682 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



100 islands, cays, and rocks which are known as the Virgin Islands. The 

 bank is terminated on the south by the Anegada trough, named from the 

 passage, previously mentioned. Taber (1922) points out that the south 

 side of the trough near the island of St. Croix is a great escarpment which 

 descends 14,130 feet in less than 5 miles, and thus has an average slope of 

 30 degrees. This he regards as a fault scarp due to vertical movement. As 

 will be related later, Hess believes it is due chiefly to horizontal move- 

 ment. On the basis of biogeographic data, Schuchert thinks the Virgin 

 Islands were joined to St. Croix across the Anegada trough during either 

 the Miocene or the Pliocene, and that they have been separated due to 

 block faulting along the Anegada Passage in fairly recent times. 



It appears that Puerto Rico, the Virgin Islands, and the island of 

 St. Croix developed as a unit, and that the geology of the Virgin Is- 

 lands, if only partially exposed, fits that of Puerto Rico. See chart, Fig. 

 42.3. 



After the igneous activity and deformation of the older series of Puerto 

 Rico that is also believed to form the foundation of the Virgin Islands bank, 

 a mountainous upland probably existed. According to Meyerhoff (1927), 

 fluvial erosion reduced the mountainous upland to an imperfect peneplain 

 in early Eocene time. The relatively level summits of the upland of Saint 

 John, 1000 feet above sea level, are a remnant of the old surface. Uplift 

 in late Eocene time resulted in dissection of the old surface, and all but 

 the central cores of the present larger islands were reduced to a late 

 mature of old surface about 800 feet below. Only a few remnants of this 

 second surface have been preserved, because a second uplift in early 

 Oligocene time was followed by about 500 feet of downcutting. 



The third cycle of erosion formed the lower peneplain of Puerto Rico, 

 as well as the mature to old surface which extends beneath the coastal 

 plain on St. Croix and Vieques, and which underlies remnants of the 

 coastal plain on the submarine platform. Formation of the lower peneplain 

 was followed by subsidence and deposition of coastal plain sediments in 

 the middle Tertiary, and during late Tertiary time uplift exposed the 

 coastal plain marls and limestones to dissection. The Tertiary deposits 

 collected in the entire area now constituting the submarine platforms of 

 the islands. Toward the close of the Tertiary, differential movement or 



warping caused submergence of the eastern Puerto Rico and Virgin 

 Islands region, while western Puerto Rico remained elevated. 



Bahama Islands 



Physiography. The Bahama Islands stretch for 900 miles in a north- 

 west-southeast direction in front of southern Florida, Cuba, and Haiti, and 

 include some 29 inhabited islands, 661 keys, and 2387 rocks. The Bahamas 

 are all very low, flat islands and resemble most the coast and keys of 

 southern Florida. All the islands, keys, and rocks rise from a platform thai 

 is roughly triangular, with the narrow base of the triangle on the north- 

 west. See map of Fig. 42.1. It is bounded on the west by the Florida Chan- 

 nel, which separates it from Florida by a distance of 50 miles; on the south 

 by the Bahama Channel, which separates it from Cuba by an equal dis- 

 tance; and on the east by the Atlantic Ocean. The greater part of the plat- 

 form is covered by water only 3 or 4 fathoms deep, but in part it emerges 

 slighty above sea level, forming low islands. Great submarine valleys, 

 such as the Tongue of the Ocean, Exuma Sound, and the Providence 

 Channels, form deep indentations in the platform. On the east, the plat- 

 form drops off abruptly to oceanic depths (2600 fathoms, 15,600 feet). 

 The extensive shallow banks are remarkable for their white lime oozes. 

 See Fig. 43.4. 



Submarine Canyons. The great submarine valleys, which are mani- 

 festly a very important character of the Bahama platform, are reviewed as 

 follows by Hess (1933): 



(1) The longitudinal valleys have a general NW-SE trend for the greater 

 part of their lengths, but short steep cross valleys at right angles to this trend 

 connect the longitudinal valleys with the ocean. 



(2) So far as the information goes, it appears that the valleys slope continu- 

 ously from the shallowest parts of their upper reaches (720 fathoms, 4,320 feet 

 below sea level) to the floor of the ocean basin proper (2,500 fathoms, 15,000 

 feet). The longitudinal valleys have gradients of approximately 15 to 20 feet to 

 the mile, and apparently have gendy sloping undulating bottoms, from 720 

 fathoms (4,320 feet) to about 1,000 fathoms (6,000 feet). 



(3) The cross valleys have steeper gradients, 100 feet to the mile, from 1,000 

 fathoms (6,000 feet) to the floor of the ocean at 2,500 fathoms (15,000 feet). 

 They have the typical V-shaped cross profile of a youthful river valley, and 

 some have a distinct inner gorge near the center. 



