SECT. 2] 



TOPOGRAPHY OF THE DEEP-SEA FLOOR 



243 



related to a sea-level lowered to within about 5 fm of the break (Dietz and 

 Menard, 1951). It is clear, from several lines of reasoning, that the sea-level has 

 recently risen eustatically, and that, in general, this recent rise has taken place 

 more rapidly than changes of land level affected by erosion, sedimentation, or 

 diastrophism. However, the shelf break falls at different depths in different 

 areas, ranging from 12 fm to 250 fm. The mean depth of the shelf break is 

 approximately 73 fm. Heezen et al. (1959) suggested that the lack of agreement 

 in the depth of the shelf break is due to the fact that shelf break does not 

 everywhere represent a single ancient eustatic level. Thus, the most prominent 

 break may locally be a Pliocene or Miocene structural bench, but elsewhere, 

 late Pleistocene or Recent strata may form the shelf break. In some areas the 



800 



900 



100O 



1100 



1200 



100 



200 



300 



400 



East 



West 



Fig. 11. Shelf break in the continental slope off New York. The record was made with the 

 Precision Depth Recorder. (After Heezen et al., 1959.) 



late Pleistocene eustatic low level may truncate the continental slope and there 

 the shelf break may truly represent a late-stage eustatic sea-level. 



The deeper structure of the continental margin indicates a fundamental 

 structural discontinuity at the base of the continental slope (Fig. 8). It would 

 seem a small extrapolation to attribute a fault origin to the continental slope. 

 Although faulting may have played a part in the earhest history of the Category 

 II provinces, alternate periods of sedimentation and marine planation on the 

 continental shelf, and long-continued erosion by slumps, turbidity currents 

 and deep-sea currents on the continental slope, together with a general sub- 

 sidence of the area, could alone have produced the characteristic form of the 

 continental terrace. 



d. Submarine canyons 



Submarine canyons were early recognized and have captured the imagination 

 of several generations of geologists and hydi"ographers (see Chapter 20). 



