256 HEEZEN AN D MENARD [CHAP. 12 



canyons cut in the continental slope and continental rise lead out to deep-sea 

 fans, A\hicli in turn are crossed by channels extending for some distance on the 

 abyssal ])lains. In short, these are all elements of a drainage system reaching to 

 a depth of more than 2| miles. An enormous turbidity current swept down 

 from the (Jrand Bank area off Newfoundland and broke a large number of 

 submarine cables in 1929 (Heezen and Ewing, 1952). Ancient turbidity currents 

 of this scale have been reported as existing in the geological record and they may 

 be relatively common in some places and at some times. However, much of the 

 dejiosition by modern turbidity currents has been in great fans at the mouths 

 of submarine canyons. Turbidity currents arising from slumps on the con- 

 tinental slope between the canyons cannot be expected to play a large role in 

 shaping the topography, since their deposits are not concentrated into in- 

 dividual large features but spread over a broad front. Hence, their effect is 

 limited to smoothing the topography. In many regions the sediment available 

 to turbidity currents is dumped into the heads of submarine canyons by long- 

 shore drift. Hence, the sediment slumped into the canyon heads emerges on the 

 fans in turbidity currents. In other parts of the world, rivers, such as the Congo 

 and the Magdalena, lead directly to a submarine canyon. These submarine 

 canyons carry sediment from the river delta directly to the deep sea. At the 

 mouths of the Congo and Magdalena Canyons huge bulging abyssal cones 

 blanket the sea floor. Seaward of the abyssal cones, turbidit}^ currents follow 

 distributory channels onto the abyssal plains (see Chapter 27). 



In addition to their occurrence on the ocean-basin floor, abyssal plains are 

 also foiuid in the marginal trenches, in marginal basins, and in epicontinental 

 marginal seas, and features of exactly the same morphology and origin are 

 found in certain lakes, 



c. Abyssal hills 



An abyssal hill is a small hill that rises from the ocean-basin floor and is 

 from a few fathoms to a few hundred fathoms in height, and from a few hundred 

 feet to a few miles in width (Figs. 18, 19 and 20). The term "abyssal-hills 

 province" is applied to those areas of the ocean-basin floor in wliich nearly 

 the entire area is occupied by hills, i.e. the province lies at approximately the 

 depth of the adjacent abyssal plain but lacks a smooth floor. Isolated abyssal 

 hills and groups of abyssal hills also occur in the abyssal plains. 



Abyssal hills are found along the seaward margin of most abyssal plains and 

 occur in profusion in basins isolated from adjacent land areas by ridges, rises, 

 or trenches. In the North Atlantic the abyssal hills form two strips parallel to 

 the Mid-Atlantic Ridge for virtually its entire length. The Bermuda Rise is 

 bordered on the southeast by abyssal hills which join with the strips adjoining 

 the Mid-Atlantic Ridge. Southeast of the Bermuda Rise, the abyssal plain is 

 absent ; consequently, the western Atlantic Abyssal-Hills Province exceeds 

 500 miles in width. 



In the North Atlantic the axes of maximum depth on the eastern and western 

 sides of the Mid- Atlantic Ridge lie in the abyssal-hills province (Fig. 3). This 



