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
Archaean ("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 Archaean 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 Archaean 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 nebulae 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 Archaean 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 Archaean 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.]