Anguilla is the most northerly of the
Leeward Islands in the West Indies. It
lies one hundred and fifty miles east of Puerto Rico and eleven miles north of
the Dutch/French island of St Maarten/St Martin. The island is narrow and low-lying. It is some fifteen miles long and two to
three miles wide (see illus 1). The
total area is thirty-six square miles.
There are no mountains, rivers or forests.
It is an arid island, with little agricultural
potential. The highest point at Crocus
Hill is just over two hundred feet above sea level. The island runs from east to west. Its surface is tilted towards the south, so
that most of the island slopes in that direction. At the foot of the slopes on the south, the
limestone surface passes gently below sea level. At the higher end of the slope on the north,
there are cliffs broken by the occasional beach.
1. Map of Anguilla
The greater part of the surface of the
island has no topsoil. It consists of
fissured slabs of limestone. A low,
tough scrub covers most of it. What
little weathered limestone soil exists is found in the hollows and valleys
called ‘bottoms’ that dot the island.[1] This soil is frequently coloured a bright
red, like clay, and can be quite fertile.
The soil is not clay, but weathered limestone coloured by the insoluble
salts left behind when rainfall dissolved the white limestone.
The main crops for many years in the past
were pigeon peas, corn or maize, and sweet potatoes. Pigeon peas were grown in the fissures in the
solid limestone from which the scrub was previously cut down and burned. The
pigeon pea shrub is nominally a biannual growing to a height of ten feet or
more. Those of Anguilla bear fruit, but
they seldom grow higher than waist level unless planted in good, deep soil. During periods of low rainfall, they almost
never survive the first year’s dry season to produce a second crop.
Geologically, Anguilla consists of a cap
of upwards of two hundred feet of sedimentary limestone lying on an igneous
base. In only two places, at Crocus Bay
and Road Bay, is the volcanic basement of the island exposed to view. This volcanic basement is best visible at the
northern end of the beach at Road Bay. There,
a large dark boulder, black, brown or purple in colour, traversed by white veins
of calcite, can be seen protruding from the cliff at sea level. It rises to a height of some 20 feet above
the sea and is easy to find. Above it is
the limestone cap that covers the entire island.
The second place to see evidence of the volcanic
birth of Anguilla is at Crocus Bay to the south west of the beach, or to the
left as you stand on the sand facing the sea.
There is a breccia layer visible at the foot of the cliff about one
hundred feet after the end of the beach.
It must be reached by clambering over the rocks and stones that take the
place of the beach. The layer of breccia
is covered by several almost indistinct layers of coloured clays. The breccia was laid down as volcanic ash
from nearby eruptions during a geologically violent time. The clays were deposited in shallow water as
the volcanic ash and rock were eroded by ancient rivers and flowed down to the
sea as silt.
Some two hundred million years ago, at the
start of the Jurassic period, what is now Anguilla was a small part of an old
prehistoric continent known to geologists as Pangaea (see illus 2).[2] This ancient land mass was covered during the
earlier Carboniferous period in swamps and forests, and animal life was then not
yet significant.[3] During the Jurassic period, Pangaea began to
break up. Parts of this pre-historic
continent sank below the sea. Other
parts began to draw away from one another to become the present-day continents,
separated by the Atlantic and other Oceans.
Europe and Africa lie to one side of the Mid-Atlantic Ridge, and North
and South America lie to the other. Running
the length of the Mid-Atlantic Ridge, small amounts of magma ooze out of the
earth onto the sea bottom. As the magma
hardens it pushes Africa apart from South America at the rate of about 2.5 cm
per year. The cooled magma to the west
of the Mid-Atlantic Ridge forms the South American tectonic plate which as it
advances westward pushes up against the lighter Caribbean plate causing
volcanic activity and earthquakes from time to time. Anguilla and the other lesser Antilles were born
on the lip of the Caribbean plate that rests uneasily on the heavier Atlantic
plate that forms the ocean bottom to the east of the archipelago.
