Since the Cretaceous Period, tlie eroded roots of 

 the old coastal highlands have experienced suc- 

 cessive invasions of the sea, and a large wedge of 

 sediment has been deposited on their surfaces. 

 The most recent submergence was between about 

 4,000 and 20,000 years ago. (For discussions of 

 terrestrial conditions on the shelf in the recent 

 past see Emery, 1966a ; Emery, Wigley, and Rubin, 

 1966 ; and Wigley. 1966) . The presently submerged 

 surface of this sedimentary wedge is the Conti- 

 nental Shelf of eastern North America. 



General Sediment Distribution 



The surface of the Middle Atlantic Continental 

 Shelf is covered only in part by contemporary 

 sediments. These are mainly in a narrow near- 

 shore zone (Emery, 1961; Uchupi, 1963). The 

 largest part of the shelf is covered by relict de- 

 posits formed during lower stands of sea level in 

 the Ice Age. 



Relict shelf features and sediments were recog- 

 nized as early as 1850 by Austen who suggested 

 that the English Channel was once a subaerial 

 river valley. Dana (1863) extended Austen's idea 

 to the Middle Atlantic Continental Shelf by his 

 discovery on an 1852 Coast Survey chart of both 

 the Hudson and Block submarine channels which, 

 he concluded, were once occupied by the Hudson 

 and Connectidut Rivers. Taylor (1872) later sug- 

 gested that evidence of dry land, rivers, and shore- 

 line features should be found within the 100-fm. 

 line, and during the same period Louis Agassiz 

 taught his Harvard classes that the offshore fish- 

 ing banks consisted superficially of glacial drift 

 (Upham, 1894). Most recent authors accept the 

 idea of relict deposits, although emphasis shifted 

 somewhat after Gulliver (1899) and Jolmson 

 (1919) introduced the concept of an inner shelf 

 which had been cut by waves and an outer shelf 

 which had been built up by wave deposition (for 

 a discussion of this concept see Dietz, 1963b, 1964 ; 

 and Moore and Curray. 1964). 



The Middle Atlantic Shelf is covered by modi- 

 fied glacial outwash and moraines, river channel 

 and flood plain deposits, ancient deltas, offshore 

 bars, and old coastal beach-lagoon complexes 

 (Uchupi, 1968). Some smaller areas may contain 

 materials formed in place by submarine chemical 

 processes (Uchupi, 1963; Emery, 1966b). Super- 

 imposed on these primai'y sediments are patches 

 of both contemporary and ancient shell debris 



(Merrill, Emery, and Rubin, 1965; Emery, Mer- 

 rill, and Trumbull, 1965). 



The subsurface sediments of the Shelf and 

 Coastal Plain consist of layer after layer of much 

 the same type of deposit that occurs today on their 

 surface (with additions of other types such as 

 peat and limestone). These sediments have been 

 accumulating at least since the Cretaceous Period 

 and now form a thick prism which ranges from 

 a few feet at the landward border of the Coastal 

 Plain to over 15,000 feet (4.6 km.) thick at the 

 edge of the shelf. An even greater thickness has 

 accumulated at the foot of the Continental Slope, 

 and some 25,000 feet (7.6 km.) of sediments now 

 lie under the Continental Rise (for a discussion of 

 this deeper structure see Dietz, 1952 ; Drake et al., 

 1959; Heezen et al, 1959; Murray, 1961; Emery, 

 1966b ; Krause, 1966 ; Hoskins, 1967 ; and Uchupi 

 and Emery, 1967). 



Geomorphic Processes 



The nearshore breaking of waves is the most 

 important cause of erosion on the landward edge 

 of the shelf, but is apparently effective only above 

 about 5 to 10 fm. (Dietz, 1963b; see also the dis- 

 cussion by Moore and Curray, 1964 and the answer 

 Ijy Dietz, 1964). Some controversy exists, however, 

 concerning the ability of contemporary processes 

 to alter significantly the relict terrains and sedi- 

 ments seaward of the surf-zone. Several authors 

 have thought that present waves and currents can 

 scour the shelf intensely to great depths (Dana, 

 1890; Gulliver, 1899; Johnson, 1919; Alexander, 

 1934; and Jones, 1941). Other workers have sug- 

 gested that the preseiat shelf surface is drowned 

 and entirely out of adjustment with present condi- 

 tions (Lindenkohl, 1891; Dietz, 1963b, 1964). Still 

 others have believed that a thin surface layer 

 (6-24 inches, or 15-60 cm.) is in adjustment with 

 contemporary sea level (Donahue, Allen, and 

 Heezen, 1966), or that fine sediments are being 

 either moved across the shelf or dejiosited in cer- 

 tain restricted areas (Shaler, 1881; Shepard and 

 Cohee, 1936; Stetson, 1938b; and Emery, 1966b). 

 Uchupi (1968) suggested that some linear sand 

 bodies on the inner shelf may ha^-e been formed 

 by large modern storm waves. It is also possible 

 that with a long-continued stand of the sea at its 

 present level, the shoreline would build out over 

 a large portion of the inner shelf (Curray, 1964; 

 Emery, 1966b) or that existing bottom sediments 



BATHYMETRIC MAPS AND GEOMORPHOLOGY OF MIDDLE ATLAN/TIC CONTINENTAL SHELF 



51 



