Before giving further account of the physical geography of the county
it is necessary to learn somewhat of its geology, as the physical
conditions are to a large extent dependent upon geological structure.
By Geology we mean the study of the rocks, and we must at the outset
explain that the term rock is used by the geologist without any
reference to the hardness or compactness of the material to which the
name is applied; thus he speaks of loose sand as a rock equally with
a hard substance like granite.
Rocks are of two kinds, (1) those laid down mostly under water;
(2) those due to the action of heat.
The first kind may be compared to sheets of paper one over the
other. These sheets are called beds, and such beds are usually formed
of sand (often containing pebbles), mud or clay, and limestone, or
mixtures of these materials. They are laid down as flat or nearly
flat sheets, but may afterwards be tilted as the result of movement
of the earth's crust, just as you may tilt sheets of paper, folding
them into arches and troughs, by pressing them at either end. Again,
we may find the tops of the folds so produced worn away as the result
of the constant action of rivers, glaciers, and sea-waves upon them,
as one might cut off the tops of the folds of the paper with a pair
of shears. This has happened with the ancient beds forming parts
of the earth's crust, and we therefore often find them tilted, with
the upper parts removed. Tilted beds are said to dip, the direction
of dip being that in which the beds plunge downwards, thus the beds
of an arch dip away from its crest, those of a trough towards its
middle. The dip is at a low angle when the beds are nearly horizontal,
and at a high angle when they approach the vertical position. The
horizontal line at right angles to the direction of the dip is called
the line of strike. Beds form strips at the surface, and the portion
where they appear at the surface is called the outcrop. On a large
scale the direction of outcrop generally corresponds with that of the
strike. Beds may also be displaced along great cracks, so that one
set of beds abuts against a different set at the sides of the crack,
when the beds are said to be faulted.
The other kinds of rocks are known as igneous rocks, which have been
melted under the action of heat and become solid on cooling. When in
the molten state they have been poured out at the surface as the lava
of volcanoes, or have been forced into other rocks and cooled in the
cracks and other places of weakness. Much material is also thrown out
of volcanoes as volcanic ash and dust, and is piled up on the sides
of the volcano. Such ashy material may be arranged in beds, so that it
partakes to some extent of the qualities of the two great rock groups.
The production of beds is of great importance to geologists, for by
means of these beds we can classify the rocks according to age. If
we take two sheets of paper, and lay one on the top of the other on
a table, the upper one has been laid down after the other. Similarly
with two beds, the upper is also the newer, and the newer will remain
on the top after earth-movements, save in very exceptional cases
which need not be regarded by us here, and for general purposes we
may regard any bed or set of beds resting on any other in our own
country as being the newer bed or set.
The movements which affect beds may occur at different times. One set
of beds may be laid down flat, then thrown into folds by movement, the
tops of the beds worn off, and another set of beds laid down upon the
worn surface of the older beds, the edges of which will abut against
the oldest of the new set of flatly deposited beds, which latter may
in turn undergo disturbance and removal of their upper portions.
Again, after the formation of the beds many changes may occur in
them. They may become hardened, pebble-beds being changed into
conglomerates, sands into sandstones, muds and clays into mudstones
and shales, soft deposits of lime into limestone, and loose volcanic
ashes into exceedingly hard rocks. They may also become cracked,
and the cracks are often very regular, running in two directions at
right angles one to the other. Such cracks are known as joints, and
the joints are very important in affecting the physical geography
of a district. As the result of great pressure applied sideways,
the rocks may be so changed that they can be split into thin slabs,
which usually, though not necessarily, split along planes standing at
high angles to the horizontal. Rocks affected in this way are known
as slates.
If we could flatten out all the beds of England, and arrange them
one over the other and bore a shaft through them, we should see
them on the sides of the shaft, the newest appearing at the top and
the oldest at the bottom. Such a shaft would have a depth of between
50,000 and 100,000 feet. The beds are divided into three great groups
called Primary or Palaeozoic, Secondary or Mesozoic, and Tertiary or
Cainozoic, and at the base of the Primary rocks are the oldest rocks
of Britain, which form as it were the foundation stones on which the
other rocks rest, and are termed Precambrian rocks. The three great
groups are divided into minor divisions known as systems.
