Fringing reef

“ The fringing reefs immediately surrounding the island, or a portion of it, might be confounded with the barrier reefs we have been describing, if they only differed in their smaller breadth; but the circumstance that they abut immediately on the coast in place of being separated by a channel or lagoon more or less deep and continuous, proves that they are in direct communication with the slope of the submarine soil, and permits of their being distinguished from the barrier reefs. The dangerous breakers which surround the Mauritius are a striking example of the fringing reef. ”

“ Coral formations may be classed as fringing or shore reefs, barrier reefs and atolls. Fringing reefs are platforms of coral rock extending no great distance from the shores of a continent or island. The seaward edge of the platform is usually somewhat higher than the inner part, and is often awash at low water. It is intersected by numerous creeks and channels, especially opposite those places where streams of fresh water flow down from the land, and there is usually a channel deep enough to be navigable by small boats between the edge of the reef and the land. ”

“ No theory worthy of notice has been advanced to account for those barrier-reefs, which encircle islands of moderate dimensions. The great reef which fronts the coast of Australia has been supposed, but without any special facts, to rest on the edge of a submarine precipice, extending parallel to the shore. The origin of the third class or of fringing-reefs presents, I believe, scarcely any difficulty, and is simply consequent on the polypifers not growing up from great depths, and their not flourishing close to gently shelving beaches where the water is often turbid. ”

“ From the description now given, we must conclude that the dimensions and structure of fringing-reefs depend entirely on the greater or less inclination of the submarine slope, conjoined with the fact that reef-building polypifers can exist only at limited depths. It follows from this, that where the sea is very shallow, as in the Persian Gulf and in parts of the East Indian Archipelago, the reefs lose their fringing character, and appear as separate and irregularly scattered patches, often of considerable area. ”

“ In the plan of Sandwich harbour, the headlands are represented as fringed; coloured red.—Aurora and Pentecost Islands, according to Bougainville, apparently have no reefs; nor has the large island of S. Espiritu, nor Bligh Island or Banks’ Islands, which latter lie to the N.E. of the Hebrides. But in none of these cases, have I met with any detailed account of their shores, or seen plans on a large scale; and it will be evident, that a fringing-reef of only thirty or even a few hundred yards in width, is of so little importance to navigation, that it will seldom be noticed, excepting by chance ”

“ This is the natural result of the causes described when dealing with the boat channel of a fringing reef; it is the same thing on a much larger scale. Seeing that the rate of growth of coral masses is always only the excess of growth over destruction and solution, the presence of growing corals is no evidence against the fact that the lagoon shores may be undergoing destruction, and that such coral growth as is present may add nothing to the inner sides of the reef in the end. No more does the accumulation of great quantities of mud prove that the lagoon will in time be quite filled in. ”

“ Pearl-shell is collected on the reefs, but not as yet in sufficient quantities to establish a lucrative industry. In this respect the island is not so fortunate as Alphonse, for there are no sheltered rock-pools in which the shell can be collected by swimming-divers without danger from the sharks; the sea everywhere flowing in over the outer edge of the broad fringing reef, and the great reef to the southward only drying in patches at low tide. ”

“ The interior portion of the reef may be regarded as almost wholly an accumulation of dead skeletons. Where a river comes down from the land there is a break in the reef, for the reasons which have been already mentioned.
The origin and mode of formation of a fringing reef, such as that just described, are plain enough. The embryos of the coral polypes have fixed themselves upon the submerged shore of the island, as far out as they could live, namely, to a depth of twenty or twenty-five fathoms. One generation has succeeded another, building itself up upon the dead skeletons of its predecessor. ”

“ In very many instances, however, atolls and barrier reefs may be explained without subsidence. Thus a barrier reef may be formed by the seaward growth of a fringing reef upon the talus of its sea face. In Figure 162 f is a fringing reef whose outer wall rises from about one hundred and fifty feet, the lower limit of the reef-building species. At the foot of this submarine cliff a talus of fallen blocks t accumulates, and as it reaches the zone of coral growth becomes the foundation on which the reef is steadily extended seaward. ”

“ It cannot be that the corals will not grow close to the land; for the shores within the lagoon-channel, when not surrounded by alluvial soil, are often fringed by living reefs; and we shall presently see that there is a whole class, which I have called Fringing Reefs from their close attachment to the shores both of continents and of islands. Again, on what have the reef-building corals, which cannot live at great depths, based their encircling structures? This is a great apparent difficulty, analogous to that in the case of atolls, which has generally been overlooked. ”

“ Their peculiar shape, a narrow seaward channel and a broad landward expansion, is due to the relation of hard and soft rocks which generally prevail along the coast. Wherever the conditions are favorable for the growth of corals, a fringing reef is built upon whatever rocks happen to be{9} at sea level, and as the land rises or sinks this rock reef forms a veneer of varying thickness upon the seaward land surface. ”

“ Much the same in make are the Fringing Reefs and Barrier Reefs, except that the former skirt the shore more closely, while the latter may lie at a distance of many miles from island or mainland.
It is impossible for the polyps to raise their stony structure above the sea. Some kinds can endure exposure to the air for a short time without dying, but only for a short time. A coral island, when first built, is merely level with the ocean-surface at low tide. ”

“ Have we reason to believe, that where there are fringing-reefs, there has not lately been subsidence; or, for it is almost our only way of ascertaining this point, are there frequently proofs of recent elevation? Can we by this means account for the presence of certain classes of reefs in some large areas, and their entire absence in others? Do the areas which have subsided, as indicated by the presence of atolls and barrier-reefs, and the areas which have remained stationary or have been upraised, as shown by fringing-reefs, bear any determinate relation to each other ”

