Produced by Tom Cosmas and the Online Distributed
Proofreading Team at http://www.pgdp.net (This file was
produced from images generously made available by The
Internet Archive/American Libraries.)
THE GEOLOGICAL STORY OF
THE ISLE OF WIGHT.
[Illustration: _Photo by J. Milman Brown, Shanklin._]
GORE CLIFF--UPPER GREENSAND WITH CHERT BEDS
The Geological Story
of the
Isle of Wight
BY THE
Rev. J. CECIL HUGHES, B.A.
_With Illustrations of Fossils by
MAUD NEAL_
LONDON:
EDWARD STANFORD, LIMITED
12, 13, & 14 LONG ACRE, W.C. 2.
1922
PREFACE
No better district could be chosen to begin the study of Geology than
the Isle of Wight. The splendid coast sections all round its shores,
the variety of strata within so small an area, the great interest of
those strata, the white chalk cliffs and the coloured sands, the
abundant and interesting fossils to be found in the rocks, awaken in
numbers of those who live in the Island, or visit its shores, a desire
to know something of the story written in the rocks. The Isle of Wight
is classic ground of Geology. From the early days of the science it
has been made famous by the work of great students of Nature, such as
Mantell, Buckland, Fitton, Sedgwick, Owen, Edward Forbes, and others,
who have carried on the study up to the present day. Many of the
strata are known to geologists everywhere as typical; several bear the
names of the Island localities, where they occur; some--and those not
the least interesting--are not found beyond the limits of the Island.
Though studied for so many years, there is no exhausting their
interest: new discoveries are constantly made, and new questions arise
for solution. To those who have become interested in the rocks of the
Island, and the fossils they have found in them, and who wish to learn
how to read the story they tell, and to know something of that story,
this book is addressed. It is intended to be an introduction to the
science of Geology, based on the Geology of the Isle of Wight, yet
leading on to some glimpse of the history presented to us, when we
take a wider outlook still, and try to trace the whole wondrous path
of change from the world's beginning to the present day.
I wish to express my warmest thanks to Miss Maud Neal for the
beautiful drawings of fossils which illustrate the book, and to
Professor Grenville A. J. Cole, F.R.S., for his kindness in reading
the manuscript, and for valuable suggestions received from him. I have
also to acknowledge my indebtedness to Mr. H. J. Osborne White's new
edition of the _Memoir of the Geological Survey of the Isle of Wight_,
1921; and to thank Mr. J. Milman Brown, of Shanklin, for the three
photographs of Island scenery, showing features of marked geological
interest, and Mr. C. E. Gilchrist, Librarian of the Sandown Free
Library, for kindly reading the proofs of the book.
J. CECIL HUGHES.
Mar., 1922.
CONTENTS
Chap. Page
I. The Rocks and Their Story 1
II. The Structure of the Island 10
III. The Wealden Strata: The Land of the Iguanodon 15
IV. The Lower Greensand 23
V. Brook and Atherfield 29
VI. The Gault and Upper Greensand 37
VII. The Chalk 42
VIII. The Tertiary Era: The Eocene 54
IX. The Oligocene 63
X. Before and After: The Ice Age 70
XI. The Story of the Island Rivers; and How the Isle of
Wight Became An Island 86
XII. The Coming of Man 97
XIII. The Scenery of the Island: Conclusion 105
ILLUSTRATIONS OF FOSSILS
_PLATE I.--Facing page 20._
Wealden Cyrena Limestone
Vertebra of Iguanodon
Lower Greensand Perna Mulleti
Meyeria Vectensis (Atherfield Lobster)
Panopaea Plicata
Terebratula Sella
_PLATE II.--Facing page 23._
Lower Greensand Trigonia Caudata
Trigonia Daedalea
Gervillia Sublanceolata
Upper Greensand (Ammonite) Mortoniceras Rostratum
Nautilus Radiatus
_PLATE III.--Facing page 45._
Lower Greensand Thetironia Minor
Rhynchonella Parvirostris
Upper Greensand (Pecten) Neithea Quinquecostata
Chalk (Ammonite) Mantelliceras Mantelli
(Sea Urchins)
Micraster Cor-Anguinum
Echinocorys Scutatus
(Internal cast in flint)
