The Great Bank Around the Virgin Islands

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The Great Bank Around the Virgin Islands

Cinnamon Beach, Virgin Island National Park, St. John Photographic Print
Schwabel, Jim
12 in. x 16 in.
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Framed Mounted

The bank around the Virgin Islands is unlike any other bank thus far described, in that a considerable part of its area may be underlain by lowlands worn down on relatively weak continental rocks, remnants of which are seen in some slender islands north of the passage between St. Thomas and St. John, as well as farther east on some of the larger islands. The subsidence by which these worn-down lowlands were submerged is for two reasons inferred to be of smaller measure than the subsidence which the other members of the Lesser Antillean chain have suffered: first, because the Virgin group, the northernmost member of the chain in which subsidence has long been a dominant movement, lies nearest to Porto Rico, the first member of the Greater Antilles where upheaval appears long to have been equally dominating; second, because on the one hand none of the other members of the chain exhibit any continental rocks although all the members are supposed to have their volcanic cones based on a former land belt of such rocks, while on the other hand the Virgin group exhibits a considerable area of such rocks on which its volcanic rocks are visibly based.

There is, however, little probability that the worndown lowlands, which since their submergence form the foundation for much of the bank, had been worn down before their submergence to nearly so smooth a surface as that which the bank presents today. The submerged part of the lowlands probably had a hilly surface, though of less altitude and relief than those other parts of it still standing above sea level; its hilly surface was probably smoothed by aggradation while it was submerged in a reef-enclosed lagoon; after the lagoon floor had been smoothed in this way, it was probably somewhat planed down by low-level abrasion in the Glacial epochs; and since its abrasion it has presumably been more or less aggraded. This interpretation is manifestly very speculative; but it appears to be in accord with all relevant facts of observation. The bank now has a depth about 40 fathoms along the northern border and of 20 or 30 fathoms along the southern.

This would seem to imply that it has suffered some change of depth, either by tilting or by unequal aggradation, since it was planed off by the lowered Glacial ocean. The aggradation of so large an area as that of this great bank by marine organic detritus without significant aid from inorganic detritus outwashed from an extensive land area may seem at first thought improbable, and, if the organic detritus were wholly derived from an encircling barrier reef, it might indeed be difficult to account for a sufficient supply; but Vaughan's studies have shown that in wide lagoons a vastly greater supply of detritus comes from organisms living in the lagoon waters than from the enclosing reef. When it is thus understood that the chief supply of organic detritus for lagoon aggradation is proportionate to the area of the lagoon, the smooth upbuilding of the vast lagoon floor now represented by the bank does not seem unreasonable.