LANDWARD SLOPES OF TRENCHES: This term was introduced to set apart the landward slopes of marginal trenches from the usual continental slopes found off trench-less coasts. These steep slopes (> 1:40) drop from depths of a few hundred fathoms near a continent to a depth of a few thousand fathoms in a marginal trench. In the North Atlantic the one example is north of Puerto Rico. A large part of the circumference of the Pacific is bounded by such features.
Category III provinces.—At the base of the continental slope a gentle gradient continues to the local level of the abyssal floor. This seaward gradient ranges from 1:100 to 1:700 and extends over a strip from a few miles to a few hundred miles in width. On many topographic profiles made at right angles to the slope of the continental margin three major breaks are visible: the shelf break, the base of the continental slope, and the point where the near-by level of the ocean-basin floor intersects the more steeply sloping continental margin. Since we have limited the continental slope to gradients greater than 1:40, we split off this lower portion of the continental margin into a separate province, the continental rise. In the older "bathymetric" classification of relief the ocean basin—continental slope boundary was along the 2000- or 2500-fathom contour, an arbitrary division which cut across the then-unrecognized continental rise. At the base of the Blake Escarpment lies an enclosed basin, and north of Puerto Rico the sea floor drops almost directly to the floor of a marginal trench. These seemingly diverse provinces of continental rise, marginal escarpments, enclosed marginal basins, marginal trenches, and outer ridges are placed in category III because of their similarity in position with respect to the continents and ocean floor and because of other similarities shown in the deeper structure of the continental rise.
CONTINENTAL RISE: The continental rise, where present, lies at the base of the continental slope. Gradients on the continental rise generally range from 1:100 to 1:700, while the width ranges from a few score to a few hundred miles. However, gradients as high as 1:50 are observed in segment 3 of the upper continental rise, and gradients as low as 1:2500 are locally present in segment 2 of the lower continental rise (Tables 1 and 2). The seaward limit of the continental rise is generally abrupt, and at this point regional gradients decrease to less than 1:1000. The depth on the continental rise ranges from 750 to 2800 fathoms. Local relief is moderate to low, and, except for infrequent seamounts and fairly frequent submarine canyons, the local relief of the continental rise rarely exceeds 20 fathoms.
The continental rise is well developed over most of the area covered by the physiographic diagram. The major exceptions are north of the Iberian Peninsula where the rise is present, but extremely narrow, and in the southwestern third of the map in the area south of Cape Hatteras, where it is not present. In this latter area the geographical position usually filled by the continental rise is occupied by the low, broad outer ridge and the enclosed marginal basin and marginal trench.
OUTER RIDGE: 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.
MARGINAL BASIN: 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. It is separated from the ocean floor by a low outer ridge which rises 100-500 fathoms above the level of the adjacent ocean floor. The bottom of a trench is generally rugged except near the deepest spots where it is flat.
The combination of marginal basin and outer ridge replaces the continental rise east of the Blake Plateau. North of Puerto Rico this combination is replaced by a marginal trench-outer ridge complex. The reason for this grouping is discussed in a later section.