Reef point

“ Formerly, vessels bound eastward on the flood, taking the eastern channel, in the endeavor to get around the reef at Hallet’s Point, ran the risk of being carried upon the Frying Pan, near Ward’s Island, while vessels bound westward on the ebb, were thrown so much off by the reef at Hallet’s Point as to be endangered by the set of the tide directly upon the middle reef. ”

“ The reef commences at Cape Bougainville, and trends round to Point Gibson, where it terminates. This part of the coast is fronted by extensive reefs, which render the approach to it very dangerous: at sixteen miles to the northward of the cape there is a range, the HOLOTHURIA BANKS, that extend in an east and west direction for twenty-three miles; their north-east extent was not ascertained, but the western end, in latitude 13 degrees 32 minutes, and longitude 125 degrees 46 minutes 45 seconds, is narrow, and not more than five or six miles broad. ”

“ The reef is in 15 degrees 57 minutes South, and 123 degrees 4 minutes 45 seconds East.
Six miles south of Caffarelli Island, is a rocky island, surrounded by a reef; and eight miles farther are several small rocky islands, forming the north extremity of a range, which, extending to the South by East for ten miles, form the eastern side of Sunday Strait, which is the best, and in fact the only safe communication with the deep opening between Point Cunningham and the islands to the eastward. ”

“ About a mile more or less off shore, there is as a rule a rocky reef, on which the breakers may be seen to dash violently in many places, the depth above the reef being in several places, 1, 11⁄2 and even 2 fathom, so that pinnaces and boats may get over it for the purpose of landing, there being deeper water close inshore, but all of it with a rocky, sharp coral-bottom, so that it is difficult to land there, and much harder still to keep a pinnace at anchor with a drag ”

“ The plain is formed of gravel from the high hills, its yellow border seawards being coral limestone, and the islands also. In the sea are numerous reefs, here of very intricate plan, lines of white breakers separating the deep blue black water from large areas of green and brown shoals in waveless lagoons. There are deep channels between these reefs and the shore, which is itself fringed by a shallow reef with its edge at low water level but bearing perhaps one or two fathoms of water on its surface within. ”

“ 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. ”

“ 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. ”

“ At one time I thought it convenient thus to restrict the term, but as these reefs are similar in structure, and in position relatively to the land, to those, which, like a wall with a deep moat within, encircle many smaller islands, I have classed them together. The reef, also, on the west coast of New Caledonia, circling round the extremities of the island, is an intermediate form between a small encircling reef and the Australian barrier, which stretches for a thousand miles in nearly a straight line. ”

“ These ever-growing reefs would surround the whole coast were it not for the fresh water that oozes out from the land and prevents the coral from growing at certain points, thus keeping open narrow passages through the reef, or wider stretches along the coast free from rocks. These basins form good anchorages for small craft, as the swell of the open sea cannot cross the reef; only the entrances are often crooked and hard to find. ”

“ The engineers propose, by a jetty from the south beach to a group of rocks forming the south side of the middle channel, to extend the narrow deep channel inside, and the consequent force of concentrated tidal and river water, up to the rocky reef itself. They judge that the tidal force is ample to scour away clean all the sand deposited both in and outside the reef. ”

“ These reefs, however, vary in construction; some are called encircling reefs, when they appear at a distance from the shore, and a lagoon intervenes; others are called fringing reefs, which are joined to the land, and extend out from it without any lagoon. Others are denominated lagoon islands, when the reef itself, raised above the surface of the ocean, forms the land generally in a circular shape, and surrounds a lake or lagoon, which has sometimes a passage to the sea, and at others is completely closed. ”

“ Where the water is perfectly tranquil, as within a lagoon, the reefs generally grow up perpendicularly, and sometimes even overhang their bases; on the other hand, on the leeward side of Mauritius, where the water is generally tranquil, although not invariably so, the reef is very gently inclined. ”

“ As far as the actual reef is concerned, there is no essential difference between it and that forming a barrier or an atoll: it is, however, generally of less width, and consequently few islets have been formed on it. From the corals growing more vigorously on the outside, and from the noxious effect of the sediment washed inwards, the outer edge of the reef is the highest part, and between it and the land there is generally a shallow sandy channel a few feet in depth. ”

“ But there is a third kind of reef, of very common occurrence in the Pacific and Indian Oceans, which goes by the name of "atoll." This is, to all intents and purposes, an encircling reef, without anything to encircle; or, in other words, without an island in the middle of its lagoon. The atoll has exactly the appearance of a vast, irregularly oval, or circular, breakwater, enclosing smooth water in its midst. The depth of the water in the lagoon rarely exceeds twenty or thirty fathoms, but, outside the reef, it deepens with great rapidity to two hundred or three hundred fathoms. ”

“ At New Caledonia (Plate II., Figure 5.) the barrier-reefs extend for 150 miles on each side of the submarine prolongation of the island; and at their northern extremity they appear broken up and converted into a vast atoll-formed reef, supporting a few low coral-islets: we may imagine that we here see the effects of subsidence actually in progress, the water always encroaching on the northern end of the island, towards which the mountains slope down, and the reefs steadily building up their massive fabrics in the lines of their ancient growth. ”

