Grenada, The Grenadines and Their Long Bank

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Grenada, The Grenadines and Their Long Bank

Grand Anse Beach, Grenada, Caribbean Islands Photographic Print
16 in. x 12 in.
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Framed Mounted

Grenada and the Grenadines, far southern members of the chain, surmount the second largest bank of the Lesser Antilles; it measures nearly 100 miles in length, north-northeast and south-southwest, and from 10 to 17 miles in breadth. Grenada is a high and relatively young volcanic island and will therefore be described after the smaller and apparently older Grenadines, which are well subdued volcanic residuals scattered over the northern 60 miles of the bank. I saw them only from a passing steamer. The larger ones are 15 in number, varying from one to eight miles in diameter and rising from moderate measures up to 1000 feet in height; many islets and rocks are also charted.

The members of this group that were seen by me repeated on a somewhat larger scale the features of the Saints, south of Guadeloupe, already described as the second member of a simple sequence. Carriacou, one of the more northern islands, is briefly described by Clark, who visited the group in 1903 to study its birds, as "in the main composed of beds of fine-grained volcanic sands and tuffs. On the eastern slopes of the island, and at Belair (in the center), at an altitude of 600 feet, the tuffs of which the hills are composed are covered with layers of a shallow-water foraminiferal limestone, from ten to twenty inches in thickness.

It therefore appears to consist of layers of volcanic ash which were deposited in the sea, and afterwards covered with the shallow-water deposit. Later this was subjected to upheaval, with the result that part of the limestone was raised to at least 600 feet above sea level. The rest of the Grenadines are geologically much like Carriacou, but appear to lack the limestone capping of that island."

None of the islands now have the forms of young volcanic cones; all appear to be residual forms of subdued outlines, except for occasional sharp, necklike peaks, and abrupt, spur-end sea cliffs. The larger members have irregular shore lines, with open valley-head embayments separated by cliffed headlands. Soundings on a large-scale chart suggest that the cliffs plunge below sea level. the abrupt change not infrequently seen from a steep cliff at the end of a headland to a non-cliffed shore along the side of the headland, notwithstanding the open exposure of the side to present wave attack, suggests that the cliff was cut on a shore line of different pattern from that of today, and thus confirms the inference from the chart that the cliffs plunge below sea level.

At the same time, the dimensions of the embayments are such that the period required for the erosion of the valley heads they occupy must have been long compared to the period required for the abrasion of the cliffs. Hence these small islands support the interpretation demanded by the larger islands above described, to the effect that, while erosion has been a long-lasting process, abrasion has been of short duration and, therefore, that during much of the long erosional period the Grenadines, like the larger island of St. Lucia, must have been protected, presumably by an encircling coral reef, from wave attack.

Union Island stands near the middle of the group; Cannouan Island, not far away, may be taken as typical of the others. In the smaller islands embayments are wanting, as they are in the small knobs of southeastern St. Kitts, and for the sufficient reason that such islands are mere hill-top residuals, too small for the retention of embayable valley heads. Some of the smallest islands are, indeed, almost or quite reduced to cliff-faced stacks, because a moderate measure of cliff recession, which consumes only a small fraction of a larger island, consumes nearly all or all the upland surface of a very small island.

The amount of submergence that has taken place here is not easily determined. If each of the islands represents an independent center of eruption, the post-erosional submergence may have been of moderate amount; but if the present discontinuity of the islands means that an originally continuous ancestral island, some 30 miles in length, has been maturely eroded and then separated by submergence into the present number of its small descendents, the measure of submergence must have been great.

THE YOUNGER ISLAND OF GRENADA

Grenada (pronounced Grenayda by its inhabitants) 17 by 8 miles across and 2749 feet high, stands on the northwestern side of the bank at a quarter of the bank length from its Southwestern end and falls off on that side into deep water. The bank is from three to six miles wide on the other side of the island. The axial range of the island, trending with the bank, is elaborately and, for the most part, maturely dissected and is therefore of Preglacial origin. Its southern end is extended westward by a trailing series of subdued volcanic hills, several miles in length.

The range is of unsymmetrical cross section: the coastal slopes are of moderate declivity on the southeast and of more rapid descent on the northwest; moreover, the gentler southeastern slopes are strongly embayed and moderately cliffed, the more rapid slopes on the northwest are less embayed and much more cliffed. As on the other islands, the embayed valleys are here ascribed to prolonged erosion with respect to normal ocean level and not simply to relatively brief low-level erosion; and their embayment is ascribed to regional subsidence and not only to Postglacial ocean rise, because here as elsewhere the embayed valleys are too widely opened to be explained as the work of lowlevel erosion during the Glacial epochs. None of the many bays that I saw during an automobile excursion around the island appeared to be volcanic craters, although some of them are popularly so explained.

The unsymmetrical form of Grenada seems to be due to its unsymmetrical position on the bank, which, as above noted, is believed to have gained very much of its present extent before this island was built up on it. The southeast side of the island, where its lavas and mud flows found a bank with shoal water on which to spread, appears to have gained moderate slopes like those of Nevis, though perhaps not so very gentle as the basal part of that island. The valleys eroded in these slopes must have had a gentle fall near the shore; and hence, when submergence took place, long embayments were produced there; the cliffs here, still later cut back, are not high because the spurs in which they are cut are relatively low and of gradual declivity.

On the northwest side of the island the lavas and mud flows may have perhaps at first been spread out with moderate slopes on the bank; but, as the island grew in height and area, they must have run over the bank border into deep water, and thereafter their slopes must have been steep, like those of the Soufrières of Montserrat. The valleys here eroded have steep fall; the bays are therefore relatively short and are now largely filled with deltas; the inter-bay spurs have high cliffs because the spurs have steep-pitching crests. The cliffs appear to plunge below sea level but only to a moderate depth; the cliff-base platform is probably buried by a considerable amount of detritus from the high and steep cliff faces.

Most of these cliffed spur ends descend to a fairly even shore line; but one of the spurs, known as Bois Morice, about three miles north of the trail of hills at the southwest end of the island, is cliffed but moderately and stands forth from the rest of the shore line in a long-sloping salient. This appears to result from the former presence here of a peninsular extension of the spur--perhaps formed by a lava flow of late date or of unusually large volume--farther seaward than the rest of the western slope, so that the low-level abrasion of the peninsula occupied so much time that little time was left for cutting back the main body of the spur. The extension of a bank nearly two miles beyond the end of the salient gives some confirmation to this suggestion.