2. Pangaea 400
million years ago
Kenneth Earle writing long before plate tectonics was understood
described the geology of Anguilla as follows,[4]
Geologically, the island of
Anguilla forms one of the sedimentary group of islands constituting the eastern
and south-eastern half of the Lesser Antillean chain . . . At only one spot on
the island have I found this igneous basement exposed in the sea cliff, viz,
immediately north of Road Village, where it rises in a short distance from sea
level to about 25 feet and then suddenly drops again to sea level and
disappears. This rock is dark, black,
brown or purple in colour and is traversed by veins of calcite.
The rocks are immediately
overlain by white or cream-coloured marly limestone with fossils . . .
In Crocus Bay a rather
different state of affairs is seen.
There is no true igneous basement, but at the base of the limestone
series at Pelican Point is seen a series of clays, grits, volcanic breccias,
etc . . . At the western side of Crocus Bay there are also blue, white, red and
yellow clays at the base of the cliff - the blue clays crop out in the beach -
but no volcanic grits. It is associated
with these blue clays that the lignite and "amber" occur . . .
Limestone of Upper Oligocene
age forms the whole of the surface rock of Anguilla . . .
The occurrence of lignite or
brown coal on the shores of Crocus Bay has been known for a long time. This is to be picked up from the sea itself
at certain times of the year (April - September), and some 50 lbs was collected
in November 1921 from the surf . . . It is a soft, black, woody coal, showing
distinct stratification with traces of pyritous material and plant remains on a
bedding plane. It has a distinctly brown
appearance when cut with a knife . . . At certain times of the year the sea is
said to sweep the sand away and expose a clear bed of lignite, but no one seems
to have ever been on the spot at the time to make critical investigations of
its dip, thickness, etc. In my presence,
a man went three or four paces into the sea at two distinct points, and, after
removing boulders and sand from the sea floor, brought up fragments of the
material, and I have seen lumps as big as a man's head collected on a previous
occasion. The only record of the lignite
in writing is that in 1871 indications of coal were found in a blue clay, 30
feet down, at Crocus Bay, while "much coal" was still being obtained
in 1872 from the opening made. It was
also reported to occur at Chalvilles, in the centre of the island.
Acting on these data, I had two
excavations made in the beach just above water mark exactly at the spots where
I saw the other pieces taken from the sea.
The first excavation - made in a blue clay, cropping out in the beach -
had to be abandoned owing to the influx of the sea. The second was carried down to a depth of
eight and nine feet (beach sloping).
Shortly below beach level, clay was encountered, in which was found a
thin string, nowhere more than an inch thick, of the lignite. Associated with it were two or three nodules,
the interior of which showed a transparent honey-coloured fossil resin allied
to amber, which had also been reported by previous observers. Further excavation only revealed clay, black,
white, yellow and deep red in colour, but no coal.
This layer of lignite dates to the
Carboniferous Era, when coal and oil originate, not only does Anguilla have no
coal, but there is no oil either. The
crude oil and natural gas that other West Indian islands find around their
shores, date back to this period. Nor
would there be gold or silver in the rocks of Anguilla, as these valuable
minerals were deposited billions of years ago in the ancient plates that form
the continental land masses. Anguilla
and other coral islands are much too young for this.
During long periods of time, as the South
American plate collided with the Caribbean plate causing it sometimes to rise
and sometimes to fall Anguilla was deep under water.[5] It was then that Anguilla’s limestone cap was
laid down. Thomas Wayland Vaughan
described the limestone thus,[6]
This formation is uppermost
Oligocene, if the Aquitanian of Europe is correctly referred to the Oligocene. In the opinion of some palaeontologists it
would be classified as earliest Miocene.
