Barrier reef

“ It is remarkable how little attention has been paid to encircling barrier-reefs; yet they are truly wonderful structures. The following sketch represents part of the barrier encircling the island of Bolabola in the Pacific, as seen from one of the central peaks. In this instance the whole line of reef has been converted into land; but usually a snow-white line of great breakers, with only here and there a single low islet crowned with cocoa-nut trees, divides the dark heaving waters of the ocean from the light-green expanse of the lagoon-channel. ”

“ These islets may coalesce and form a strip of dry land lying some hundred yards or less from the extreme outer edge of the reef, and separated by a wide channel from the mainland. Where the barrier reef is not far from the land there are always gaps in it opposite the mouths of rivers or considerable streams. The outer wall of a barrier reef is steep, and frequently, though not always, descends abruptly into great depths. ”

“ Barrier reefs are met with at greater distances from the land, and often rise from profound depths. The barrier reef which extends along the north-east coast of Australia, often at a distance from the land of fifty or sixty miles, stretches, with interruptions, for about 1250 miles, with a breadth varying from ten to ninety miles. In some places, the depth of the sea immediately outside of this reef exceeds 1800 feet. ”

“ Well characterised atolls and encircling barrier-reefs, where several occur in a group, or a single barrier-reef if of large dimensions, leave scarcely any doubt on the mind respecting the movement by which they have been produced; and even a small amount of subsequent elevation is soon betrayed. The evidence from a single atoll or a single encircling barrier-reef, must be received with some caution, for the former may possibly be based upon a submerged crater or bank, and the latter on a submerged margin of sediment, or of worn-down rock. ”

“ In barrier-reefs it has long been remarked with surprise that the passages through the reef exactly face valleys in the included land, even in cases where the reef is separated from the land by a lagoon-channel so wide and so much deeper than the actual passage itself, that it seems hardly possible that the very small quantity of water or sediment brought down could injure the corals on the reef. ”

“ Of all the wonderful animal structures in the world the Great Barrier Reef of Australia is the most remarkable. It consists of a chain of coral islands and reefs parallel to the east coast of Queensland. This great reef is about twelve hundred miles long, and the distance from the mainland to its outer border is from ten to more than one hundred miles. It is far enough off the coast to leave a wide channel between the reef and the shore. ”

“ Where the channel is narrow, the agency, which on unprotected coasts is most productive of sediment, namely the force of the breakers, is here entirely excluded, and the reef being breached in the front of the main valleys, much of the finer mud from the rivers must be transported into the open sea. As a current is formed by the water thrown over the edge of atoll-formed reefs, which carries sediment with it through the deep-water breaches, the same thing probably takes place in barrier-reefs, and this would greatly aid in preventing the lagoon-channel from being filled up. ”

“ 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. ”

“ 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. ”

“ As in the Red Sea the level rarely alters by more than a foot once in the twenty-four hours, and often the rise or fall is much less, the action of tidal currents is at a minimum, yet even so they produce well-marked effects.
[117] Barrier reefs may be formed from fringing reefs by the enlargement of the boat channel, while the reef is extending seawards.
The island of Zanzibar, 60 miles long by 20 wide, and 20 miles from the mainland of Africa, seems to be a part of the East African barrier system, and it certainly was separated from the mainland by the destruction of the intervening land ”

“ Myriads of these inhabit the Pacific, constructing entire islands, and throwing up mighty barriers of rock, and threading over vast areas of the sea with inosculating lines of coral reef. The calcareous accumulation, known as the Great Barrier Reef, extends for about a thousand miles in length, by about thirty in mean breadth, filling up, with its various reticulations, the whole intermediate space betwixt the coast of Australia and Bristow Island, off the coast of New Guinea. The works of these minute creatures thus occupy an area which may be roughly estimated at thirty thousand square miles ”

“ The captain shook his head.
"The charts say that it is completely surrounded by a barrier reef. It appears to be a volcanic rock about which the coral builders have been busy. But it is the nearest land; the only land we can possibly make in our present condition; and, at least, we won't drown on it. We can save enough from the Stephanie to support life, and I have no doubt we can find some means of getting away or communicating with other ships," continued the veteran sailor confidently, although he knew, and everyone else realized, more or less, that the chance of either was very slim. ”

