Abstract Although the study of Atlantic continental margin physiog- raphy (Plate la) started with lead-line sounding data centuries ago, it was the development and automation of precision echo sounding in the second quarter of the twentieth century that fostered modern investigation and interpretation (Veatch and Smith, 1939; Emery and Uchupi, 1972). Bottom samples ga- thered in the last four decades have strongly influenced the ex- planations for the development of submarine topography (Field and others, 1979; Hollister, 1973; Knebel, 1981; Milliman and others, 1972; Schlee and Pratt, 1970). High-resolution seismic profiling, side-scan sonar, bottom photography, current meters, and submersibles have allowed more detailed examination and interpretation of selected areas. Key recent articles cited in this chapter, from the vast literature about the Atlantic margin, pro- vide references for an up-to-date understanding of the major physiographic features of the shelf, slope, and rise. In addition, controversies and unresolved questions have been identified. For readers less familiar with the location of physiographic features discussed in the text, the map provided should be of considerable assistance. Because the nature of the shelf physiography and the proc- esses responsible for creating the submarine landforms differ markedly along the U.S. Atlantic shelf (Burk and Drake, 1974; Nairn and Stehli, 1974; Emery and Uchupi, 1972; and Uchupi, 1968), six areas are recognized. From north to south they are: 1) Gulf of Maine, 2) Georges Bank, 3) Southern New England,4) Mid-Atlantic from Rhode Island to Cape Hatteras, 5) South-Atlantic from Cape Hatteras to Southern Florida, and 6) the Bahamas. A seventh section introduces the recent literature on microtopography and related sediment transpont.

The epicenter of the 1929 “Grand Banks” earthquake (M s = 7.2) was on the continental slope above the Laurentian Fan. The zone in which cables broke instantaneously due to the earthquake is characterized by surface slumping up to 100 km from the epicenter as shown by sidescan sonographs and seismic reflection profiles. The uppermost continental slope, however, is almost undisturbed and is underlain by till deposited from grounded ice. The Eastern Valley of the Laurentian Fan contains surficial gravels molded into large sediment waves, believed to have formed during the passage of the 1929 turbidity current. Sand sheets and ribbons overlie gravel waves in the lower reaches of Eastern Valley. Cable-break times indicate a maximum flow velocity of 67 km/hr (19 m/s). The occurrence of erosional lineations and gravel on valley walls and low intravalley ridges suggest that the turbidity current was several hundred meters thick. The current deposited at least 175 km 3 of sediment, primarily in a vast lobe on the northern Sohm Abyssal Plain where a bed more than 1 m thick contains material ranging in size from gravel to coarse silt. There is no apparent source for so much coarse sediment on the slumped areas of the muddy continental slope. We therefore infer that there was a large volume of sand and gravel available in the upper fan valley deposits before the earthquake. This coarse sediment was discharged from sub-glacial meltwater streams when the major ice outlet through the Laurentian Channel was grounded on the upper slope during middle Wisconsinan time. This sediment liquefied during the 1929 event, and the resulting flow was augmented by slumping of proglacial silts and gas-charged Holocene mud on the slope. Although earthquakes of this magnitude probably have a recurrence interval of a few hundred years on the eastern Canadian margin, we know of no other deposits of the size of the 1929 turbidite off eastern Canada. For such convulsive events, both a large-magnitude earthquake and a sufficient accumulation of sediment are required.