Goose Green

Wednesday, August 06, 2008

Chapter 1 Section I.III

Map of Chagos-Archipelago in the Indian Ocean.Image via Wikipedia


Maldiva Archipelago.--Ring-formed reefs, marginal and central.--Great
depths in the lagoons of the southern atolls.--Reefs in the lagoons all
rising to the surface.--Position of islets and breaches in the reefs, with
respect to the prevalent winds and action of the waves.--Destruction of
islets.--Connection in the position and submarine foundation of distinct
atolls.--The apparent disseverment of large atolls.--The Great Chagos
Bank.--Its submerged condition and extraordinary structure.

Although occasional references have been made to the Maldiva atolls, and to
the banks in the Chagos group, some points of their structure deserve
further consideration. My description is derived from an examination of
the admirable charts lately published from the survey of Captain Moresby
and Lieutenant Powell, and more especially from information which Captain
Moresby has communicated to me in the kindest manner.

The Maldiva Archipelago is 470 miles in length, with an average breadth of
about 50 miles. The form and dimensions of the atolls, and their singular
position in a double line, may be seen, but not well, in the greatly
reduced chart (Figure 6) in Plate II. The dimensions of the longest atoll
in the group (called by the double name of Milla-dou-Madou and
Tilla-dou-Matte) have already been given; it is 88 miles in a medial and
slightly curved line, and is less than 20 miles in its broadest part.
Suadiva, also, is a noble atoll, being 44 miles across in one direction, and
34 in another, and the great included expanse of water has a depth of between
250 and 300 feet. The smaller atolls in this group differ in no respect from
ordinary ones; but the larger ones are remarkable from being breached by
numerous deep-water channels leading into the lagoon; for instance, there
are 42 channels, through which a ship could enter the lagoon of Suadiva.
In the three southern large atolls, the separate portions of reef between
these channels have the ordinary structure, and are linear; but in the
other atolls, especially the more northern ones, these portions are ring-
formed, like miniature atolls. Other ring-formed reefs rise out of the
lagoons, in the place of those irregular ones which ordinarily occur there.
In the reduction of the chart of Mahlos Mahdoo (Plate II., Figure 4), it
was not found easy to define the islets and the little lagoons within each
reef, so that the ring-formed structure is very imperfectly shown; in the
large published charts of Tilla-dou-Matte, the appearance of these rings,
from standing further apart from each other, is very remarkable. The rings
on the margin are generally elongated; many of them are three, and some
even five miles, in diameter; those within the lagoon are usually smaller,
few being more than two miles across, and the greater number rather less
than one. The depth of the little lagoon within these small annular reefs
is generally from five to seven fathoms, but occasionally more; and in Ari
atoll many of the central ones are twelve, and some even more than twelve
fathoms deep. These rings rise abruptly from the platform or bank, on
which they are placed; their outer margin is invariably bordered by living
coral (Captain Moresby informs me that Millepora complanata is one of the
commonest kinds on the outer margin, as it is at Keeling atoll.) within
which there is a flat surface of coral rock; of this flat, sand and
fragments have in many cases accumulated and been converted into islets,
clothed with vegetation. I can, in fact, point out no essential difference
between these little ring-formed reefs (which, however, are larger, and
contain deeper lagoons than many true atolls that stand in the open sea),
and the most perfectly characterised atolls, excepting that the ring-formed
reefs are based on a shallow foundation, instead of on the floor of the
open sea, and that instead of being scattered irregularly, they are grouped
closely together on one large platform, with the marginal rings arranged in
a rudely formed circle.

