304 Henry C. Stetson 



the entire removal of strata deposited near shore during the previous onlap, but it con- 

 tinues to grow in width because deposition is uninterrupted in the deeper water over 

 the seaward slope. Equilibrium, or still stand, will hkewise produce a forward growth 

 of the slope; httle or nothing is added to the thickness, although there is no loss. 



Through all the oscillations of the strand, as well as during periods when sea-level 

 remained constant, the face of the slope always progresses steadily seaward into 

 deeper water. Since Cretaceous time the basement has been sinking with occasional 

 reversals, and the sum total of these oscillatory movements has been downwards. 

 The result has been a terrace constructed of a huge series of overlapping lenses of 

 sediments of diverse Uthology. Consequently, although both terraces have grown 

 intermittently in thickness with many depositional breaks, they have grown con- 

 tinuously in width. For example, total thickness of the sedimentary formations along 

 the axis of the geosynchne at the coast near the Texas-Louisiana line has been esti- 

 mated at more than 40,000 feet (Lowman, 1949), but the maximum width of the 

 whole terrace is measured by the 700 odd miles lying between Cairo, lUinois (the high 

 water mark of the Upper Cretaceous seas) and the present continental slope. On 

 the Atlantic side the sedimentary wedge at the shoreline is sUghtly less than 10,000 

 feet thick, as logged in an oil well drilled at Cape Hatteras. Offshore, seismic profiles 

 run by the Lament Geological Observatory indicate a maximum thickness of the 

 order of 16,000 feet in a basin southeast of Delaware Bay, with a rise in the basement 

 seaward near the present continental slope (Ewing et ai, 1950). Its greatest width 

 is about 175 miles lying on a traverse through Cape May from the Fall Line to the 

 break-in slope. The thickness of the sedimentary prism offshore in the Gulf is, at 

 present, unknown. 



TURBIDITY CURRENTS AND SLOPE TOPOGRAPHY 



The underwater topography of the Atlantic slope shows a dendritic drainage 

 pattern (Veatch and Smith, 1939), characteristic of many land surfaces, of main 

 stream valleys with tributaries, although the streams may have been submarine flows 

 of muddy water known as turbidity currents, and not rivers flowing under the air. 

 Numerous canyons gash the continental slope from Georges Bank to the Chesapeake, 

 and some of them, such as the Hudson, are of considerable size. Perhaps a brief 

 explanation of these submarine streams of muddy water is in order, as they are thought 

 by many geologists to be the erosive agents responsible for the spectacular and enig- 

 matic submarine canyons, which we are now discovering to be world wide. The 

 density of any water mass is increased by a suspended load of sediment. Consequently 

 muddy water will flow under clear water of the same temperature and salinity. Many 

 of the silts and clays of the continental slope have a very high water content, fre- 

 quently running over 150% of dry weight, and are consequently very unstable and 

 will liquify easily by jarring such as could be produced by a submarine earthquake. 

 Theoretically, the resultant flow may start out as a hquid slump and quickly change 

 into a dense flow of turbid water, thought by some to attain considerable velocity, 

 and hence great erosive power, as it slides down the steep continental slope. Flows 

 of this type have never been observed in the ocean, but gentle turbidity currents are 

 known to flow along the bottom of the whole length of man-made Lake Mead, 

 where the muddy Colorado River plunges beneath the clear waters of that Lake. 

 They have also been observed in some glacial lakes such as Geneva. 