NAMES OF SUBDIVISIONS CHARACTERS OF ROCKS
SYSTEMS
{ { Metal Age Deposits }
{ Recent { Neolithic ,, } Superficial Deposits
{ Pleistocene { Palaeolithic ,, }
{ { Glacial ,, }
T {
E { { Cromer Series }
R { { Weybourne Crag }
T { Pliocene { Chillesford and Norwich Crags } Sands chiefly
I { { Red and Walton Crags }
A { { Coralline Crag }
R {
Y { Miocene Absent from Britain
{
{ { Fluviomarine Beds of Hampshire}
{ { Bagshot Beds }
{ Eocene { London Clay } Clays and Sands
{ { Oldhaven Beds, Woolwich and } chiefly
{ { Reading Groups }
{ { Thanet Sands }
{ { Chalk }
{ Cretaceous { Upper Greensand and Gault } Chalk at top
{ { Lower Greensand } Sandstones, Mud and
{ { Weald Clay } Clays below
{ { Hastings Sands }
{
{ { Purbeck Beds }
S { { Portland Beds }
E { { Kimmeridge Clay }
C { { Corallian Beds }
O { Jurassic { Oxford Clay and Kellaways Rock} Shales, Sandstones
N { { Cornbrash } and Oolitic
D { { Forest Marble } Limestones
A { { Great Oolite with Stonesfield }
R { { Slate }
Y { { Inferior Oolite }
{ { Lias--Upper, Middle, and Lower}
{
{ { Rhaetic }
{ { Keuper Marls }
{ Triassic { Keuper Sandstone } Red Sandstones and
{ { Upper Bunter Sandstone } Marls, Gypsum and
{ { Bunter Pebble Beds } Salt
{ { Lower Bunter Sandstone }
{ { Magnesian Limestone and }
{ Permian { Sandstone } Red Sandstones and
{ { Marl Slate } Magnesian Limestone
{ { Lower Permian Sandstone }
{
{ { Coal Measures } Sandstones, Shales
{ Carboniferous { Millstone Grit } and Coals at top
{ { Mountain Limestone } Sandstones in middle
{ { Basal Carboniferous Rocks } Limestone and Shales
{ { } below
{
{ { Upper } } Red Sandstones,
{ Devonian { Mid } Devonian and Old } Shales, Slates and
P { { Lower } Red Sandstone } Limestones
R {
I { { Ludlow Beds } Sandstones, Shales
M { Silurian { Wenlock Beds } and Thin Limestones
A { { Llandovery Beds }
R {
Y { { Caradoc Beds } Shales, Slates,
{ Ordovician { Llandeilo Beds } Sandstones and
{ { Arenig Beds } Thin Limestones
{
{ { Tremadoc Slates }
{ Cambrian { Lingula Flags } Slates and
{ { Menevian Beds } Sandstones
{ { Harlech Grits and Llanberis }
{ { Slates
{
{ { Sandstones,
{ Pre-Cambrian No definite classification { Slates and
{ yet made { Volcanic Rocks
In the preceding table (p. 29) a representation of the various great
subdivisions or 'systems' of the beds which are found in the British
Islands is shown. The names of the great divisions are given on the
left-hand side, in the centre the chief divisions of the rocks of each
system are enumerated, and on the right-hand the general characters
of the rocks of each system are given.
Berkshire is now part of an island and is a long way from the sea,
but there have been times when the arrangement of land and sea on
the globe was very different from what it is now. Our district has
during some periods been part of a continent, and in others it has
been overflowed by the sea.
These changes in the distribution of land and water were due to
movements of the crust of the earth, and very largely to movements
of compression from the sides, causing folding of the strata of which
the crust of the earth is composed.
After many and great changes, at a time geologically recent, but
still long before the beginning of history in the usual sense of the
word, the district now known as Berkshire rose above the sea for the
last time.
