58 vaughan: virgin and leeward islands 



The third type of profile (that showing submarine terraces 

 around islands) will now be discussed in some detail. From 

 shore line characters and other evidence the conclusion was 

 reached that the Virgin Islands, the members of the St. Martin 

 Group, and Antigua and Barbuda have recently undergone 

 submergence to an amount of about 20 fathoms. 3 Assuming 

 this conclusion to be correct, should the sea level have remained 

 stationary for a period of appreciable length before this submer- 

 gence, there should be a submerged scarp or facet indicating 

 its former stand; should there have been a succession of tem- 

 porary stands there should be a series of submarine terrace 

 flats separated by scarps. The available sources of informa- 

 tion were the charts of the U. S. Hydrographic Office and of the 

 British Admiralty. The Virgin Bank and the St. Martin Plateau 

 were selected for special study. The charts of the former, on 

 a scale of slightly more than 1 mile to an inch, and that of the 

 latter, on a scale of about 2\ miles to an inch, were contoured 

 on a 2 fathom interval from the shore to a depth of 40 fathoms, 

 and on an interval of 10 fathoms from 40 to 100 fathoms. 



The Virgin Group will be described first. The shore line shows 

 indentations indicative of submergence, and that the sea has 

 stood at its present level long enough for alluvial filling of the 

 heads of harbor digitations, while sea-cliffs occur at the ends of 

 promontories. The chart of the nearby sea-bottom shows 

 that south of St. John, Tortola, and Virgin Gorda there are two 

 distinct submerged terraces and a less definite third terrace 

 (see fig. 3). The outer terrace flat lies at depths between 26 

 and 28 fathoms on its landward and between 28 and 30 fathoms 

 on its seaward margin, and it ranges in width from \ mile to 3 

 miles. On its sea-front is a ridge which is inferred to be a sub- 

 merged barrier coral reef. On its landward side a scarp rises 

 from a depth of 26 to 28 fathoms to about 17 fathoms. Above 

 this scarp is a second terrace flat, which has a depth of 14 to 

 15 fathoms on its landward and a depth of 14 to 20 fathoms on 

 its seaward face, and ranges in width from one-third of a mile 

 to 2 miles. Apparently the outer margin of this flat also bears 



1 Bull. Am. Geog. Soc, 46: 426-429. 1914. 



