SECT. 2] ABYSSAL PLAINS 359 



its fluidity while traversing distances of 500-1000 miles, particularly if re- 

 quired to start in abyssal depths instead of high on the continental slope ; 

 (3) in every plain where the slope has been determined, the slope is downward 

 from an area adjacent to an obvious source of abundant sediments rather than 

 from a source of vulcanism. 



E. Areas of Loruj-Continued Sedimentation 



Koczy (1954) suggests that the abyssal plains are merely "areas in which 

 the sedimentation process has been in action for a long geological period". He 

 apparently means to impty that the small-scale gravity movements — compac- 

 tion, folding, slumping, etc.— would eventually produce a flat surface if given 

 enough time. For sedimentation of the particle-by-particle type alone should 

 cause no significant alterations of the topography, leaving the uj^per surface 

 of the sediments parallel to the rock surfaces below. The theory of long- 

 continued sedimentation woidd not explain the gradients or the type of sedi- 

 ments found on the plain. It is difficult to understand how small-scale gravity 

 movements alone could produce such a flat plain. The study of sub-bottom 

 reflections from known horizons on Precision Depth Recorder records has 

 shown that even in very deep water the sediment is eroded from the crests of 

 very small irregularities, even those a few hundred feet high, and that this 

 erosion causes beds to thin over the rises and thicken in depressions. Thus, it is 

 entirely conceivable that given a long period of time and a complete cessation 

 of diastrophism, sediments slowly redistributed by currents or gravity dis- 

 locations could produce a smooth surface. But it seems unlikely that such a 

 process could produce the exponential shape leading away from obvious sources 

 of sediment, the continuous down-slope gradients, the shallow-water detritus 

 and the other characteristics mentioned above. 



F. Uniquely Fine Sediments Distributed by Bottom Currents 



Browne and Cooper (1950) recorded a rough gravitational field over an 

 abyssal plain west of the Bay of Biscay and inferred the existence of large 

 topographic features in the buried rock surface. They concluded that on the 

 abyssal plains "detrital mud is so finely divided that it can be carried in 

 suspension for a very long time and gradually spread by bottom currents so as 

 to fill in the hollows in the original topography". Browne (1954) abandons this 

 hypothesis, for he states, "turbidity currents provide a natural explanation of 

 how the ooze came to be contained in the trough of basement rocks and hence 

 almost obliterated the surface indications of the underlying topography". The 

 sands and silt occurring on all abyssal plains studied to date contradict the 

 hypothesis that the fiatness was produced by extremely fine sediments through 

 the action of deep-ocean currents. However, as we stated in the previous 

 paragraph, the action of deep-ocean currents can be important in producing 

 smooth topography. Yet it seems improbable that this mechanism could have 



