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Title: The Geological Story of the Isle of Wight
Author: Hughes, J. Cecil
Language: English
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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)
Panopæa Plicata
Terebratula Sella
_PLATE II.--Facing page 23._
Lower Greensand Trigonia Caudata
Trigonia Dædalea
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 Lævigatus
(Fusus) Leiostoma Pyrus
Oligocene Limnæa 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°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.
Chapter II.
THE STRUCTURE OF THE ISLAND.
The best place to begin the study of the Geology of the Isle of Wight
is in Sandown Bay. North of Sandown, beyond the flat of the marshes,
are low cliffs of reddish clay, which has slipped in places, and is
much covered by grass. At low tide we shall see the coloured clays on
the shore, unless the sand has covered them up. Variegated marls they
are called--_marl_ means a limy clay, _loam_ a sandy clay; and very
fine are the colours of these marls, rich reds and purples and browns.
Beyond the little sea wall below Yaverland battery we come to a
different kind of clay forming the cliff. It is in thin layers. Clay
in thin layers like this is called _shale_. Some of these shales are
known as paper shales, for the layers are thin almost like the leaves
of a book. The junction of the shales with the marls is quite sharp,
and we see that the shales rest on the coloured marls, not
horizontally, but sloping down towards the North. Bands of limestone
and sandstone running through the shales, and a hard band of brown
rock which runs out on the shore as a reef, slope in the same
direction. As we pass on by the Red Cliff to the White Cliffs we
notice that the strata slope more steeply the further North we go. We
have seen that these strata were laid down layer by layer at the
bottom of the sea. If we find a lot of things lying one on top of
another, we may generally conclude that the ones at the bottom were
put there first, then the next, and so on to the top. And this will
generally be true with regard to the rocks. The lowest rocks must have
been laid down first, then the next, and so on. But these layers of
shale with shells in them, and layers of limestone made of shells,
must have been laid down at first fairly flat on the sea floor; but as
they were upheaved out of the sea they have been tilted, so that we
now see them in an inclined position. And when we come to the chalk,
we should see, if we looked at the end of the Culver Cliffs from a
boat, that the lines of black flints that run through the chalk are
nearly vertical. The strata there have been tilted up on end.
[Illustration: FIG. 1.]
DIAGRAM OF COAST, SANDOWN BAY, DUNNOSE TO CULVER CLIFF.
W _Wealden._
P _Perna Bed._
LG _Lower Greensand._
Cb _Clay Bands._
S _Sandrock and Carstone._
g _Gault._
UG _Upper Greensand._
C _Chalk._
Sc _Shanklin Chine._
Lc _Luccombe Chine._
In describing how strata lie, we call the inclination of the strata
from the horizontal the _dip_. The direction of a horizontal line at
right angles to that of the dip is called the _strike_. If we compare
the sloping strata to the roof of a house, a line down the slope of
the roof will mark the direction of the dip, the ridge of the roof
that of the strike. The strata we are considering dip towards the
North; the line of strike is East and West.
Returning towards Sandown we see the strata dipping less and less
steeply, till near the Granite Fort the rocks on the shore are
horizontal. Continuing our walk past Sandown to Shanklin we pass the
same succession of rocks we have been looking at, but in reverse
order, and sloping the other way. It is not very easy to see this at
first, for so much is covered by building; but beyond Sandown we see
Sandstone Cliffs like the Red Cliff again, the strata dipping gently
now to the south, and in the downs above Shanklin we see the chalk
again. So we have the same strata north and south of Sandown, forming
a sort of arch. But the centre of the arch is missing. It must have
been cut away. We saw that the land was all being eaten away by rain
and rivers. Now we see what they have done here. Go up on to the
Downs, and look over the central part of the Island. We see two ranges
of downs running from east to west,--the Central Downs of the Island,
a long line of chalk down 24 miles from the Culver Cliff on the east
to the Needles on the west; and the Southern Downs along the South
Coast from Shanklin to Chale. In the Central Downs the chalk rises
nearly vertically, and turns over in the beginning of an arch towards
the South. Then comes a big gap, and the chalk appears again in the
Southern Downs nearly horizontal, sloping gently to the south. The
chalk was once joined right across the central hollow, where now we
see the villages of Newchurch, Godshill, and Arreton. All that
enormous mass of rock that once filled the space between the downs has
been cut away by running water.
An arch of strata like this [Inverted-U], such as the one we are looking
at, is called an _anticline_. When the arch is reversed, like this [U],
it is called a _syncline_. Looking north from the Central Downs over the
Solent we are looking at a syncline. The chalk, which dips down at the
Culvers and along the line of the Central Downs, runs like a trough
under the Solent, and rises again, as we see it on the other side, in
the Portsdown Hills.
We might suppose the top of an anticlinal arch would be the highest
part of the country; that, even if rain and running water have worn
the country down, that would still stand highest, and be worn down
least. But there are reasons why this need not be so. For one thing,
when the horizontal strata are curved over into an arch, they
naturally crack just at the top of the curve, so and into the cracks
the rain gets, and so a stream is started there, which cuts down and
widens its channel, and so eats the land away. Again, the rising land
only emerges gradually from the sea, and the sea may cut off the top
of the arch before it has risen out of its reach. Moreover on the
higher land the fall of rain and snow is greater, and the frosts are
more severe; so that it is just there that the forces wearing down the
land are most effective.
[Illustration: curve with two v-shaped marks at center]
We must notice another thing which happens when rocks are being
upheaved and bent into curves. The strain is very great, and sometimes
the strata crack and one side is pushed up more than the other. These
cracks are called _faults_. At Little Stairs, about half way between
Sandown and Shanklin, two or three faults may be seen in the cliff.
The effect of two of the faults may be easily seen by noticing the
displacement of a band of rock stained orange by water containing
iron. The strata are thrown down towards the north about 8 ft. A third
fault, the effect of which is not so evident at first sight, throws
the strata down roughly 50 ft. to the south. These are only small
faults, but sometimes faults occur, in which the strata have been
moved on opposite sides of the fault thousands of feet away from one
another. We might think we should see a wall of rock rising up on the
surface of the ground where a fault occurs; but the faults have mostly
taken place ages ago; and, when they do happen, the rocks are
generally moved only a little way at a time. Then after a while
another push comes on the rocks, and they shift again at the same
place, and go a bit further. All this time frost and rain and rivers
are working at the surface, and planing it down; so that the
unevenness of the surface caused by faults is smoothed away; and so
even a great fault does not show at the surface.
As we follow the Sandown anticline westward it gradually dies away,
the upheaved area being actually a long oval--what we may call a
turtle-back. As the Sandown anticline dies out, it is succeeded by
another a little further south, the Brook anticline. There are in fact
a series of these east and west anticlines in the Island and on the
adjacent mainland, caused by the same earth movement. As a consequence
of the arching of the strata we find the lowest beds we saw in Sandown
Bay running out again on the west of the Island in Brook Bay, and a
general correspondence of the strata on the east and west of the
Island; while, as we travel from Sandown or Brook northward to the
Solent, we come to continually more recent beds overlying those which
appear to the south of them.
When, as in the south side of our central downs, the strata are
sharply cut away by denudation, we call this an _escarpment_. The
figure shows the structure of the Sandown anticline we have described.
We must now examine the rocks more closely, beginning with the lowest
strata in the Island, and try to read the story they have to tell.
Chapter III
THE WEALDEN STRATA: THE LAND OF THE IGUANODON
The lowest strata in the Isle of Wight are the coloured marls and
blue-grey shales we have already observed in Sandown Bay, which run
through the Island to Brook Bay. They are known as the Wealden Strata,
because the same strata cover the part of Kent and Sussex called the
Weald. They consist of marls and shales with bands of sandstone and
limestone. The marls and shales in wet weather become very soft, and
flow out on to the shore, causing large slips of land.[1] Now, what we
want to find out is what the world was like ages ago, when these
Wealden Strata were being formed. We have learnt something of how
clays and sandstones and limestones are formed: to learn more we must
see what sort of fossils we can find in these rocks. "Fossil" means
something dug up; and the word is generally used for remains of
animals or plants which we find buried in the rocks. We have seen
shells in these strata. These we must examine more closely. And as we
walk on the shore we shall find other fossils. In the marls and shales
exposed on the shore we are pretty sure to see pieces of wood, black
as coal, sometimes quite large logs, often partly covered with shining
iron pyrites. Perhaps you say--I hope you do--there must have been
land not far away when these marls and shales were forming. Always try
to see what the things we find have to tell us. The sort of place
where we should be most likely to find wood floating in the sea to-day
would be near the mouth of a great river like the Mississippi or the
Amazon,--rivers which bring down numerous logs of wood from the forest
country through which they flow.
Examine the shales and limestone bands. On the surface of some of the
paper-shales are numbers of small round or oval white spots. They are
the remains of shells of a very minute crustacean, Cypris and
Cypridea, from which the shales are known as Cyprid shales. In other
bands of shale are quantities of a bivalve shell called _Cyrena_.
There is a band of limestone made up of Cyrena shells, containing also
little roundish spiral shells called _Paludina_.[2] This limestone
resembles that called Sussex or Petworth Marble, which is mainly
composed of shells of Paludina, but some layers also contain bivalve
shells. It is hard enough to take a good polish, and may be seen, like
the similar Purbeck marble, in some of our grand old churches. Another
band of limestone running through the shales is made up of small
oysters (_Ostrea distorta_).
We shall see fossil shells best on the _weathered_ surfaces of rocks,
_i.e._, surfaces which have been exposed to the weather. One
beginning geological study will probably think we shall find fossils
best by looking at fresh broken surfaces of rock. This is not so. If
you want to find fossils, look at the rock where it has been exposed
to the weather. The action of the weather--rain, carbonic dioxide in
the rain water, etc.--is to sculpture the surface of the rock, so that
the fossils stand out in relief. A weathered surface is often seen
covered with fossils, when a new broken one shows none at all.
Many of the shells in the limestones are very like shells which are
found at the present day. We must know where they are found now. Well,
these Paludinas are a kind of freshwater snail; and, in fact, all the
shells we find in the Wealden strata are freshwater shells, till we
come near the top, and find the oysters, which live in salt or
brackish water. There were quantities in Brading Harbour in old days,
before it was reclaimed from the sea. Now, this is a very important
point, that our Wealden shells are freshwater shells. For what does it
tell us? Why, we see that the first strata we have come to examine
were not laid down in the sea at all. Then where were they formed?
They seem to be the Delta of a great river, long since passed away,
like the Nile, the Amazon, or the Niger at the present day. When these
great rivers near the sea, they spread out in many channels, and
deposit the mud they have brought down over a wide area shaped like a
V, or like the Greek letter $Delta$ (Delta). Hence we speak of the
Delta of the Nile. Some river deltas are of immense size. That of the
Niger, for instance, is 170 miles long, and the line where it meets
the sea is 300 miles long. Our old Wealden river must have been a
great river like the Niger, for the Wealden strata stretch,--often
covered up for a long way by later rocks, then appearing again,--as
far as Lulworth on the Dorset coast to the west, into Buckinghamshire
on the north, while to the north east they not only cover the Weald,
but pass under the Straits of Dover into Belgium, and very similar
strata are found in Westphalia and Hanover. The ancient river delta
must have been 200 miles or more across.
You must not think this great river flowed in the Island of England as
it is to-day. England was being made then. This must have been part of
a great continent in those days, for such a great river to flow
through, and form a delta of such size. We cannot tell quite what was
the course of this river. But to the north of where we are now must
have stretched a great continent, with chains of lofty mountains far
away, from which the head waters of the river flowed. Near its mouth
the river broke up into many streams, separated by marsh land; while
inside the sand banks of the sea shore would be large lagoons as in
the Nile delta at the present day. In these waters lived the shellfish
whose shells we are finding. And flowing through great forests the
river carried down with it logs of wood and whole trees, and left them
stuck in the mud near its mouths for us to find to-day.
What kind of trees grew in the country the river came from? Well,
there were no oaks or beeches, no flowering chestnuts or apples or
mays. But there were great forests of coniferous trees; that is trees
like our pines and firs, cedars and yews, and araucarias; and there
were cycads--a very different kind of tree, but also bearing
cones--which you may see in a greenhouse in botanical gardens. They
have usually a short trunk, sometimes nearly hemispherical, with
leaves like the long leaves of a date palm. They are sometimes called
sago trees, for the trunk has a large pith, which, like some palms,
gives us sago. Stems of cycads, covered with diamond-shaped scars,
where the leaf stalks have dropped off, are found in the Wealden
deposits. Most of the wood we find is black and brittle. Some,
however, is hard as stone, where the actual substance of the wood has
been replaced by silica, preserving beautifully the structure of the
wood. Specially noteworthy are fragments of a tree called
_Endogenites_ (or _Tempskya_) _erosa_, because it was at first
supposed to belong to the endogens,--the class to which the palm
bamboo belong; it is now considered to be a tree-fern. Many specimens
of this wood are remarkably beautiful, when polished, or in their
natural condition. Here, by the way, it may be well to explain how we
name animals and plants scientifically. We have English names only for
the commoner varieties. So we have to invent names for the greater
number of living and extinct animals and plants. And the best way is
found to be this. We give a name, generally formed from the Latin--or
the Greek--to a group of animals or plants, which closely resemble one
another; the group we call a _genus_. Then for the _species_, the
particular kind of animal or plant of the group, we add a second name
to the first. Thus, if we are studying the apple and pear group of
fruit trees, we call the general name of the group _Pyrus_. Then the
crab apple is _Pyrus malus_, the wild pear _P. communis_, and so on.
So that when you arrange any of your species, and put down the
scientific names, you are really doing a bit of classification as
well. You are arranging your specimens with their nearest relations.
To return to our ancient river. With the logs and trunks of trees,
which the river brought down, came floating down also the bodies of
animals, which had lived in the country the river flowed through. What
kind of animals? Very wonderful animals, some of them, not like any
living creature that lives to-day. By the time they reached the mouth
of the river the bodies had come to pieces, and their bones were
scattered about the river mouth. On the shore where we are walking we
may find some of these bones. But it is rather a chance whether we
find any in any one walk we take. The best time to find them is when
rough seas in winter have washed some out of the clay, and left them
on the shore. It is only rarely that large bones are found here; but
you should be able to find some small ones fairly often. The bones are
quite as heavy as stone, for all the pores and cavities have been
filled with stone, generally carbonate of lime, in the way we
explained in describing the formation of beds of limestone. This makes
them quite different from any present-day bones that may happen to lie
on the shore. So that you cannot mistake them, if once you have seen
them. They are bones of great reptiles,--the class of creatures to
which lizards and crocodiles belong. But these were much larger than
crocodiles, and quite peculiar in their appearance. The principal one
was the Iguanodon. He stood on his hind legs like a kangaroo, with a
great thick tail, which may have helped to support him. When full
grown he stood about 14 ft. high. You may find on the shore vertebræ,
_i.e._, joints of the backbone, sometimes large, sometimes quite small
if they come from the end of the tail. I have found several here about
5 inches long by 4 or 5 across. A few years ago I found the end of a
leg bone almost a foot in diameter. Dr. Mantell, a great geological
explorer in the days when these reptiles were first discovered about
80 years ago, estimated from the size of part of a bone found in
Sandown Bay that one of these reptiles must have had a leg 9 ft. long.
It was a long time after the bones of these creatures were first found
before it was known what they really looked like. The animals lived a
long way from here, and by the time the river had washed them down to
its mouth the skeletons were broken up, and the bones scattered. At
last a discovery was made, which told us what the animals were like.
In a coal mine at Bernissart in Belgium the miners found the coal seam
they were following suddenly come to an end, and they got into a mass
of clay. After a while it was seen what had happened. They had struck
the buried channel of an old river, which in the Wealden days had
flowed through and cut its channel in the coal strata, which are much
older still than the Wealden. And in the mud of the ancient buried
river what should they come upon but whole skeletons of Iguanodons. In
the days of long ago the great beasts had come down to the river to
drink, and had got "bogged" in the soft clay. The skeletons were
carefully got out, and set up in the Museum at Brussels. Without going
so far as that, you may see in the Natural History Museum in London,
or the Geological Museum at Oxford, a facsimile of one of these
skeletons, large as life, and have some idea of the sort of beast the
Iguanodon was. I should tell you why he was so named. Before it was
known what he was like in general form, it was found that his teeth,
which are of a remarkable character, were similar to those of the
Iguana, a little lizard of the West Indies. So he was called
Iguanodon,--an animal with teeth like the Iguana (fr. _Iguana_, and
Gk. $odous$ g. $odontos$ a tooth). He was quite a harmless beast,
though he was so large. He was a vegetarian. There were other great
reptiles, more or less like him, which were also vegetable feeders.
But there were also carnivorous reptiles, generally smaller than the
herbivorous, whose teeth tell us that they preyed on other animals.
[Illustration: PL. I]
Perna Mulleti Meyeria Vectensis
(Atherfield Lobster)
Panopæa Plicata Terebratula Sella
Cyrena Limestone Iguanodon Vertebra
WEALDEN AND LOWER GREENSAND
Those were the days of reptiles. Now the earth is the domain of the
mammalia. But then great reptiles like the Iguanodon wandered over the
land; great marine reptiles, such as the Plesiosaurus, swam the
waters; and wonderful flying reptiles, the Pterodactyls, flew the air.
Some species of these were quite small, the size of a rook: one large
species found in the Isle of Wight had a spread of wing of 16 feet.
Imagine this strange world,--its forests with pines and monkey puzzles
and cycads,--ferns also, of which many fragments are found,--its great
reptiles and little reptiles, on land, in the water and the air. Were
there no birds? Yes, but they were rare. From remains found in Oolitic
strata,--somewhat older than the Wealden,--we know that birds were
already in existence; and they were as strange as anything else. For
they had jaws with teeth like the reptiles. They had not yet adopted
the beak. And instead of all the tail feathers starting from one
point, as in birds of the present day, these ancient birds had long
curving tails like reptiles, with a pair of feathers on each joint.
Birds of similar but slightly more modern type have been found in
Cretaceous strata (to which the Wealden belongs) in America, but so
far not in strata of this age in Britain.
Among other objects of interest along this Wealden shore may be
noticed a curious transformation which has affected the surface of
some of the shell limestones after they were formed, which is known as
cone-in-cone structure. It has quite altered the outer layer of the
rock, so that all trace of the shells of which it consists is
obliterated. Numerous pieces of iron ore from various strata lie on
the shore. Through most of English history the Weald of Kent and
Sussex was the great iron-working district of England. The ore from
the Wealden strata was smelted by the help of charcoal made from the
woods that grew there, and gave the district its name;--for _Weald_
means "forest." This industry gradually ceased, as the much larger
supplies of iron ore found near the coal in the mines of the North of
England came to be worked. Iron pyrites, sulphide of iron in
crystalline form, was formerly collected on the Sandown shore, and
sent to London for the manufacture of sulphuric acid. This mineral is
often found encrusting fossil wood. It also occurs as rounded nodules
(mostly derived from the Lower Chalk) with a brown outer coat, and
often showing a beautiful radiated metallic structure, when broken.
(This form is called marcasite.)
As we walk by the edge of the water, we shall see what pretty stones
lie along the beach. When wet with the ripples many look like polished
jewels. Some are agates, bright purple and orange in colour, some
clear translucent chaldedony. We shall have more to say about these
later on. They do not come from the Wealden, but from beds of flint
gravel, and are washed along the shore. But there are also jaspers
from the Wealden. These are opaque, generally red and yellow. There
are also pieces of variegated quartz, and other beautiful pebbles of
various mineral composition. These are stones from older rocks, which
have been washed down the Wealden rivers, and buried in the Wealden
strata, to be washed out again after hundreds of thousands of years,
and rolled about on the shore on which we walk to-day.
[Footnote 1: Blue clays of various geological age, which in wet
weather become semi-liquid, and flow out on to the shore, are
known in the Island by the local name of _Blue Slipper_.]
[Footnote 2: The name now adopted is _Viviparus_. There is also
a band of ferruginous limestone mainly composed of _Viviparus_.]
[Illustration: PL. II]
Trigonia Caudata Trigonia Dædalea
Gervillia Sublanceolata
(Ammonite) Nautilus Radiatus
Mortoniceras Rostratum
LOWER AND UPPER GREENSAND
Chapter IV
THE LOWER GREENSAND
For ages the Wealden river flowed, and over its vast delta laid down
its depth of river mud. The land was gradually sinking; for
continually strata of river mud were laid down over the same area, all
shallow-water strata, yet counting hundreds of feet in thickness in
all. At last a change came. The land sank more rapidly, and in over
the delta the sea water flowed. The sign of coming change is seen in
the limestone band made up of small oysters near the top of the
Wealden strata. Marine life was beginning to appear.