_PLATE IV.--Facing page 61._
Eocene Cardita Plarnicosta
Turritella Imbricataria
Nummulites Laevigatus
(Fusus) Leiostoma Pyrus
Oligocene Limnaea Longiscata
Planorbis Euomphalus
Cyrena Semistriata
DIAGRAMS
Facing page
1. Coast, Sandown Bay 10
2. Coast, Atherfield 29
3. Coast, Whitecliff Bay 56
4. Section Through Headon Hill and High Down.
(Strata Seen at Alum Bay) 58
5. St George's Down 79
6, 7. Development of River Systems 86
8. The Old Solent River 94
9. Shingle at Foreland 79
PHOTOGRAPHS
Facing page
1. Gore Cliff. _Frontispiece._
2. Chalk at the Culver Cliffs. 46
3. Chalk at Scratchell's Bay. 51
GEOLOGICAL MAP OF THE ISLE OF WIGHT 112
Chapter I
THE ROCKS AND THEIR STORY
Walking along the sea shore, with all its varied interest, many must
from time to time have had their attention attracted by the shells to
be seen, not lying on the sands, or in the pools, but firmly embedded
in the solid rock of the cliffs and of the rock ledges which run out
on to the shore, and have, it may be, wondered sometimes how they got
there. At almost any point of the coast of the Isle of Wight, in bands
of limestone and beds of clay, in cliffs of sandstone or of chalk, we
shall have no difficulty in finding numerous shells. But it is not
only in the rocks of the sea coast that shells are to be found. In
quarries for building stone and in the chalk pits of the downs we see
shells in the rock, and may often notice them in the stones of walls
and buildings. How did they get there? The sea, we say, must once have
been here. It must have flowed over the land at some time. Now let us
think. We are going to read a wonderful story, written not in books,
but in the rocks. And it will be much more valuable if we learn to
read it ourselves, than if we are just told what other people have
made out. We know a thing much better if we see the answers to
questions for ourselves than if we are told the answers, and take some
one else's word for it. And if we learn to ask questions of Nature,
and get answers to them, it will be useful in all sorts of ways all
through life. Now, look at the shells in the rock of cliff and quarry.
How are they there? The sea cannot have just flowed over and left
them. The rock could not have been hard, as it is now, when they got
in. Some of the rocks are sandstone, much like the sand on the sea
shore, but they are harder, and their particles are stuck together.
Does sand on a sea shore ever become hard like rock, so that shells
buried in it are found afterwards in hard rock? Now we are getting the
key to a secret. We are learning the way to read the story of the
rocks. How? In this way. Look around you. See if anything like this is
happening to-day. Then you will be able to read the story of what
happened long, long ago, of how this world came to be as it is to-day.
We have asked a question about the sandstone. What about the clays and
the limestone? As before, what is happening to-day? Is limestone being
made anywhere to-day, and are shells being shut up in it? Are shells
in the sea being covered up with clay,--with mud,--and more shellfish
living on the top of that; and then, are they, too, being covered up?
So that in years to come they will be found in layers of clay and
stone like those we have been looking at in quarry and sea cliff?
We have asked our questions. Now we must look around, and see if we
can find the answers. After it has been raining heavily for two or
three days go down to the marshes of the Yar, and stand on one of the
bridges over the stream. We have seen it flowing quite clear on some
days. Now it is yellow or brown with mud. Where did the mud come from?
Go into a ploughed field with a ditch by the side. Down the ditch the
rain water is pouring from the field away to the stream. It is thick
with mud. Off the ploughed field little trickles of water are running
into the ditch. Each brings earth from the field with it. Off all the
country round the rain is trickling away, carrying earth into the
ditches and on into the stream, and the stream is carrying it down
into the sea. Now think. After every shower of rain earth is carried
off the land into the sea. And this goes on all the year round, and
year after year. If it goes on long enough--? Look a long way ahead, a
hundred years,--a thousand,--thousands of years. We shall be talking
soon of what takes many thousands of years to do. Why, you say, if it
goes on long enough, all the land will be carried into the sea. So it
will be. So it must be. You see how the world is changing. You will
soon see how it has changed already, what wonderful changes there have
been. You will see that things have happened in the world which you
never guessed till you began to study Geology.
Now, let us go a bit further. What becomes of all the mud the streams
and rivers are carrying down into the sea? Look at a stream coming
steeply down from the hills. How it rushes along, rolling pebbles
against one another, sweeping everything before it, clearing out its
channel, polishing the rocks, and carrying all it rubs off down
towards the sea. Now look at a river near its mouth in flat lowland
country. It flows now much slower; and so it has not power to bear
along all the material it swept down from the hills. And so it drops
a great deal; it is always silting up its own channel, and in flood
time depositing fresh layers of mud on the flat meadow land,--the
alluvial flat,--through which it generally flows in the last part of
its course. But a good deal of sediment is carried by the river out to
sea. The water of the river, moving slower as it enters the sea, has
less and less power to sweep along its burden of sand and mud, and it
drops it on the sea bottom,--first the bigger coarser particles like
the sand, then the mud; farther out, the finer particles of mud drop
to the bottom.
