42 



HISTORY OF THE OCEANS 



(a) 



Fig. 1. Sections of the continental shelf. 



and Tertiary rocks crop out in the walls of canyons within a short 

 distance of the edge of the shelf, and similar results have been 

 found by M. N. Hill in the mouth of the English Channel. It 

 seems that all the recent sediments are being carried off the slope 

 by submarine landslides and turbidity currents and are coming to 

 rest either in the hummocky area just to seaward of the slope or 

 on the abyssal plains. It may even be that the continental slope 

 is being eroded by the cutting back of canyons. If the structure is 

 indeed as in Fig. lb, and not as in Fig. la, it is difficult to see how 

 it was formed. Heezen et al. (1959) have recently suggested that 

 the continental shelf off the eastern seaboard of the United States 

 started as a trench, comparable to that north of Puerto Rico, and 

 that this became filled. In the course of filling the trench, the shelf 

 is supposed to have formed so that the original landward edge of 

 the trench lies at about the present edge of the shelf. The edge 

 itself would then be a fault scarp caused by vertical movements 

 on the edge of the buried trench. 