Above the Wealden shales in Sandown Bay may be seen a band of brown
rock. It is in places much covered by slip, but big blocks lie about
the shore, and it runs out to sea as a reef before we come to the Red
Cliff. The blocks are seen to consist of a hard grey stone, but the
weathered surfaces are soft and brown. They are full of fossils, all
marine, sea shells and corals. The sea has washed in well over our
Wealden delta, and with this bed the next formation, the Lower
Greensand, begins. The bed is called the Perna bed, from a large
bivalve shell (_Perna mulleti_) frequently to be found in it, though
it is difficult to obtain perfect specimens showing the long hinge of
the valve, which is a marked feature of the shell. Among other shells
are a large round bivalve _Corbis_ (_Sphæra_) _corrugata_, a flatter
bivalve _Astarte_,--and a smaller oblong shell _Panopæa_,--also a
peculiar shell of triangular form, _Trigonia_,--one species _T.
caudata_ has raised ribs running across it, another _T. dædalea_ has
bands of raised spots. A pretty little coral, looking like a
collection of little stars, _Holocystis elegans_, one of the Astræidæ,
is often very sharply weathered out.
Above the Perna bed lies a mass of blue clay, weathering brown, called
the Atherfield clay, because it appears on a great scale at Atherfield
on the south west of the Island. It is very like the clay of the
Wealden shales, but is not divided into thin layers like shale.
Next we come to the fine mass of red sandstone which forms the
vertical wall of Red Cliff. Not many fossils are to be found in these
strata. Let us note the beauty of colouring of the Red Cliff--pink and
green, rich orange and purple reds. And then let us pass to the other
side of the anticline, and walk on the shore to Shanklin. Here we see
the red sandstone rocks again, but now dipping to the south. You
probably wonder why these red cliffs are called Greensand. But look at
the rocks where they run out as ledges on the shore towards Shanklin.
Here they are dark green. And this is really their natural colour.
They are made of a mixture of sand and clay coloured dark green by a
mineral called glauconite. Grains of glauconite can easily be seen in
a handful of sand,--better with a magnifying glass. This mineral is a
compound of iron, with silica and potash, and at the surface of the
rock it is altered chemically, and oxide of iron is formed--the same
thing as rust. And that colours all the face of the cliff red. The
iron is also largely responsible for our finding so few fossils in
these strata. By chemical changes, in which the iron takes part, the
material of the shells is destroyed.[3] Near Little Stairs hollows in
the rock may be seen, where large oyster shells have been. In some you
may find a broken piece of shell, but the shells have been mostly
destroyed. Nearer Shanklin we shall find large oysters, _Exogyra
sinuata_, in the rock ledges exposed at low tide. Some are stuck
together in masses. Evidently there was an oyster bank here. And here
the shells have not been destroyed like those in the cliff.
From black bands in the cliff water full of iron oozes out, staining
the cliff red and yellow and orange, and trickling down, stains the
flint stones lying on the shore a bright orange. At the foot of the
cliff you may sometimes see what looks like a bed of conglomerate,
_i.e._, a bed of rounded pebbles cemented together. This does not
belong to the cliff, but is made up of the flint pebbles on the shore,
and the sand in which they lie, cemented into a solid mass by the iron
in the water which has flowed from the cliff. It is a modern
conglomerate, and shows us how old conglomerates were formed, which we
often find in the various strata. The cement, however, in these is not
always iron oxide. It may be siliceous or of other material. The
iron-charged water is called chalybeate; springs at Shanklin and Niton
at one time had some fame for their strengthening powers. The strata
we have been examining are known as the Ferruginous sands, _i.e._,
iron sands (Lat. _ferrum_, "iron"). Beyond Shanklin is a fine piece of
cliff. Look up at it, but beware of going too close under it. The
upper part consists of a fine yellow sand called the Sandrock. At the
base of this are two bands of dark clay. These bands become filled
with water, and flow out, causing the sandrock which rests on them to
break away in large masses, and fall on to the beach.
It is clay bands such as these which are the cause of our Undercliffs
in the Isle of Wight. Turn the point, and you see exactly how an
undercliff is formed. You see a wide platform at the level of the
clay, which has slipped out, and let down the sandrock which rested on
it. Beyond Luccombe Chine a large landslip took place in 1910, a great
mass of cliff breaking away, and leaving a ravine behind partly filled
with fallen pine trees. The whole fallen mass has since sunk lower and
nearer to the sea. The broken ground overgrown with trees called the
Landslip, as well as the whole extent of the ground from Ventnor and
Niton, has been formed in a similar way. But the clay which by its
slip has produced these is another clay called the Gault, higher up in
the strata. At the top of the high cliff near Luccombe Chine a hard
gritty stratum of rock called the Carstone is seen above the Sandrock,
and above it lies the Gault clay, which flows over the edge of the
cliff.
In the rock ledges and fallen blocks of stone between Shanklin and
Luccombe many more fossils may be found than in the lower part of the
Ferruginous sands. Besides bands of oysters, blocks of stone are to be
found crowded with a pretty little shell called _Rhynchonella_. There
are others with many _Terebratulæ_, and others with fragments of sea
urchins. The Terebratulæ and Rhynchonellæ belong to a curious group
of shells, the Brachiopods, which are placed in a class distinct from
the Mollusca proper. They were very common in the very ancient seas of
the Cambrian period,--the period of the most ancient fossils yet
found,--and some, the Lingulæ, have lived on almost unchanged to the
present day. One of the two valves is larger than the other, and near
the smaller end you will see a little round hole. Out of this hole,
when the creature was alive, came a sort of neck, which attached it to
the rock, like the barnacles. There is a very hard ferruginous band,
of which nodules may be found along the shore, full of beautifully
perfect impressions of fossils, though the fossils themselves are
gone. Casts of a little round bivalve shell, _Thetironia minor_, may
easily be got out. The nodules also contain casts of Trigonia,
Panopoea, etc. A stratum is sometimes exposed on the shore
containing fossils converted into pyrites. A long shell, _Gervillia
sublanceolata_, is the most frequent.
All the shells we have found are of sea creatures, and show us that
the Greensand was a marine formation. But the strata were formed in
shallow water not far from the shore. We have learnt that coarse
sediment like sand is not carried by the sea far from the coast. And a
good deal of the Greensand is coarser than sand. There are numerous
bands of small pebbles. The pebbles are of various kinds; some are
clear transparent quartz, bits of rock-crystal more or less rounded by
rolling on the shore of the Greensand period. These go by the name of
Isle of Wight diamonds, and are very pretty when polished. Another
mark of the nearness of the shore when these beds were laid down is
the current bedding, of which a good example may be seen in the cliff
at the north of Shanklin parade. It is sometimes called false bedding,
for the sloping bands do not mark strata laid down horizontally at the
bottom of the sea, but a current has laid down layers in a sloping
way,--it may be just over the edge of a sandbank. Again notice how
much wood is to be seen in the strata. Land was evidently not far off.
All along the shore you may find hard pieces of mineralised wood, the
rings of growth often showing clearly. Frequently marine worms have
bored into them before they were locked up in the strata; the holes
being generally filled afterwards with stone or pyrites.
The wood is mostly portions of trunks or branches of coniferous trees.
We also find stems of cycads. There has been found at Luccombe a very
remarkable fruit of a kind of cycad. We said that in the Wealden
period none of our flowering plants grew. But these specimens found at
Luccombe show that cycads at that time were developing into flowering
plants. Wonderful specimens of what may almost be called cycad flowers
have been found in strata of about this age in Wyoming in America; and
this Luccombe cycad,--called Benettites Gibsonianus,--shows what these
were like in fruit. Remains of various cycadeous plants have been
found in the corresponding strata at Atherfield; and possibly by
further research fresh knowledge may be gained of an intensely
interesting story,--the history of the development of flowering
plants.
On the whole the vegetation of the period was much the same as in the
Wealden. But these flowering cycads must have formed a marked addition
to the landscape,--if indeed they did not already exist in the Wealden
times. The cones of present day cycads are very splendidly
coloured,--orange and crimson,--and it can hardly be doubted that the
cycad flowers were of brilliant hues.
The land animals were still like the Wealden reptiles. Bones of large
reptiles may at times be found on the shore at Shanklin. Several have
been picked up recently. From the prevalence of cycads we may conclude
that the climate of the Wealden and Lower Greensand was sub-tropical.
The existing Cycadaceæ are plants of South Eastern Asia, and
Australia, the Cape, and Central America. The forest of trees allied
to pines and firs and cedars probably occupied the higher land.
Turtles and the corals point to warm waters. The existing species of
Trigonia are Australian shells. This beautiful shell is found
plentifully in Sydney harbour. It possesses a peculiar interest, as
the genus was supposed to be extinct, and was originally described
from the fossil forms, and was afterwards found to be still living in
Australia.
[Footnote 3: Carbonate of lime has been replaced by carbonate of
iron, and the latter converted into peroxide of iron. At Sandown
oxidation has gone through the whole cliff.]
[Illustration: FIG. 2]
COAST ATHERFIELD TO ROCKEN END
Wl _Wealden Beds._
P _Perna Bed._
A _Atherfield Clay._
Ck _Cracker Group._
Lg _Lower Gryphæa Beds._
Sc _Scaphite. "_
Lc _Lower Crioceras "_
W _Walpen Clay._
Uc _Upper Crioceras Beds._
WS _Walpen and Ladder Sands._
Ug _Upper Gryphæa Beds._
Ce _Cliff End Sands._
F _Foliated Clay._
SU _Sands of Walpen Undercliff._
Fer _Ferruginous Bands of Blackgang Chine._
B _Black Clay._
S _Sandrock and Clays._
Wh _Whale Chine._
L _Ladder Chine._
Wp _Walpen Chine._
Bg _Blackgang Chine._
Chapter V
BROOK AND ATHERFIELD
To most Sandown Bay is by far the most accessible place in the Island
to study the earlier strata; and for our first geological studies it
has the advantage of showing a succession of strata so tilted that we
can pass over one formation after another in the course of a short
walk. But when we have learnt the nature of geological research, and
how to read the record of the rocks, and examined the Wealden and
Greensand strata in Sandown Bay, we shall do well, if possible, to
make expeditions to Brook and Atherfield, to see the splendid
succession of Wealden and Greensand strata shown in the cliffs of the
south-west of the Island. It is a lonely stretch of coast, wild and
storm-swept in winter. But this part of the Island is full of
interest and charm to the lover of Nature and of the old-world
villages and the old churches and manor houses which fit so well into
their natural surroundings. The villages in general lie back under the
shelter of the downs some distance from the shore; a coastguard
station, a lonely farm house, or some fishermen's houses as at Brook,
forming the only habitations of man we come to along many miles of
shore. Brook Point is a spot of great interest to the geologist. Here
we come upon Wealden strata somewhat older than any in Sandown Bay.
The shore at the Point at low tide is seen to be strewn with the
trunks of fossil trees. They are of good size, some 20 ft. in length,
and from one to three feet in diameter. They are known as the Pine
Raft, and evidently form a mass of timber floated down an ancient
river, and stranded near the mouth, just as happens with great
accumulations of timber which float down the Mississippi at the
present day. The greater part of the wood has been replaced by stone,
the bark remaining as a carbonaceous substance like coal, which,
however, is quickly destroyed when exposed to the action of the waves.
The fossil trees are mostly covered with seaweed. On the trunks may
sometimes be found black shining scales of a fossil fish, _Lepidotus
Mantelli_. (A stratum full of the scales of _Lepidotus_ has been
recently exposed in the Wealden of Sandown Bay.) The strata with the
Pine Raft form the lowest visible part of the anticline. From Brook
Point the Wealden strata dip in each direction, east and west. As the
coast does not cut nearly so straight across the strata as in Sandown
Bay, we see a much longer section of the beds. On either side of the
Point are coloured marls, followed by blue shales, as at Sandown. To
the westward, however, after the shales we suddenly come to variegated
marls again, followed by a second set of shales. There was long a
question whether this repetition is due to a fault, or whether local
conditions have caused a variation in the type of the beds. The
conclusion of the Geological Survey Memoir, 1889, rather favoured the
latter view, on the ground of the great change which has taken place
in the character of the beds in so short a distance, assuming them to
be the same strata repeated. The conjecture of the existence of a
fault has, however, been confirmed; for during the last years a most
interesting section has been visible at the junction of the shales and
marls, where a fault was suspected. The shales in the cliff and on the
shore are contorted into the form of a Z. The section appears to have
become visible about 1904 (it was in the spring of that year that I
first saw it), and was described by Mr. R. W. Hooley, F.G.S. (_Proc.
Geol. Ass._, vol. xix., 1906, pp. 264, 265). It has remained visible
since.
The Wealden of Brook and the neighbouring coast is celebrated for the
number of bones of great reptiles found here, from the early days of
geological research, the '20's and '30's of last century, when
admirable early geologists, such as Dr. Buckland and Dr. Mantell, were
discovering the wonders of that ancient world, to the present time.
Various reptiles have been found besides the Iguanodon--the
Megalosaurus, a great reptile somewhat similar, but of lighter build,
with sabre-shaped teeth, with serrated edges: the Hylæosaurus, a
smaller creature with an armour of plates on the back, and a row of
angular spines along the middle of the back; the huge _Hoplosaurus
hulkei_, probably 70 or 80 feet in length; the marine Plesiosaurus and
Ichthyosaurus, and several more; also bones of a freshwater turtle and
four types of crocodiles. In various beds a large freshwater shell,
_Unio valdensis_, occurs, and in the cliffs of Brook have been found
many cones of Cycadean plants. In bands of white sandy clay are
fragments of ferns, _Lonchopteris Mantelli_. In the shales are bands
of limestone with Cyrena, Paludina, and small oysters, and paper
shales with cyprids, as at Sandown. The shore near Atherfield Point is
covered with fallen blocks of the limestones.
The Lower Greensand is seen in Compton Bay on the northern side of the
Brook anticline. Here is a great slip of Atherfield clay. The beds
above the clay are much thinner than at Atherfield, and fossils are
comparatively scarce. On the south of the anticline the Perna bed
slopes down to the sea about 150 yards east of Atherfield Point, and
runs out to sea as a reef. Large blocks lie on the shore, where
numerous fossils may be found on the weathered surfaces. The ledges
which here run out to sea form a dangerous reef, on which many vessels
have struck. There is now a bell buoy on the reef. On the headland is
a coastguard station, and till lately there has been a sloping wooden
way from the top of the cliff to bring the lifeboat down. This was
washed away in the storms of the winter 1912-13.
Above the Perna bed lies a great thickness of Atherfield clay. Above
this lies what is called the Lower Lobster bed, a brown clay and sand,
in which are numerous nodules containing the small lobster _Meyeria
vectensis_,--known as Atherfield lobsters. Many beautiful specimens
have been obtained.
We next come to a great thickness of the Ferruginous Sands, some 500
feet. The Lower Greensand of Atherfield was exhaustively studied in
the earlier days of geology by Dr. Fitton, in the years 1824-47, and
the different strata are still referred to according to his divisions.
The lowest bed is the Crackers group about 60 ft. thick. In the lower
part are two layers of hard calcareous boulder-shaped concretions,
some a few feet long. The lower abound in fossils, and though hard
when falling from the cliffs are broken up by winter frosts, showing
the fossils they contain beautifully preserved in the softer sandy
cores of the concretions. _Gervillia sublanceolata_ is very frequent,
also _Thetironia minor_, the Ammonite _Hoplites deshayesi_, and many
more. Beneath and between the nodular masses caverns are formed, the
resounding of the waves in which has given the name of the "Crackers."
In the upper part of this group is a second lobster bed.
The most remarkable fossils in the Lower Greensand are the various
genera and species of the ammonites and their kindred. The Ammonite,
through many formations, was one of the largest, and often most
beautiful shells. There were also quite small species. The number of
species was very great. Now the whole group is extinct. They most
resembled the Pearly Nautilus, which still lives. In both the shell is
spiral, and consists of several chambers, the animal living in the
outer chamber, the rest being air-chambers enabling it to float. The
class Cephalopoda, which includes the Ammonites, the Nautilus, and
also the Cuttle-fish, is the highest division of the Mollusca. The
animals all possess heads with eyes, and tentacles around the mouth.
They nearly all possess a shell, either external, as in the Nautilus,
or internal, as in the cuttle-fishes, the internal shell of which is
often washed ashore after a rough sea. The Cephalopods are divided
into two orders. The first includes the Cuttle-fish and the Argonaut
or Paper Nautilus. Their tentacles are armed with suckers, and they
have highly-developed eyes. They secrete an inky fluid, which forms
sepia. The internal shell of extinct species of cuttle-fish, of a
cylindrical shape, with a pointed end, is a common fossil in various
strata, and is known as a Belemnite (Gr. $belemnon$ "a dart".) The
second order includes the Pearly Nautilus of the present day, and the
numerous extinct Nautiloids and Ammonoids. The tentacles of the Pearly
Nautilus have no suckers; and the eyes are of a curiously primitive
structure,--what may be called a pin-hole camera, with no lens. The
shells of the Nautilus and its allies are of simpler form, while the
Ammonites are characterised by the complicated margins of the partition
walls or septa, by which the shells are sub-divided. The chambers of
the fossil Ammonites have often been filled with crystals of rich
colours; and a polished section showing the chambers is then a most
beautiful object.[4]
Continuing along the shore, we come to the Lower Exogyra group, where
_Terebratula sella_ is found in great abundance. A reef with _Exogyra
sinuata_ runs out about 350 yards west of Whale Chine. The group is 33
ft. thick, and is followed by the Scaphites group, 50 ft. The beds
contain _Exogyra sinuata_, and a reef with clusters of Serpulæ runs
out from the cliff. In the middle of the group are bands of nodules
containing _Macroscaphites gigas_. The Lower Crioceras bed (16 ft.)
follows, and crosses the bottom of Whale Chine. The Scaphites and
Crioceras are Cephalopoda, related to the Ammonites; but in this Lower
Cretaceous period a remarkable development took place; many of the
shells began to take curious forms, to unwind as it were. Crioceras, a
very beautiful shell, has the form of an Ammonite, but the whorls are
not in contact; thus making an open spiral like a ram's horn, whence
its name (Gk. $keras$, ram, $krios$, horn). Ancyloceras begins like
Crioceras, but from the last whorl continues for some length in a
straight course, then bends back again; Macroscaphites is similar, but
the whorls of the spiral part are in contact. In Scaphites, a much
smaller shell, the uncoiled part is much shorter, and its outline more
rounded. It is named from its resemblance to a boat (Gk. $skaphê$).[5]
The Walpen and Ladder Clays and Sands (about 60 ft.) contain nodules
with Exogyra and the Ammonite _Douvilleiceras martini_. The
dark-green clays of the lower part form an undercliff, on to which
Ladder Chine opens. The Upper Crioceras Group (46 ft.), like the
Lower, contains bands of Crioceras? also _Douvilleiceras martini_,
Gervillia, Trigonia, etc. It must be stated that there is some
uncertainty with regard to the ammonoids found in this neighbourhood,
Macroscaphites having been described as Ancyloceras, and also
sometimes as Crioceras. The discovery of the true Ancyloceras
(_Ancyloceras Matheronianum_) at Atherfield is described (and a figure
given) by Dr. Mantell; but what is the characteristic ammonoid of the
"Crioceras" beds requires further investigation. The neighbourhood of
Whale and Walpen Chines is of great interest. Ammonites may be found
in the bottom of Whale Chine fallen out of the rock. Red ferruginous
nodules with Ammonites lie on the shore, in the Chines, and on the
Undercliff, some of the ammonites more or less converted into
crystalline spar. Hard ledges of the Crioceras beds run into the sea.
The shore is usually covered deep with sand and small shingle; but there
are times when the sea has washed the ledges clear; and it is then that
the shore should be examined.
The Walpen and Ladder Sands (42 ft.); the Upper Exogyra Group (16
ft.); the Cliff End Sand (28 ft.); and the Foliated Clay and Sand (25
ft.), consisting of thin alternations of greenish sand and dark-blue
clay, follow. Then the Sands of Walpen Undercliff (about 100 ft.);
over which lie the Ferruginous Bands of Blackgang Chine (20 ft.). Over
these hard beds the cascade of the Chine falls. Cycads and other
vegetable remains are found in this neighbourhood. Throughout the
Atherfield Greensand fragments of the fern _Lonchopteris_
(_Weichselia_) _Mantelli_ are found. 220 ft. of dark clays and soft
white or yellow sandrock complete the Lower Greensand. In the upper
beds of the Greensand few organic remains occur. A beautiful section
of Sandrock with the junction of the Carstone is to be seen inland at
Rock above Bright-stone. The Sandrock here is brightly coloured like
the sands of Alum Bay,--though it belongs to a much older
formation,--and shows current bedding very beautifully. The junction
of the Sandrock and Carstone is also well seen in the sandpit at
Marvel.