“ At half-past 5 P.M. tacked to the northward, reefs still in sight. At 9 A.M. tacked after the Commodore, a reef of coral rock bearing east to east-north-east distant 4 or 4 1/2 miles. At noon the Investigator bore north by east distant 1 1/2 miles, a shoal of sand apparently bearing north distant 5 or 6 miles, another bearing north-east by north distant 4 miles and a small rock on an extensive shoal of sand east by south distant 1 1/2 miles, this shoal seems to stretch a long way from east-south-east to north-east. Latitude observed 20 degrees 54 minutes 42 seconds south. ”

“ This outer Reef and channel are, however, in a much less advanced state than the preceding ones; only here and there a sand-flat large enough to afford a foundation for a beacon or a lighthouse shows that this Reef also is gradually coming to the surface, and that a series of islands corresponding to the Keys must eventually be formed upon its summit. Some of my readers may ask why the Reef does not rise evenly to the level of the sea, and form a continuous line of land, instead of here and there an island. ”

“ The ring-shaped reef, or atoll, is broken in one or more places, generally on the leeward side, and built up higher on the windward side. The reason for such omissions and buildings is obvious when we remember that the coral animal cannot move from its fixed position to seek food, but must depend upon the waves to bring it within reach. The water dashing up against the reef on the windward side brings an abundance of food, while the slight movement of the waves on the leeward side brings but little food. ”

“ An harbour might be made by cutting a channel through the reef about four hundred feet long, but it would be necessary to blow up some sunken rocks to facilitate the entry: if it should ever be thought proper to do this, five vessels of seven feet draught might lie all the year round in security within the reef: they will not be able to enter but in the finest weather, with the wind from north-east to north-west, and then they must warp in: perhaps less difficulty will be found when I am informed of the state of the weather during the Summer months. ”

“ On the ocean side, on the mounds of the steep beach, over all the width of the reef right out to where the surf is bursting, in every cranny, under every scattered fragment of the coral, an incredible plenty of marine life displays the most wonderful variety and brilliancy of hues. The reef itself has no passage of colour but is imitated by some shell. ”

“ All the high islands appear to be fronted by distant reefs rising abruptly from the sea, and within some of which there is reason to believe that the water is deep. I have therefore little doubt they are of the barrier class.—In the southern part of the group we have Elizabeth Island, which is surrounded by a reef at the distance of a mile; and two miles eastward of it (Krusenstern, “Append.” 1835, p. 42) there is a little island containing a lagoon.—Near here, also lies Circular-reef (Horsburgh, “Direct.,” vol. i, p. 691, 4th ed.) ”

“ LOUISIADE.—The dangerous reefs which front and surround the western, southern, and northern coasts of this so-called peninsula and archipelago, seem evidently to belong to the barrier class. The land is lofty, with a low fringe on the coast; the reefs are distant, and the sea outside them profoundly deep. Nearly all that is known of this group is derived from the labours of D’Entrecasteaux and Bougainville: the latter has represented one continuous reef ninety miles long, parallel to the shore, and in places as much as ten miles from it ”

“ 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. ”

“ The shores of the Red Sea are little indented; good harbours are almost wanting in the desert regions of the north, while in the south the chief inlets are at Massawa, and at Kamaran, almost directly opposite. Coral formations are abundant; immense reefs, both barrier and fringing, skirt both coasts, often enclosing wide channels between the reef and the land. The reefs on the eastern side are the more extensive; they occur in places as much as 25 m. ”

“ Thirdly, there was the fact that the coral reefs rise suddenly like a wall out of oceanic depths, soundings of a thousand fathoms and more being constantly found close up to the barrier-reefs. Fourthly, there is the fact that on the inner side, next the island or the continent which they inclose or protect, the lagoon or the sheltered area is often very deep close to the reef, not indeed affording oceanic soundings, but nevertheless soundings of twenty to thirty fathoms. ”

“ Recapitulation.—In the three first chapters, the principal kinds of coral-reefs were described in detail, and they were found to differ little, as far as relates to the actual surface of the reef. An atoll differs from an encircling barrier-reef only in the absence of land within its central expanse; and a barrier-reef differs from a fringing-reef, in being placed at a much greater distance from the land with reference to the probable inclination of its submarine foundation, and in the presence of a deep-water lagoon-like space or moat within the reef. ”

“ Reef-building polyps do not live below the depth of one hundred or one hundred and twenty feet, and hence require a [61] foundation near the surface. This is supplied by submarine mountains and plateaus, or the slopes of those volcanic cones which form the high islands.
Growing vertically, the reefs repeat at the surface the outlines of their bases, which fact gives rise to the circular figure both of atolls and reefs in mid-ocean, and to the elongated, wall-like form of reefs adjacent to the continents, like those of Florida and of Australia. ”

“ Fringing-reefs, like barrier-reefs, both surround islands, and front the shores of continents. In the charts of the eastern coast of Africa, by Captain Owen, many extensive fringing-reefs are laid down; thus, for a space of nearly forty miles, from latitude 1° 15′ to 1° 45′ S., a reef fringes the shore at an average distance of rather more than one mile, and therefore at a greater distance than is usual in reefs of this class ”