In the North Atlantic one may distinguish several types of topography, each of which is characteristic of certain portions of the ocean floor.
The continental slopes are now known to show in many cases a steplike succession of horizontal or imperceptibly sloping shelves or terraces. They are dissected by systems of submarine canyons, the pattern of which is such as to suggest to many a subaerial origin. A recent investigation of the Hudson submarine valley by the Atlantis has shown that this valley extends at least as far as the 2500 fathom curve, 300 miles out to sea, and shows features suggesting a subaerial origin.
The broad basins are characterized by smooth floors or plains covering areas of more than 200,000 square miles out of which there rise large sea mounts, isolated or in groups. Thus plains at a depth of 2900 fathoms occupy the floors of the North American and the North Canary basins.
The floor of the northwestern part of the North American basin, north of Bermuda, is characterized by a plain at 2650 fathoms. Sea mounts of various sizes and shapes rise out of these otherwise smooth plains. There is a conspicuous group of flat-topped sea mounts rising from the 2650-fathom plain of the western North American basin. They rise approximately to 800 fathoms and string out toward the southeast, roughly from the direction of Cape Cod. They show evidence of terracing on their flanks.
The central part of the Atlantic Ocean is characterized by a long submarine mountain range, extending from Iceland to a point southwest of the Cape of Good Hope, known as the Mid-Atlantic Ridge. Numerous fathograms across the Ridge have shown that it is characterized by a high central zone, or Main Range, consisting of parallel ridges following the general trend of the Ridge and rising in many places to less than 800 fathoms below the surface of the ocean.
Between 1600 and 2500 fathoms the flanks of the Ridge are characterized by a succession of flats which for the lack of any better term have been called terraces. Reflection-shooting studies by Ewing and Press and others have shown that these terraces are or have been areas of greater deposition.
Finally, between this Terraced Zone and the 2900-fathom plain, one may usually recognize a mountainous area standing out as a distinct physiographic province.