It is paleontologically characterised by certain foraminifera, described
by JA Cushman in a report not yet published, by numerous species of corals,
among which are the general Stylophora, Stylocaenia, Antillia, Orbicella,
Siderastrea and Goniopora, by echoids described by Guppy or by Cotteau; among
which are Echinolampas Semiorbis Guppy, E. Lycopersicus Cotteau and Agasizzia
Clevei Cotteau; and by a number of species of mollusca, described in a
manuscript by CW Cooke. The mollusca
include Amusium Lyonii Gabb and Orthaulax Pugnax (Heilprin). I obtained no specimens of Lepidocyclina in
Anguilla. The type exposure is along the
south-east and south shore of Crocus Bay.
The material consists of calcareous clay, argillaceous limestone and
more or less pure limestone. The
formation unconformably overlies basic igneous rock.
3. Subdivisions of the Paleogene period
according to the IUGS, as of July 2009.
The
name Oligocene (see illus 3) comes from the Greek ὀλίγος (oligos, few)
and καινός (kainos, new). This
refers to the sparsity of mammalian faunas found in this geologic epoch after a
burst of evolution during the Eocene. The Oligocene epoch started about 33.9
million years ago and lasted for some 10 million years, ending about 23 million
years ago. Translated into ordinary
English Dr Vaughan’s quotation above means, therefore, that Anguilla’s 200 foot
limestone cap, which lies on top of the igneous rock and breccia that make up
the basement of the island, was laid down between 30 and 20 million years ago,
with the youngest fossil shells and corals lying at the surface and the oldest
at the bottom of the cap.
Limestone consists of the remains of
ancient life forms. Vast numbers of sea
creatures died in the sea over eons of time.
Their bones fell to the floor of the sea, joining conchs, corals and sea
urchins. Gradually, layers of these
remains were built up. The animal
remains were changed by time and the pressure of the sea and their own
accumulated weight into limestone.[7]
When Anguilla surfaced for the last time,
during more recent geological times, it was capped with this layer of
limestone.[8] To this day, one can walk about the island
picking up the fossil shells, corals and sea urchins of this era. They are loosely embedded in the limestone
that now forms the surface of the island and wash out in large numbers ready
for collection.
Famous among the extinct mammals of
Anguilla is Amblyrhiza inundata
(see illus 4). This great rat was
first described in 1868 by Professor Edward Drinker Cope.[9] Fragments of fossil bone were dug up in a
phosphate mine on Anguilla. A shipment
of it was sent to Professor Cope in the United States of America for
analysis. He identified the bones he
found in it as coming from this huge prehistoric rodent.
4. Amblyrhiza inundata (reconstruction)
The
fossils were shipped to him accompanied by an Amerindian conch chisel. The sciences of geology and biology were not
in Professor Cope's time as advanced as they are now. He entertained the possibility that the Amblyrhiza remains were associated with artefacts
of early man. That was an error. It is now known that Amblyrhiza pre-dated
the entry of man into Anguilla by many thousands of years. The Amblyrhiza
remains are 125,000 years old, long before humans entered the American
Continent from Asia.
In a series of articles in the Anguilla
Life magazine, botanist Mary Walker has described the source of Anguilla’s
plants. She says that the story begins
some two million years ago in the late Pliocene time. Then, there was extensive land uplift due to
continental plate shifting. During the
Pleistocene glacial age that followed, much ocean water was tied up in the great
continental ice sheets. Sea level was
lowered by some two hundred feet. At
that time, Anguilla and the Leeward Islands formed part of a larger land mass
extending perhaps westward to Puerto Rico from Antigua in the east to
Guadeloupe in the south. At low
elevations the land constituted a dry zone.
This was covered by a type of vegetation now called an evergreen bush
land.
Some of Anguilla’s common shrubs and trees
date from this period. They are the bearded
fig, fustic, cedar, masswood, cinnamint, alexanders, and loblolly. Anguilla shares them with Puerto Rico and the
Lesser Antilles. Anguilla has about five
hundred species of plants, as do the other limestone islands of the Lesser
Antilles. More than two hundred of
Anguilla’s native plants are found on Puerto Rico too.