We have now come to the end of the Lower Cretaceous, in which are
included the Wealden and the Lower Greensand. Judged by the character
of the flora and fauna, the two form one period, the main difference
being the effect of the recession of the shore line, due to the
subsidence which let in the sea over the Wealden delta, so that we
have marine strata in place of freshwater deposits. But that the
plants and animals of the Wealden age still lived in the not distant
continent is shown by the remains borne down from the land. These
strata are an example of a phenomenon often met with in geology,--that
of a great thickness of deposits all laid down in shallow water. The
Wealden of the Isle of Wight are some 700 feet thick, in Kent a good
deal thicker, the Hastings Sands, the lower part of the formation,
being below the horizon occurring in the Island: the Lower Greensand
is some 800 feet thick. In the ancient rocks of Wales, the Cambrian
and Silurian strata, are thousands of feet of deposits, mostly laid
down in fairly shallow water. In such cases there has been a
long-continued deposition of sediment, while a subsidence of the area
in which it was laid down has almost exactly kept pace with the
deposit. It is difficult not to conclude that the subsidence has been
caused by the weight of the accumulating deposit,--continuing until
some world-movement of the contracting globe has produced a
compensating elevation of the area.
[Footnote 4: Some fine ammonites may be seen at the Clarendon
Hotel, Chale,--one about 5 ft. in circumference.]
[Footnote 5: _See Guide to Fossil Invertebrata_, Brit. Mus. Nat.
Hist.]
Chapter VI
THE GAULT AND UPPER GREENSAND
We have seen how the continent through which the great Wealden river
flowed began to sink below the sea level, and how the waters of the
sea flowed over what had been the delta of the river, laying down the
beds of sandstone with some mixture of clay which we call the Lower
Greensand. The next stratum we come to is a bed of dark blue clay more
or less sandy, called the Gault. In the upper beds it becomes more
sandy and grey in colour. These are known as the "passage beds,"
passing into the Upper Greensand. The thickness of the Gault clay
proper varies from some 95 to 103 feet. Compared to the mainland the
Gault is of small thickness in the Island, though the dark clay bands
in the Sandrock mark the oncoming of similar conditions. The fine
sediment forming the clay points to a further sinking of the sea bed.
In general, we find very few fossils in the Gault in the Island,
though it is very fossiliferous on the mainland at Folkestone. North
of Sandown Red Cliff the Gault forms a gully, down which a footpath
leads to the shore. It is seen at the west of the Island in Compton
Bay, where in the lower part some fossil shells may be found.
The Upper Greensand is not very well named, as the beds only partially
consist of sandstone, in great part of quite other materials. Some
prefer to call the Lower Greensand Vectian, from Vectis, the old name
of the Isle of Wight, and the Upper Greensand Selbornian, a name
generally adopted, because it forms a marked feature of the country
about Selborne in Hampshire.[6] But, though the Upper Greensand covers
a less area in the Isle of Wight than the Lower, it forms some of the
most characteristic scenery of the Island. One of the most striking
features of the Island is the Undercliff, the undulating wooded
country from Bonchurch to Niton, above the sea cliff, but under a
second cliff, a vertical wall which shelters it to the North. This
wall of cliff consists of Upper Greensand. In a similar way to the
small undercliffs we saw at Luccombe, the Undercliff has been formed
by a series of great slips, caused here by the flowing out of the
Gault clay, which runs in a nearly horizontal band through the base of
all the Southern Downs of the Island, the Upper Greensand lying above
it breaking off in masses, and leaving vertical walls of cliff. These
walls are seen not only in the Undercliff, but also on the northern
side of the downs, where they form the inland cliff overhanging a
pretty belt of woodland from Shanklin to Cook's Castle, and again
forming Gat Cliff above Appuldurcombe. We have records of great
landslips at the two ends of the Undercliff, near Bonchurch and at
Rocken End, about a century ago. But the greater part of the
Undercliff was formed by landslips in very ancient times, before
recorded history in this Island began. The outcrop of the Gault is
marked by a line of springs on all sides of the Southern Downs. The
strata above, Chalk and Upper Greensand, are porous and absorb the
rainfall, which permeates through till it reaches the Gault Clay,
which throws it out of the hill side in springs, some of which furnish
a water supply for the surrounding towns and villages.
Where the Upper Greensand is best developed, above the Undercliff, the
passage beds are followed by 30 feet of yellow micaceous sands, with
layers of nodules of a bluish-grey siliceous limestone known as Rag.
The nodules frequently contain large Ammonites and other fossils. Next
follow the Sandstone and Rag beds, about 50 feet of sandstone with
alternating layers of rag. The sandstones are grey in colour,
weathering buff or reddish-brown, tinged more or less green by grains
of glauconite. Near the top of these strata is the Freestone bed, a
thick bed of a close-grained sandstone, weathering a yellowish grey,
which forms a good building stone. Most of the churches and old manor
and farm houses in the southern half of the Island are built of this
stone. Then forming the top of the series are 24 feet of chert
beds,--bands of a hard flinty rock called chert alternating with
siliceous sandstone, the sandstone containing large concretions of rag
in the same line of bedding. The chert beds are very hard, and where
the strata are horizontal, as above the Undercliff, project like a
cornice at the top of the cliff. Perhaps the finest piece of the Upper
Greensand is Gore Cliff above Niton lighthouse, a great vertical wall
with the cornice of dark chert strata overhanging at the top. The
thickness in the Undercliff, including the Passage Beds, is from 130
to 160 ft.
The Upper Greensand may be studied at Compton Bay, and at the Culvers;
and along the shore west of Ventnor the lower cliff by the sea
consists largely of masses of fallen Upper Greensand, many of which
show the chert strata well. In numerous walls in the south of the
Island may be seen stone from the various strata--sandstone, blue
limestone or rag, and also the chert.
Let us think what was happening when these beds were being formed. The
sandstone is much finer than that of the Lower Greensand; and we have
limestones now,--marine, not freshwater as in the Wealden. Marine
limestones are formed by remains of sea creatures living at some depth
in clear water. And now we come to a new material, chert. It is not
unlike flint, and flint is one of the mineral forms of silica. Chert
may be called an impure or sandy flint. The bands of chert appear to
have been formed by an infiltration of silica into a sandstone,
forming a dense flinty rock, which, however, has a dull appearance
from the admixture of sand, instead of being a black semi-transparent
substance like flint. But where did the silica come from? In the
depths of the sea many sea creatures have skeletons and shells formed
of silica or flint, instead of carbonate of lime, which is the
material of ordinary shells and of corals. Many sponges, instead of
the horny skeleton we use in the washing sponge, have a skeleton
formed of a network of needles of silica, often of beautiful forms.
Some marine animalcules, the Radiolaria, have skeletons of silica. And
minute plants, the Diatoms, have coverings of silica, which remain
like a little transparent box, when the tiny plant is dead. Now, much
of the chert is full of needles, or spicules, as they are called, of
sponges, and this points to the source from which some at least of the
silica was derived. To form the chert much of the silica has been in
some manner dissolved, and deposited again in the interstices of
sandstone strata. We shall have more to say of this process when we
come to speak of the origin of the flints in the chalk. Sponges
usually live in clear water of some depth; so all shows that the sea
was becoming deeper when these strata were being formed.
Along the shore of the Undercliff, Upper Greensand fossils may be
found nicely weathered out. Very common is a small curved bivalve
shell,--a kind of small oyster,--_Exogyra conica_, as are also
serpulæ, the tubes formed by certain marine worms. Very pretty pectens
(scallop shells) are found in the sandstone. Many other shells,
_Terebratulæ_, _Trigonia_, _Panopæa_, etc., occur, and several species
of ammonite and nautilus.[7] A frequent fossil is a kind of sponge,
Siphonia. It has the form of an oblong bulb, supported by a long stem,
with a root-like base. It is often silicified, and when broken shows
bundles of tubular channels.
In the chert may often be seen pieces of white or bluish chalcedony,
generally in thin plates filling cracks in the chert. This is a very
pure and hard form of silica, beautifully clear and translucent.
Pebbles which the waves have worn in the direction of the plate are
very pretty when polished, and go by the name of sand agates. They may
sometimes be picked up on the shore near the Culvers.
[Footnote 6: Names proposed by the late A. J. Jukes-Browne.]
[Footnote 7: Of Ammonites, _Mortoniceras rostratum_ and
_Hoplites splendens_ may be mentioned: and of Pectens, _Neithea
quinquecostata_ and _quadricostata_, _Syncyclonema orbicularis_,
and _Æquipecten asper_.]
Chapter VII
THE CHALK
As we have traced the world's history written in the rocks we have
seen an old continent gradually submerged, a deepening sea flowing
over this part of the earth's surface. Now we shall find evidence of
the deepening of the sea to something like an ocean depth. We are
coming to the great period of the Chalk, the time when the material
was made which forms the undulating downs of the south-east of England,
and of which the line of white cliffs consists, which with sundry
breaks half encircles our shores, from Flamborough Head in Yorkshire,
by Dover and the Isle of Wight, to Bere in Devon. Across the Channel
white cliffs of chalk face those of England, and the chalk stretches
inland into the Continent. Its extent was formerly greater still.
Fragments of chalk and flint are preserved in Mull under basalt, an
old lava flow, and flints from the chalk are found in more recent
deposits (Boulder Clay) on the East of Scotland, pointing to a former
great extension northward, which has been nearly all removed by
denudation. In the Isle of Wight the chalk cliffs of Freshwater and
the Culvers are the grandest features of the Island; while all the
Island is dominated by the long lines of chalk downs running through
it from east to west. Now what is the chalk? And how was it made? The
microscope must tell us. It is found that this great mass of chalk is
made up principally of tiny microscopic shells called Foraminifera,
whole and in crushed fragments. There are plenty of foraminifera in
the seas to-day; and we need not go far to find similar shells. On the
shore near Shanklin you will often see streaks of what look like tiny
bits of broken shell washed into depressions in the sand. These,
however, often consist almost entirely of complete microscopic shells,
some of them of great beauty. The creature that lives in one of these
shells is only like a drop of formless jelly, and yet around itself it
forms a complex shell of surprising beauty. The shells are pierced
with a number of holes, hence their name (fr. Lat. _foramen_, a hole,
and _ferre_, to bear). Through these holes the animal puts out a
number of feelers like threads of jelly, and in these entangles
particles of food, and draws them into itself. Now, do we anywhere
to-day find these tiny shells in such masses as to build up rocks? We
do. The sounding apparatus, with which we measure the depths of the
sea, is so constructed as to bring up a specimen of the sea bottom.
This has been used in the Atlantic, and it is found that the really
deep sea bottom, too far out for rivers and currents to bring sand and
mud from the land, is covered with a white mud or ooze. And the
microscope shows this to be made up of an unnumerable multitude of the
tiny shells of foraminifera. As the little creatures die in the sea,
their shells accumulate on the bottom, and in time will be pressed
into a hard mass like chalk, the whole being cemented together by
carbonate of lime, in the way we explained in describing the making of
limestones. So we find chalk still forming at the present day. But
what ages it must take to form strata of solid rock of such tiny
shells! And what a vast period of time it must have required to build
up our chalk cliffs and downs, composed in large part of tiny
microscopic shells! With the foraminifera the microscope shows in the
chalk a multitude of crushed fragments, largely the prisms which
compose bivalve shells, flakes of shells of Terebratula and
Rhynchonella, and minute fragments of corals and Bryozoa. Scattered in
the chalk we shall also find larger shells and other remains of the
life of the ancient sea. The base of the cliffs and fallen blocks on
the shore are the best places to find fossils. Much of the base of the
cliffs is inaccessible except by boat. The lower strata may be
examined in Sandown and Compton Bays, and the upper in Whitecliff Bay.
A watch should always be kept on the tide. The quarries along the
downs are not as a rule good for collecting, as the chalk does not
become so much sculptured by weathering.
The deep sea of the White Chalk did not come suddenly. In the oncoming
of the period we find much marl--limy clay. As the sea deepened,
little reached the bottom but the shells of foraminifera and other
marine organisms. How deep the sea became is uncertain: there is
reason to believe that it did not reach a depth such as that of the
Atlantic.
It is difficult to draw the line between the Upper Greensand and the
Chalk strata. Above the Chert beds is a band a few feet thick known as
the Chloritic Marl, which shows a passage from sand to calcareous
matter. It is named from the abundance of grains of green colouring
matter, now recognised as glauconite; so that it would be better
called Glauconitic Marl. It is also remarkable for the phosphatic
nodules, and for the numerous casts of Ammonites, Turrilites, and
other fossils mostly phosphatized, which it contains. This band is one
of the richest strata in the Island for fossils. It differs, however,
in different localities both in thickness and composition. It is best
seen above the Undercliff, and in fallen masses along the shore from
Ventnor to Niton. It is finely exposed on the top of Gore Cliff, where
the flat ledges are covered with fossil Ammonites, Turrilites,
Pleurotomaria, and other shells. The Ammonite (_Schloenbachia
varians_) is especially common. The sponge (_Stauronema carteri_) is
characteristic of the Glauconitic Marl. As the edge of the cliff is a
vertical wall, none should try this locality but those who can be
trusted to take proper care on the top of a precipice. When a high
wind is blowing the position may be especially dangerous.
[Illustration: PL. III]
(Pecten) Neithea Quinquecostata
Thetironia (Ammonite) Rhynchonella
Minor Mantelliceras Mantelli Parvirostris
(Sea Urchins)
Micraster Cor-Anguinum Echinocorys Scutatus
(Internal cast in flint)
LOWER AND UPPER GREENSAND AND CHALK
The Chloritic Marl is followed by the Chalk Marl, of much greater
thickness. This consists of alternations of chalk with bands of Marl,
and contains glauconite and also phosphatic nodules in the lower part.
Upwards it merges into the Grey Chalk, a more massive rock, coloured
grey from admixture of clayey matter. These form the Lower Chalk, the
first of the three divisions into which the Chalk is usually divided.
Above this come the Middle and Upper, which together form the White
Chalk. They are much purer white than the lower division, which is
creamy or grey in colour. The Chalk Marl and Grey Chalk are well seen
at the Culver Cliff, and run out in ledges on the shore. The lower
part of this division is the most fossiliferous, and contains various
species of Ammonities, Turrilites, Nautilus, and other Cephalopoda.
(Of Ammonites _Schloenbachia varians_ is characteristic. Also may be
named _S. Coupei_, _Mantelliceras mantelli_, _Metacanthoplites
rotomagensis_, _Calycoceras naviculare_, the small Ammonoid Scaphites
æqualis; and of Pectens, _Æquipecten beaveri_ and _Syncyclonema
orbicularis_ may be mentioned). White meandering lines of the sponge
_Plocoscyphia labrosa_ are conspicuous in the lower beds. The Chalk
Marl is well shown at Gore Cliff, sloping upwards from the flat ledges
of the Chloritic Marl. It may be studied well, and fossils found, in
the cliff on the Ventnor side of Bonchurch Cove,--which has all
slipped down from a higher level.
The uppermost strata of the Lower Chalk are known as the Belemnite
Marls. They are dark marly bands, in which a Belemnite, _Actinocamax
plenus_, is found. The hard bands known as Melbourn Rock and Chalk
Rock, which on the mainland mark the top of the Lower and Middle Chalk
respectively, are neither of them well marked in the Isle of Wight. In
the Middle Chalk _Inoceramus labiatus_, a large bivalve shell, occurs
in great profusion; and in the Upper flinty Chalk are sheets of
another species, _I. Cuvieri_. It is hardly ever found perfect, the
shells being of a fibrous structure, with the fibres at right angles
to the surface, and so very fragile.
There is a striking difference between the Middle and Upper Chalk,
which all will observe. It consists in the numerous bands of dark
flints which run through the Upper Chalk parallel to the strata. The
Lower Chalk is entirely, and the Middle Chalk nearly, devoid of flint.
Though the line at which the commencement of the Upper Chalk is taken
is rather below the first flint band of the Upper Chalk, and a few
flints occur in the highest beds of the Middle Chalk; yet, speaking
generally, the great distinction between the Middle and Upper Chalk,
the two divisions of the White Chalk, may be considered to be that of
flintless chalk and chalk with flints.
Early in our studies we noticed the great curves into which the
upheaved strata have been thrown, and that on the northern side of the
anticline the strata are in places vertical. This can be well observed
in the Culver Cliffs and Brading Down, where the strata of the Upper
Chalk are marked by the lines of black flints. In the large quarry on
Brading Down the vertical lines of flint can be clearly seen; and by
walking at low tide at Whitecliff Bay round the corner of the cliff,
or by observing the cliff from a boat, we may see a beautiful section
of the flinty chalk, the lines of black flints sloping at a high
angle. The flints in general form round or oval masses, but of
irregular shape with many projections, and the masses lie in regular
bands parallel to the stratification. The tremendous earth movement
which has bent the strata into a great curve has compressed the
vertical portion into about half its original thickness, and has made
the chalk of our downs extremely hard. It has also shattered the
flints in the chalk into fragments. The rounded masses retain their
form, but when pulled out of the chalk fall into sharp angular
fragments, and we find they are shattered through and through.
[Illustration: _Photo by J. Milman Brown, Shanklin._]
CULVER CLIFFS--HIGHLY INCLINED CHALK STRATA
Now, what are flints, and how were they formed? Flints are a form of
silica, a purer form than chert, as the chalk in which they are
embedded was formed in the deep sea, and so we have no admixture of
sand. Flints, as we find them in the chalk, are generally black
translucent nodules, with a white coating, the result of a chemical
action which has affected the outside after they were formed. Flint is
very hard,--harder than steel. You cannot scratch it with a knife,
though you may leave a streak of steel on the surface of the flint.
This hardness is a property of other forms of silica, as quartz and
chalcedony. The question how the flints were formed is a difficult
one. As to this much still remains obscure. The sea contains mineral
substances in solution. Calcium sulphate and chloride, and a small
amount of calcium carbonate (carbonate of lime) are in solution in the
sea. From these salts is derived the calcium deposited as calcium
carbonate to form the shells of the Foraminifera and the larger
shells in the Chalk. There is also silica in small quantity in sea
water. From this the skeletons of radiolaria and diatoms and the
spicules of sponges are formed. Now, many flints contain fossil
sponges, and when broken show a section of the sponge clearly marked.
Especially well can this be seen in flints which have lain some time
in a gravel bed formed of flints worn out of the chalk by denudation.
Hard as a flint seems, it is penetrated by numerous fine pores. The
gravel beds are usually stained yellow by water containing iron, and
this has penetrated by the pores through the substance of the flints,
staining them brown and orange. Many of the stained flints show
beautifully the sponge markings,--a wide central canal with fine
thread-like canals leading into it from all sides.
The Chalk Sea evidently abounded in siliceous organisms, and it cannot
be doubted that it is from such organisms that the silica was derived,
which has formed the masses of flint. Silica occurs in two forms--in a
crystalline form as quartz or rock crystal, and as amorphous, _i.e._,
formless or uncrystalline (also called opaline) silica. The siliceous
skeletons of marine organisms are formed of amorphous silica. Flint
consists of innumerable fine crystalline grains, closely packed
together. Amorphous silica is less stable than crystalline, and is
capable of being dissolved in alkaline water, _i.e._, water containing
carbonate of sodium or potassium in solution. If the silica so
dissolved be deposited again, it is generally in the crystalline form.
It seems probable, therefore, that the amorphous silica of the
skeletal parts of marine organisms has been dissolved by alkaline
water percolating through the strata, and re-deposited as flint.