The perfect series which can be traced from portions of simple linear reef,
to others including long linear lagoons, and from these again to oval or
almost circular rings, renders it probable that the latter are merely
modifications of the linear or normal state. It is conformable with this
view, that the ring-formed reefs on the margin, even where most perfect and
standing furthest apart, generally have their longest axes directed in the
line which the reef would have held, if the atoll had been bounded by an
ordinary wall. We may also infer that the central ring-formed reefs are
modifications of those irregular ones, which are found in the lagoons of
all common atolls. It appears from the charts on a large scale, that the
ring-like structure is contingent on the marginal channels or breaches
being wide; and, consequently, on the whole interior of the atoll being
freely exposed to the waters of the open sea. When the channels are narrow
or few in number, although the lagoon be of great size and depth (as in
Suadiva), there are no ring-formed reefs; where the channels are somewhat
broader, the marginal portions of reef, and especially those close to the
larger channels, are ring-formed, but the central ones are not so; where
they are broadest, almost every reef throughout the atoll is more or less
perfectly ring-formed. Although their presence is thus contingent on the
openness of the marginal channels, the theory of their formation, as we
shall hereafter see, is included in that of the parent atolls, of which
they form the separate portions.

The lagoons of all the atolls in the southern part of the Archipelago are
from ten to twenty fathoms deeper than those in the northern part. This is
well exemplified in the case of Addoo, the southernmost atoll in the group,
for although only nine miles in its longest diameter, it has a depth of
thirty-nine fathoms, whereas all the other small atolls have comparatively
shallow lagoons; I can assign no adequate cause for this difference in
depth. In the central and deepest part of the lagoons, the bottom
consists, as I am informed by Captain Moresby, of stiff clay (probably a
calcareous mud); nearer the border it consists of sand, and in the channels
through the reef, of hard sand-banks, sandstone, conglomerate rubble, and a
little live coral. Close outside the reef and the line joining its
detached portions (where intersected by many channels), the bottom is
sandy, and it slopes abruptly into unfathomable depths. In most lagoons
the depth is considerably greater in the centre than in the channels; but
in Tilla-dou-Matte, where the marginal ring-formed reefs stand far apart,
the same depth is carried across the entire atoll, from the deep-water line
on one side to that on the other. I cannot refrain from once again
remarking on the singularity of these atolls,--a great sandy and generally
concave disc rises abruptly from the unfathomable ocean, with its central
expanse studded and its border symmetrically fringed with oval basins of
coral-rock, just lipping the surface of the sea, sometimes clothed with
vegetation, and each containing a little lake of clear water!

In the southern Maldiva atolls, of which there are nine large ones, all the
small reefs within the lagoons come to the surface, and are dry at low
water spring-tides; hence in navigating them, there is no danger from
submarine banks. This circumstance is very remarkable, as within some
atolls, for instance those of the neighbouring Chagos group, not a single
reef comes to the surface, and in most other cases a few only do, and the
rest lie at all intermediate depths from the bottom upwards. When treating
of the growth of coral I shall again refer to this subject.

Although in the neighbourhood of the Maldiva Archipelago the winds, during
the monsoons, blow during nearly an equal time from opposite quarters, and
although, as I am informed by Captain Moresby, the westerly winds are the
strongest, yet the islets are almost all placed on the eastern side of the
northern atolls, and on the south-eastern side of the southern atolls.
That the formation of the islets is due to detritus thrown up from the
outside, as in the ordinary manner, and not from the interior of the
lagoons, may, I think be safely inferred from several considerations, which
it is hardly worth while to detail. As the easterly winds are not the
strongest, their action probably is aided by some prevailing swell or

