THE GREAT FAULT TROUGHS OF THE ANTILLES 107 



THE ANEGADA TROUGH 



The Anegada Trough, separating Porto Rico and the Virgin 

 Islands group from St. Croix and the Lesser Antilles is the deepest 

 of the many passages connecting the Caribbean Sea with 

 the Atlantic Ocean. A description of this trough accompanied 

 by a map and transverse profile was given in a previous 

 paper.^ 



It is deepest midway between St. Croix and Vieques where a 

 sounding of 2,501 fathoms (4,574 m.) was obtained; here it extends 

 nearly east and west for 100 km. Between St. Croix and the islands 

 of St. Thomas and St. John the depth is over 2,000 fathoms. 

 Farther east the trough rises until the depth is a little over 1,000 

 fathoms and there it bifurcates. One branch seems to extend 

 northeast and join the Brownson Trough about 60 km. northeast 

 of Anegada Island, where the depth is over 3,000 fathoms; the 

 other extends eastward in the direction of St. Martin but is not 

 well defined. In its deeper parts the trough is about 40 km. in 

 width between the tops of the inclosing scarps, which show abrupt 

 changes in slope at both top and bottom. The floor of the trough 

 is relatively fiat. 



The south side of the trough near St. Croix and westward 

 therefrom is bounded by a tremendous fault scarp (see profile 

 Fig. i). Near Harms Bluff at the northwest corner of St. Croix 

 the scarp descends 4,348 m. in 8 km., an average slope of 30°. 

 For a distance of 4.4 km. the slope averages over 37°, and for shorter 

 distances it is much steeper. Vaughan states that the faulting has 

 taken place so recently that the sea has barely cut a niche into the 

 fault plain." The northern scarp of the trough does not touch the 

 coast of any of the islands; it Kes 15 km. south of St. Thomas and 

 6 or 7 km. from Vieques. It is not so precipitous as the opposing 

 scarp, the average slope being about 12°, though in places it is much 

 steeper. 



I Harry Fielding Reid, and Stephen Taber, "The Virgin Islands Earthquake of 

 1867-1868," Bull. Seis. Soc. Amer., Vol. X (1920), pp. 20-25. 



= T. W. Vaughan, "Some Features of the Virgin Islands of the United States," 

 Asso. Amer. Geog. Ann., Vol. IX (1920), pp. 78-82. 



