Postglacial Features of Antigua and Its Bank

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Postglacial Features of Antigua and Its Bank

The delta plains by which many of the Antigua embayments have been much shortened, the beaches swinging in concave curves between the headlands-and composed of calcareous sands even where the headlands are made of volcanic rocks--and the discontinuous fringing and bank reefs charted around the island, all seem to have been largely formed in Postglacial time during and since the rise of the ocean to its present level. This must be wholly true for the bank reefs, and must be also true in the main for the beaches and deltas; part of their volume may be of Preglacial or Interglacial formation, but Postglacial additions sufficient to make good the effects of low-level erosion in the Glacial epochs must have been of a considerable volume.

The limestones of Barbuda are, according to an unpublished report by Earle, of Pliocene or Pleistocene age and probably correspond to certain horizontal limestones of small thickness and late date which unconformably overlie the worn-down Oligocene limestones near the northeastern shore of Antigua. Both these late-date limestones indicate that the subsidence which accompanied or followed the advanced erosion of the tilted atoll has been recently reversed by a slight uplift; and since this recent uplift erosion has not only continued the work previously well advanced on the uptilted part of the atoll but has also attacked the horizontal limestones deposited at the time of the greatest subsidence. How far this attack was aided by low-level erosion and abrasion is not determined.

If the headland cliffs of Antigua are the work of low-level abrasion in the Glacial epochs, as seems highly probable, then it is also probable that a large part of the present bank was more or less planed down in its shallower areas at that time and that a detrital embankment was added around it with a marginal depth about 40 fathoms below the level of the lowered ocean. But if so, the bank has since then, like the Saba bank, been significantly aggraded; for much of its surface has such depths as 15 or 20 fathoms, while its margin is only about 40 fathoms below present ocean level. This bank, with an island at each end of it, would be an excellent place for studying the nature of the deposits by which a rimless bank is built up.

Antigua was the most instructive island I visited in the Lesser Antilles: it not only exhibits in its physiographic features an interesting stage in a second-cycle sequence of island development but presents in its geological structure a most encouraging confirmation of the scheme of a first-cycle sequence. Indeed, it reveals the deep under-structure of coral-reef lagoon deposits better than any of the 35 reef-encircled islands seen by me in the Pacific in 1914, for most of those islands had not been elevated, and those which had been elevated gave no such exhibition of their under-structure as Antigua affords.

Its well displayed series of volcanic and calcareous strata is far more demonstrative of the conditions under which atolls are formed than is the famous Royal Society boring in the reef of Funafuti, an atoll in the Ellice group of the Pacific; for while that boring, which was but little more than 1000 feet deep and did not reach a volcanic foundation, yielded only a slender rock core whose interpretation has given rise rather to dispute than to agreement, the beveled section of the Antigua monocline is broadly open to deliberate observation in many outcrops, road cuts, and quarries in its surface of scores of square miles, as well as in many headland cliffs around its shores; and the evidence that its tuffs and limestones give for long continued and great subsidence during this deposition is not to be questioned.

To be sure, an island that is supposed to be a tilted and degraded atoll, the encircling reef of which is lost, may be regarded as an uncertain witness as to the origin of atolls by those who have not seen it; but even they may still find some value in the evidence given by the island as to the origin of a former pelagic bank; for, while such a bank may in some cases be built up to small depth by the accumulation of organic detritus on a deep, non-subsiding foundation, as Rein and Murray supposed, it is clear that this bank had no such origin.

Its upper 1500 feet of strata show conclusively, by their fossils as well as by their structure, that they were built up as persistently shallow-water deposits on a bank or lagoon floor by the slow accumulation of organic detritus on a slowly subsiding foundation; and the presence of a reef around the bank while it was building is made so highly probable by various converging lines of evidence that I am persuaded the bank will, in time, come to be accepted as representing the deposits of a former barrier-reef lagoon and later of an atoll lagoon, and indeed as the most instructive section of an atoll thus far known in the oceans.