SECT. 2] ABYSSAL PLAINS 313 



Characteristically, the abyssal plains lie at the base of the continental rise. 

 The abyssal -liills provinces lie on the seaward edge of the abyssal plains. The 

 local axis of maximum depth in an ocean basin is generally found within the 

 abyssal-hills province or at the seaward edge of the abyssal plains. The seaward 

 edge of the continental rise is generally marked by an abrupt change in slope. 

 Here gradients change from values between 1 : 100 and 1 : 700, characteristic 

 of the continental rise, to values of less than I : 1000. In some areas no 

 distinct break is seen and the continental rise grades in an exponential form 

 into the abyssal plain. In these cases the 1 : 1000 isopleth is taken as a boundary 

 between the continental rise and the abyssal plain. Gradients of abyssal plains 

 range from 1 : 1000 to 1 : 10,000. 



Near the seaward edges of the continental rise, there is often a gradual 

 merging of the abyssal cones and deep-sea fans with abyssal plains. In some 

 areas deep-sea channels at the seaward ends of submarine canyons extend 

 across the boundary onto the abyssal plains. Seamounts that rise in the abyssal 

 plain do so abruptly in a manner suggesting that their bases have been buried. 



Abyssal plains are also found in the marginal seas (western Mediterranean, 

 Gulf of Mexico, Caribbean and doubtless in many others) and features with 

 aU the characteristics of abyssal plains are found in the floors of lakes, an 

 excellent example being the Lake of Geneva (Dussart, 1957). The basins 

 lying in the continental margin of Southern California are floored with abyssal 

 plains, and deep-sea trench plains are found in the floors of the major ocean 

 trenches. 



An abyssal plain has been defined as "an area of the ocean-basin floor in 

 which the bottom is flat . . .". What exactly is meant by "flat"? In the rela- 

 tively well mapped Atlantic abyssal plains, variations in depth do not exceed 

 one fathom jjer mile. Irregularities of less than one fathom are gentle, never 

 sharp. Although abyssal hiUs and seamounts break through the surface of the 

 plains as islands protrude above the surface of the sea, they do not otherwise 

 disturb the nearly level surface of the abyssal plains. A typical P.D.R. record 

 from the Cape Verde Abyssal Plain is shown in Fig. Ic. 



Superflcially, the smooth topography of the continental rise (see Chapter 12) 

 is quite similar to the abyssal plains. However, when studied with the aid of 

 P.D.R. , the two are quite easily distinguished. Local areas on the continental 

 rise are of abyssal-plain flatness and in many instances have gradients which 

 would allow them to qualify under the definition of abyssal plains. However, 

 these local perched plains are interrupted by areas of more irregular relief, often 

 of only a few fathoms in amplitude (Fig. la) but distinctly diff"erent from the 

 monotonous, featureless topography of the abyssal plains. The authors consider 

 the term "abyssal plain" restricted to areas "of the ocean floor in which the 

 bottom is flat and the slope to the bottom is less than 1 : 1000". It is clear that 

 the smooth surface of the continental rise owes its origin to much the same 

 processes that produced the abyssal plains. However, this fact should not 

 obscure the marked morphological difference between these two features of 

 depositional topography. 



