W. Maurice Ewing

“ BERMUDA RISE: In the center of the North America Basin and surrounding the island of Bermuda is an oval asymmetrical arch about 300 by 600 miles with the long axis oriented northeast. The topography of the Bermuda Rise is relatively gentle as compared to the Mid-Atlantic Ridge but considerably more rugged than the continental rise (Pl. 27) . The rise is usually distinctly marked on the west by a 10- to 100-fathom decrease in depth and a change from the monotonously smooth abyssal plain on the west to gently rolling hills, 20-50 fathoms high and 2-10 miles wide (Pl. 16) . ”

“ An outer ridge is a broad ridge generally more than 100 miles wide that rises from less than 100 fathoms to about 1000 fathoms above the adjacent floor. It lies parallel to the continental margin and may enclose a basin or trench on the landward side. The local relief of an outer ridge is generally a little greater than that of the continental rise but much more subdued than that on the oceanic rises and mid-oceanic ridges. ”

“ On the west the outer ridge forms a continuous barrier to turbidity-current sediments coming from the near-by Bahamas so that the only source of turbidity-current sediments is from the Cape Hatteras region to the north or possibly from the Hudson Canyon via the suspected abyssal gap mentioned above. The type of sediment is entirely consistent with the gradient of the plain which slopes southward from the Hatteras region. ”

“ A marginal plateau is a shelflike feature which lies at greater depths than the continental shelf and is separated from the continental shelf by an incipient continental slope. These features generally lie at depths greater than 100 fathoms and less than 1200 fathoms. They can be distinguished from epicontinental marginal seas by their lack of a seaward barrier or sill. The surface of a marginal plateau is generally quite similar to the continental shelf in slope and in the frequency and magnitude of minor relief features. ”

“ The Hispaniola-Caicos Channel in particular contains an abyssal plain which, although much smaller than the large, deep-sea abyssal plains, has all the characteristics of slope flatness and sediment composition. This is called the Hispaniola-Caicos Abyssal Plain.
The smallest, yet perhaps the most striking, Atlantic abyssal plains are those at the bottom of the Puerto Rico Trench at depths of about 4358 fathoms (4585 corrected) . They exhibit the proper degree of flatness, low gradient (Ewing and Heezen, 1955) , and the shallow-water clastic sediments typical of abyssal plains. ”

“ A marginal basin, where present, lies at the foot of the continental slope or at the base of a marginal escarpment. It is slightly shallower than the general level of the ocean basins and is bounded on the seaward side by an outer ridge. Part of its floor is generally occupied by an abyssal plain.
MARGINAL TRENCH: A marginal trench is a narrow, steep-sided feature running closely parallel to the trend of the continental margin; it is generally at least 1000 fathoms below the general level of the adjacent ocean floor. ”

“ MADEIRA AND CAPE VERDE ABYSSAL PLAINS: From Gibraltar to Cape Verde and extending southward to the Sierra Leone Rise at 10° N. a vast abyssal plain parallels the coast of Africa. For most of its length the western boundary of this plain is 750 miles from the coast. The average width of the plain is 200 miles, and it occasionally reaches 300 miles. An eastward extension of the abyssal hills approaches within 200 miles of the Canary Islands, forming a natural constriction which divides the plain into a northern part, the Madeira Abyssal Plain, and a southern part, the Cape Verde Abyssal Plain. ”

“ As far as we know, individual abyssal hills are not discernibly different from the smaller hills of the steps of the Mid-Atlantic Ridge or much of the topography of the oceanic rises, and thus there is no reason to assume that the abyssal hills have a different origin. ”

“ The fact that the same levels are found along coasts of diverse geology and tectonic development supports the eustatic origin of terraces between sea level and 70-100 fathoms. The gradients of the continental shelf are so low and the benches are so persistent that block faulting and slump scars are excluded as general explanations. The benches of the continental slope extend to depths of 1500 fathoms and vary in depth from point to point along the continental slope. These cannot be Pleistocene eustatic levels unless we consider that they were formed prior to recent large crustal deformations. ”

“ The lack of sub-bottom reflections over the parts of the abyssal plains close to the continents is attributed to the numerous sand and silt layers found in the cores which reflect most of the sound. The occurrence of good reflections beneath the outer edges of the abyssal plains could be explained by either assuming that for a long geologic time no sand-or silt-carrying turbidity current has reached this area, or that red clay is deposited here much faster than elsewhere. ”

“ Mid-ocean canyons.—Whereas most submarine canyons are furrows cut into the continental-margin provinces, a class of canyon is found in the abyssal plains, generally associated with abyssal gaps.
A mid-ocean canyon is a steep-walled, flat-floored persistent linear depression, 1-5 miles wide and a few to more than 100 fathoms deep, which occurs in an abyssal plain.
Each mid-ocean canyon discovered in the North Atlantic leads to an abyssal gap in a manner which suggests a genetic relationship. All recognized mid-ocean canyons parallel the adjacent continental margin. ”

“ The setting up of a lower limit for the gradient marks a departure from the older usage in which the continental slope was defined as "the slopes leading from the outer edge of the continental shelves down to the great depths of the ocean" (Shepard, 1948) . This older definition included the continental rise, marginal plateaus, and marginal escarpments. The continental slopes are a world-wide phenomenon. ”

“ Early charts of the western Atlantic showed an extensive bank of 20,000 square miles rising from the center of Newfoundland Abyssal Plain. At about the center of this area an exceptionally high peak has been discovered by cable ships, and the old name Milne Seamount has been assigned to this peak. It rises more than 2500 fathoms above the abyssal floor. Two seamounts of similar size have been found north and south of the Milne Seamount by workers on the R. V. Atlantis (Fig. 30) . ”

“ Only a few areas of the world are sufficiently well sounded to provide data for a study of structural benches. One cannot expect to see identical structural benches in each profile even across a slope composed of a laterally uniform series of horizontal beds of contrasting lithologies. The exact mode of erosion, the local system of jointing, and chance variations in a number of other variables make it necessary to have a large number of closely spaced, accurately located profiles. ”

“ The lens of sediments that has filled the marginal trench off northeastern United States is truly geosynclinal in thickness (Drake et al., in press) . The sediments of the shelf lens are similar in lithology to the orthoquartzite suite of a [Pg 54] mio-geosyncline (cf. Kay, 1951) , and the thicker continental-rise lens is probably similar in lithology to the graywacke-volcanic suite of a eugeosyncline. (Kay, 1951) . It seems virtually certain that ancient orthogeosynclines were, before deformation, closely analogous to the continental margins. ”

“ An abyssal gap is a constricted passage connecting two abyssal plains which, in the vicinity of the gap, lie at different levels. The sea floor slopes down continuously through the gap from the higher to the lower abyssal plain, at a gradient considerably greater than that of either adjacent abyssal plain. ”