vaughan: virgin and leeward islands 59 



a coral reef. These are the two principal terrace flats. The 

 scarp separating them is indicated by crowded contours, and 

 chart No. 1832, U. S. Hydrographic Office, shows its continuity 

 for 36 nautical miles or about l£ land miles farther than from 

 Washington to BaRimore. A third, still higher terrace flat is 

 suggested between depths of 6 and 10 fathoms, above which a 

 fourth terrace may now be in process of formation, but the in- 

 formation regarding these is at present not definite enough to 

 warrant a positive statement. The continuity of the upper one 

 of the two well-marked flats needs to be emphasized. It should 

 be noted that east of Virgin Gorda there has been an uptilt. 



On the windward (northern) side of St. Thomas, there is an 

 extensive outer flat, bounded on its landward side by a steep 

 escarpment which in places is nearly 160 feet high (see fig. 3). 

 The landward margin of the plain is between 26 and 28 fathoms 

 in depth; the seaward margin has a depth between 30 and 34 

 fathoms; the width is as great as 10 miles and for distances as 

 great as 8| miles, in depths between 29 and 31 fathoms, the 

 range in relief of the surface may be as small as 2 fathoms. Its 

 outer margin is cut by reentrants which have bottoms about 40 

 fathoms deep and simulate hanging valleys. There are also 

 near the outer margin of this flat, banks or ridges the upper 

 surfaces of which are relatively flat, between 17 and 20 fathoms 

 in depth. One of these banks has a total basal width of about 4 

 miles and a length of more than 5 miles. As its form is not 

 that of a coral reef, it can only be the base of what was an island, 

 which had been reduced almost to a smooth surface by marine 

 planation and then, as indicated by other evidence, submerged. 

 As all the other shoals with one exception are truncated at nearly 

 the same level it seems that most of them should be ascribed 

 to a similar origin. These shoals usually show escarpments 

 between 20 and 30 fathoms on their windward sides and more 

 gradual slopes on the leeward sides. The outer flat on the north 

 side of St. Thomas corresponds to the lower flat on the south 

 side of St. John, Tortola, and Virgin Gorda. Both are sub- 

 marine plains, which several lines of evidence show were de- 

 veloped when sea-level was about 20 fathoms or slightly more, 



