Postglacial Coral Reefs on the Virgin Bank

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Postglacial Coral Reefs on the Virgin Bank

Playa Lagun, Curacao, Caribbean Photographic Print
Westmorland,...
16 in. x 12 in.
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Framed Mounted

The bay-head deltas and the cove-filling beaches of the Virgin Islands appear to be of Postglacial date, as are also the discontinuous coral reefs which now rise from certain parts of the bank towards or to sea level. One of the largest sea-level reefs is the Horseshoe, which extends several miles southward from Anegada, two or three miles back from the eastern border of the bank; but whether this is a true coral reef or simply a trail of detritus swept away from Anegada has not been determined.

Two other reefs are of greater length; one is a submarine reef, which is charted with a width of an eighth of a mile and a length of 40 miles, near the southeastern border of the bank, thus resembling a true barrier barrier reef in its position, although failing to rise, as such a reef should, to the ocean surface. It is shown on H. O. charts 1002 and 3903 by a pair of continuous and parallel, 20-fathom contour lines, enclosing soundings of from 14 to 20 fathoms; but its continuity seems open to question.

The other long reef is partly emerged in a string of low calcareous islets stretching 15 miles eastward from the northeastern cape of Porto Rico toward the subdued volcanic island of Culebra, the physiographic dependence of St. Thomas briefly described above; this reef stands 10 or 15 miles back from the northern border of the bank. Discontinuous fringing or bank reefs are charted at several points, but they are of small dimensions. Vaughan has shown that the Virgin bank as a whole is broadly benched at slightly different levels, apparently the result of abrasion by the changing ocean of Postglacial time.

The Unavoidably Hypothetical Nature of this Account

The scheme here adopted, under which the Virgin Islands are given a certain place in association with their neighbors in a systematic sequence, is very hypothetical. There can be no question of that. But this hypothetical scheme seems more reasonable in its postulates, more coherent in its processes, more comprehensive in its reach, and more competent in bringing order out of confusion than any other scheme of Lesser Antillean evolution that has been announced.

As with all hypothetical schemes which seek to recover the unobservable past--such, for example, as those that explain fossil-bearing strata in lofty mountains as former sea-floor deposits, uplifted and eroded; or that explain the observed repetition of similar series of strata by faulting and erosion--it is irrelevant to object to them because they are hypothetical; for they are unavoidably hypothetical.

It is more relevant to consider how far they are consistent with other hypothetical schemes which are regarded as established and how far they are therefore able to give acceptable explanation for the observed facts with which they deal. When the scheme here set forth is thus examined, the Virgin Islands with their great bank may be regarded as a well-advanced member of the sequence in which the other islands of the Lesser Antilles, already described and yet to be described, take their place.