SECT. 2] CONTIiraJNTAX SHELF AND SLOPE 293 



since this area was flat and not mountainous, as are Norway and Scotland. 

 Tidal scour does not seem to be quite satisfactory in explaining the Hurd Deep, 

 even if it is entirely cut into Mesozoic and not Palaeozoic strata (as it appears 

 on King's map, 1954), because it does not coincide exactly with the area where 

 the tidal currents are strongest off the Cotentin peninsula. The Ushant trough 

 might perhaps result from faulting, but more data are needed. In the North 

 Sea, the deeps were perhaps carved by streams flowing under pressure beneath 

 an ice-cap, a process which can account for their unusual depth and their 

 complex pattern. An alternative suggestion is that they have been lowered by 

 subsidence since they have been cut by subaerial rivers. The influence of 

 subsidence on shelves is examined below. 



Erosion by the various processes which have just been reviewed does not 

 explain the nature of the continental margins. It may have occurred in rocks 

 of the same structure as those of the adjacent continent, and, then, the con- 

 tinental shelf might be related to a progressive bevelling of the continent by 

 erosional processes alone. But it may be also that erosion has affected only 

 a superflcial veneer of the continental margins, so that the shelf, as a whole, 

 would result either from tectonic movements or from sedimentation. The true 

 nature of the continental margins (and borderlands) is a problem which may be 

 approached by seismic prospecting (see Chapter 1), or by drilling or by 

 consideration of the structural geology of the nearby continent and of data 

 from dredgings and corings on the shelf. 



B. The Constructional, Subsiding Type (Fig. 10a) 



Detailed information is not yet available about the structure of the larger part 

 of the continental margins of the world. The areas where investigations are 

 most accurate are eastern North America, the Bahama platform, the northern 

 part of the Gulf of Mexico, North-west Europe (Celtic Sea, English Channel, 

 North Sea), Southern California, and South France and Corsica. These shelves 

 fall into a constructional, subsiding type, except for the last and the last but one. 



In eastern North America, it is now ascertained that the submerged conti- 

 nental margin belongs to the same structiu'al unit as the Atlantic Coastal Plain, 

 which consists of Mesozoic and Cenozoic sediments covering the dowaiwarped 

 Appalachian basement. In the coastal plain, drillings for oil research in the 

 Cape Hatteras area (Swain, 1947; Kuenen, 1950a) "show a prism of Tertiary 

 to Lower Cretaceous strata resting on a weathered surface of granite. Each 

 stratum tliickens regularly seaward", and the basement, which is found at 

 about 800 m below sea-level at 70 km to the WNW of Cape Hatteras, is en- 

 countered at 3200 m below the cape. In the coastal plain of New Jersey, the 

 older the deposits are, the steeper the seaward dip is, the values averaging 

 (Johnson and Richards, 1952) 11 ft per mile for the Cohansey formation 

 (Miocene), 30 ft per mile for the Upper Cretaceous, 40 ft per mile for the top 

 of the Raritan formation (base of Upper Cretaceous), and 65 to 102 ft per mile 

 for the buried surface of the basement (see also Anderson, 1951 ; Spangler and 



