OCEANOGRAPHIC INFORMATION FOR SUBMARINE CABLES 1063 



2.4.1 Continental Margins 



The first area considered is the continental shelf which may be char- 

 acterized as the shallow (0-100 fathoms) submarine terrace bordering 

 the continents, extending seaward hundreds of miles in some localities. 

 The shelf terminates where the bottom gradient increases suddenly 

 from the shelf average of approximately 0.1° to the continental slope 

 average of more than 4°. This change occurs in depths ranging from 

 20 to more than 100 fathoms. The shelf is a continuation of the coastal 

 plain and displays the same numerous small irregularities as the plain. 

 For example, off the United States east coast — from Cape Cod south 

 — the shelf is relatively flat, but with many small (10-fathom) hills and 

 ridges. This shelf varies in width from about 150 miles off Cape Cod to 

 its virtual disappearance off the east coast of Florida. This region of 

 smooth shelf terminates in depths of about 60 fathoms between Cape 

 Cod and Cape Hatteras, but in only 20-30 fathoms south of Hatteras. 

 It is crossed by at least three submerged valleys, off the Hudson Rivei 

 and Delaware and Chesapeake Bays. 



North of Cape Cod the shelf presents a somewhat different pattern 

 with many basins and troughs pitting the shelf whose width increases 

 to 240 miles off Newfoundland. Characteristic of this region are the 

 extensive off-shore shoals (the "Banks"), with depths of about 30 

 fathoms, which are found to seaward of areas with depths up to 100 

 fathoms. 



Outside the continental shelf the bottom drops comparatively rapidly 

 down the continental slope. The top of the continental slope usually 

 lies near the 100-fathom contour but the base lies in depths varying 

 from 700-2,000 fathoms, depending on the area. The typical slope has 

 a gradient of approximately 1:13 (about 4j°). The base is marked by a 

 sharp change in the seaward gradient, from values greater than 1:50 

 (1°) to values less than 1:200 (|°). The continental slope is dissected 

 by numerous submarine canyons, comparable in dimensions to the 

 canyons of mountain slopes. In some cases these canyons connect with 

 shelf channels but generally just slightly indent the shelf edge. Some of 

 the canyons continue across the continental rise (at the foot of the slope), 

 but most of them apparently flatten out and disappear in the rise. 



The continental rise located at the foot of the continental slope is a 

 gently sloping apron with gradients varying from about 1:200 (0°17') 

 to nearly 1:1,000 (0°3|')- Elinor relief features are rare although there 

 are submarine canyons and occasional protruding seamounts. The sub- 

 marine canyons having steep, ^'-shaped walls which may approach the 



