The Bays and Cliffs of Antigua

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The Bays and Cliffs of Antigua

Antigua, Caribbean Photographic Print
Nesbitt,...
16 in. x 12 in.
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Framed Mounted

The work of abrasion, seen in the immature cliffs by which the headlands of Antigua are a little cut back, is very small as compared with the great work of erosion by which the island has been so well degraded. This is conspicuously the case at several parts of the coast; for example, the well degraded or subdued tuff ridges of cuesta-like form advancing eastward on the south of the broad and shallow embayment, Willoughby Bay, that enters the southeastern end of the medial subsequent lowland, are so little cut back that their headland cliffs are inconspicuous features. Similarly the subdued agglomerate hills advancing westward on the south of the broad and shallow embayment that serves the chief town, St. John, as a harbor at the other end of the medial lowland, are little cliffed.

The same is true of the headlands farther south. The higher hills of agglomerates enclosing Falmouth and English harbors on the mid-south coast are but moderately cliffed, in spite of the fact that the submarine bank there has, according to the chart, a width of only about a mile before deep water begins; and farther west on the south coast, where the highest summits of the island are maintained on lavas and agglomerates, the slopes descend to sea level, where they are fronted by a narrow alluvial flat without any sign of wave work at their present base. Whatever cliffs were cut here by low-level abrasion are now wholly submerged. It is therefore clear that the small work of abrasion on the headlands demands that Antigua shall have been protected from the attack of the ocean waves during most of the long period of second-cycle erosion; but, on the other hand, the immature cliffs of the headlands demand that that protection shall have been lately withdrawn for a brief period.

If the island were everywhere surrounded by as wide a bank as that on the north, that would afford it some protection from wave attack; but, as a matter of fact, it has only a very narrow bank along the middle of its southern side; and yet there as elsewhere the headland cliffs are almost insignificant as measures of abrasion, compared with the intermediate embayed valleys as measures of erosion. It cannot possibly be because of the narrow bank along this part of the coast that the cliffs there are so little developed; some more effective coast protection must have long been present there; and by far the most effective protection for such a coast is a neighboring barrier reef, as has already been inferred in the case of St. Lucia. But, if such a reef existed there, it probably encircled the whole of the second-cycle Antigua-Barbuda bank, except for certain lapses of development on the leeward border, such as are usually seen in present-day barrier and atoll reefs in the Pacific, presumably because the drift of fine sediments from the lagoon floor is unfavorable to coral growth there.

Not only is a barrier reef thus seen to have afforded the most effective protection against abrasion, but that protection is easily removed, during the brief period of abrasion that is attested by the headland cliffs, by the chilling of the ocean waters and the resultant killing of the reef-building corals in the Glacial epochs. That the abrasion took place during a time of lowered ocean level is also suggested by the plunge of the south-coast cliffs below present sea surface. Certain cliffs, however, in the weak limestones on the northwest coast of the island give less evidence of plunging; hence they have probably been more cut back from their original position by abrasion at present sea level than have the cliffs in volcanic rocks on the south.

The protecting barrier reef here inferred is a wholly second-cycle product, a reef of second generation, begun during the late Tertiary subsidence of the island and cut away by low-level abrasion in the Glacial epochs; it is therefore not to be confused with the inferred first-cycle barrier and atoll reef, which is believed to have grown up during the Oligocene or first-cycle subsidence of the original island and to have been destroyed by second-cycle erosion after the atoll had been tilted up, probably in Miocene time. If the late-Tertiary barrier reef encircled almost the entire bank area, as just suggested, its northern loop must have been built up as the northern part of the Oligocene atoll was tilted down; and that northern part of the late Tertiary reef may therefore have grown up from the top of the Oligocene reef: there the first-cycle reef would have merged into the second-cycle reef.

The imagined northlooping reef thus constituted would have resembled, although constructed on a larger scale, the actual reef loop around the Fiji island of Lakemba. That reef today runs close around an island at one end of its oval circuit and loops around a lagoon at the other end; and, according to Foye, the unsymmetrical form of the Lakemba reef with respect to its island is the result of down-tilting and upgrowth at the looped end, just as is here inferred to be the case with the imagined late Tertiary reef loop of Antigua.