10G4 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1957 



vertical generally resemble land canyons cut in the sides of mountain 

 ranges. These canyons, some with tri])utaries, usually follow gently cur\- 

 ing to straight courses down the slope from their origins on the shelf and 

 gradually disappear on the ocean floor. Most exhibit sediment hlling 

 on their floors, and many have rocky walls. In some areas the base of 

 the continental rise is marked by a range of hills 100 fathoms or less in 

 height and one mile wide or less at the base. 



2.4.2 Ocean Basins 



The continental rise gives way to the abyssal plains which occupy a 

 sizable proportion of the ocean basins. The smooth, nearly flat topog- 

 raphy of the abyssal plains was apparently produced by deposition of 

 sands and silts which were carried by turbidity currents from the 

 continental margins via the submarine canyons. 



Sea mounts which rise from the abyssal plain have an appearance of 

 being partially buried. The small sea mounts and hills which protrude 

 from the abyssal plain increase in number toward the seaward limit of 

 the plain. The seaward extremity of the abyssal plains frequently occurs 

 where the small hills become so numerous that they occupy the entire 

 area. The margin of the abyssal plain along certain positive features 

 such as the east or west margin of the Bermuda Rise is marked by a 

 sharp rise of the sea floor where the depositional floor has built up 

 against the topographic rise. The larger sea mounts that occur scattered 

 through the abyssal plain also show the same partially biu'ied appearance. 



Besides the two lines of large sea mounts which parallel the Mid- 

 Atlantic Ridge on the east and west, there is another major trend of 

 sea mounts, the Kelvin Group, running southeast from New England. 



The eastern and western basins of the Atlantic are quite similar but 

 there are several significant differences. The European continental 

 slopes are in general higher, steeper, and more rugged and irregular 

 than those oft" the North American coast. The continental rise is often 

 absent or poorly developed; in some areas the continental slope descends 

 almost directly to the abyssal plain. In the area north of the Azores the 

 abyssal plains are less well developed than on the west side of the ridge. 

 Rockall, Bill Bailey's, and Lousy Banks, rocky spines nuuiing south 

 from the Iceland-Faeroe Ridge, represent features which have no 

 direct analogies in the western basin. In the area between the Azores 

 and Gibraltar numerous sea mounts of large size are encountered more 

 frequently than in the western basin. The northwest margin of Africa 

 bears the closest similarity to the northeast coast of the United States, 

 with an extensive abyssal plain and a well-developed continental rise. 