In groups of atolls, exposed to a trade-wind, the ship-channels into the
lagoons are almost invariably situated on the leeward or less exposed side
of the reef, and the reef itself is sometimes either wanting there, or is
submerged. A strictly analogous, but different fact, may be observed at
the Maldiva atolls--namely, that where two atolls stand in front of each
other, the breaches in the reef are the most numerous on their near, and
therefore less exposed, sides. Thus on the near sides of Ari and the two
Nillandoo atolls, which face S. Male, Phaleedoo, and Moloque atolls, there
are seventy-three deep-water channels, and only twenty-five on their outer
sides; on the near side of the three latter named atolls there are fifty-
six openings, and only thirty-seven on their outsides. It is scarcely
possible to attribute this difference to any other cause than the somewhat
different action of the sea on the two sides, which would ensue from the
protection afforded by the two rows of atolls to each other. I may here
remark that in most cases, the conditions favourable to the greater
accumulation of fragments on the reef and to its more perfect continuity on
one side of the atoll than on the other, have concurred, but this has not
been the case with the Maldivas; for we have seen that the islets are
placed on the eastern or south-eastern sides, whilst the breaches in the
reef occur indifferently on any side, where protected by an opposite atoll.
The reef being more continuous on the outer and more exposed sides of those
atolls which stand near each other, accords with the fact, that the reef of
the southern atolls is more continuous than that of the northern ones; for
the former, as I am informed by Captain Moresby, are more constantly
exposed than the northern atolls to a heavy surf.

The date of the first formation of some of the islets in this Archipelago
is known to the inhabitants; on the other hand, several islets, and even
some of those which are believed to be very old, are now fast wearing away.
The work of destruction has, in some instances, been completed in ten
years. Captain Moresby found on one water-washed reef the marks of wells
and graves, which were excavated when it supported an islet. In South
Nillandoo atoll, the natives say that three of the islets were formerly
larger: in North Nillandoo there is one now being washed away; and in this
latter atoll Lieutenant Prentice found a reef, about six hundred yards in
diameter, which the natives positively affirmed was lately an island
covered with cocoa-nut trees. It is now only partially dry at low water
spring-tides, and is (in Lieutenant Prentice's words) "entirely covered
with live coral and madrepore." In the northern part, also, of the Maldiva
Archipelago and in the Chagos group, it is known that some of the islets
are disappearing. The natives attribute these effects to variations in the
currents of the sea. For my own part I cannot avoid suspecting that there
must be some further cause, which gives rise to such a cycle of change in
the action of the currents of the great and open ocean.

Several of the atolls in this Archipelago are so related to each other in
form and position, that at the first glance one is led to suspect that they
have originated in the disseverment of a single one. Male consists of
three perfectly characterised atolls, of which the shape and relative
position are such, that a line drawn closely round all three, gives a
symmetrical figure; to see this clearly, a larger chart is required than
that of the Archipelago in Plate II.; the channel separating the two
northern Male atolls is only little more than a mile wide, and no bottom
was found in it with 100 fathoms. Powell's Island is situated at the
distance of two miles and a half off the northern end of Mahlos Mahdoo (see
Figure 4, Plate II.), at the exact point where the two sides of the latter,
if prolonged, would meet; no bottom, however, was found in the channel with
200 fathoms; in the wider channel between Horsburgh atoll and the southern
end of Mahlos Mahdoo, no bottom was found with 250 fathoms. In these and
similar cases, the relation consists only in the form and position of the
atolls. But in the channel between the two Nillandoo atolls, although
three miles and a quarter wide, soundings were struck at the depth of 200
fathoms; the channel between Ross and Ari atolls is four miles wide, and
only 150 fathoms deep. Here then we have, besides the relation of form, a
submarine connection. The fact of soundings having been obtained between
two separate and perfectly characterised atolls is in itself interesting,
as it has never, I believe, been effected in any of the many other groups
of atolls in the Pacific and Indian seas. In continuing to trace the
connection of adjoining atolls, if a hasty glance be taken at the chart
(Figure 4., Plate II.) of Mahlos Mahdoo, and the line of unfathomable water
be followed, no one will hesitate to consider it as one atoll. But a
second look will show that it is divided by a bifurcating channel, of which
the northern arm is about one mile and three-quarters in width, with an
average depth of 125 fathoms, and the southern one three-quarters of a mile
wide, and rather less deep. These channels resemble in the slope of their
sides and general form, those which separate atolls in every respect
distinct; and the northern arm is wider than that dividing two of the Male
atolls. The ring-formed reefs on the sides of this bifurcating channel are
elongated, so that the northern and southern portions of Mahlos Mahdoo may
claim, as far as their external outline is concerned, to be considered as
distinct and perfect atolls. But the intermediate portion, lying in the
fork of the channel, is bordered by reefs less perfect than those which
surround any other atoll in the group of equally small dimensions. Mahlos
Mahdoo, therefore, is in every respect in so intermediate a condition, that
it may be considered either as a single atoll nearly dissevered into three
portions, or as three atolls almost perfect and intimately connected. This
is an instance of a very early stage of the apparent disseverment of an
atoll, but a still earlier one in many respects is exhibited at Tilla-dou-
Matte. In one part of this atoll, the ring-formed reefs stand so far apart
from each other, that the inhabitants have given different names to the
northern and southern halves; nearly all the rings, moreover, are so
perfect and stand so separate, and the space from which they rise is so
level and unlike a true lagoon, that we can easily imagine the conversion
of this one great atoll, not into two or three portions, but into a whole
group of miniature atolls. A perfect series such as we have here traced,
impresses the mind with an idea of actual change; and it will hereafter be
seen, that the theory of subsidence, with the upward growth of the coral,
modified by accidents of probable occurrence, will account for the
occasional disseverment of large atolls.