As the silica was deposited, chalk was removed. The large irregular
masses of flint lying in the Chalk strata have clearly taken the place
of chalk which has been removed. Water charged with silica soaking
through the strata has deposited silica, and at the same time
dissolved out so much carbonate of lime. Bivalve shells, originally
carbonate of lime, are often replaced, and filled up by flint, and
casts of sea urchins in solid flint are common, and often beautiful
fossils. This process of change took place after the foraminiferal
ooze had been compacted into chalk strata; and to some extent at any
rate, there has been deposition of silica after the chalk had become
hard and solid; for we find flat sheets, called tabular flint, lying
along the strata, or filling cracks cutting through the strata at
right angles. But in all probability the re-arrangement of the
constituents of the strata took place in the main during the first
consolidation, as the strata rose above the sea-level, and the
sea-water drained out. A suggestion has been made by R. E. Liesegang,
of Dresden, to explain the occurrence of the flints in the bands with
clear interspaces between, which are such a marked feature of the
Upper Chalk. He has shown how "a solution diffusing outward and
encountering something with which it reacts and forms a precipitate,
moves on into this medium until a concentration sufficient to cause
precipitation of the particular salt occurs. A zone of precipitation
is thus formed, through which the first solution penetrates until the
conditions are repeated, and a second zone of precipitate is thrown
down. Zone after zone may thus arise as diffusion goes on." He
suggests that the zones of flint may be similar phenomena, water
diffusing through the masses of chalk taking up silica till such
concentration is reached that precipitation takes place, the water
then percolating further and repeating the process.[8]
The precipitation of silica and replacement of the chalk occurs
irregularly along the zone of precipitation, forming great irregular
masses of flint, which enclose the sponges and other marine organisms
that lay in the chalk strata. Where a deposit of silica has begun, it
will probably have determined the precipitation of more silica, in the
manner constantly seen in chemical precipitation; and it would seem
that siliceous organisms as sponges have to some extent served as
centres around which silica has been precipitated, for flints are very
commonly found, having the evident external form of sponges.
It will be well to say something here of the history of the flints as
the chalk which contains them is gradually denuded away. Rain water
containing carbonic dioxide has a great effect in eating away all
limestone rocks, chalk included. A vast extent of chalk, which
formerly covered much of England has thus disappeared. The arch of
chalk connecting our two ranges of downs has been cut through, and
from the top of the downs themselves a great thickness of chalk has
been removed. The chalk in the downs above Ventnor and Bonchurch is
nearly horizontal. It consists of Lower and Middle Chalk; and probably
a small bit of the Upper occurs. But the top of St. Boniface Down is
covered with a great mass of angular flint gravel, which must have
come from the Upper Chalk. The gravel is of considerable thickness,
perhaps 20 ft., and on the spurs of the down falls over to a lower
level like a table-cloth. It is worked in many pits for road metal.
This flint gravel represents the insoluble residue which has been left
when the Chalk was dissolved away.
On the top of the cliffs between Ventnor and Bonchurch, at a point
called Highport, is a stratum of flint gravel carried down from the
top of the down. The shore here is strewn with large flints fallen
from the gravel. The substance of many of the flints has undergone a
remarkable change. Instead of black or dull grey flint it has become
translucent agate, of splendid orange and purple colours, or has been
changed into clear translucent chalcedony. In the agate the forms of
fossil sponges can often be beautifully seen. The colours are due to
iron-charged water percolating into the flint in the gravel bed, but
further structural changes have altered the form of the silica;
chalcedony having a structure of close crystalline fibres, revealed by
polarized light: when variously stained and coloured, it is usually
called agate. Many of these flints, when cut through and polished, are
of great beauty. The main force of the tides along these shores is
from west to east; and so there is a continual passage of pebbles on
the shore in that direction. The flints in Sandown Bay have in the
main travelled round from here; and towards the Culvers small handy
specimens of agates and chalcedonies rounded by the waves may be
collected.
[Illustration: _Photo by J. Milman Brown, Shanklin._]
SCRATCHELL'S BAY--HIGHLY INCLINED CHALK STRATA
The extensive downs in the centre of the Island are largely overspread
with angular flint gravel similarly formed to that of St. Boniface. Of
other beds of gravel, which have been washed down to a lower level by
rivers or other agency we shall have more to say later.
The Chalk strata in the Isle of Wight are of great thickness. In the
Culver Cliff there are some 400 feet of flintless Chalk (Lower and
Middle Chalk), and then some 1,000 feet of chalk with flints. There is
some variation in the thickness of the strata in different parts of
the Island, and the amount of the Upper strata, which has been
removed by denudation, varies considerably. The average thickness of
the white chalk in the Island is about 1,350 feet.[9] Including the
Lower Chalk, the maximum thickness of the Chalk strata is 1,630 ft.
The divisions of the chalk we have so far considered depend on the
character of the rock: we must say a word about another way of
dividing the strata. It is found that in the chalk, as in other
strata, fossils change with every few feet of deposit. We may make a
zoological division of the chalk by seeing how the fossils are
distributed. The Chalk was first studied from this point of view by
the great French geologist, M. Barrois, who divided it into zones,
according to the nature of the animal life, the zones being called by
the name of some fossil specially characteristic of a particular zone.
More recently Dr. A. W. Rowe has made a very careful study of the
zones of the White Chalk, and is now our chief authority on the
subject. The strata have been grouped into zones as follows:--
Zones. Sub-Zones.
{ Belemnitella mucronata.
{ Actinocamax quadratus.
{ { Offaster pilula.
Upper { Offaster pilula. { Echinocorys depressus.
Chalk. {
{ Marsupites { Marsupites.
{ testudinarius. { Uintacrinus.
{ Micraster cor-anguinum.
{ Micraster cor-testudinarium.
{ Holaster planus.
Middle { Terebratulina lata.
Chalk. { Inoceramus labiatus.
{ Holaster subglobosus. { Actinocamax
Lower { { plenus (at top).
Chalk. { Schloenbachia varians.{ Stauronema
{ { carteri (at base).
The method of study according to zoological zones is of great
interest. The period of the White Chalk was of long duration, and the
physical conditions remained very uniform. So that by studying the
succession of life during this period we may learn much about the
gradual change of life on the earth, and the evolution of living
things.
We have seen that the whole mass of the chalk is made up mainly of the
remains of living things,--mostly of the microscopic foraminifera. We
have seen that sponges were very plentiful in that ancient sea. Of
other fossils we find brachiopods--different species of Terebratula
and Rhynchonella--a large bivalve _Inoceramus_ sometimes very common;
the very beautiful bivalve, _Spondylus spinosus_, belemnites, serpulæ;
and different species of sea-urchin are very common. A pretty
heart-shaped one, _Micraster cor-anguinum_, marks a zone of the higher
chalk, which runs along the top of our northern downs. Other common
sea urchins are various species of _Cidaris_, of a form like a turban
(Gk. _cidaris_, a Persian head-dress); _Cyphosoma_, another circular
form; the oval _Echinocorys scutatus_, which, with varieties of the
same and allied species, abounds in the Upper Chalk, and the more
conical _Conulus conicus_. The topmost zone, that of _B. Macronata_,
would yield a record of exuberant life, were the chalk soft and
horizontal. There was a rich development of echinoderms (sea urchins
and star fishes), but nothing is perfect, owing to the hardness of the
rock (Dr. Rowe). The general difference in the life of the Chalk
period is the great development of Ammonites and other Cephalopods in
the Lower Chalk, and of sea urchins and other echinoderms in the
Upper, while the Middle Chalk is wanting in the one and the other.
Shark's teeth tell of the larger inhabitants of the ocean that flowed
above the chalky bottom.
Many quarries have been opened on the flanks of the Chalk Downs, of
which a large number are now disused. They occur just where they are
needed for chalk to lay on the land, the pure chalk on the north of
the Downs to break up the heavy Tertiary clays, which largely cover
the north of the Island; the more clayey beds of the Grey Chalk on the
south of the downs to stiffen the light loams of the Greensand.[10]
[Footnote 8: See _Common Stones_, by Grenville A. J. Cole,
F.R.S. 1921.]
[Footnote 9: 1,472 ft. at the western end of the Island, 1,213
ft. at the eastern.--Dr. Rowe's measurements.]
[Footnote 10: Dr. A. W. Rowe.]
Chapter VIII
THE TERTIARY ERA: THE EOCENE
Ages must have passed while the ocean flowed over this part of the
world, and the chalk mud, with its varied remains of living things,
gradually accumulated at the bottom. At last a change came. Slowly the
sea bed rose, till the chalk, now hardened by pressure, was raised
into land above the sea level. As soon as this happened, sea waves and
rain and rivers began to cut it down. There is evidence here of a wide
gap in the succession of the strata. Higher chalk strata, which
probably once existed, have been washed away, while the underlying
strata have been planed off to an even surface more or less oblique to
the bedding-planes. The highest zone of the chalk in the Island (that
of _Belemnitella macronata_) varies greatly in thickness, from 150 ft.
at the eastern end of the Island to 475 at the western. The latest
investigations give reason to conclude that this is due to gentle
synclines and anticlines, which have been planed smooth by the erosion
which preceded the deposition of the next strata,--the Eocene.[11] At
Alum Bay the eroded surface of the chalk may be seen with rolled
flints lying upon it, and rounded hollows or pot-holes, the appearance
being that of a foreshore worn in a horizontal ledge of rock, much
like the Horse Ledge at Shanklin.
The land sank again, but not to anything like the depth of the great
Chalk Sea. We now come to an era called the Tertiary. The whole
geological history is divided into four great eras. The first is the
Eozoic, or the age of the Archæan,--often called Pre-Cambrian--rocks;
rocks largely volcanic, or greatly altered since their formation,
showing only obscure traces of the life which no doubt existed. Then
follow the Primary era, or, as it is generally called, the Palæozoic;
the Secondary or Mesozoic; and the Tertiary or Kainozoic. Palæozoic is
used rather than Primary, as this word is ambiguous, being also used
for the crystalline rocks first formed by the solidification of the
molten surface of the earth. But Secondary and Tertiary are still in
constant use. These long ages, or eras, were of very unequal duration;
yet they mark such changes in the life of animal and plant upon the
earth that they form natural divisions. The Palæozoic was an immense
period during which life abounded in the seas,--numberless species of
mollusca, crustaceans, corals, fish are found,--and there were great
forests, which have formed the coal measures, on land,--forests of
strange primeval vegetation, but in which beautiful ferns, large and
small, flourished in great numbers. The Secondary Era may be called
the age of reptiles. To this era all the rocks we have so far studied
belong. Now we come to the last era, the Tertiary, the age of the
mammals. Instead of reptiles on land, in sea and air, we find a
complete change. The earth is occupied by the mammalia; the air
belongs to the birds such as we see to-day. The strange birds of the
Oolitic and Cretaceous have passed away. Birds have taken their modern
form. In some parts of the world strata are found transitional between
the Secondary and Tertiary.
The Tertiary is divided into four divisions,--the Eocene, the
Oligocene (once called Upper Eocene), the Miocene, and the Pliocene;
which words signify,--Pliocene the more recent period, Miocene the
less recent, Eocene the dawn of the recent.
In the Eocene we shall find marine deposits of a comparatively shallow
sea, and beds deposited at the mouth of great rivers, where remains of
sea creatures are mingled with those washed down from the land by the
rivers. These strata run through the Isle of Wight from east to west,
and we may study them at either end of the Island, in Whitecliff and
Alum Bays. The strata are highly inclined, so that we can walk across
them in a short walk. Some beds contain many fossils, but many of the
shells are very brittle and crumbly; and we can only secure good
specimens by cutting out a piece of the clay or sand containing them,
and transferring them carefully to boxes, to be carried home with
equal care. Often much of the face of the cliff is covered with slip
or rainwash, and overgrown with vegetation. Sometimes a large slip
exposes a good hunting ground.
Now let us walk along the shore, and try to read the story these
Tertiary beds tell us. We will begin in Whitecliff Bay. Though easily
accessible, it remains still in its natural beauty. The sea washes in
on a fine stretch of smooth sand sheltered by the white chalk wall
which forms the south arm of the bay. North of the Culver downs the
cliffs are much lower, and consist of sands and clays of varying
colour, following each other in vertical bands. Looking along the line
of shore we notice a band of limestone, at first nearly vertical like
the preceding strata, then curving at a sharp angle as it slopes to
the shore, and running out to sea in a reef known as Bembridge Ledge.
This is the Bembridge limestone; and the beginning of the reef marks
the northern boundary of Whitecliff Bay, the shore, however,
continuing in nearly the same line to Bembridge Foreland, and showing
a continuous succession of Eocene and Oligocene strata. The strata
north of the limestone are nearly horizontal, dipping slightly to the
north. In the Bembridge limestone we see the end of the Sandown
anticline, and the beginning of the succeeding syncline. The strata
now dip under the Solent, and rise into another anticline in the
Portsdown Hills. North and south of the great anticline of the Weald
of Kent and Sussex are two synclinal troughs known as the London and
Hampshire basins. Nearly the whole of our English Eocene strata lies
in these two basins, having been denuded away from the anticlinal
arches. The Oligocene only occur in the Hampshire basin, the higher
strata only in the Isle of Wight.
[Illustration: FIG. 3.]
COAST SECTION, WHITECLIFF BAY.
BM _Bembridge Marls._
BL _Bembridge Limestone._
O _Osborne Beds._
H _Headon Beds._
BS _Barton Sand._
B _Barton Clay._
Br _Bracklesham Beds._
Bg _Bagshot Beds._
L _London Clay._
R _Reading Beds._
Ch _Chalk._
P _Pebble Beds._
S _Sandstone Band._
Above the Chalk we come first to a thick red clay called Plastic clay.
It is much slipped, and the slip is overgrown. The only fossils found
in the Island are fragments of plants; larger plant remains on the
mainland show a temperate climate. This clay was formerly worked at
Newport for pottery. The clay is probably a freshwater deposit formed
in fairly deep water. On the mainland we find on the border shallow
water deposits called the Woolwich and Reading beds. (The clay is 150
to 160 ft. thick at Whitecliff Bay, less than 90 ft. at the Alum Bay.)
We come next to a considerable thickness of dark clay with sand, at
the surface turned brown by weathering. This is the London clay, so
called because it underlies the area on which London is built. At the
base is a band of rounded flint pebbles, which extends at the base of
the clay from here to Suffolk. In it, as well as in a hard sandstone
18 inches higher up, are tubular shells of a marine worm, _Ditrupa
plana_. The sandstone runs out on the shore. About 35 ft. above the
basement bed is a zone of _Panopæa intermedia_ and _Pholadomya
margaritacea_, at 50 ft. another band of _Ditrupa_, and at about 80
ft. a band with a small _Cardita_. In the higher part of the clay are
large septaria,--rounded blocks of a calcareous clay-ironstone, with
cracks running through them filled with spar. _Pinna affinis_ is found
in the septaria. The thickness of the clay in Whitecliff Bay is 322
feet. It can be seen on the shore, when the tide happens to have swept
the sand away. Otherwise the lower beds are hardly visible, there
being no cliff here, but a slope overgrown with vegetation.
In Alum Bay the London clay, about 400 ft. in thickness, consists of
clays, chiefly dark blue, with sands, and lines of septaria. In the
lower part is a dark clay with _Pholadomya margaritacea_, still
preserving the pearly nacre. There are also _Panopæa intermedia_, and
in septaria _Pinna affinis_. All these with their pearly lustre, are
beautiful fossils. A little higher is a zone with _Ditrupa_, and
further on a band of _Cardita_. Other shells also are found in the
clay, especially in the lower part. They are all marine, and indicate
a sub-tropical climate. Lines of pebbles show that we are near a
beach. In other parts of the south of England remains from the land
are found, borne down an ancient river in the way we found before in
the Wealden deposits.
But times have changed since the Wealden days, and the life of the
Tertiary times has a much more modern appearance. From leaves and
fruits borne down from the forest we can learn clearly the nature of
the early Eocene land and climate. Leaves are found at Newhaven, and
numerous fossil fruits at Sheppey. The character of the vegetation
most resembled that now to be seen in India, South Eastern Asia, and
Australia. Palms grew luxuriantly, the most abundant fruit being that
of one called Nipadites, from its resemblance to the Nipa palm, which
grows on the banks of rivers in India and the Philippines. The forests
also included plants allied to cypresses, banksia, maples, poplars,
mimosa, custard apples, gourds, and melons. The rivers abounded in
turtle--large numbers of remains of which are found in the London clay
at the mouth of the Thames--crocodiles and alligators. With the
exception of the south east of England, all the British Isles formed
part of a continental mass of land covered with a tropical vegetation.
The mountain chains of England, Scotland, and Wales rose as now, but
higher. Long denudation has worn them down since. In the south-east of
England the coast line fluctuated; and sea shells, and the remains of
the plant and animal life of the neighbourhood of a great tropical
river alternate in the deposits.
[Illustration: FIG. 4]
SECTION THROUGH HEADON HILL AND HIGH DOWN.
SHOWING STRATA SEEN AT ALUM BAY.
G _Gravel Cap._
Bm _Bembridge Limestone._
O _Osborne Beds._
UH _Upper Headon._
MH _Middle " ._
LH _Lower Headon._
BS _Barton Sand._
B _Barton Clay._
Br _Bracklesham Beds._
Bg _Bagshot Sands._
L _London Clay._
R _Reading Beds._
Ch _Chalk._
The London clay is succeeded by a great thickness of sands and clays
which form the Bagshot series. These are divided in the London basin
into Lower, Middle, and Upper Bagshot. In the Hampshire basin the
strata are now classified as Bagshot Sands, Bracklesham Beds, Barton
Beds, the last comprising the Barton Clay and the Barton Sand,
formerly termed Headon Hill Sands. There is some uncertainty as to the
manner in which these correspond to the beds of the Bagshot district,
as the Tertiary strata have been divided by denudation into two
groups, and differ in character in the two areas. It is possible that
the Barton Sand represents a later deposit than any in the London
area.
Almost the only fossil remains in the Bagshot Sands are those of
plants, but these are of great interest. In Whitecliff Bay the beds
consist for the most part of yellow sands, above which is a band of
flint pebbles, which has been taken as the base of the Bracklesham
series, for in the clay immediately above marine shells occur. The
Bagshot Sands, in Whitecliff Bay, are about 138 feet thick, in Alum
Bay, 76 feet, according to the latest classification. In Alum Bay the
strata consist of sands, yellow, grey, white, and crimson, with clays,
and bands of pipe clay. This is remarkably white and pure, as though
derived from white felspar, like the China clay in Cornwall. The pipe
clay contains leaves of trees, sometimes beautifully preserved.
Specimens are not very easy to obtain, as only the edges of the leaves
appear at the surface of the cliff. They have been found chiefly in a
pocket, or thickening of the seam of pipe clay, which for forty years
yielded specimens abundantly, afterwards thinning out, when the leaves
became rare. The leaves lie flat, as they drifted and settled down in
a pool. With them are the twigs of a conifer, occasionally a fruit or
flower, or the wing case of a beetle. The leaves show a tropical
climate. The flora is a local one, differing considerably from those
of Eocene deposits elsewhere. The plants are nearly all dicotyledons.
Of palms there are only a few fragments, while the London clay of
Sheppey is rich in palm fruits, and many large palms are found in the
Bournemouth leaf beds, corresponding in date to the Bracklesham. The
differences may be largely due to conditions of locality and
deposition. The Alum Bay flora is characterised by a wealth of
leguminous plants, and large leaves of species of fig (_Ficus_);
simple laurel and willow-like leaves are common, of which it is
difficult to determine the species, and there is abundance of a
species of _Aralia_. The character of the flora resembles most those
of Central America and the Malay Archipelago.
[Illustration: PL. IV]
Nummulites Lævigatus
Turritella Limnæa
Imbricataria Longiscata
Cardita Planicosta
(Fusus) Planorbis
Leiostama Pyrus Euomphalus
Cyrena Semistriata
EOCENE AND OLIGOCENE
The Bracklesham Beds in Alum Bay (570 ft. thick) consist of clays,
with lignite forming bands 6 in. to 2 ft. thick; white, yellow, and
crimson sands; and in the upper part dark sandy clays, with bands
showing impressions of marine fossils. Alum Bay takes its name from
the alum formerly manufactured from the Tertiary clays. The coloured
sands have made the bay famous. The colours of the sands when freshly
exposed, and of the cliffs when wet with rain, are very rich and
beautiful,--deep purple, crimson, yellow, white, and grey. Some of the
beds are finely striped in different shades by current bedding. The
contrast of these coloured cliffs with the White Chalk, weathered to a
soft grey, of the other half of the bay is very striking and
beautiful. About 45 ft. from the top is a conglomerate of flint
pebbles, some of large size, cemented by iron oxide. In Whitecliff Bay
the Bracklesham Beds (585 ft.) consist of clays, sands, and sandy
clays, mostly dark, greenish and blue in colour, containing marine
fossils and lignite. Sir Richard Worsley, in his History of the Isle
of Wight, tells that in February, 1773, a bed of coal was laid bare in
Whitecliff Bay, causing great excitement in the neighbourhood. People
flocked to the shore for coal, but it proved worthless as fuel. It
has, however, been worked to some extent in later years. In some of
the beds are many fossils. Numbers have lately been visible where a
large founder has taken place. There are large shells of _Cardita
planicosta_ and _Turritella imbricataria_. They are, however, very
fragile. In a stratum just above these are numbers of a large
Nummulite (_Nummulites lævigatus_). These are round flat shells like
coins,--hence the name (Lat. _nummus_, a coin). They are a large
species of foraminifera. We may split them with a penknife; and then
we see a pretty spiral of tiny chambers. A smaller variety, _N.
variolarius_, occurs a little further on, and a tiny kind, _N.
elegans_, in the Barton clay. One of the most striking features of the
later Eocene is the immense development of Nummulite limestones--vast
beds built up of the delicate chambered shells of Nummulites,--which
extend from the Alps and Carpathians into Thibet, and from Morocco,
Algeria, and Egypt, through Afghanistan and the Himalaya to China. The
pyramids of Egypt are built of this limestone.