“ In the West Indies there are reefs, which I should probably have arranged under both these classes, had not the existence of large and level banks, lying a little beneath the surface, ready to serve as the basis for the attachment of coral, been occasionally brought into view by the entire or partial absence of reefs on them, and had not the formation of such banks, through the accumulation of sediment now in progress, been sufficiently evident. Fringing-reefs sometimes coat, and thus protect the foundations of islands, which have been worn down by the surf to the level of the sea. ”

“ On quitting Port Jackson, the course to be steered is N. E. by E. by compass, to longitude about 1551⁄2°, when the land will be fifty leagues off; then North, also by compass, as far as latitude 24°. Thus far no danger lies in the way; but there is then the Cato's Bank, a dry sand frequented by birds and surrounded with a reef (Atlas, Plate X) , and further northward is Wreck Reef, both discovered in the future part of this voyage. Wreck Reef consists of six distinct patches of coral, extending twenty miles east and west ”

“ The land at the back of Point Waterhouse is higher than that of the island, and is composed of grassy, woody hills, rising over each other by gentle ascents. Upon the point there is a sandy hillock, and a reef of rocks extends out from it a quarter of a mile. We had 8 fathoms, whilst rounding this reef; and in steering through the passage, the soundings were 8, 7, 6, 5, 4, 5, 6 fathoms; the sandy bottom being visible under the sloop. At the further end of the channel, a rocky islet and a small reef were passed, leaving them on the starbord hand. ”

“ To be constantly covered with water, seems necessary to the existence of the animalcules, for they do not work, except in holes upon the reef, beyond low-water mark; but the coral sand and other broken remnants thrown up by the sea, adhere to the rock, and form a solid mass with it, as high as the common tides reach. That elevation surpassed, the future remnants, being rarely covered, lose their adhesive property; and remaining in a loose state, form what is usually called a key, upon the top of the reef. ”

“ The stormy appearance of the night induced me to stretch in, under this reef; and finding there was shelter from the east winds, we came to an anchor in 5 fathoms, coarse sand: the Nautilus followed; but not liking the place, captain Bishop preferred keeping the sea. On sounding round the sloop, I found the bottom every where foul, and that there were no means of escape in case of a shift of wind; therefore, after killing a few seals upon the granitic rocks, we weighed the anchor, ran two leagues to the southward, and then hauled the wind under storm sails for the night. ”

“ At two o'clock, the eastern reef, which was a mile distant to leeward and nearly dry, was seen to terminate, whilst the northern reefs extended out of sight to the north-east; the opening between them was a mile and a half wide, and full of ripplings; but having the whale boat a-head, we bore away E.S.E., to go through the least agitated part. Having little wind, and a flood tide making against us, the boat was called back to tow, and the brig directed to take its station by means of her sweeps. Our soundings were irregular in the narrow part, between 24 and 9 fathoms, on rocky ground ”

“ The breakers roared as loudly as before over the coral reefs, but, still unharmed, the British frigate flew quickly by them. A graze almost from the outer point of the rugged surface of a reef might hurl her to destruction; but neither coral reef, nor rock, nor sandbank stopped her course. Day came at last, and what a wide expanse of troubled waters broke upon the sight of the weary seamen! No one had that night turned in; all kept the deck, steady at their stations, ready to do what men might do to save the ship or their lives; at all events to obey their officers to the last. ”

“ Several low islands, on which cocoa-nut trees grew, were seen on the way. Having reached the south-east end of Otaheite, the two ships being together, at daybreak they found themselves not half a league from a reef, towards which the send of the sea, the wind having fallen, was drifting them rapidly. To anchor was impossible. A passage was discovered through the reef, but a boat being sent ahead to sound, it was found there was not sufficient water for the ships to pass on. The horrors of shipwreck threatened the explorers. Closer and closer they drove to the reef. ”

“ In the morning [WEDNESDAY 17 NOVEMBER 1802] , a fresh land wind at south-east favoured our course, the water deepened to 10 fathoms, and at eight o'clock to no ground with 13, near the south end of a reef extending out from the hill. On coming into 5 fathoms behind the reef, the anchor was dropped on a muddy bottom, with the hill bearing N. 15° E., one mile and a quarter, and the dry extremity of the reef S. E. 1⁄2 E. The coast to the southward was scarcely visible from the mast head, but land was seen to extend westward from the hill, as far as nine or ten miles ”

“ Murray's Islands should not be passed, or quitted if the ship have anchored there, later than ten or eleven o'clock in the morning; because the sun will be getting ahead and obscure the sight before another good anchorage can be secured. On passing the islands, keep the reef which lies five miles to the north about a mile on the starbord hand, steering W. 1⁄2 S. by compass, with a boat ahead; for in this part there are many tide ripplings scarcely to be distinguished from the reefs. Having passed the ripplings, haul a point more to the southward ”

“ High, precipitous, perpendicular, level, and dotted with farm-houses, this singular bit of land stretches several miles out southward to sea, bordered with a rocky beach, and tapered off into the wide ocean with Duxbury Reef—a dangerous rocky reef, curving down to the southward and almost always white with foam, save when the sea is calm, and then the great lazy green waves eddy noiselessly over the half-hidden rocks, or slip like oil over the dreadful dangers which they hide. ”

“ On the 27th, the search was continued, without success; but on the 28th, a boat was despatched to examine an opening in the reef; and the ship stood off and on, waiting the result. At five in the evening, the boat made a signal for a passage being found; but fearing to venture through, so near sunset, without more particular information, captain Edwards called the boat on board. In the mean time, a current, or tide, set the Pandora upon the reef; and, after beating there till ten o'clock, she went over it into deep water; and sunk in 15 fathoms, at daylight of the 29th. ”