“ Outside the Barrier Reef there is always a swell, in which the small boats rolled heavily, and the passengers grumbled proportionately. On the return journey the prevailing strong winds heap up heavy seas, and are apt to drive a ship past the narrow opening in the Reef. Small boats of about 800 tons now run directly from Thursday Island to Port Moresby. But though we had all the inclination, we had no time to make this most interesting journey.
Our fellow-passenger had an interest in the Pearl-fishing Industry carried on in the neighbourhood of Thursday Island, by the “Island boys.” ”

“ How I wish I could shew it to you! These islands are of very various character, and many of them like the garden of Eden for natural loveliness; shewing almost every kind of scenery within a small area. Most of them are girdled more or less entirely by what is called a barrier reef—an outside and independent coral formation, sometimes narrow, sometimes miles in width, on the outer edge of which the sea breaks in an endless line of white foam. Within the reef the lagoon, as it is called, is perfectly still and clear ”

“ After the formation of these flats and islands corals grew upon them. The recent reefs have been elevated at least ten feet, and do not owe their contours to subsidence, yet they form true atolls. The inner channels of the Barrier Reef are maintained free of corals by the great amount of silt held in suspension in the water or deposited to form a blue mud over the bottom. Thus there appears to be nothing in the Great Barrier Reef region to lend support to Darwin's theory of coral reefs. ”

“ With the débris from all these sources the reef is constantly built up until it rises to low-tide level. Higher than this the corals cannot grow, since they are killed by a few hours’ exposure to the air.
Fig. 159. Patch of Growing Corals exposed at an Exceptionally Low Tide, Great Barrier Reef, Australia
When the reef has risen to wave base, the waves abrade it on the windward side and pile to leeward coral blocks torn from their foundation, filling the interstices with finer fragments. ”

“ The stretch of smooth water that forms a natural harbour for a thousand miles inside the great barrier reef was unimagined. The northern coast land was believed to be uninhabitable by white men; the interior was supposed to be an almost waterless area of intolerable heat, while experts affirmed that if sheep survived at all beneath the tropical suns of Queensland, they would grow hair instead of wool. ”

“ Within the barrier the water is as smooth as a lake, but the trade winds, which blow for nine months in the year upon the shore, send the long rollers of the Pacific against the reef, which varies from 5ft. to 30ft. in width. Dashing upon this impregnable barrier, they rise in columns of rosy foam often to a height of from 20ft. to 50ft., and, glittering in the rays of the tropical sun, fall like obelisks of diamonds. A long line of silent ripples is often at first the only indication of the presence of one of these spines of coral and volcanic rock ”

“ On this view every difficulty vanishes; fringing reefs are thus converted into barrier-reefs; and barrier-reefs, when encircling islands, are thus converted into atolls, the instant the last pinnacle of land sinks beneath the surface of the ocean. ”

“ As the reef-building corals require food, are preyed upon by other animals, are killed by sediment, cannot adhere to a loose bottom, and may be easily carried down to a depth whence they cannot spring up again, we need feel no surprise at the reefs both of atolls and barriers becoming in parts imperfect. The great barrier of New Caledonia is thus imperfect and broken in many parts; hence, after long subsidence, this great reef would not produce one great atoll 400 miles in length, but a chain or archipelago of atolls, of very nearly the same dimensions with those in the Maldiva archipelago. ”

“ Reefs having intermediate character between those of the fringing and barrier classes do exist; for instance, on the south-west coast of Madagascar, a reef extends for several miles, within which there is a broad channel from seven to eight fathoms deep, but the sea does not deepen abruptly outside the reef. ”

“ First of all, the north and south central band of Torres Straits, or submarine continuation of the eastern chain, forms the western boundary of the Great Barrier reefs, just as much as does the dry land to the southward. No true independent coral reefs infringe on this band though they come close up to it. There are no wide spread coral reefs round its islets as there are round Darnley and Murray Islands, but only small fringing reefs clinging to the rocks. ”