SECT. 2] CONTINENTAL SHELF AND SLOPE 295 



between Florida and Newfoundland Banks. Between Jacksonville, Florida and 

 Cape Henry, Virginia, the basement beneath the shelf lies at depths ranging 

 from 2800 to 20,000 ft. Off Chesapeake Bay, the slope of the basement is 

 relatively simple down to 12,000 ft below sea-level. Off New York (Fig. 8a), 

 the thickest part of the sedimentary basin lies under the middle shelf, where 

 the contact with the basement reaches 16,000 ft ; the contact rises again at the 

 edge of the shelf where it is found at about 12,000 ft, a feature which points to 

 a maximum subsidence located under the shelf and not extending far into the 

 Ocean. In the Gulf of Maine, the basement is shallow (1000 to 2000 ft below sea- 

 level or even less), and the sedimentary rocks seem to be Triassic in age ; 

 farther offshore, under Georges Bank, the basement slope lowers first very 

 quickly, and then more slowly and irregularly beneath the continental slope. 

 Off Nova Scotia (Fig. 8b), the basement slope is irregular and dips very 

 steeply beneath the continental slope ; a Triassic trough seems to exist off 

 Halifax, and the banks around Sable Island consist only partly of glacial 

 deposits, under which probably lie Cenozoic and Mesozoic beds. Between 

 Misaine Bank and Saint Pierre Bank, the seismic prospection under the wide 

 U-shaped Laurentian Trough, at the opening of Cabot Strait, indicates a 

 considerable thickening of consolidated sediments upon the basement, so that 

 the depression is likely to be initially a tectonic graben ; but an important over- 

 deepening by ice has probably occurred during the Pleistocene, explaining the 

 present surface forms, as Shepard suggested in 1931. In a north-south section 

 off Avalon Peninsula, Newfoundland (Fig. 8c), the maximum depth of the base- 

 ment is found outside the continental slope, and the thickness of the sediments 

 above it reaches about 40,000 ft ; but further to the south the contact rises to 

 30,000 ft, as it does off New York. An interesting feature is the relatively great 

 thickness of the consolidated sediments under the Grand Banks (about 6000 ft), 

 which seems to indicate a Triassic, or possibly Palaeozoic, age for a very large 

 part of the banks. Whatever the age, the Grand Banks as a whole are certainly 

 not made up of glacial sediments, and came into existence well before Pleis- 

 tocene times. 



Summarizing these results, we may conclude that subsidence has been 

 general along eastern North America, the continental margin of which is a huge 

 sedimentary basin. Irregularities occur in this basin, since usually, especially 

 in the northern part, the maximum subsidence has probably migrated from 

 place to place. 



In the Great Bahama Bank, a drilling has been made at Andros Island down 

 to 14,587 ft : it reached Lower Cretaceous rocks at 12,480 ft after it cut through 

 exclusively calcareous formations. The Quaternary limestones may be as much 

 as 400 ft thick under Andros Island. The Eocene is found at 2200 ft and the 

 Upper Cretaceous at 8760 ft. It is believed that the Bahama Banks result from 

 marine deposition on a basement having the same general contours as the 

 banks have today, and that this basement has subsided slowly, the platform 

 remaining shallow as a consequence of sedimentation, whereas the latter had 

 little effect in the deep channels between the banks (filing, 1954; Newell and 