During the exploring cruise of the _Challenger_, under the direction
of Sir Wyville Thomson, in 1872-6, the most extensive exploration of
the depths of the sea that has been made up to the present time, it
was found that everything in the nature of gravel or sand was laid
down within a very few miles, only the finer muddy sediments being
carried as far as 20 to 50 miles from the land, the very finest of
all, under most favourable conditions, rarely extending beyond 150,
and never exceeding 300 miles from land into the deep ocean. So
gradually layer after layer of sand and mud cover the sea bed round
our coasts; and shells of cockles and periwinkles, of crabs and sea
urchins, and other sea creatures that have lived on the bottom of the
sea are buried in the growing layers of sand and mud. As layer forms
on layer, the lower layers are pressed together, and become more and
more solid. And so we have got a good way towards seeing the making of
clay and sandstone with shells in them, such as we saw in the sea
cliffs and the quarries.
But it is not only rain and rivers that are wearing the land away. All
round the coasts the sea is doing the same work. We see the waves
beating against the shores, washing out the softer material, hollowing
caves into the cliffs, eating away by degrees even the hardest rock,
leaving for a while at times isolated rocks like the Needles to mark
the former extension of the land. Most people see for themselves the
work of the sea, but do not notice so much what the rain and the
frost, the streams and the rivers are doing. But these are wearing
away the ground over the whole country, while the sea is only eating
away at the coast line. So the whole of the land is being worn away,
and the sand and mud carried out into the sea, and deposited there,
the material of new land beneath the waters.
How do these beds rise up again, so that we find them with their sea
shells in the quarry? Well, we look at the sea heaving up and down
with the tides, and we think of the land as firm and fixed. And yet
the land also is continually heaving up and down--very slowly,--far
too slowly for it to be noticed, but none the less surely. The exact
causes of this are not yet well understood, because we know but little
about the inside of the earth. The deepest mine goes a very little
way. We know that parts of the interior are intensely hot. The
temperature in a mine becomes hotter, about 1 deg.F. for every 60 ft. we go
down on the average. We know that there are great quantities of molten
rock in places, which, in a volcanic eruption is poured out in sheets
of lava over the land. There are great quantities of water turned into
steam by the heat, and in an eruption the steam pours out of the
crater of the volcano like the clouds of steam out of the funnel of a
locomotive. The people who live about a volcano are living, as it
were, on the top of the boiler of a steam engine; and their country is
sometimes shaken up and down like the lid of a kettle by the escaping
steam. In such a country the land is often changing its level. A few
miles from Naples at Pozzuoli, the ancient Puteoli, may be seen
columns of what appears to be an ancient market hall, though it goes
by the name of the Temple of Serapis. About half way up the columns
are holes bored by boring shellfish, such as we may find on the shore
here at low tide. We see from this that since the building was
constructed in Roman times the land has sunk, and carried the columns
into the sea, and shellfish have bored into them. Then the land has
risen, and lifted the columns out of the sea again.
But it is not only in the neighbourhood of volcanoes that the land is
moving. Not suddenly and violently, but slowly and gradually great
tracts of land rise and sink. Sometimes the land may remain for a long
time nearly stationary. The Southern coasts of England seem to stand
at much the same level as in the time of the Romans 1,500 or 2,000
years ago. On the other hand there is evidence which seems to show
that the coast of Norway has for some time been gradually rising.
It was thought at one time that the interior of the earth was liquid
like molten lava, and that the land we see was a comparatively thin
crust over this like the crust of a pie. But it is now believed for
various mathematical reasons, that the main mass of the earth is rigid
as steel. Still underneath the surface rocks there must be a quantity
of semi-fluid matter, like molten rock, and on this the solid land
sways about, as we see the ice on a pond sway with the pressure of the
skaters on it. So the solid land, pressed by internal forces, rises
and falls like the elastic ice, sometimes sinking and letting the sea
flow over, then rising again, and bringing up the land from beneath
the sea.
Again, as the heated interior of the earth gradually cools by the
radiation of the earth's heat into space, it will tend to shrink away
from the cooler rocks of the crust. This then, sinking in upon the
shrinking interior, will be thrown into folds, like the skin on a
shrivelled apple. Seeing, as we often do, layers of rock thrown into
numerous folds, so as to occupy a horizontal space far less than that
in which they were originally laid down, we can hardly resist the
conclusion that shrinkage of the cooling interior of the earth has
been a chief cause of the greatest movements of the surface, and of
the lateral pressure we so often find the strata to have undergone.
As we study geology we shall find plenty to show that the land does
rise and fall, that where now is land the sea has been, that land once
stretched where now is sea, though there is still much which is not
well understood about the causes of its movements. We have seen how
many of the rocks are made in the sea,--the sandstones and the
clays,--but there are two other kinds of rocks, about which we must
say a little. The first are the Igneous rocks, which means rocks made
by fire. These rocks have solidified, most frequently in crystalline
forms, from a molten mass. Lava, which flows hot and fluid, from a
volcano, and cooling becomes a sheet of solid rock, is an igneous
rock. Some igneous rocks solidify under ground under great pressure,
and become crystalline rocks such as granite. We shall not find these
rocks in the Isle of Wight. We should find them in Cornwall, Wales,
and Scotland; and, if we could go deep enough, we should find some
such rock as granite underneath the other rocks all the world over.