The Bracklesham beds are followed by the Barton clay, famous for the
number of beautiful fossil shells found at Barton on the Hampshire
coast. At Whitecliff Bay the fossils are, unfortunately, very friable.
At Alum Bay the pathway to the shore is in a gully in the upper part
of the Barton clay. The strata consist of clays, sands, and sandy
clays. The base of the beds is marked by the zone of _Nummulites
elegans_. Numerous very pretty shells of the smaller Barton types may
be found, with fragments of larger ones; or a whole one may be found.
Owing to the cliff section cutting straight across the strata, which
are nearly vertical, there is far less of the beds open to observation
than at Barton, which probably accounts for the list of fossils being
much smaller. The shells are chiefly several species of _Pleurotoma_,
_Rostellaria_, _Fusus_, _Voluta_, _Turritella_, _Natica_, a small
bivalve _Corbula pisum_, a tubular shell of a sand-boring mollusc
_Dentalium_, _Ostroea_, _Pecten_, _Cardium_, _Crassatella_. The
fauna is like a blending of Malayan and New Zealand forms of marine
life. Throughout the Eocene from the London clay onward the shells are
such as abound in the warm sea south east of Asia. Similarly the plant
remains take us into a tropic land, where fan palms and feather palms
overshadowed the country, trees of the tropics mingling with trees we
still find in more Northern latitudes. The general character of the
flora as of the shells was Oriental and Malayan; both being succeeded
in later strata by a flora and fauna with greater analogy to that now
existing in Western North America.
In Alum Bay the Barton clay is suddenly succeeded by the very fine
yellow and white sands which run along the western base of Headon
Hill, the curve of the syncline bringing them round from a nearly
vertical to an almost horizontal position. These are now known as the
Barton Sand. They are 90 ft. thick, the whole of the Barton beds being
338 ft. in Alum Bay, 368 ft. in Whitecliff. The sands were formerly
extensively used for glass making. They are almost unfossiliferous.
The passage from Barton clay to the sands in Whitecliff Bay is more
gradual. The sands here show some fine colouring which reminds us of
the more celebrated sands of Alum Bay.
[Footnote 11: See Memoir of Geological Survey of I. W. by H. J.
Osborne White, F.G.S. 1921, p. 90.]
Chapter IX
THE OLIGOCENE
We pass on to strata which used to be called Upper Eocene, but are now
generally classified as a period by themselves, and called the
Oligocene. They are also known as the Fluvio-marine series. Large part
was deposited in freshwater by rivers running into lagoons, or in the
brackish water of estuaries, while at times the sea encroached, and
beds of marine origin were laid down.
The west of the Island is much the best locality for the lower strata,
those which take their name from Headon Hill between Alum and Totland
Bays. There are three divisions of the Headon strata, marine beds in
the middle coming between upper and lower beds formed in fresh and
brackish water. Light green clays are very characteristic of these
beds, and at the west of the Island thick freshwater limestones, which
have died out before the strata re-appear in Whitecliff Bay. The
strongest masses of limestone in Headon Hill belong to the Upper
division. The limestones are full of freshwater shells, nearly all the
long spiral Limnæa and the flat spiral disc of Planorbis, perhaps the
most abundant species being _L. longiscata_ and _P. euomphalus_. The
limestones themselves are almost entirely the produce of a freshwater
plant _Chara_, which precipitates lime on its tissues, in the same
manner as the sea weeds we call corallines. On the shore round the
base of Headon Hill lie numerous blocks of limestone, the débris of
strata fallen in confusion, in which are beautiful specimens of Limnæa
and Planorbis. The shells, however, are very fragile. The marine beds
of the Middle Headon are best seen in Colwell Bay, where a few yards
north of How Ledge they descend to the beach, and a cliff is seen
formed of a thick bed of oysters, _Ostrea velata_. The oysters occupy
a hollow eroded in a sandy clay full of _Cytherea incrassata_, from
which the bed is known as the "Venus" bed, the shell formerly being
called _Venus_, later _Cytherea_, at present _Meretrix_. The marine
beds contain many drifted freshwater shells as Limnæa and Cyrena. The
How Ledge limestone forms the top of the Lower Headon. It is full of
well-preserved Limnæa and Planorbis.
The Upper and Lower Headon are mainly fresh or brackish water
deposits. The purely freshwater beds contain _Limnæa_, _Planorbis_,
_Paludina_, _Unio_, and land-shells. In the brackish are found
_Potamomya_, _Cyrena_, _Cerithium_ (_Potamides_), _Melania_ and
_Melanopsis_. _Paludina lenta_ is very abundant throughout the
Oligocene. A large number of the marine shells of the Headon beds are
species also found in the Barton clay. _Cytherea_, _Voluta_,
_Ancillaria_, _Pleurotoma_, _Natica_ are purely marine genera.
In White Cliff Bay the beds are mostly estuarine. Most of the fossils
are found in two bands, one about 30 ft. above the base of the series,
the other a stiff blue clay, about 90 feet higher, which seems to
correspond with the "Venus Bed" of Colwell Bay. Many of the fossils
are of Barton types.
The Headon beds are about 150 feet thick at Headon Hill, 212 ft. in
Whitecliff Bay; and are followed by beds varying from about 80 to 110
ft. in thickness, known as the Osborne and St. Helens series. They
consist mainly of marls variously coloured, with sandstone and
limestone. In Headon Hill is a thick concretionary limestone, which
almost disappears northward. The Oligocene strata often vary
considerably within short distances. The Osborne beds are exposed
along the low shore between Cowes and Ryde, and from Sea View to St.
Helens. In Whitecliff Bay they are not well seen, occurring in
overgrown slopes. They consist mostly of red and green clays. A band
of cream-yellow limestone a foot thick is the most conspicuous
feature. The fossils resemble those from the Headon beds, but are much
less plentiful. The marls seem to have been mostly deposited in
lagoons of brackish water, which at the present day are favourite
places for turtles and alligators, and of these many remains are found
in the Osborne beds. The beds are specially noted for the shoals of
small fish, _Diplomystus vectensis_ (_Clupea_), first observed by Mr.
G. W. Colenutt, F.G.S., and prawns found in them, and also remains of
plants. The beds that appear in the neighbourhood of Sea View and St.
Helens are divided into Nettlestone Grits and St. Helen's Sands, the
former containing a freestone 8 feet thick.
Above these beds lies the Bembridge limestone, which is so
conspicuous in Whitecliff Bay, and forms Bembridge Ledge. On the north
shore of the Island the strata rise slightly on the northern side of
the syncline. There are also minor undulations in an east and west
direction. The result is to bring up the Bembridge limestone at
various points along the north shore, where it forms conspicuous
ledges--Hamstead Ledge at the mouth of the Newtown river, ledges in
Thorness Bay, and Gurnard Ledge. In Whitecliff Bay the limestone,
about 25 feet thick, forms the conspicuous reef called Bembridge
Ledge. The Bembridge limestone consists of two or more bands of
limestone with intercalated clays. It is usually whiter than the
Headon limestones, and the fossils occur as casts, the shells being
sometimes replaced by calc-spar. The limestone has been much used as a
building stone for centuries, not only in the Island, but for
buildings on the mainland. The most famous quarries were those near
Binstead, from which Quarr, the site of the great Abbey, now almost
entirely disappeared, derives its name. From these quarries was
obtained much of the stone for Winchester Cathedral and many other
ancient buildings. In the old walls and buildings of Southampton the
stone may be recognised at once by the casts of the Limnæae it
contains. The quarries at Quarr were noted in more ways than one. In
later times the remains of early mammalia,--Palæotherium,
Anoplotherium, and others--have been found. The quarries are now
abandoned and overgrown. The limestone may be seen inland at Brading,
where it forms the ridge on which the Church stands.
The limestone is a freshwater formation, and the fossils are mostly
freshwater shells, of the same type as the Headon, Limnæa and
Planorbis the most common. There are also land shells, especially
several species of Helix, the genus which includes the common
snail,--_H. globosa_, very large,--and great species of _Bulimus_
(_Amphidromus_) and _Achatina_ (_B. Ellipticus_, _A. costellata_).
These interesting shells were chiefly obtained in the limestone at
Sconce near Yarmouth, a locality now inaccessible, being occupied by
fortifications. The land shells have an affinity to species now found
in Southern North America. The limestone also abounds in the so-called
"seeds" of Chara. The reproductive organs,--the "seeds,"--of this
curious water-plant, allied to the lower Algæ, are, like the rest of
the plant, encased in carbonate of lime, and are very durable. Large
numbers are found in the Oligocene strata. Under the microscope they
are seen to be beautifully sculptured in various designs, with a
delicate spiral running round them. Above the limestone lie the
Bembridge marls, varying in thickness in different localities from 70
to 120 feet. North of Whitecliff Bay they stretch on to the Foreland.
They are in the main a freshwater formation, but a few feet above the
limestone is a marine band with oysters, _Ostrea Vectensis_. It runs
out along the shore, where the oysters may be seen covering the
surface. The Lower Marls consist chiefly of variously-coloured clays
with many shells, chiefly _Cyrena pulchra_, _semistriata_, and
_obovata_, _Cerithium mutabile_, and _Melania muricata_ (_acuta_); and
red and green marls, in which are few shells, but fragments of turtle
occur. A little above the oyster bed is a band of hard-bluish
septarian limestone. Sixty years ago Edward Forbes remarked on the
resemblance of this band to the harder insect-bearing limestones of
the Purbeck beds. In a limestone exactly resembling this, and
similarly situated in the lower part of the marls in Gurnard and
Thorness Bays, numerous insects were afterwards found,--beetles,
flies, locusts, and dragonflies, and spiders. Leaves of plants,
including palms, fig, and cinnamon, have also been found in this bed,
showing that the climate was still sub-tropical. The upper Marls
consist chiefly of grey clays with abundance of _Melania turritissima_
(_Potamaclis_). The chief shells in the marls are _Cyrena_, _Melania_,
_Melanopsis_ and _Paludina_ (_Viviparus_). They are often beautifully
preserved; the species of Cyrena often retain their colour-markings.
Bembridge Foreland is formed by a thick bed of flint gravel resting on
the marls, which are seen again in Priory Bay, where in winter they
flow over the sea-wall in a semi-liquid condition. They lie above the
limestone at Gurnard, Thorness, and Hamstead. West of Hamstead Ledge
the whole of the beds crop out on the shore, where beautifully
preserved fossils may be collected. Large pieces of drift wood occur,
also seeds and fruit. Many fragments of turtle plates may be found.
Large crystals of selenite (sulphate of lime) occur in the Marls.
Last of the Oligocene in the Isle of Wight are the Hamstead beds.
These strata are peculiar to the Isle of Wight. The Bembridge beds
also are not found on the mainland, except a small outlier at
Creechbarrow Hill in Dorset. The Hamstead beds consist of some 250
feet of marls, in which many interesting fossils have been found. They
cover a large area of the northern part of the Island, largely
overlaid by gravels, and are only seen on the coast at Hamstead, where
they form the greater part of the cliff, which reaches a height of 210
ft., the top being capped by gravel. In winter the clays become
semi-liquid, in summer the surface may be largely slip and rainwash,
baked hard by the sun. The lower part of the strata may be best seen
on the shore. The strata consist of 225 ft. of freshwater, estuarine,
and lagoon beds, with _Unio_, _Cyrena_, _Cyclas_, _Paludina_,
_Hydrobia_, _Melania_, _Planorbis_, _Cerithium_ (rare), and remains of
turtles, crocodiles, and mammals, leaves and seeds of plants; and
above these beds 31 feet of marine beds with _Corbula_, _Cytherea_,
_Ostrea callifera_, _Cuma_, _Voluta_, _Natica_, _Cerithium_, and
_Melania_.
Except for the convenience of dividing so large a mass of strata, it
would not be necessary to divide these from the Bembridge beds, as no
break in the character of the life of the period occurs at the
junction. The basement bed of the Hamstead strata is known as the
Black Band, 2 feet of clay, coloured black with vegetable matter, with
_Paludina lenta_ very numerous, _Melanopsis carinata_, _Limnæa_,
_Planorbis_, a small _Cyclas_ (_C. Bristovii_), seed vessels, and
lumps of lignite. It rests on dark green marls with _Paludina lenta_
and _Melanopsis_, and full of roots. This evidently marks an old land
surface. About 65 feet higher is the White Band,--a white and green
clay full of shells, mostly broken. There are bands of tabular
ironstone containing _Paludina lenta_. Clay ironstone was formerly
collected on the shore between Yarmouth and Hamstead and sent to
Swansea to be smelted. The strata consist largely of mottled green and
red clays, probably deposited in brackish lagoons. These yield few
fossils except remains of turtle and crocodile and drifted plants. The
blue clays are much more fossiliferous. Among other plants are leaves
of palm and water-lily. The strata gradually become more marine
upwards. The marine beds were called by Forbes the Corbula beds, from
two small shells, _C. pisum_ and _C. vectensis_, of which some of the
clays are full. Remains of early mammalia are found in the Hamstead
beds, the most frequent being a hog-like animal, of supposed aquatic
habits, Hyopotamus, of which there are more than one species.
The fauna and flora of the Oligocene strata show that the climate was
still sub-tropical, though somewhat cooling down from the Eocene.
Palms grew in what is now the Isle of Wight. Alligators and crocodiles
swam in the rivers. Turtle were abundant in river and lagoon.
Specially interesting in the Eocene and Oligocene are the mammalian
remains. They show us mammals in an early stage before they branched
off into the various families as we know them to-day. The Palæotherium
was an animal like the tapir, now an inhabitant of the warmer regions
of Asia and America. Recent discoveries in Eocene strata in Egypt show
stages of development between a tapir-like animal and the elephant
with long trunk and tusks. There were in those days hog-like animals
intermediate between the hogs and the hippopotami. There were
ancestors of the horse with three toes on each foot. There were
hornless ancestors of the deer and antelopes. Many of the early
mammals showed characters now found in the marsupials, the order to
which the Kangaroo and Opossum belong, members of which are found in
rocks of the Secondary Era, and are the only representatives of the
mammalia in that age. Some of the early Eocene mammalia are either
marsupials, or closely related to them. In the Oligocene we find the
mammalian life becoming more varied, and branching out into the
various groups we know to-day; while the succeeding Miocene Period
witnesses the culmination of the mammalia--mammals of every family
abounding all over the earth's surface, in a profusion and variety not
seen before--or since, outside the tropics.
Chapter X
BEFORE AND AFTER.--THE ICE AGE.
We have read the story written in the rocks of the Isle of Wight. What
wonderful changes we have seen in the course of the long history!
First we were taken back to the ancient Wealden river, and saw in
imagination the great continent through which it flowed, and the
strange creatures that lived in the old land. We saw the delta sink
beneath the sea, and a great thickness of shallow water deposits laid
down, enclosing remains of ammonites and other beautiful forms of
life. Then long ages passed away, while in the waters of a deeper sea
the great thickness of the chalk was built up, mainly by the
accumulation of microscopic shells. In time the sea bed rose, and new
land appeared, and another river bore down fruits to be buried with
sea shells and remains of turtles and crocodiles in the mud deposited
near its mouth to form the London clay. We followed the alternations
of sea and land, and the changing life of Eocene and Oligocene times.
We have heard of the early mammalia found in the quarries of Quarr,
and have learnt from the leaf beds of Alum Bay that at that time the
climate of this part of the world was tropical. Indeed, I think
everything goes to prove that through the whole of the times we have
been studying,--except perhaps the earliest Eocene, that of the
Reading beds,--the climate was considerably warmer than it is at the
present day. After all these changes do you not want to know what
happened next? Well, at this point we come to a gap in the records of
the rocks, not only in the Isle of Wight, but also in the British
Isles. The British Isles, or even England and Wales alone, are almost,
if not quite unique in the world in that, in their small extent, they
contain specimens of nearly every formation from the most ancient
times to the present day. In other parts of the world we may find
regions many times this area, where we can only study the rocks of
some one period. But just at this point in the story comes a
period,--a very important one, too,--the Miocene--of which we have no
remains in our Islands. We must hear a little of what happened before
we come back to the Isle of Wight again in comparatively recent times.
But, first, perhaps, I had better tell,--just in outline,--something
of the earlier history of the world, before any of our Isle of Wight
rocks were made. For, if I do not, quite a wrong idea may be formed of
the world's history. The time of the Wealden river has seemed to us
very ancient. We cannot say how many hundreds of thousands, or rather
millions of years have passed since that ancient Wealden age. And you
may have thought that we had got back then very near the world's
birthday, and were looking at some of the oldest rocks on the globe.
But no. We are not near the beginning yet. Compared with the vast ages
that went before, our Wealden period is almost modern. We cannot tell
with any certainty the comparative time; but we may compare the
thickness of strata formed to give us some sort of idea. Now to the
first strata in which fossil remains of living things are found we
have in all a thickness of strata some 12 times that of all the rocks
we have been studying from Wealden to Oligocene, together with the
later rocks, Miocene and Pliocene, not found in the Isle of Wight. And
before that there is, perhaps, an equal thickness of sedimentary
deposits; though the fossils they, no doubt, once contained have been
destroyed by changes the rocks have undergone.
Now let me try to give you some idea of the world's history up to the
point where we began in the Isle of Wight. If we could see back
through the ages to the furthest past of geological history, we should
see our world,--before any of the stratified rocks were laid down in
the seas,--before the seas themselves were made,--a hot globe, molten
at least at the surface. How do we know this? Because under the rocks
of all the world's surface we find there is granite or some similar
rock,--a rock which shows by its composition that it has crystallised
from a molten condition. Moreover we have seen that the interior of
the earth is intensely hot. And yet all along the earth must be
radiating off heat into the cold depths of space, and cooling like any
other hot body surrounded by space cooler than itself. And this has
gone on for untold ages. Far enough back we must come to a time when
the earth was red hot,--white hot. In imagination we see it
cooling,--the molten mass solidifies into Igneous rock,--the clouds of
steam in which the globe is wrapped condense in oceans upon the
surface. The bands of crystalline rock that rise above the primeval
seas are gradually worn down by rain and rivers and waves, and the
first sedimentary deposits laid down in the waters. And in the waters
and on the land life appeared for the first time,--we know not how.
A vast thickness of stratified rocks was formed, which are called
Archæan ("ancient"). They represent a time, perhaps, as great as all
that has followed. These rocks have undergone great changes since
their formation. They have been pressed under masses of overlying
strata, and have come into the neighbourhood of the heated interior of
the earth; they have been burnt and baked and compressed and folded,
and acted on by heated water and steam, and their whole structure
altered by heat and chemical action. Limestones, _e.g._, have become
marble, with a crystalline structure which has obliterated any fossils
they may have once contained. Yet it is probable that, like nearly all
later limestones, they are of organic origin. These Archæan rocks
cover a large extent of country in Canada. We have some of them in our
Islands, in the Hebrides, and north-west of Scotland and in Anglesey,
and rising from beneath later rocks in the Malvern Hills and Charnwood
Forest.[12]
The Archæan rocks are succeeded by the most ancient fossiliferous
rocks, the great series called the Cambrian, because found, and first
studied, in Wales. They consist of very hard rocks, and contain large
quantities of slate. They are followed by another series called the
Ordovician; and that by another the Silurian. These three great
systems of rocks measure in all some 30,000 ft. of strata. They form
the hills of Wales and the English Lake District. They contain large
masses of volcanic rocks. We can see where were the necks of old
volcanoes, and the sheets of lava which flowed from them. The
volcanoes are worn down to their bases now; and the hills of Wales
and the Lakes represent the remains of ancient mountain chains, which
rose high like the Alps in days of old, long before Alps or Himalayas
began to be made. These ancient rocks contain abundant remains of
living things, chiefly mollusca, crustaceans, corals, and other marine
organisms, showing that the waters of those ages abounded with life.
We must pass on. Next comes a period called the Devonian, or Old Red
Sandstone, when the Old Red rocks of Devon and Scotland were laid
down. These contain remains of many varieties of very remarkable fish.
