1072 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1957 



Fig. 13 — Continental-slope photograph taken at 550-fathom depth at 44°43'N, 

 54°30'W. Area shown in each photograph in Figs. 13-16 is about 40 square feet. 



3.2.3 The Bottom in the North Atlantic 



The continental shelf at depths less than about 70 fathoms was dry 

 land for a considerable period prior to 11,000 years ago. Thus, the sedi- 

 ments of the continental shelf resembled the sediments of the coastal 

 plain from Cape Hatteras to Cape Cod. A deposit of sand continues 

 along the shelf edge and is generally thought to be an old beach. Land- 

 ward of this is a series of irregularities generally considered to be old 

 dunes and beaches. Photographs of the continental shelf are shown in 

 Fig. 12. 



Hardened sandstones and limestone have been recovered from the 

 walls of submarine canyons off Georges, Browns and Banquero Banks. 

 A rock outcrop has been photographed at a depth of 500 fathoms in a 

 small gully south of Block Island. In other areas the continental slope 

 is covered with low-density gray clay in which the coring rig completely 

 buries itself. Gravel and sand form the floors of some continental slope 

 canyons while others are deeply covered with low density mud. In many 

 areas ancient, partly consolidated clay crops out on canyon walls. 



The Western Union Company, when plowing in their continental- 

 slope cables, had widely different experience along closely parallel 

 cables. '^ Presumably the differences in the depth to which the plow would 

 penetrate were due to differences between ancient and recent compaction 

 of sediments. Although rock was probably not en(^ountered on these 

 runs, it is known from dredging experience that rock can be expected. 



A photograph (Fig. 13) of the bottom at 550 fathoms depth south of 

 the Grand Banks reveals huge ripple marks. It is not difficult to imagine 

 that cable chafe would be appreciable in such an area. 



Beneath the nearly flat abyssal plains alternate layers of sand, silt, 



