This paper presents primarily the soundings obtained by the Research Vessel ATLANTIS during the summer field season of 1947. It also utilizes some of the data obtained by later expeditions of the ATLANTIS and by several other vessels. The ATLANTIS is equipped with a continuously recording fathometer with a range up to 4000 fathoms.

The outstanding features of the area investigated in 1947 (ATLANTIS cruise #150) are:

  • (1) A conspicuously flat plain at a depth of 2900 fathoms occupies the deeper sections of the North Atlantic basin between Bermuda and the Azores. More recent data indicate that this horizontal stretch of sea floor extends at least as far south as Lat. 29°N. and possibly as far north as Lat. 40°N., between Long. 50°W. and 56°W., and also that at the same depth a similar plain is found east of the Ridge in the Northern Canary basin. They are dotted with small sea mounts of variable height whose exact shapes have not yet been determined. The size and number of these appear to increase toward the south, and the southern extremities of the plains consist of smooth horizontal stretches interrupted by pronounced elevations and depressions.

  • (2) The Mid-Atlantic Ridge is characterized by two strongly contrasting types of topography: (a) The central backbone of the Ridge, or Main Range, which is shoaler than 1600 fathoms, consists of a series of parallel ranges trending NE.–SW. Several of these rise to less than 800 fathoms. Their trend follows roughly that of the Main Range. The portion of the Main Range to which the most attention was devoted is between Lat. 30° and 34°N. In this area its width is of the order of 150 miles, and it is of the order of several hundred miles in the area of the Azores.

  • (b) The flanks, between the 1600- and 2500-fathom isobaths, consist of a succession of smooth shelves, each from 1 to 50 nautical miles or more in width. This zone is 200–300 miles in width.

  • (c) The region between the limit of the 2900-fathom plain and the foot of the first of the series of shelves characterizing the flanks of the Ridge, at 2500 fathoms, in some localities stands out as a distinct physiographic province.

  • (3) The following local features are noteworthy:

  • (a) Close to 31°N.Lat. a deep east-west trench extends from about 41° to 43°W.Long. and cuts deep into the Main Range. Its deepest point is at 2800 fathoms. Crushed and metamorphosed ultra-basics were brought to the surface by dredging its flanks.

  • (b) At 30°15'W., 34°N., a flat-topped sea mount rises to 180 fathoms, about 100 miles southeast of the Main Range. The dredge brought up calcareous discs of probably Cenozoic age off its top. They were about 15 cm in diameter and about 4 cm thick.

  • (c) Another sea mount was found at 33°43'N., 62°30'W., 150 miles NE. of Bermuda, shoaling to 780 fathoms, having a NW.-SE. elongation, an unknown total length, and a width of 4 miles across its flat top.

  • (d) Another flat suboceanic plain occupies the bottom of the basin separating Bermuda from the Northeastern United States, at a depth of 2650 fathoms.

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