A long period of coral seas succeeded, when coral reefs flourished
over what was to be England; and their remains formed the
Carboniferous Limestone of Derbyshire and the Mendip Hills. A period
followed of immense duration, when over pretty well the whole earth
there seem to have been comparatively low lands covered with a
luxuriant and very strange vegetation. The remains of these ancient
forests have formed the coal measures, which tell of the most
widespread and longest enduring growth of vegetation the world has
seen. Strange as some of the plants were--gigantic horsetails and
club-mosses growing into trees--many were exquisitely beautiful. There
were no flowering plants, but the ferns, many of them tree ferns, were
of as delicate beauty as those of the present day. Many of the ferns
bore seeds, and were not reproduced by spores, such as we see on the
fronds of our present ferns. That is a wonderful story of plant
history, which has only been read in recent years.
After the long Carboniferous period came to an end followed periods in
which great formations of red sandstone were made,--the Permian, and
the New Red Sandstone or Trias. During much of this time the
condition of the country seems to have resembled that of the Steppes
of Central Asia, or even the great desert of Sahara--great dry sandy
deserts--hills of bare rock with screes of broken fragments heaped up
at their base,--salt inland lakes, depositing, as the effect of
intense evaporation, the beds of rock salt we find in Cheshire or
elsewhere, in the same manner as is taking place to-day in the Caspian
Sea, in the salt lakes of the northern edge of the Sahara, and in the
Great Salt Lake of Utah.
At the close of the period the land here sank beneath the sea--again a
sea of coral islands like the South Pacific of to-day. There were many
oscillations of level, or changes of currents; and bands of clay, when
mud from the land was laid down, alternate with beds of limestone
formed in the clearer coral seas. These strata form a period known as
the Jurassic, from the large development of the rocks in the Jura
mountains. In England the period includes the Liassic and Oolitic
epochs. The Liassic strata stretch across England from Lyme Regis in
Dorset to Whitby in Yorkshire. Most of the strata we are describing
run across England from south-west to north-east. After they were laid
down a movement of elevation, connected with the movement which raised
the Alps in Europe, took place along the lines of the Welsh and Scotch
mountains and the chain of Scandinavia, which raised the various
strata, and left them dipping to the south-east. Worn down by
denudation the edges are now exposed in lines running south-west to
north-east, while the strata dip south-east under the edges of the
more recent strata. The Lias is noted for its ammonites, and
especially for its great marine reptiles, Ichthyosaurus and
Plesiosaurus. The Oolitic Epoch follows--a long period during which
the fine limestone, the Bath freestone, was made; the limestones of
the Cotswolds, beds of clay known as the Oxford and Kimmeridge clays;
and again coral reefs left the rock known as coral rag. In the later
part of the period were formed the Portland and Purbeck beds, marine
and freshwater limestones, which contain also an old land surface,
which has left silicified trunks of trees and stems of cycads.
And now following on these came our Wealden strata, the beginning of
the Cretaceous period. You see what ages and ages had gone before, and
that when Wealden times came, far back as they are, the world's
history was comparatively approaching modern times. We must remember
that all these formations, of which we have given a rapid sketch, are
of great thickness,--thousands of feet of rock,--and represent vast
ages of time. See what we have got to from looking at the shells in
the sea cliff! We have come to learn something of the world's old
history. We have been carried back through ages that pass our
imagination to the world's beginning, to the time of the molten globe,
before ever it was cool enough to allow life--we know not how--to
begin upon its surface. And Astronomy will take us back into an even
more distant past, and show us a nebulous mist of vast extent
stretching out into space like the nebulæ observed in the heavens
to-day, before sun and planets and moons were yet formed. So we are
carried into the infinite of time and space, and questions arise
beyond the power of human mind to solve.
Now we have, I hope, a better idea of the position the strata we have
been specially studying occupy in the geological history, and shall
understand the relation the strata we may find elsewhere bear to those
in the Isle of Wight and the neighbouring south of England.
After this sketch of what went before our Island story, we must see
what followed at the end of the Oligocene period. We said that there
are no strata in the British Isles representing the next period, the
Miocene. But it was a period of great importance in the world's
history. Great stratified deposits were laid down in France and
Switzerland and elsewhere, and it was a great age of mountain
building. The Alps and the Himalaya, largely composed of Cretaceous
and Eocene rocks, were upheaved into great mountain ranges. It is
probable that during much of the period the British Isles were dry
land, and that great denudation of the land took place. But in the
first part of the period at all events this part of the world must
have been under water, and strata have been laid down, which have
since been denuded away. For our soft Oligocene strata, if exposed to
rain and river action during the long Miocene period and the time
which followed, would surely have been entirely swept away. The
Miocene was succeeded by the Pliocene, when the strata called the
Crag, which cover the surface of Norfolk and Suffolk, were formed.
They are marine deposits with sea shells, of which a considerable
proportion of species still survive.
We have seen that through the ages we have been studying the climate
was mostly warmer than at the present day. The climate of the Eocene
was tropical. The Miocene was sub-tropical and becoming cooler. Palms
become rarer in the Upper strata. Evergreens, which form three-fourths
of the flora in the Lower Miocene, divide the flora with deciduous
trees in the Upper. And through the Pliocene the climate, though still
warmer than now, was steadily becoming cooler; till in the beginning
of the next period, the Pleistocene, it had become considerably colder
than that of the present day. And then followed a time which is known
as the great Ice Age, or the Glacial Period,--a time which has left
its traces all over this country, and, indeed all over Northern Europe
and America, and even into southern lands. The cold increased, heavy
snowfalls piled up snow on the mountains of Wales, the Lake District,
and Scotland; and the snow remained, and did not melt, and more fell
and pressed the lower snow into ice, which flowed down the valleys in
glaciers, as in Switzerland to-day. Gradually all the vegetation of
temperate lands disappeared, till only the dwarf Arctic birch and
Arctic willows were to be seen. The sea shells of temperate climates
were replaced by northern species. Animals of warm and temperate
climates wandered south, and the Arctic fox, and the Norwegian
lemming, and the musk ox which now lives in the far north of America
took their place; and the mammoth, an extinct elephant fitted by a
thick coat of hair and wool for living in cold countries, and a
woolly-haired rhinoceros, and other animals of arctic regions occupied
the land. When the cold was greatest, the glaciers met and formed an
ice-sheet; and Scotland, northern England and the Midlands, Wales, and
Ireland were buried in one vast sheet of ice as Greenland is to-day.
How do we know this? To tell how the story has been read would be to
tell one of the most interesting stories of geology. Here we can only
give the briefest sketch of this wonderful chapter of the world's
history. But we must know a little of how the story has been made out.
We have already seen that the changes in plant and animal life point
to a change from a hot climate, through a temperate, at last to arctic
cold. Again, over the greater part of Northern England the rocks of
the various geological periods are buried under sheets of tough clay,
called boulder clay, for it is studded with boulders large and small,
like raisins in a plum pudding. No flowing water forms such a deposit,
but it is found to be just like the mass of clay with stones under the
great glaciers and ice sheets of arctic regions; and just such a
boulder clay may be seen extending from the lower end of glaciers in
Spitzbergen, when the glacier has temporarily retreated in a
succession of warm summers. The stones in our boulder clay are
polished and scratched in a way glaciers are known to polish and
scratch the stones they carry along, and rub against the rocks and
other stones. The rock over which the glacier moves is similarly
scratched and polished, and just such scratching and polishing is
found on the rocks in Wales and the Lake District. Again, we find
rocks carried over hill and dale and right across valleys, it may be
half across England. We can trace for great distances the lines of
fragments of some peculiar rock, as the granite of Shap in
Westmorland; and even rocks from Norway have been carried across the
North Sea, and left in East Anglia. This will just give an idea how we
know of this strange chapter in the history of our land. For, by this
time it was our land--England--much as we know it to-day; though at
times the whole stood higher above sea level, so that the beds of the
Channel and the North Sea were dry land. But, apart from variation of
level, the geography was in the main as now.
[Illustration: FIG. 9]
SHINGLE AT FORELAND
Bm _Bembridge Marls._
S _Shingle._
b _Brick Earth._
Cf _Old Cliff in Marls._
[Illustration: FIG. 5]
DIAGRAM OF STRATA BETWEEN SOUTHERN DOWNS AND ST. GEORGE'S DOWN.
Dotted Lines _Former extension of Strata._
Broken Line _Former Bed of Valley sloping to St. George's Down._
The ice sheet did not come further south than the Thames valley. What
was the country like south of this? Well, you must think of the land
just outside the ice sheet in Greenland, or other arctic country. No
doubt the winters must have been very severe,--hard frosts and heavy
snows,--the ground frozen deep. Some arctic animals would manage to
live as they do now just outside the ice sheet in Greenland. Now, have
we any deposits formed at that time in the Isle of Wight? I think we
have. A large part of the surface of the Island is covered by sheets
of flint gravel. The gravels differ in age and mode of formation. We
have already considered the angular gravels of the Chalk downs,
composed of flints which have accumulated as the chalk which once
contained them was dissolved away. But there are other gravel beds,
which consist of flints which, after they were set free by the
dissolution of the chalk, have been carried down to a lower level by
rivers or other agency, and more or less rounded in the process. Many
of these beds occur at a high level; and, as they usually cap
flat-topped hills, they are known as Plateau Gravels. Perhaps the
most remarkable is the immense sheet of gravel which covers the flat
top of St. George's Down between Arreton and Newport. Gravel pits show
upwards of 30 feet of gravel, consisting of flints with some chert and
ironstone, and the greatest thickness is probably considerably more
than this. The southern edge of the sheet is cut off straight like a
wall. To the north it runs out on ridges between combes which have cut
into it. In places in the mass of flints occur beds of sand, which
have all the appearance of having been laid down by currents of water.
The base of the gravel where it is seen on the steep southern slope of
the down has been cemented by water containing iron into a solid
conglomerate rock. The flints forming this gravel have not simply sunk
down from chalk strata dissolved away; for they lie on the upturned
edges of strata from Lower Greensand to Upper Chalk, which have been
planed off, and worn into a surface sloping gently to the north; and
over this surface the gravel has somehow flowed. The sharp wall in
which it ends at the upper part of the slope shows that it once
extended to the south over ground since worn away. Clearly, the gravel
was formed before denudation had cut out the great gap between the
central and southern downs of the Island. The down where the gravel
lies is 363 ft. above sea level, 313 ft. above the bottom of the
valley below. So that, though the gravel sheet is much newer than the
strata we have been studying, it must nevertheless be of great
antiquity.
It seems that at the top of St. George's Down we are standing on what
was once the floor of an old valley. In the course of denudation the
bottom of a river valley often becomes the highest part of a district.
For the bed of the valley is covered by flint gravel, and flint is
excessively hard, and the bed of flints protects the underlying rock;
so that, while the rocks on each side are worn away, what was the
river bed is eventually left high above them. Thus the highest points
of a district are often capped by flint gravel marking the beds of old
streams. Tracing up this old valley to the southward, at a few miles
distance it will have reached the chalk region on the south of the
anticline: and the flints carried down the valley may have come from
beds of angular flints already dissolved out of the chalk such as we
find on St. Boniface Down.
But how have these great masses of flints been swept along? Can the
land have been down under the sea; and have sea waves washed the
stones along? But these flints, though water-worn, are not rounded as
we find beach shingle. What immense rush of water can have spread
these flints 30 feet deep along a river valley? We must go to mountain
regions for torrents of this character. And then, mountain torrents
round the stones in their bed while these are mostly angular. The
history of these gravels is a difficult one. I can only give what
seems to me the most probable explanation. It appears to me probable
that in the Ice Age, south of the ice sheet, the ground must have been
both broken up by frosts, and also held together by being frozen hard
to some depth. Then when thaws came in the short but warm summers, or
when an intermission of the severe cold took place, great floods would
flow down the valleys in the country south of the ice sheet, and
masses of ice with frozen earth and stones would be borne along in a
sort of semi-liquid flow. In this way Mr. Clement Reid explains the
mass of broken-up chalk with large stones found on the heads of cliffs
on the South coast, and known by the name of "combe-rock" or "head."
The Ice Age was not one simple period, and it is still difficult to
fit together the history we read in different places, and in
particular to correlate the gravels of the south of England with the
boulder clays of the glaciated area. There were certainly breaks in
the period, during which the climate became much milder, or even
warm; and these were long enough for southern species of animals and
plants to migrate northward, and occupy the lands where an arctic
climate had prevailed. There were moreover considerable variations in
the relative level of land and sea. So that we have a very complex
history, which is gradually coming into clearer light.
That the gravels of the south of England belong largely to the age of
ice, is shown by remains of the mammoth contained in many. These,
however, are found in later gravels than those we have considered so
far, gravels laid down after the land had been cut down to much lower
levels. These lower gravels are known as Valley gravels, because they
lie along the course of existing valleys, the Plateau gravels having
been laid down before the present valleys came into existence. Teeth
of the mammoth are found in the Thames valley, and on the shores of
Southampton Water, in gravels about 50 to 70 feet above sea level, and
have been found also in the Isle of Wight at Freshwater Gate, at the
top of the cliffs near Brook, and in other places. The gravels near
Brook with the clays on which they rest have been contorted, and the
gravel forced into pockets in the clay, in a manner that suggests the
action of grounding ice ploughing into the soil.
The high level gravels must belong to an early stage of the Glacial
Epoch. We get some idea of the great length of time this age must have
lasted, as we look from St. George's Down over the lower country of
the centre of the Island. After the formation of the St. George's Down
gravel the vast mass of strata between this and the opposite downs of
St. Boniface and St. Catherine's was removed by denudation; and
gravels were then laid down on the lower land, along Blake Down, at
Arreton, over Hale common, and along the course of the Yar. Patches of
gravel occur on the Sandown and Shanklin cliffs. At Little Stairs a
gravel, largely of angular chert, reaches a thickness of 12 feet, and
in parts are several feet of loam above gravel.
At the west of the Island a great sheet of gravel covers the top of
Headon Hill, reaching a height of 390 feet. It appears sometimes to
measure 30 feet in thickness. Like that on St. George's Down it slopes
towards the Solent, resting on an eroded surface, in this case of
Tertiary strata; and here too the upper part of the sheet has been
removed by the wearing out of the deep valley between the Hill and the
Freshwater Downs. The sheet lies on an old valley bottom, which sloped
from the chalk downs on the south, then much higher and more extensive
than now. Here too we may see something of the length of the Glacial
Period. For at Freshwater Gate is a much later gravel, in which teeth
of the mammoth have been found. It was probably derived from older
gravels that once lay to the south, as the flints are rounded by
transport. But the formation of all these gravels appears to belong to
the Glacial Period; and as we stand in Freshwater Gate, and look at
this great gap in the downs worn out by the Western Yar, and think of
the time when a river valley passed over the tops of the High Downs
and Headon Hill, we receive a strong impression of the length of the
great Ice Age.
Now surely the question will be asked, what caused these changes of
climate in the world's past history--so that at times a tropical
vegetation spread over this land, and vegetation flourished sufficient
to leave beds of coal within the Arctic circle, and in the Antarctic
continent, and at another the climate of Greenland came down to
England, and an ice sheet covered nearly the whole country? This still
remains one of the difficult problems of Geology. An explanation has
been attempted by Astronomical Theory, according to which the varying
eccentricity of the earth's orbit--that is to say a slight change in
the elliptic orbit of the Earth, by which at times it becomes less
nearly circular--a change which is known to take place--may have had
the effect of producing these variations of climatic conditions. The
theory is very alluring, for if this be the cause, we can calculate
mathematically the date and duration of the Glacial Period. But,
unfortunately, supposing the astronomical phenomena to have the effect
required, the course of events given by the astronomical theory would
be entirely different to that revealed by geological research.
Geographical explanations have usually failed through being of too
local a character to explain a phenomenon which affected the whole
northern hemisphere, and the effects of which reached at least as far
south as the Equator,[13] and are seen again in the southern hemisphere
in Australia, New Zealand, and South America. It is now believed that
great world-movements take place, due to the contraction by cooling of
the Earth's interior, and the adjustment of the crust to the
shrinkage.[14] Possibly some explanation might be found in these
world-wide movements; but their effect seems to last through too long
periods of time to suit our Ice Ages. Again, while the geographical
distribution of animals and plants in the present and past seems to
imply very great changes in the land masses and oceanic areas,[15]
these changes appear to bear no relation to glacial epochs. The cause
of the Ice Ages remains at present an unsolved problem. More than one
Ice Age has occurred during the long geological history. The marks of
such a period are found in Archæan rocks, in the Cambrian, when
glaciers flowed down to the sea level in China and South Australia
within a few degrees of the tropics, and above all in early Permian
times. The Dwyka conglomerate of the Karroo formation of South Africa
(deposits of Permo-Carboniferous age) show evidence of extensive
glaciation; deposits of the same age in Northern and Central India,
even within the tropics, a glacial series of great thickness in
Australia, and deposits in Brazil, appear to show a glaciation greater
than that of the recent glacial period. Yet these epochs formed only
episodes in the great geological eras. On the whole the climate
throughout geological time would seem to have been warmer than at the
present day. It may, perhaps, be doubted whether the earth has yet
recovered what we may call its _normal_ temperature since the Glacial
Epoch.
Note on Astronomical Theory.--If the Ice Age be due to the increased
eccentricity of the Earth's orbit, the theory shows that a long
duration of normal temperature will be followed by a group of Glacial
Periods alternating between the northern and southern hemispheres, the
time elapsing between the culmination of such a period in one
hemisphere and in the other being about 10,500 years. While one
hemisphere is in a glacial period, the other will be enjoying a
specially mild,--a "genial" period. Now, according to the record of
the rocks, the "genial" periods were far from being those breaks in
the Glacial which we know as Inter-glacial periods. We have the
immensely long warm period of the Eocene and Oligocene, the Miocene
with a still warm but reduced temperature, and then the gradual
cooling during the Pliocene, till the drop in temperature culminates
in the Ice Age. Moreover, the duration of each glaciation during this
Ice Age is usually considered to have been much longer than the 10,000
years or so given by the Astronomical Theory. Add to this that the
periods of high eccentricity of the Earth's orbit, though occurring
at irregular intervals, are, on the scale of geological time, pretty
frequent; so that several of such periods would have occurred during
the Eocene alone. Yet the geological evidence shows unbroken
sub-tropical conditions in this part of the world throughout the
Eocene.
[Footnote 12: The older division of the Archæan rocks--the
Lewisian gneisse--consists entirely of metamorphic and igneous
rocks; a later division--the Torridonian sandstones--is
comparatively little altered, but still unfossiliferous.]
[Footnote 13: The great equatorial mountains Kilimanjaro and
Ruwenzori show signs of a former extension of glaciers.]
[Footnote 14: For an account of such movements, see Prof.
Gregory's _Making of the Earth_ in the Home University Library.]
[Footnote 15: See The _Wanderings of Animals_. By H. Gadow,
F.R.S., Cambridge Manuals.]
Chapter XI
THE STORY OF THE ISLAND RIVERS; AND HOW
THE ISLE OF WIGHT BECAME AN ISLAND
We must now consider the history of the river system of the Isle of
Wight, to which our study of the gravels has brought us. For rivers
have a history, sometimes a most interesting one, which carries us
back far into the past. Even the little rivers of the Isle of Wight
may be truly called ancient rivers. For though recent in comparison
with the ages of geological time, they are of a vast antiquity
compared with the historical periods of human history.
To understand our river systems we must go back to the time when
strata formed by deposit of sediment in the sea were upheaved above
the sea level. To take the simplest case, that of a single anticlinal
axis fading off gradually at each end, we shall have a sort of turtle
back of land emerged from the sea, as in figure 6, _aa_ being the
anticlinal axis. From this ridge streams will run down on either side
in the direction of the dip, their course being determined by some
minor folds of the strata, or difference of hardness in the surface,
or cracks formed during elevation. On each side of the dip-streams
smaller ones will flow, more or less in the direction of the strike,
and run into the main streams. Various irregularities, such as started
the flow of the streams, will favour one or another. Consider three
streams, _a_, _b_, _c_, and let us suppose the middle one the
strongest, with greatest flow of water, and cutting down its bed most
rapidly. Its side streams will become steeper and have more erosive
force, and so will eat back their courses most rapidly until they
strike the line of the streams on either side. Their steeper channels
will then offer the best way for the upper waters of the streams they
have cut to reach the sea; and these streams will consequently be
tapped, and their head waters cut off to flow to the channel of the
centre stream. We shall thus have for a second stage in the history a
system such as is shown in fig. 7. The same process will continue till
one river has tapped several others; and there will result the usual
figure of a river and its tributaries, to which we are accustomed on
our maps. We shall observe that tributaries do not as a rule gradually
approach the central stream, but suddenly turn off at nearly a right
angle from the direction in which they are flowing, and, after a
longer or shorter course, join at another sharp angle a river flowing
more or less parallel to their original direction.
