Category I provinces.—The ocean overflows its basin onto the edge of the continents. The principal physiographic province of this submerged portion of the continental platform is the continental shelf which is present off all the lands of the world. The continental shelf is a smooth area with very low relief and is nearly everywhere limited to depths less than 250 fathoms. The continental blocks are also flooded by epicontinental seas. Some of these have rough bottoms, as the Gulf of Maine; others are relatively smooth-floored. Marginal plateaus, where present, lie in depths of 500-1200 fathoms, and many are nearly as smooth as the continental shelves.
CONTINENTAL SHELF: The continental shelf is a shallow (averaging less than 100 fathoms), gently sloping (less than 1:1000) surface of low local relief (less than 10 fathoms) which extends from the shore line to the shelf break where the seaward gradient sharply increases to greater than 1:40. Its width ranges from a few miles to more than 200 miles.
Continental shelves border all land areas. Because of their proximity to shore, their shallow depth, and their importance in navigation the continental shelves are now the best-known part of the oceans (Veatch and Smith, 1939; Shepard, 1948).
The transition from the continental shelf to the continental slope is generally abrupt and is known as the shelf break. This feature ranges in depth from 20 to more than 100 fathoms and in form from a sharp edge to a rounded shoulder. The change in the gradient is from less than 1:1000 to greater than 1:40.
EPICONTINENTAL MARGINAL SEAS: The epicontinental marginal seas are those shallow seas (less than 1500 fathoms) which lie on the continental block and can be distinguished from the continental shelves by their greater depth (usually > 100 fathoms) and possibly greater topographic irregularity. Most of them are enclosed by shallow banks (< 50 fathoms) of the continental shelf and by land. The Gulf of Maine, the Gulf of St. Lawrence, and the channels of the Bahamas belong to this class.
MARGINAL PLATEAUS: 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 Blake Plateau is the only well-expressed representative of this morphologic type in the area of the diagram. Well-developed marginal plateaus are also found off the coast of southern Argentina and east of New Zealand.
Category II provinces.—The steep slopes which border the continental block are grouped into category II provinces. Loosely speaking, we are referring to the continental slope but, because of the complications imposed by such features as marginal plateaus and marginal trenches, we distinguish three province types.
CONTINENTAL SLOPE: The continental slope is that relatively steep (3°-6°) portion of the sea floor which lies at the seaward border of the continental shelf. It typically drops from depths of 50-100 fathoms to depths of 750-1750 fathoms. The top of the slope is usually well marked by a relatively sharp shelf break. The base of the slope, although less definite, is generally abrupt. As a basis of classification in those few areas where no abrupt change is noted, we have set the gradient of 1:40 as the lowest gradient of the continental slope. 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. The details of their regional distribution in the North Atlantic are covered in a later section.
MARGINAL ESCARPMENTS: A marginal escarpment is a precipitous escarpment which forms the seaward slope of a marginal plateau. Such escarpments begin in depths of 500-1500 fathoms and are 1000 to 2000 fathoms high. The base of the escarpment is well marked by an abrupt change in slope. Gradients of marginal escarpments exceed 1:10. The Blake Escarpment is a marginal escarpment. Similar features are found in the Gulf of Mexico and off the southeast coast of Argentina.