About sixty percent of Anguilla’s plants
are native or wild. That is, they become
established and grow without any help from people. Some plants arrived by water and others by
air. The Amerindians first introduced
papaya, cassava, corn, cotton, tobacco, and the calabash tree whose dried fruit
was used for making pots, pans and jugs.
About thirty five percent of Anguilla’s
plants are exotic or introduced. Most
are cultivated as food plants or ornamentals.
About forty of them are grasses and weeds which have naturalised, that
is they grow on their own. A few, like
pomegranates, tamarinds, acacias and pommesurettes have escaped from gardens
and joined the wild vegetation.
In the year 1985, Dr Richard A Howard of
Harvard University collected plants on Anguilla while doing field work for his
six-volume Flora of the Lesser Antilles.
He made a chance stop along the road in Chalvilles. One of the plants in the bush caught his
eye. It was not like any other plant he saw
before. From its flowers, he identified
it as a member of the genus Rondeletia. Botanists call this a ‘plastic’ genus as its
plants have evolved readily into many different species in order to adapt to
the different habitats that were being created in the Pliocene and Pleistocene
times. Five species are endemic to the
Leeward Islands, meaning they grow only there.
The plant that Dr Howard discovered he named Rondeletia anguillensis (see illus 5).
5. Rondeletia Anguillensis (photo by the
author)
It is Anguilla’s one endemic plant we know
of. It evolved to grow in the dissolved
limestone pavement that covers the northern and eastern parts of the
island. It is a low shrub reaching only
to the knee in the specimens I have seen.
It has the small tubular flowers typical of the Rubiaceae or coffee
family to which it belongs. Other
members of the family that are abundant on Anguilla are the fustic, coughbush,
black torch, wild guava, and five-finger bush.
At first glance Rondeletia looks much like a diminutive five-finger
bush. On closer examination, its flowers
are smaller, and pink rather than white.
Its branches are sharp tipped like the five-finger, but the branching
pattern is different. It has tinier
leaves which minimise water loss and help conserve moisture in this dry
environment. The sharp tips and leathery
leaves make it not attractive to goats, which should ensure its survival.[10]
[1] One of the curiosities that never ceases
to amuse the discerning person is that for such a flat island there is an
exceptionally large number of places named Valley and Hill. Almost half of the place names on the island
are named one or the other.
[3] It
is this primal vegetation that formed the coal that is mined today deep in the
earth, and that, as Dr S B Jones recorded in his book Annals of Anguilla,
is occasionally washed up on Anguilla's shores as fragments of lignite and
fossil resin.
[4] Kenneth W Earle, The Geology of
Anguilla (1922) quoted by Katherine J Burdon, A
Handbook of St Kitts-Nevis, Chapter 28.
[5] The nesting Green Turtles of Ascension
Island spend long periods of time feeding off the coast of Brazil. During the Carboniferous Era, they got into
the habit of swimming into a small stream to a nearby island to lay their eggs.
Over the ensuing period of 300 million
years, their descendants have continued to cross the same stream every 3 or 4
years to lay their eggs. Only, now the distance
from the coast of Brazil to Ascension Island is 2,500 km, and the original
stream has become the Atlantic Ocean. It
takes the turtles 5 to 6 weeks to make the trip, but turtles notoriously return
to the beach where they were born to lay their eggs.
[6] Thomas
Wayland Vaughan, Correlation of the Tertiary Geological Formations of the
South-Eastern United States, Central America and the West Indies in the
Journal of the Washington Academy of Sciences, Vol VIII, No 9, May 4th 1918,
cited by Katherine J Burdon.
[8] JS Beard, Natural
Vegetation of the Windward and Leeward Islands (1949), p.18: “The rock sequence in Anguilla indicates
submergence from Eocene to Miocene, emergence in the Pliocene, a slight
depression during the Pleistocene, and subsequent reemergence which still
continues.”
[9] Professor Cope's report may be accessed
online: https://books.google.com.ai/books?id=nJA4rYDQT-4C&pg=PA183&lpg=PA183&dq=&hl=en#v=onepage&q&f=false
