FORCES AND PROCESSES IN OCEAN BASINS 43 



This suggested history does not seem plausible. Trenches are 

 normally associated with island arcs for whose past presence on 

 the east coast of the United States there is no evidence. It also 

 seems difhcult to suppose such very extensive faults to have 

 occurred after the most recent sediments outcropping in the 

 canyons, particularly if the same mechanism is to be extended to 

 other coasts which have similar shelves. The filling of a series of 

 faulted basins, as off the coast of California today or of a gentle 

 down warp, would seem a more likely early stage of a continental 

 shelf than does an island arc. 



All that can be deduced from the observations is that the shelf 

 was formed in at least two stages. In the first, more or less flat 

 lying sediments were laid on a subsiding basement, then a scarp 

 was formed at the outer edge, and the existing regime set in. The 

 scarp could be found by normal faulting, as Heezen et al. (1959) 

 suggest, or by erosion or, conceivably, by continental drift splitting 

 a relatively narrow sediment-filled basin and converting it into 

 the shelves of two opposing continents. All such suggestions are 

 guesses with little evidence in their favor and no great chance of 

 being right. However, they do suggest what evidence we should 

 look for. The crucial question is whether the sediments in the basin 

 on the seaward side of the slope contain the missing extension of 

 the shelf, including the foreset beds at its original edge, or whether 

 it is filled with sediments younger than the youngest exposed in 

 the canyons. This can presumably be decided by drilling. 



Much information on the structure of the shelf must have been 

 acquired by the extensive reflection shooting by oil companies in 

 the Gulf of Mexico and elsewhere. It is to be hoped that this will 

 some day be made available. 



The suggestion that the thick pile of sediments beneath a 

 continental shelf will become in time a geosyncline is a natural 

 one. One could suppose that the blanket of sediments caused a 

 rise of temperature at depth, reduced the yield strength of the 

 rocks, and initiated the crumpling under horizontal compressive 

 stresses. There are certain difficulties in this view. The typical 

 continental shelf does not receive sediment from both sides as 

 many geosynclines are supposed to have done in the past. Perhaps 

 the evidence for the conditions of formation of sediments in geo- 