The Great Chagos bank alone remains to be described. In the Chagos group
there are some ordinary atolls, some annular reefs rising to the surface
but without any islets on them, and some atoll-formed banks, either quite
submerged, or nearly so. Of the latter, the Great Chagos Bank is much the
largest, and differs in its structure from the others: a plan of it is
given in Plate II., Figure 1, in which, for the sake of clearness, I have
had the parts under ten fathoms deep finely shaded: an east and west
vertical section is given in Figure 2, in which the vertical scale has been
necessarily exaggerated. Its longest axis is ninety nautical miles, and
another line drawn at right angles to the first, across the broadest part,
is seventy. The central part consists of a level muddy flat, between forty
and fifty fathoms deep, which is surrounded on all sides, with the
exception of some breaches, by the steep edges of a set of banks, rudely
arranged in a circle. These banks consist of sand, with a very little live
coral; they vary in breadth from five to twelve miles, and on an average
lie about sixteen fathoms beneath the surface; they are bordered by the
steep edges of a third narrow and upper bank, which forms the rim to the
whole. This rim is about a mile in width, and with the exception of two or
three spots where islets have been formed, is submerged between five and
ten fathoms. It consists of smooth hard rock, covered with a thin layer of
sand, but with scarcely any live coral; it is steep on both sides, and
outwards slopes abruptly into unfathomable depths. At the distance of less
than half a mile from one part, no bottom was found with 190 fathoms; and
off another point, at a somewhat greater distance, there was none with 210
fathoms. Small steep-sided banks or knolls, covered with luxuriantly
growing coral, rise from the interior expanse to the same level with the
external rim, which, as we have seen, is formed only of dead rock. It is
impossible to look at the plan (Figure 1, Plate II.), although reduced to
so small a scale, without at once perceiving that the Great Chagos Bank is,
in the words of Captain Moresby (This officer has had the kindness to lend
me an excellent MS. account of the Chagos Islands; from this paper, from
the published charts, and from verbal information communicated to me by
Captain Moresby, the above account of the Great Chagos Bank is taken.),
"nothing more than a half-drowned atoll." But of what great dimensions,
and of how extraordinary an internal structure? We shall hereafter have to
consider both the cause of its submerged condition, a state common to other
banks in the group, and the origin of the singular submarine terraces,
which bound the central expanse: these, I think, it can be shown, have
resulted from a cause analogous to that which has produced the bifurcating
channel across Mahlos Mahdoo.

Zemanta Pixie

Labels: , , , , , , ,


Post a Comment

<< Home