The volume of Triassic and younger sediment on and offshore from the Atlantic coast between Virginia and Nova Scotia can be estimated from the isopach map by Drake, Ewing, and Sutton (1959) of Atlantic coastal and offshore sediments. This volume is compared with that which would have been derived from the probable source area at present rates of erosion. It is found that the average rate of erosion in Triassic and later time was probably not less than three fourths and perhaps equal to the present rate.

The arrangement of the sedimentary troughs identified by Drake, Ewing, and Sutton suggests that the Continental Shelf was formed in part by isostatic sinking of the offshore crust beneath the sedimentary load supplied by the rivers. But the presence of a nearly continuous median ridge in the sedimentary basin suggests that isostatic sinking is not alone responsible for the downwarp and in fact could not suffice to bring it about. The density of the sediment must be less than two thirds that of the subcrustal material displaced. If the basement had been originally horizontal or sloping uniformly seaward, the basin landward of the median ridge must have sunk by a mechanism other than isostatic depression beneath an additional 5000 feet of sediment, for the seafloor is roughly at the same depth over both ridge and inner basin. This differential depression of the basement, as well as the sinking of a former land area to form such basins, requires thinning of the crust; it is here suggested that subcrustal erosion by currents beneath the M discontinuity caused the thinning. Possibly the median ridge and the basins are due partly to instabilities produced by such currents, although they seem too large to be attributed to drag.

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