302 Henry C. Stetson 



are, therefore, considerably broader. But the same depositional pattern of over- 

 lapping lenses is common to each. As Stephenson (1926) observed many years ago 

 in commenting on the stratigraphy of both areas, " The different kinds of materials 

 do not form separate, uniform sheets extending throughout the entire length of the 

 Atlantic and Gulf Plain, for the sediments laid down at any given time differed from 

 place to place and the conditions of sedimentation constantly shifted from time to 

 time. Briefly stated, this means that no two columnar sections, unless closely adjacent 

 to each other, are identical in lithologic succession ". In other words, major and minor 

 transgressions and regressions followed each other with startHng rapidity. Deposition 

 has been punctuated by many hiatuses, but the net result has been a growth in thick- 

 ness and in width, until at present these terraces, coastal plain and continental plat- 

 forms have attained tremendous proportions, particularly in the Gulf. Since Lower 

 Cretaceous time, then, when both terraces began to assume their present shape, 

 conditions governing transportation and deposition have remained virtually un- 

 changed. The parallel development of these sedimentary wedges affords an excellent 

 illustration of what will happen when a major sea is forced to deal for long periods 

 of time with large supplies of sediment of varying texture over a fluctuating strand- 

 line. The same structure will be built along any continental margin; the details will, 

 of course, be modified, depending on the proportions of the variables, but the overall 

 pattern should be recognized. It is not often that the oceanographic and geologic 

 environment has remained so constant for such a long period to permit such a full 

 development. 



By way of illustration let us glance briefly at the contemporary oceanographic 

 and sedimentary picture. The Atlantic shelf is, at present, an area of non-deposition, 

 and in places is even undergoing erosion. From New Jersey southwards reworked 

 material from outcropping Coastal Plain formations furnished much of the bottom 

 material, as is evident from the fossils and from the lithology (Stetson, 1936). Con- 

 sequently, the shelf is predominantly sandy. True, the northern rivers carry little sedi- 

 ment, but although those of the middle and south Atlantic states which do have a 

 large suspended load debouch into bays and sounds, all of the sediment is not trapped 

 in them. What fine material is being delivered to the sea is by-passed to the slope. 

 Cores taken in the silts and clays of varying textures on the slope from Cape Hatteras 

 to New England and in the nearby Atlantic basin show that deposition has been of 

 the order of 50-70 cm since the last cold period, which presumably can be correlated 

 with the last advance of the Wisconsin ice (Stetson, 1949). South of Hatteras the 

 slope and Blake Plateau have generally hard bottoms {Ibid, 1949). Temperature and 

 salinity data indicate that the Gulf Stream impinges strongly against this portion of 

 the slope (Iselin, 1936) and sweeps over the surface of the Blake Plateau, preventing 

 any deposition in these two areas. 



Turning now to the Gulf shelf, we find what is evidently an area of deposition. 

 Silts of varying textures predominate except for a narrow, sandy strip close to the 

 beach. This is the more usual condition around the world today; it is the large, 

 sandy expanses that are unusual, and the only shelf that is in any way comparable 

 to the one off the eastern United States is found off Argentina. In the case of the Gulf 

 it is probable that more material is being supplied to the sea than it can transport, 

 and the result is that the continental terrace is at present growing at the expense of 

 the Gulf, a process that has been predominant since the beginning of the Tertiary. 