Since that date deposits of clay, sand, etc., have been formed in our
area, and their formation is indeed still going on to some extent,
but though these are true geological deposits they are of no great
thickness, seldom as much as 20 feet. They are, however, at or near
the surface of the ground, and consequently exercise considerable
influence on the character of the country. We will, however, leave
them out of account for the moment and consider the deposits formed
before the district finally rose above the sea.
These deposits are usually spoken of as forming the solid geology
of the area, and the three divisions, into which as we have said
Berkshire is divided, are characterised as follows:--
1. In the northern part of the county, including the Vale of White
Horse, the geological strata are older than the chalk formation.
2. In the central part of Berkshire the chalk formation is at or
near the surface of the ground.
3. In the forest country of south and east Berkshire, the surface
is formed of geological formations newer than the chalk, but the
chalk is always to be found underground if one goes deep enough.
If we look at a sectional plan of geological strata we shall see that
none of the formations which come to the surface in our county are
of any great antiquity, but somewhere deep down, say over a thousand
feet below us, there is a platform of much older rocks, upon which
those that come to the surface rest in an irregular manner. What these
old rocks may be we do not know, but probably New Red Sandstone and
possibly beds of coal may occur amongst them.
Speaking generally, we pass from older to newer geological formations
as we go from the north-west towards the south-east, and we find that
the Oxford Clay is the oldest formation which comes to the surface
in Berkshire.
The Oxford Clay forms a strip of low land along the banks of the Isis
from the Cole to the Cherwell near Oxford. It was originally mud
deposited in a sea which extended over a great part of England. It
is dark coloured, often shaley, with a little clayey limestone. A
large oyster is one of its common fossils. Its thickness is about 450
feet, and it is not a water-bearing formation. The Oxford Clay dips
underground to the east and is covered by newer rocks, the first of
which is the Corallian.
The Corallian forms a very well-marked band running across the
county from the Cole to the Thames. Wytham Hill is formed of it, and
Shrivenham, Coleshill, Faringdon, Buckland, Fyfield, Appleton, and
Cumnor are situated upon it. It is essentially a calcareous formation
with some hard limestone beds, and has a thickness of from 50 to 80
feet. It was formed in the sea; probably a shallow sea with shoals,
sand, and coral banks. Fossil corals are abundant, and many specimens
of Ammonites and other marine shells are to be found. There are some
good examples of these from Marcham in the Reading Museum. Supplies
of good water may often be obtained from this formation. The Corallian
beds are quarried for building stone and road material in many places.
The Kimmeridge Clay, which comes above the Corallian, is, like the
Oxford Clay, a bed of hardened marine mud. It has now become a shaley
clay, and is about 140 feet thick. It forms a narrow east and west band
across the county. Much of the Vale of White Horse is on this clay,
and the town of Abingdon stands upon it. It is not a water-bearing
formation.
The Portland Beds. A small patch of this formation is found resting
upon the Kimmeridge Clay in Berkshire. It caps the rising ground
south of Shrivenham, and the village of Bourton stands upon it. Its
thickness is about 20 feet.
After the deposition of the Portland rocks, which are of marine origin,
there is reason to believe that our district became land and a part
of a continent, but no relics of this period remain here. They were
all swept away when the land sank again and the Cretaceous sea flowed
over Berkshire.
The Lower Greensand--our next deposit--was formed after a long
interval, and, owing to earth movements which had taken place during
that interval, it rests upon the older rocks in an irregular manner. It
is a marine formation, and only occurs in patches, the largest of which
extends from Uffington to near Faringdon. Its greatest thickness is
about 60 feet, and it consists of sand with some ironstone and chert,
pebble beds, and a calcareous sponge gravel. The sponge gravel,
so-called from the number of fossil sponges it contains, is dug for
garden paths and walks, and is exported to long distances. The fossil
sponges in the gravel are abundant and beautifully preserved, and
they seem to have lived on the spot. The ironstone was at one time
worked near Faringdon. At New Lodge, in the parish of Winkfield, the
Lower Greensand was reached in a boring at a depth of 1234 feet. A
good supply of water was obtained, but it contains a large quantity
of common salt.
