The Bahama Banks appear to consist of a slab superimposed on the same surface which forms the Blake Plateau. The Blake Escarpment merges with the lower part of the eastern slope of the Bahamas. The slopes of the Bahamas are precipitous; gradients are of the order of 1:4 to 1:8. Vertical cliffs, which lie just below the 50-fathom curve, have been reported by lead soundings (Armstrong, 1953). The Tongue of the Ocean and the Northeast and Northwest Providence channels form a network of submarine canyons (Hess, 1933). The floor of this canyon system has a continuous down-slope gradient to the floor of the Blake-Bahama Basin. Sediment cores from the floor of the Blake-Bahama Basin (2525 fathoms) (Ericson, Ewing, and Heezen, 1952) contained thick (1-3 m) beds of graded calcareous sand. The steep slopes of the Bahamas are generally rocky, and cores here reveal a variety of Tertiary and Cretaceous sediments. Exuma Sound also is linked by submarine canyons to the Blake-Bahama Basin. The graded calcareous sands of the Blake-Bahama Basin were probably carried through this submarine canyon system by turbidity currents. The topographic benches of Exuma Sound have been described by Lee (1951).

The southeastern Bahamas from Great Inagua to Navidad Bank consist of more numerous isolated banks and greater expanses of ocean floor in the depth range of 1700-2400 fathoms. The basins behind the southern Bahamas lie below the sill depth between the line of banks. Thus an abyssal plain lies entrapped in the Hispaniola-Caicos Channel and the southeastern portion of the Old Bahama Channel. Profile W-29 (Pl. 24) shows much irregular relief between 1000 and 1500 fathoms.

Puerto Rico Trench Sector.—With the disappearance of the Bahama Banks at the eastern edge of Navidad Bank, the continental margin assumes its third mode of expression: the marginal trench-outer ridge complex. The outer ridge, which nearly disappeared in the southeastern Bahama sector, again becomes a prominent feature. The last traces of the marginal plateau merge with the continental slope of Hispaniola and Puerto Rico. The Puerto Rico Trench develops rapidly east of Navidad Bank; it lies between the outer ridge and the continental slope or landward trench slope of the Greater Antilles. The relief of the outer ridge is somewhat greater than that observed on the outer ridge farther northwest near the Blake Plateau. The floor of the Puerto Rico Trench is divided into two parts by a longitudinal ridge. The deepest parts of both are floored by nearly level trench plains. The deeper one on the outer or northern side maintains a nearly constant depth at 4358 fathoms (4585 fathoms corrected) for 150 miles (Ewing and Heezen, 1955). The southern or inner trench is more variable in depth, and the trench plain lies intermittently along its length. Its depth ranges from 3600 to 4300 fathoms (uncorrected). The walls of the Puerto Rico Trench are formed by a series of extremely steep segments which show a remarkable persistence along the trench. Profiles W-32 and W-33 illustrate the typical trench profile north of Puerto Rico. Breaks in slope are observed at 700 fathoms and at 1500 fathoms; at 2000 fathoms the gradient steepens to > 1:6. In this region all soundings are side echoes. The outer or seaward wall of the trench is also characterized by a succession of laterally persistent slope segments. A bench at 3800 fathoms at the top of a scarp which drops to the bottom of the trench is characteristic of several profiles.

Anegada Passage.—The Virgin Islands Bank extends 30 miles east of Puerto Rico along the south side of the Puerto Rico Trench. Between the Virgin Islands Bank and St. Croix a deep passage cuts through from the Atlantic to the Caribbean. This deep passage is 130 miles long and runs along an e-ne-w-sw line. Its walls are extremely steep (9°-43°) (Frassetto and Northrop, 1957). The structure of this feature has been studied by Shurbet and Worzel (1957) and by J. Ewing et al. (1957).

Heezen et al., PL. 7

[TRACINGS OF PDR RECORDS ACROSS CONTINENTAL SLOPE AND PART OF THE BLAKE PLATEAU]

SOUTHWESTERN EUROPE AND NORTHWEST AFRICA: The continental margin of Europe and Africa is illustrated in Plate 25 by only 23 profiles, and therefore our description of this area cannot be as detailed as that for North America. For purposes of description we have broken the area into four sectors of contrasting type: (1) the Anglo-French sector: (2) the Iberian sector; (3) the Gibraltar sector; and (4) the North African sector. In all but the three profiles in the Gibraltar sector there is a well-defined continental shelf. The continental slope is everywhere present but ranges widely in height and gradient. The continental rise is extremely well developed off Africa but virtually absent in the Bay of Biscay. Abyssal plains are shown on almost every profile, but their depth ranges from 2550 to 3075 fathoms, and their width from 50 to 250 miles.

Anglo French Sector.—Profiles E-1, E-2, and E-3 are representative of the Anglo-French sector which extends from 45° N. to 60° N. Only the southern part of this sector is shown on the physiographic diagram. The continental slope is broken by a prominent bench or marginal plateau at 1000-1200 fathoms. The northwest corner of the physiographic diagram south to 42° N. is included in Hill's (1956) contour chart. According to this chart the prominent 1200-fathom bench extends for more than 900 miles along the continental margin from 45° to 60° North Latitude.

The continental slope from the shelf break to the prominent bench exhibits smaller benches and changes in slope, many of which probably will be correlatable when more profiles are obtained in this region. The general gradient of this portion of the slope ranges from 1:10 to 1:30. On the bench individual slope segments range from 1:40 to 1:80. Below the bench the sea floor drops from 1500 to 2100 fathoms at gradients of 1:15 to 1:30.