tinental rise. 



The continental slope appears to be a thinly veneered or bare outcrop of 

 Tertiary and Mesozoic sediments. The upper continental rise can in general be 

 considered a filled marginal trench and the lower continental rise is directly 

 analogous to the outer ridge observed off marginal trenches. The initial form of 

 the tyjDical Atlantic continental margin must have been remarkably similar to 

 the form of the present Pacific marginal trenches. 



The ocean-basin floor lies between the continental margin and the mid- 

 oceanic ridge in the Atlantic and Indian Oceans and comprises large parts of 

 the North Pacific. The ocean-basin floor consists of the deeper abyssal floor 

 and the elevated oceanic rises. On the abyssal floor adjacent to the continental 

 margin are found the flattest surfaces of the Earth. These Abyssal plains were 

 built by turbidity-current deposits. The unburied abyssal floor is represented 

 by abyssal hills. 



The mid-oceanic ridge is a broad fractured arch whose axis follows the 

 median line of the ocean and covers the center third of the ocean. The crest 

 provinces include: (1) a rift valley 15-30 mi wide and 500-1500 fm deep; (2) rift 

 mountains which form the sides of the rift valley ; and (3) the high fractured 

 plateau, a rugged plateau bordering the rift mountains. These characteristics 

 are found universally in the Atlantic and the Indian Oceans but the form of the 

 mid-oceanic ridge is much more subdued in the eastern Pacific and it is not yet 

 certain whether the rift vaUey system can be traced to the east Pacific. 



The flank provinces consist on each side of steps generally separated by 