The other rocks, such as the sandstones and clays, are called
Sedimentary rocks, because they are formed of sediment, material
carried by the sea and rivers, and dropped to the bottom. They are
also called Stratified rocks, because they are formed of Strata,
_i.e._, beds or layers, as we see in cliff and quarry.
But we have seen another kind of rock,--the limestones. In Sandown Bay
towards the Culvers, bands of limestone run through the dark clay
cliffs, and broken fragments lie on the shore, looking like pieces of
paving stone. Examining these we find that they are made up of shells,
one band of small oysters, the others of shells of other kinds. You
see how they have been made. There has been an oyster bed, and the
shells have been pressed together, and somehow stuck together, so
that they have formed a layer of rock. They are stuck together in this
way. The atmosphere contains a small quantity of carbonic dioxide, and
the soil a larger quantity, the result of vegetable decomposition.
Rain water absorbs some of it, and carries it into the rocks, as it
soaks into the ground. This gas has the property of combining with
carbonate of lime,--the material of which shells and limestone are
made. The bicarbonate of lime so formed is soluble in water, which is
not the case with the simple carbonate. Water containing carbonic
dioxide soaking into a limestone rock or a mass of shells dissolves
some of the carbonate of lime, and carries it on with it. When it
comes to an open space containing air, some of the carbonic dioxide is
given off, leaving the insoluble carbonate of lime again. So by
degrees the hollows are filled up, and a solid layer of rock is
formed. Even while gathering in the sea the shell-fragments may be
cemented by the deposit of carbonate of lime from sea-water containing
more of the soluble bicarbonate than it can hold.
These limestones are examples of rocks which are said to be of organic
origin, that is to say, they are formed by living things. Organic
rocks may be formed by animal or vegetable growth. Rocks of vegetable
origin are seen in the coals. A peat bog is composed of a mass of
vegetable matter, chiefly bog moss, which for centuries has been
growing and accumulating on the spot. At the bottom of the bog will
frequently be found trunks of oak, or other trees, the remains of a
forest of former days. The wood has undergone chemical changes, has
lost much of its moisture, and often become very hard, as in bog oak.
Beds of coal have been formed by a similar process, on a much vaster
scale, and continued much longer. The remains of ancient forests have
been buried under sand stones and other rocks, have undergone chemical
change, and been compressed into the hard solid mass we call coal.
Fossil wood, which has not reached the stage of hard coal, but forms a
soft brown substance, is called lignite. This is of frequent
occurrence in various strata in the Isle of Wight.
Of organic rocks of animal origin the most remarkable are the chalk,
of which we shall speak later, and the coral-reefs, which are found in
the warm waters of tropical seas. Sailing over the South Pacific you
will see a line of trees--coconut trees chiefly--looking as if they
rose up from the sea. Coming nearer you see that they grow on a low
island, which rises only a few feet above the water. These islands are
often in the form of a ring, and look "like garlands thrown upon the
waters." Inside the ring is a lagoon of calm water. Outside the heavy
swell of the Southern Ocean thunders on the coral shore. If a sounding
line be let down from the outer edge of the reef, it will be found
that the wall of coral goes down hundreds of feet like a precipice. On
an island in the Southern Sea, Funafuti, a deep boring has been made
1,114 ft. deep. As far as the boring went all was coral. All this mass
of coral is formed by living things,--polyps they are called. They are
like tiny sea anemones, only they grow attached to one another,
forming a compound animal, like a tree with stem and branches, and
little sea anemones for flowers. The whole organism has a sort of
shell or skeleton, which is the coral. Blocks are broken off by the
waves, and ground to a coral mud, which fills up the interstices of
the coral; and as more coral grows above, the lower part of the reef
becomes, by pressure and cementing, a solid coral limestone. Once upon
a time there were coral islands forming in a sea, where now is
England. These old coral reefs form beds of limestone in Devon,
Derbyshire, and other parts of England. In the Isle of Wight we have
no old coral reefs, but we shall easily find fossil corals in the
rocks. They helped to make up the rocks, but there were not enough
here to make reefs or islands all of coral.
The great branching corals that form the reefs can only live in warm
waters. So we see that when corals were forming reefs where now is
England the climate must have been warm like the tropics. That is a
story we shall often read as we come to hear more about the rocks. We
shall find that the climate has often been quite warm as the tropics
are now: and we shall also read another wonderful story of a time when
the climate was cold like the Arctic regions.