[Illustration: FIG. 6]
[Illustration: FIG. 7]
DEVELOPMENT OF RIVER SYSTEMS
The Chalk and overlying Tertiary strata were uplifted from the sea in
great folds forming a series of such turtle-backs as we have been
considering. The line of upheaval was not south-west and north-east,
as that which raised the older formations in bands across England, but
took place in an east and west direction. The main upheaval was that
of the great Wealden anticline. Other folds produced the Sandown and
Brook anticlines, and that of the Portsdown Hills. The upheaval seemed
to have been caused by pressure acting from the south, for the steeper
slope of each fold is on the northern side. Our latest Oligocene
strata are tilted with the chalk, showing that the upheaval took place
after Oligocene times. But the great movement was in the main earlier
than the Pliocene. For on the North Downs near Lenham is a patch of
Lower Pliocene deposit resting directly on the Chalk, the older
Tertiary strata having been removed by denudation, clearly due to the
uplift of the Wealden anticline. The raising of the Pliocene deposit
to its present position proves that the same movement was continued at
a later time, probably during the Pleistocene. But the greater part of
the movement may be assigned to the Miocene, the period of great
world-movements which raised the Alps and the Himalaya.
Many remarkable, and, at first sight, very puzzling features connected
with the courses of rivers find an explanation when we study the river
history. Thus, looking at the Weald of Kent and Sussex, we see that it
consists of comparatively low ground rising to a line of heights east
and west along the centre, and surrounded on all sides but the
south-east by a wall of Chalk downs. If we considered the subject, we
should suppose that the drainage of the country would be towards the
south-east, which is open to the sea. Not so. All the rivers flow from
the central heights north and south,--go straight for the walls of
chalk downs, and cut through the escarpment in deep clefts to flow
into the Thames and the Channel. This is explained when we remember
that the rivers began to flow when the great curve of strata rose
above the sea. Though eroded by the sea during its elevation, yet when
it rose above the waters the arch of chalk must have been continuous
from what are now North Downs to South. And from the centre line of
the great turtle back the streams began to flow north and south,
cutting in the course of ages deep channels for themselves. The
greater erosion in their higher courses has cut away the mass of chalk
from the centre of the Weald, but the rivers still flow in the
direction determined when the arch was still entire.
We have a similar state of things in the Isle of Wight. Any one not
knowing the geological story, and looking at the geography of the
Island, might naturally suppose that there would be a stream flowing
from west to east, through the low ground between the two ranges of
downs, and finding its way into the sea in Sandown Bay. Instead of
this the three rivers of the Island, the two Yars and the Medina, all
flow north, and cut through the chalk escarpment of the Central downs,
as if an earthquake had made rifts for them to pass, and so find their
way into the Solent. The explanation is the same as in the case of the
Weald. The rivers began to flow when the Chalk strata were continuous
over the centre of the Island; and their course was determined when
the east and west anticlinal axis rose above the sea.
We shall notice, however, that the Island rivers start from south of
the anticlinal axis. The centre of the Sandown anticline runs just
north of Sandown, but the various branches of the Yar and Medina flow
from well south of this. The explanation would appear to be that the
anticline is almost a monoclinal curve,--that is to say, one slope is
steep, the other not far from horizontal. Streams starting from the
ridge would flow with much greater force down the northern than the
southern side, and would cut back their course much more quickly.
Thus they would continually cut into the heads of the southern
streams, and turn the water supplying them into their own channels.
In its early history a river cuts out its bed, and carries along
pebbles, sand and mud to the sea. The head waters are constantly
cutting back, and the slope becoming less steep, till a time comes
when the stream in its gently inclined lower course has no more power
to excavate, and the finer sediment, which is all that now reaches the
lower river, begins to fill up the old channel. And so the alluvium is
formed which fills the lower portions of our river valleys.
Beyond this, the great rush of waters from melting snows and ice of
the Glacial Period has come to an end. The gentler and diminished
streams of a drier age have no power to roll flint stones along and
form beds of gravel. Gravel terraces border our river valleys at a
higher level than the present streams. Periods alternated during which
gravels were laid down by the river, and when the river acquiring more
erosive force, by an elevation of the land giving its bed a steeper
gradient, or a wetter climate producing a greater rush of water, cut a
new channel deeper in the old valley. So our valleys in Southern
England are frequently bordered by a succession of gravel terraces,
the higher ones being the older, dating from times when the river
flowed at a higher level than at present. Such terraces may be seen
above the Eastern Yar and its tributary streams. In the centre of the
old gravels is the alluvial flat of a later age.
The Island rivers cut out their channels when the land stood at a
higher level than at present. The old channels of the lower parts of
the rivers are now filled with alluvium, partly brought down by the
rivers and partly marine. The channels are cut down considerably below
sea level; and by the sinking of the land the sea has flowed in, and
the last parts of the river courses are now tidal estuaries. The sea
does not cut out estuaries. They are the submerged ends of river
valleys.
Some idea may be formed of the antiquity of our Island rivers by
observing the depth of the clefts they have cut through the downs at
Brading, Newport, and Freshwater. But to this we must add the depth at
which the old channels lie below the alluvium. It would be interesting
to know the thickness of the alluvium. But it is not often that
borings come to be made in river alluvia. However, in the old Spithead
forts artesian wells are sunk; and these pass through 70 to 90 feet of
recent deposits before entering Eocene strata. Under St. Helen's Fort,
at the mouth of Brading Harbour, are 80 feet of recent deposits. The
old channel of the Yar, at its mouth, must lie at least at this depth.
Before it passes through the gap in the Chalk downs the Yar has
meandered about, and formed the alluvial flat called Morton marshes.
These marshes stretch out into the flat known as Sandown Level, which
occupies the shore of the bay between Sandown and the Granite Fort.
What is the meaning of this extension of the alluvium away from the
course of the river out to the sea at Sandown? A glance at it as
pictured on a geological map will suggest the answer. We see clearly
the alluvia of two streams converging from right and left, and uniting
to pass to the sea through Brading Harbour. But the stream to the
right has been cut off by the sea encroaching on Sandown Bay: only the
last mile of alluvium is left to tell of a river passed away. We must
reconstruct the past. We see the Bay covered by land sloping up to
east and south east, the lines of downs extending eastward from
Dunnose and the Culvers, and an old river flowing northward, and
cutting through the chalk at Brading after being joined by a branch
from the west. This old river must have been the main stream. For it
was a transverse stream, flowing nearly at right angles to the ridge
of the anticline; while the Yar comes in as a tributary in the
direction of the strike. Of other tributary streams, all from the
right are gone by the destruction of the old land. On the left streams
would flow in from the combes at Shanklin and Luccombe--streams which
have now cut out Shanklin and Luccombe chines.
Passing the gap in the downs the river meandered about, and, with
marine deposit, washed in by the tides, formed the expanse of alluvium
which occupies what was Brading Harbour,--a harbour which in old times
presented at high tide a beautiful spectacle of land-locked water
extending up to Brading. Inclosures and drainings have been made from
time to time, the upper part near Yarbridge being taken in in the time
of Edward I. Further innings were made in the reign of Queen
Elizabeth; and Sir Hugh Middleton, who brought the New River to
London, made an attempt to enclose the whole, but the sea broke
through his embankment. The harbour was finally reclaimed at great
cost in 1880, the present embankment enclosing an area of 600 acres.
The history of the Western Yar is similar to that of the Eastern. The
main stream must have flowed from land now destroyed by the sea
stretching far south of Freshwater Gate. All that is left is its tidal
estuary, and the gravel terraces and alluvial flat formed in the last
part of its course. Of a tributary stream an interesting relic
remains. For more than 2 miles from Chilton Chine through Brook to
Compton Grange a bed of river gravel lies at the top of the cliff,
marking the course of an old stream, of which coast erosion has made a
longitudinal section. This was a tributary of the Yar, when the
mammoth left his remains in the gravel at Grange Chine and Freshwater
Gate. Down the centre of the gravels lies a strip of alluvium laid
down by a stream following the same course in later days. The sea had
probably by this time cut into the stream; and it most likely flowed
into the sea somewhere west of Brook. In the alluvium hazel nuts and
twigs of trees are found at Shippard's Chine near Brook.
The lower course of the Medina is a submerged river valley, the tide
flowing up to Newport. The river rises near Chale, and flows through a
strip of alluvium, overgrown with marsh vegetation, known as "The
Wilderness." This upper course of the Medina, from the absence of
gravels or brick earth, has the appearance of a comparatively modern
river. But the Medina has a further history. If you look at the map
you will see branches of the Yar running south to north as transverse
streams, but the main course is that of a lateral river. Look at the
two chief sources of the Yar--the stream from near Whitwell and Niton,
and that from the Wroxall valley. When they get down to the marshes
near Rookley and Merston, they are not flowing at all in the direction
of Sandown or Brading. They rather look as if they would flow along
the marshy flat by Blackwater into the Medina. But the Yar cuts right
across their course, and carries them off eastward to Sandown. When we
look, we find a line of river valley with a strip of alluvium running
up from the Medina at Blackwater in the direction of these two
streams--a valley which the railway up the Yar valley from Sandown
makes use of to get to Newport. There can be little doubt that these
streams from Niton and Wroxall originally ran along this line into the
Medina; but the Yar, cutting its course backward, has captured them,
and diverted their course. They probably represent the main branches
of the Medina in earlier times, the direction of flow from south-east
to north-west instead of south to north being possibly due to the
overlapping in the neighbourhood of Newport of the ends of the Brook
and Sandown anticlines. The sheet of gravel on Blake Down belongs to
this period of the river's history. The river must have diverted
between the deposition of the Plateau Gravels and that of the Valley
Gravels of the Yar. For the former follow the original valley, the
latter the new course of the river.
We must now take a wider outlook, and see what became of our rivers
after they had flowed across what is now the Isle of Wight from south
to north. We have been speaking of times when the Island was of much
greater extent than at present. Standing on the down above the
Needles, and looking westward, we see on a clear day the Isle of
Purbeck lying opposite, and we can see that the headland there is
formed by white chalk cliffs like those beneath us. In front of them
stand the Old Harry Rocks, answering to the Needles, both relics of a
former extension of the land. In fact Purbeck is just like a
continuation of the Isle of Wight. South of the Chalk lie Greensand
and Wealden strata in Swanage Bay, and north towards Poole are
Tertiaries. Clearly these strata were once continuous with those of
the Isle of Wight. We must imagine the chalk downs of the Island
continued as a long range across what is now sea, and on through
Purbeck. A great Valley must have stretched from west to east, north
of this line, along the course of the Frome, which runs through
Dorset, and now enters the sea at Poole Harbour, on by Bournemouth,
and along the present Solent Channel--a valley still much above sea
level, not yet cut down by rivers and the sea--and down the centre of
this valley a river must have flowed, which may be called the River
Solent. It received as tributaries from the south the rivers of the
Isle of Wight, and others from land since destroyed by the sea. There
flowed into it from the north the waters of the Stour and Avon, and an
old river which flowed down the line of what is now Southampton Water.
Southampton Water looks like the valley of a large river, much larger
than the present Test and Itchen. Its direction points to a river from
the north west; and it has been shown by Mr. Clement Reid that the
Salisbury rivers--Avon, Nadder, and Wily--at a former time, when they
flowed far above their present level--continued their course into the
valley of Southampton Water. For fragments of Purbeck rocks from the
Vale of Wardour, west of Salisbury, have been found by him in gravels
on high land near Bramshaw, carried right over the deep vale of the
Avon in the direction of the Water. The lower Avon would originally be
a tributary of the Solent River; and it enters the sea about mid-way
between the Needles and the chalk cliffs of Purbeck, just opposite the
point where we might suppose the sea would have first broken through
the line of chalk downs. No doubt it broke through a gap made by the
course of an old river from the south, as it is now breaking through
the gap made by the old Yar at Freshwater. When the river Solent had
been tapped at this point, the Avon just opposite would have acquired
a much steeper flow, causing it to cut back at a faster rate, till it
cut the course of the old river which ran by Salisbury to Southampton,
and, having a steeper fall, diverted the upper waters of this river
into its own channel.
[Illustration: FIG. 8
THE OLD SOLENT RIVER]
Frost and rain and rivers cut down the valleys of the river system for
hundreds of feet; the sea which had broken through the chalk range
gradually cut away the south side of the main river valley from Purbeck
to the Needles; and eventually the valley itself was submerged by a
subsidence of the land, and the sea flowed between the Isle of Wight
and the mainland.
A gravel of somewhat different character to the rest is the sheet of
flint shingle at Bembridge Foreland. It forms a cliff of gravel about
25 feet high resting on Bembridge marls, and consists of large flints,
with lines of smaller flints and sand showing current bedding, and also
contains Greensand chert and sandstone, which must have been brought
from some district beyond the Chalk. The shingle slopes to north-east.
To the south-west it ends abruptly, the dividing line between shingle
and marls running up steeply into the cliff. This evidently marks an
old sea cliff in the marls, against which the gravel has been laid
down.[16]
One or two comparatively recent deposits may be mentioned here. At the
top of the cliff in Totland Bay, about 60 ft. above the sea, for a
distance of 350 yards, is a lacustrine deposit, consisting in the main
of a calcareous tufa deposited by springs flowing from the limestone of
Headon Hill. The tufa contains black lines from vegetable matter, and
numerous land and freshwater shells of present-day species--many species
of Helix, especially H. nemoralis and H. rotundata, Cyclostoma elegans,
Limnæa palustris, Pupa, Clausilia, Cyclas, and others.
On the top of Gore Cliff is a deposit of hard calcareous mud, reaching
a thickness of about 9 feet, and forming a small vertical cliff above
the slopes of chalk marl. It extends north a few yards beyond the
chalk marl on to Lower Greensand. It has been formed by rainwash from
a hill of chalk, which must once have existed to the south. The
deposit contains numerous existing land-shells, especially _Helix
nemoralis_ and other species of Helix.
Between Atherfield and Chale at the top of the cliff is a large area
of Blown Sand. The sand is blown up from the face of the cliff below.
It reaches a thickness of 20 feet, and possibly more in places, and
forms a line of sand dunes along the edge of the cliff. The upper part
of Ladder Chine shows an interesting example of wind-erosion. The sand
driven round it by the wind has worn it into a semi-circular hollow
like a Roman theatre.
Small spits, consisting partly of blown sand, extend opposite the
mouths of the Western Yar, the Newtown river, and the most
extensive--at the mouth of the old Brading Harbour, separating the
present reduced Bembridge Harbour from the sea. This is called St.
Helen's Spit, or "Dover,"--the local name for these sand spits.
[Footnote 16: Fig. 9, p. 79.]
Chapter XII
THE COMING OF MAN.
We have watched the long succession of varied life on the earth
recorded in the rocks, and now we come to the most momentous event of
all in the history--the coming of Man. The first certain evidence of
the presence of man on the earth is found with the coming of the
Glacial Period,--unless indeed the supposed flint implements found by
Mr. Reid Moir, under the Crag in Suffolk, should prove him earlier
still. It is a rare chance that the skeleton of a land animal is
preserved; especially rare in the case of a skeleton so frail as that
of man. The best chance for the preservation of bones is in deposits
in caves, which were frequently the dens of wild beasts and the
shelters of man. But the implements used by early man were happily of
a very imperishable nature. His favourite material, if he could get
it, was flint. Flint could by dexterous blows have flake after flake
taken off, till it formed a tool or weapon with sharp point and
cutting edge. The implements, though only chipped, or flaked, were
often admirably made. They have very characteristic shapes. Moreover,
the kind of blow--struck obliquely--by which these early men made
their tools left marks which stamp them as of human workmanship. The
flake struck off shows what is called a "bulb of percussion"--a
swelling which marks the spot where the blow was struck--and from this
extends a series of ripples, producing a surface like that of a shell,
from which this mode of breaking is called conchoidal fracture. Often,
by further chipping the flake itself is worked into an implement.
Implements have also been made of chert, but it is far more difficult
to work, as it naturally breaks in an irregular way into sharp angular
fragments. Flint, on the other hand, lent itself admirably to the use
of early man, who in time acquired a perfect mastery of the material.
The working of flints is so characteristic that, once accustomed to
them, you cannot mistake a good specimen. Sea waves dashing pebbles
about will sometimes produce a conchoidal fracture, but never a series
of fractures in the methodical way in which a flint was worked by man.
And, of course, specimens may be found so worn that it is difficult to
be sure about their nature. Again early man may, especially in very
early times, have been content to use a sharp stone almost as he found
it, with only the slightest amount of knocking it into shape. So that
in such a case it will be very difficult to decide whether the stones
have formed the implements of man or not. In later times men learnt to
polish their implements, and made polished stone axes like those the
New Zealanders and South Sea Islanders used to make in modern times.
The old age of chipped or flaked implements is called the Palæolithic;
the later age when they were ground or polished the Neolithic. (Simple
implements, as knives and scrapers, were still unpolished.) The
history of early man is a long story in itself, and of intense
interest. But we must not leave our geological story unfinished by
leaving out the culmination of it all in man. In the higher
gravels--the Plateau Gravels--no remains of man are found; but in the
lower--the Valley Gravels,--of the South of England is found abundant
evidence of the presence of man. Large numbers of flint implements
have been collected from the Thames valley and over the whole area of
the rivers which have gravel terraces along their course. Over a large
sheet of gravel at Southampton, whenever a large gravel pit is dug,
implements are found at the base of the gravel.[17] The occurrence of
the mammoth and other arctic creatures in the gravels shows that in
the Glacial Period man was contemporary with these animals. Remains in
caves tell the same story. In limestone caverns in Devon, Derbyshire,
and Yorkshire, implements made by man are found in company with
remains of the cave bear, cave hyæna, lion, hippopotamus, rhinoceros,
and other animals either extinct or no longer inhabitants of this
country--remains which have been preserved under floors of stalagmite
deposited in the caves. In caves of central France men have left
carvings on bone and ivory, representing the wild animals of that
day--carvings which show a remarkable artistic sense, and a keen
observation of animal life. Among them is a drawing of the mammoth on
a piece of mammoth ivory, showing admirably the appearance of the
animal, with his long hair, as he has been found preserved in ice to
the present day near the mouths of Siberian rivers. Drawings of the
reindeer, true to life, are frequent.
Till recently very few Palæolithic implements had been recorded as
found in the Isle of Wight. In the Memoir of the Geological Survey
(1889) only one such is recorded, found in a patch of brick earth near
Howgate Farm, Bembridge.[18] A few more implements, which almost
certainly came from this brick-earth, have been found on the shore
since. In recent years a large number of Palæolithic implements have
been found at Priory Bay near St. Helen's. They were first observed on
the beach by Prof. E. B. Poulton, F.R.S., in 1886, and were traced to
their source in the gravel in the cliff by Miss Moseley in 1902. From
that time, and especially from 1904 onwards, many have been found by
Prof. Poulton, by R. W. Poulton (and others). Up to 1909 about 150
implements had been found, and there have been more finds since.[19]
The most important finds, besides those at Priory Bay, have been those
of Mr. S. Hazzledine Warren at Freshwater, especially in trial borings
in loam and clay below the surface soil in a depression of the High
Downs, south of Headon Hill, at a level of about 360 ft. O.D., in
which a number of Palæolithic tools, flakes, and cores were found[20].
Isolated implements have been found in recent years in various
localities in the Island. There are references to finds of implements
at different times in the past, but the descriptions are generally too
vague to conclude certainly to what date they belong. Much of the
gravel used in the Island comes from the angular gravel on St.
Boniface Down, or the high Plateau Gravel of St. George's Down; but in
the lower gravels and associated brick earth, it is highly probable
that more remains of Palæolithic man will yet be found in the Island,
and quite possible that such have been found in the past, but for
want of accurate descriptions of the circumstances of the finds are
lost to us.
