SECT. 2] MICROTOPOGBAPHY 469 



b. Continental slopes and submarine canyons (Fig. 21) 



Less is known about the features of the continental slopes than of any other 

 region. In places they are comparatively smooth and gentle, implying a sedi- 

 ment cover ; many photographs have shown this to be so. On the other hand, 

 the slopes are frequently cut by submarine canyons where one would expect 

 to find exposed rock. Of this it is very difficult, with the present techniques of 

 underwater photography, to take pictures. Nortlu'op and Heezen (1951) 

 found exposed Eocene rock south of Cape Cod, and McAllister (1957) took 

 photographs of the vertical w alls of the Scripps Canyon showing conglomerates. 

 The granite walls of submarine canyons in Lower California have been des- 

 cribed by Shepard (1959). A number of dives by bathyscaph have been made in 

 or near canyons south of Toulon and west of Lisbon, most of which showed 

 semiconsolidated muds, but a few revealed exposed rock (Peres and Picard, 

 1955; Peres, Picard and Ruivo, 1957; Picard and Dietz, 1957; Peres, 1958). 



c. Ocean basins (Figs. 6, 9, 11, 12, 18, 19, 33, 37) 



The ocean basins include all those deep parts of the ocean bounded by the 

 continental margin and the mid-oceanic ridges. For the purposes of the study 

 of microtoj^ography, seamounts are not included here and will be considered 

 separately. The ocean basins can be divided into three categories ; the con- 

 tinental rise leading gently up to the continental slope, the abyssal plains 

 created by the horizontal distribution of sediments by turbidity currents and 

 the gently undulating bottom where the roughness of the hard-rock topography 

 is buried by great thicknesses of pelagic sediment. 



However, the apjDearance of the bottom is very similar in these three 

 categories and no systematic differences have been found. In areas of Globi- 

 gerina ooze, there is very little to be seen other than a level sediment surface 

 disturbed by the mounds, burrows and tracks described above. No photo- 

 graphs have yet shown an exposed sand surface on an abyssal plain due to 

 recent turbidity currents. In the deeper parts of the Atlantic Ocean, where there 

 is red clay, and in the Pacific, manganese nodules are often found (Dietz, 

 1955; Elmendorf and Heezen, 1957; Menard and Shipek, 1958; Shipek, 1960). 

 In these regions the disturbance by benthic fauna is reduced. 



Where deep-sea channels converge and cut down into the sediments in an 

 abyssal gap, rocks of consolidated sediments have been exposed (Laughton, 

 1960). These are probably mudstones and sandstones derived from the 

 lithification of deep-sea oozes. 



d. Mid-oceanic ridges (Figs. 5, 10, 14, 17, 31, 32, 35, 36) 



Best known of the mid-oceanic ridges is that in the Atlantic. It is an ex- 

 tremely rugged area with sharp peaks and deep valleys. Many of the peaks are 

 very similar to typical seamounts showing patches of exposed bedrock, steep 

 boulder-covered slopes and pockets of sediment. There is evidence in some 