We must pass on to the men of the Neolithic or later stone age. The
Palæolithic age was of very great duration, much longer than all
succeeding human history. Between Palæolithic and Neolithic times
there is in England a large gap. In France various stages have been
traced showing a continual advance in culture. In England little, if
anything, has been found belonging to the intermediate stages. Such
remains may yet be found in caves, or in lower river gravels, now
buried below the alluvium. The gap between Palæolithic and Neolithic
is marked by the great amount of river erosion which took place in the
interval. Palæolithic implements are found in gravels formed when the
rivers flowed some 100 feet above their present courses. Take, _e.g._,
the Itchen at Southampton. After the 100 foot gravels were deposited
the river cut down, not merely to its present level, but to an old bed
now covered up by various deposits beneath the river. After cutting
down to that bed the river laid down gravels upon it; and then--the
land standing at a higher level than to-day--the river valley and the
surrounding country were covered by a forest, which, as the climate
altered and became damper, was succeeded by the formation of peat. The
land has since sunk, and the peat, in parts 17 ft. thick, is now found
under Southampton Water, covered by estuarine silt. The Empress Dock
at Southampton was dug where a mud bank was exposed at low water. The
mud bank was formed of river silt 12 to 17 feet thick. Below this was
the peat, resting on gravel. On the gravel horns of reindeer were
found. In the peat were large horn cores of the great extinct ox, _Bos
primigenius_, also horns of red deer, and also in the peat were found
neolithic flint chips, a circular stone hammer head, with a hole bored
through for a wooden handle, and a large needle made of horn. Here, at
a great interval of time after Palæolithic man, as we see by the
history of the river we have just traced, we come to the new race of
men, the Neolithic.
When Neolithic man appeared the land stood higher than at present,
though not so high as during great part of the Pleistocene. Britain
was divided from the Continent, but the shores were a good way out
into what is now sea round the coasts, and forests clothed these
further shores. Remains of these, known as submerged forest, are found
below the tide mark round many parts of our coast. Peat as at
Southampton Docks, is found under the estuarine mud off Netley. The
wells at the Spithead Forts show an old land surface with peat more
than 50 feet below the tide level. The old bed of the Solent river
lies much lower still--124 feet below high tide at Noman's Land Fort;
this channel was probably an estuary after the subsidence of the land
till it silted up with marine deposits to the level on which the
submerged forest grew.
When the Solent and Southampton Water were wooded valleys with rivers
flowing down the middle, the Isle of Wight rivers were tributaries to
the Solent river, and the forest, as might be expected, extended up
their valleys, and covered the low ground of the Island. Under the
alluvial flats are remains of buried forests. In digging a well at
Sandford in 1906 large trunks of hard oak were found blocking the
sinking of the well. When the land sank the sea flowed up the river
valleys, converting them into strait and estuary, and largely filling
up the channels with the silt, which now covers the peat. In the silt
of Newtown river are found bones of _Bos primigenius_, which was found
with the Neolithic remains in the peat of Southampton docks.
The remains of Neolithic man are not only found in submerged forests,
but over the present surface of the land, or buried in recent
deposits. He has left us the tombs of his chiefs, known as long
barrows--great mounds of earth covering a row of chambers made of
flat stones, such as the mounds of New Grange in Ireland, and the
cromlechs or dolmens still standing in Wales and Cornwall. These
consist of a large flat or curved stone--it may be 14 feet in
length,--supported on three or four others. Originally a great mound
of earth or stones was piled on top. These have generally been removed
since by the hand of later man. The stones have been taken for road
metal, the earth to lay on the land. The great cromlech at Lanyon in
Cornwall was uncovered by a farmer, who had removed 100 cart loads of
earth to lay on his stony land before he had any idea that it was not
a natural mound. Then he came on the great cromlech underneath.
Another form of monument was the great standing stone or menhir, one
of which, the Longstone on the Down above Mottistone still stands to
mark the tomb of some chieftain of, it may be, 4,000 years ago.
The implements of Neolithic man are found all over England, the smooth
polished axe head, commonly called a celt (Lat. _celtis_, a chisel),
the chipped arrow head, the flaked flint worked by secondary chipping
on the edge into a knife, or a scraper for skins; and much more common
than the implement, even of the simplest description, are the waste
flakes struck off in the making. Very few stone celts have been found
in the Isle of Wight. The flakes are extremely numerous, and a scraper
or knife may often be found. They are turned up by the plough on the
surface of the fields, in the earth of which they have been preserved
from rubbing and weathering. They have however, acquired a remarkable
polish, or "patina"--how is not clearly explained--which distinguishes
their surface from the waxy appearance of newly-broken flint. In
places the ground is so covered with flakes that we can have no doubt
that these are the sites of settlements. The implements were made from
the black flints fresh out of the chalk, and we can locate the
Neolithic flint workings. In our northern range of downs where the
strata are vertical the layers of flint in the Upper Chalk run out on
the top of the downs, only covered with a thin surface soil. In
places where this soil has been removed--as in digging a quarry--the
chalk is seen to be covered with flakes similar to those found on the
lower ground, save that they are weathered white from lying exposed on
the hard chalk, instead of on soft soil into which they would
gradually sink by the burrowing of worms. It is probable that these
flakes would be found more or less along the range of downs under the
surface soil.
In places on the Undercliff have been found what are known as Kitchen
Middens--heaps of shells which have accumulated near the huts of
tribes of coast dwellers, who lived on shellfish. One such was
formerly exposed in the stream below the old church at Bonchurch, and
is believed to extend below the foundations of the Church.
After a long duration of neolithic times a great step in civilisation
took place with the introduction of bronze. Bronze implements were
introduced into this country probably some time about B.C. 1800-1500;
and bronze continued to be the best material of manufacture till the
introduction of iron some two or three centuries before the visit of
Julius Caesar to these Islands. To the early bronze age belong the
graves of ancient chieftains known as round barrows, of which many are
to be seen on the Island downs. Funeral urns and other remains have
been found in these, some of which are now in the museum at
Carisbrooke Castle. Belonging to later times are the remains of the
Roman villa at Brading and smaller remains of villas in other places;
and cemeteries of Anglo-Saxon date, rich in weapons and ornaments,
which have been excavated on Chessil and Bowcombe Downs. But the study
of the remains of ancient man forms a science in itself--Archæology.
In studying the periods of Palæolithic and Neolithic man we have stood
on the borderland where Geology and Archæology meet. We have seen that
vast geological changes have taken place since man appeared on earth.
We must remember that the geological record is still in process of
being written. It is not the record of a time sundered from the
present day, but continuous with our own times; and it is by the study
of processes still in operation that we are able to read the story of
the past.
[Footnote 17: Mr. W. Dale, F.S.A.]
[Footnote 18: See figure 9, p. 79.]
[Footnote 19: See account by R. W. Poulton in F. Morey's "Guide
to the Natural History of the Isle of Wight."]
[Footnote 20: Surv. Mem., I.W., 1921, p. 174.]
Chapter XIII.
THE SCENERY OF THE ISLAND--Conclusion.
After studying the various geological formations that enter into the
composition of the Isle of Wight, and learning how the Island was
made, it will be interesting to take a general view of the scenery,
and see how its varied character is due to the nature of its geology.
It would hardly be possible to find anywhere an area so small as this
little Island with such a variety of geological formations. The result
is a remarkable variety in the scenery.
The main feature of the Island is the range of chalk downs running
east and west, and terminating in the bold cliffs of white chalk at
Freshwater and the Culvers. Here we have vertical cliffs of great
height, their white softened to grey by weathering and the soft haze
through which they are often seen. In striking contrast of colour are
the Red Cliff of Lower Greensand adjoining the Culvers, and the
many-coloured sands of Alum Bay joining on to the chalk of Freshwater.
The summits of the chalk downs have a characteristic softly rounded
form, and the chalk is covered with close short herbage suited to the
sheep which frequently dot the green surface. Where sheets of flint
gravel cap the downs, as on St. Boniface, they are covered by furze
and heather, producing a charming variation from the smooth turf where
the surface is chalk. The Lower Greensand forms most of the undulating
country between the two ranges of downs; while the Upper Greensand,
though occupying a smaller area, produces one of the most conspicuous
features of the scenery--the walls of escarpment that form the inland
cliffs between Shanklin and Wroxall, Gat Cliff above Appuldurcombe,
the fine wall of Gore Cliff above Rocken End, and the line of cliffs
above the Undercliff. To the Gault Clay is due the formation of the
Undercliff--the terrace of tumbled strata running for miles well above
the sea, but sheltered by an upper cliff on the north, and in parts
overgrown with picturesque woods. The impervious Gault clay throws out
springs around the downs, which form the headwaters of the various
Island streams. The upper division of the Lower Greensand, the
Sandrock, forms picturesque undulating foothills, often wooded, as at
Apsecastle, and at Appuldurcombe and Godshill Park. On a spur of the
Sandrock stands Godshill Church, a landmark visible for miles around.
At Atherfield we have a fine line of cliffs of Lower Greensand, while
the Wealden Strata on to Brook form lower and softer cliffs.
To the north of the central downs the Tertiary sands and clays, often
covered by Plateau gravel, form an extended slope towards the Solent
shore, much of it well wooded, and presenting a charming landscape
seen from the tops of the downs. This slope of Tertiary strata is
deeply cut into by streams, which form ravines and picturesque creeks,
as Wootton Creek, 200 feet below the level of the surrounding country.
While much of the Island coast is a line of vertical cliff, the
northern shores are of gentler aspect, wooded slopes reaching to the
water's edge, or meadow land sloping gradually to the sea level.
Opposite the mouths of streams are banks of shingle and sand dunes,
forming the spits locally known as "dovers." Some of these, in
particular, St. Helen's Spit, afford interesting hunting grounds for
the botanist.
The great variety of soil and situation renders the Isle of Wight a
place of interest to the botanist. We have the plants of chalk downs,
of the sea cliff and shore, of the woods and meadows, of lane and
hedgerow, and of the marshes. The old villages of the Island, often
occupying very picturesque situations--as Godshill on a spur of the
southern downs, Newchurch on a bluff overlooking the Yar valley,
Shorwell nestling among trees in a south-looking hollow of the downs,
Brighstone with its old church cottages and farmhouses among trees and
meadows between down and sea--the old and interesting churches, the
thatched cottages, the old manor houses of Elizabethan or Jacobean
date, now mostly farm houses, for which the Island is famous, add to
the varied natural beauty.
One of the most characteristic features of the southern coasts of the
Island, should be mentioned, the Chines,--narrow ravines which cut
inland from the coast through the sandstone and clays of the Greensand
and Wealden strata, and along the beds of which small streams flow to
the sea. Narrow and steep-sided,--the name by which they are called is
akin to _chink_--they are in striking contrast to the more open
valleys of the streams which flow into the Solent on the north shore
of the Island. The most beautiful is Shanklin Chine. The cliff at the
mouth of the chine, just inside which stands a picturesque fisherman's
cottage with thatched roof, is 100 ft. high; and the chasm runs inland
for 350 yds., to where a very reduced cascade (for the water thrown
out of the Upper Greensand by the Gault clay is tapped at its source
for the town supply) falls vertically over a ledge produced by hard
ferruginous beds of the Greensand. Above the cascade the ravine runs
on, but much shallower, for some 900 yards. The lower ravine has much
beauty, tall trees rising up the sides, and overshadowing the chasm,
the banks thickly clothed with large ferns and other verdure. Much
wilder are the chines on the south-west of the Island. The cascade at
Blackgang falls over hard ferruginous beds (to which the beds over
which Shanklin cascade falls--though on a smaller scale--probably
correspond). The chine above these beds, being hollowed out in the
soft clays and sands of the Sandrock series, is much more open. Whale
Chine is a long winding ravine between steep walls, the stream at the
bottom making its way through blocks of fallen strata.
The cause of these chines seems to be the same in all cases. It may be
noticed that Shanklin and Luccombe chines are cut in the floors of
open combes,--wide valleys with gently sloping floors; and at each
side of these chines is to be seen the gravel spread over the floor of
the old valley. It can scarcely be doubted that these combes are the
heads of the valleys of the old streams, which flowed down a gradual
slope till they joined the old branch (or, rather the old main
river)[21] of the Yar, flowing over land extending far over what is now
Sandown Bay. When the sea encroached, and cut into the course of this
old river, and on till it made a section of what had been the left
slope of the valley, the old tributaries of the Yar now fell over a
line of cliff into the sea. They thus gained new erosive power, and
cut back at a much greater rate new and deeper channels; with the
result that narrow trenches were cut in the floors of the old gently
sloping valleys. The chines on the S.W. coast are to be explained in a
similar way. They have been cut back with vertical sides, because the
encroachment of the sea caused the streams to flow over cliffs, and so
gave then power to cut back ravines at so fast a rate that the
weathering down of the sides could not keep pace with it. The
remarkable wind-erosion of these bare south-westerly cliffs by a sort
of sand-blast driven before the gales to which that stretch of coast
is exposed has already been referred to.
A few words in conclusion to the reader. I have tried to show you
something of the interest and wonder of the story written in the
rocks. We have seen something of the world's making, and of the many
and varied forms of life which have succeeded each other on its
surface. We have had a glimpse of great and deep problems suggested,
which are gradually receiving an answer. Geology has the advantage
that it can be studied by all who take walks in the country, and
especially by those who visit any part of the sea coast, without the
need of elaborate and costly scientific instruments and apparatus. Any
country walk will suggest problems for solution. I have tried to lead
you to observe nature accurately, to think for yourselves, to draw
your own conclusions. I have shown you how to try to solve the
questions of geology by looking around you at what is taking place
to-day, and by applying this knowledge to explain the records which
have reached us of what has happened in the past. You are not asked to
accept the facts of the geological story on the word of the writer, or
on the authority of others, but to think for yourselves, to learn to
weigh evidence, to seek only to find out the truth, whether it be
geology you are studying or any other subject, and to follow the truth
whithersoever it leads.
[Footnote 21: See p. 91.]
TABLE OF STRATA
Recent. Peat and River Alluvium.
Pleistocene. Plateau Gravels: Valley Gravels and Brick-Earth.
{ Pliocene} Absent from the Isle of Wight.
{ Miocene }
{ { { Marine, Corbula Beds
{ { Hamstead { Freshwater & Estuarine.
{ {
{ { { Bembridge Marls
{ { Bembridge {
{ { Beds { Bembridge Limestone
{ {
{ Oligocene { Osborne and St. Helen's Beds.
{ {
Tertiary { { { Upper. Freshwater and Brackish
{ { Headon { Middle. Marine
{ { Beds { Lower. Freshwater and Brackish
{
{ { Barton} Barton Sand.
{ { Beds} Barton Clay.
{ {
{ Eocene { Bracklesham Beds.
{ { Bagshot Sands
{ { London Clay
{ { Plastic Clay (Reading Beds)
{ { White { Upper Chalk (Chalk with flints)
{ { Chalk { Middle Chalk (Chalk
{ { { without flints)
{ {
{ { { A. plenus Marls
{ Upper { Lower { Grey Chalk
{ Cretaceous { Chalk { Chalk Marl
{ { { Chloritic Marl
{ {
{ { { Upper { Chert Beds
{ { Selbornian { Greensand { Sandstone and
{ { { Rag Beds
Mesozoic { { Gault
or {
Secondary{ { Carstone
{ { Lower { Sandrock and Clays
{ { Greensand { Ferruginious Sands
{ { { Atherfield Clay
{ Lower { { Perna Bed
{ Cretaceous {
{ { { Shales
{ { Wealden { Variegated Marls
FOR FURTHER STUDY.
Memoirs of the Geological Survey. General Memoir of the Isle of Wight,
date 1889. New edition, entitled "A short account of the Geology of
the Isle of Wight," by H. J. Osborne White, F.G.S., 1921, price 10s.
The Memoirs are the great authority for the Geology of the Island:
technical; books for Geologists. The New Edition is more condensed
than the original, but contains much later research. Mantell's
"Geological Excursions round the Isle of Wight," 1847. By one of the
great early geologists. Long out of print, but worth getting, if it
can be picked up second-hand.
Norman's "Guide to the Geology of the Isle of Wight," 1887, still to
be obtained of Booksellers in the Island. Gives details of strata,
and lists of fossils, with pencil drawings of fossils.
Other books bearing on the subject have been mentioned in the text and
foot-notes.
An excellent geological map of the Island, printed in colour, scale
1 in. to the mile, full of geological information, is published by the
Survey at 3s.
A good collection of fossils and specimens of rocks from the various
strata of the Isle of Wight has recently been arranged at the Sandown
Free Library, and should be visited by all interested in the Geology
of the Island. It should prove a most valuable aid to all who take up
the study, and a great assistance in identifying any specimens they
may themselves find.
[Illustration]
GEOLOGICAL MAP OF THE ISLE OF WIGHT
INDEX
Words in Italics refer to a page where the meaning of a
term is given.
Agates, 22, 41, 50
Alum Bay, 56-62
Ammonites, 32, 34, 39, 44
_Anticline_, 12
Astronomical Theory of Ice Age, 83, 85
Atherfeld, 29
Avon River, 94
Barrows, 102, 104
Barton, 61
Belemnites, 33
Bembridge Limestone, 65
-- shingle at, 95
Benettites, 27
"Blue Slipper," 15
Bonchurch, 50, 103
Bos primigenius, 101, 102
Botany, 106
Bracklesham, 59, 60
Brading Harbour, 90, 91
Bronze age, 103
Brook, 29
Building Stone, 39, 65
Carstone, 26, 35
Chalcedony, 22, 41, 50
Chale, 33
Chalk, divisions of, 45, 51, 52
-- Marl, 45
-- Rock, 45
Chalybeate Springs, 25
Chert, 39
Chloritic Marl, 44
Climate.
Coal, 8, 61
Colwell Bay, 64
Compton Bay, 31, 39
Conglomerate, modern, 25
"Crackers," 32
Cretaceous.
Crioceras, 34
Current Bedding, 27
Cycads.
Denudation, 3, 12, 76, 80, 82
_Dip_, 11
Echinoderms, 48, 52
Eocene, 54
Erosion, marine, 4
" pre-Tertiary, 54
_Escarpment_, 14
_Faults_, 13
Fault at Brook, 30
Flint, origin of, 47
" implements, 97
Flora, Alum Bay, 59
" Eocene, 58, 62
" Wealden, 18, 27
Foraminifera, 42, 61
Gat Cliff, 38
Gault, 37
Glacial Period, 77-85
Glauconite, 24, 39, 44
Gore Cliff, 39, 44
Greensand, Lower, 23-36
" Upper, 37
Gravels, 50, 79, 89, 93-95
Hamstead, 65, 67
Headon Hill, 62-64
Hempstead, see Hamstead.
Hyopotamus, 69
Ice Age, 77-85
Iguanodon, 20
Insect Limestone, 67
Iron Ore, 22, 24
Iron pyrites, 22
Landslips, 25, 38
Limnæa, 63, 64, 66
Lobsters, Atherfield, 32
London Clay, 57
Luccombe, Landslip at, 25
Mammalian Remains, 66, 69
Mammoth, 77, 81
Marvel, 35
Medina, 93
Melbourn Rock, 45
Miocene, 69, 71, 76
Nautilus, 32, 45
Needles, 4
Neolithic Man, 100
Newtown River, 102
Nummulites, 61
Oligocene, 63
Palæolithic Man, 97
Perna Bed, 23, 31
Pine Raft, 29
Planorbis, 63, 64, 66
Plastic Clay, 57
Priory Bay, 99
Purbeck Marble, 16
Quarr, 65
Rag, 38
Rock (place), 35
Roman Villas, 104
St. Boniface Down, 50, 100, 105
St. George's Down, 79, 100
Sandown Anticline, 11-13, 89
Sandrock, 25, 35
Scaphites, 34
Scenery, 105
Sea Urchins, 48, 52,
Shanklin Chine, 107
Solent, 94
Southampton Dock, 101
" Water, 94
Sponges in Flint, 47
Stone Age, 97
Strata, Table of, 110, 111
_Strike_, 11
Submerged Forest, 101
Swanage, 93
_Syncline_, 12
Table of Strata, 110, 111
Tertiary, 54
Totland Bay, 63, 95
Tufa, 45
Turtle, 58, 65, 68
Undercliff, formation of, 25, 38
Volcanic Action, 5
Wealden, 15
Whitcliff Bay, 56-67
Wood, Fossil, 8, 15, 18, 27, 29
Yar, Eastern, 89-91
" Western, 92
Zones of Chalk, 51, 52
_Printed by The Crypt House Press, Bell Lane, Gloucester._
Transcriber's Notes
With the exception of the changes noted below, the text in this file
is the same as that in the original printed version. These may include
alternate spelling from what may be common today (for example,
gneisse); punctuational and/or grammatical nuances. Additionally,
several missing periods were inserted; but are not listed below.
Lastly, the Index seems to be missing a few references to page numbers
and were left as originally printed.
Emphasis Encoding
_Text_ - Italicized Text
$Text$ - Greek translation
Typographical Corrections
Page 69: regious => regions
Page 101: sourrounding => surrounding
Page 102: remains In the peat => ... in ...
Page 106: surounding => surrounding
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