The Gault, the next formation, consists of grey clay in the lower part
and of a silty marl in the upper part, with a total thickness of some
220 feet. It crosses the county as a band, from one to three miles in
width, from Ashbury to the Thames between Abingdon and Wallingford. It
is a marine formation, and does not give a water-supply.
The Upper Greensand runs across the county as a narrow and irregular
band about 90 feet thick, and consists of green sands and grey marl,
with beds of stone in places. It is of marine origin, and provides
a supply of excellent water, and consequently many villages stand
upon or close to it. Ashbury, Childrey, Wantage, Hendred, and Harwell
are examples.
The Chalk. This is far the most important geological formation in
Berkshire, for it occupies a large portion of the surface of the
county, and in the eastern part, when not at the surface, it is to be
found underground. It is a light-coloured limestone, usually soft and
earthy, but in parts very hard. Its full thickness is over 700 feet,
and being a porous rock, the rain which falls on its great surface
sinks in and furnishes a water-supply over its whole area whether the
chalk be at the surface or underground. It was deposited in a sea which
not only covered our district but spread over much of Europe. There
was, however, probably land to the west which included Cornwall,
parts of Wales, and of Ireland. The upper part of the Berkshire Chalk
contains many layers and nodules of flint.
There is a long break in our geological record after the newest beds
of the Chalk found in Berkshire had been deposited, for both the top
of the Chalk and the bottom of the next series are wanting here, and
in order to fill the interval we have to study rocks in other parts
of England, in Belgium, and in Denmark. During this great interval
in time the chalk sea retired, and much of Britain became land.
The Reading Beds repose upon a water- and weather-worn surface of
chalk. They consist of clays and sands, and were deposited in the bed
of a great river. Their thickness is from 70 to 90 feet, and good
water may be obtained from the sands. In the lower part we find a
bed of oysters, and rather higher up there is in some places a bed
of leaves, known as the "Reading Leaf-Bed," a specimen of which is
shown below. It will be noticed that the leaves are crowded together,
and were no doubt buried in the mud of the river.
The Basement Bed of the London Clay comes next in order and the fossils
are marine, showing that the sea was again spreading over our area. It
is from 6 to 16 feet in thickness, and consists of loam and clay with
green sand and pebbles. A set of shells from this bed is arranged in
the Reading Museum.
The London Clay is a marine formation of very uniform character,
a stiff clay, blue underground, but becoming brown near the
surface, owing to the action of surface water. It contains layers of
cement-stones. The thickness in the east of the county is nearly 350
feet, but the formation thins to the west, and is under 50 feet thick
at Inkpen. Fossils are not uncommon, and there is a fair collection
of Berkshire London Clay fossils in the Wellington College Museum. It
is not a water-bearing formation. Most of Windsor Park is on London
Clay, and a number of places the names of which end with "field"
are upon this formation, such as Arborfield, Binfield, Burghfield,
Shinfield, Swallowfield, Warfield, and Winkfield.
The Bagshot Beds, named after Bagshot Heath, consist of sand with
a few beds of clay. The maximum thickness is nearly 350 feet. They
are probably mainly of marine origin, but formed near the estuary
of a large river. Fossils are rare in this formation in Berkshire,
but a few specimens will be found in the Museums at Reading and at
Wellington College. The Bagshot Beds are a water-bearing formation,
but the water is not always of a satisfactory character. The scenery
of the sandy Bagshot country is well shown by the view opposite.
Some indefinite time after the deposition of the Bagshot Beds
considerable earth movements took place in the south of England,
and Berkshire became, and has since remained, dry land. The Bagshot
Beds are consequently the last marine formation in our district,
and we thus complete our account of the solid geology of the county.
The solid strata are, however, to a considerable extent covered
with a variety of geological deposits due to rain, frost, streams,
and rivers. These deposits, often termed Drift, though not marked
on the majority of geological maps, have a great importance for the
dwellers in our county, simply because they form the actual surface
and determine the character of the soil.
Clay with Flints is a formation covering a good deal of our Chalk. It
is partly débris of the chalk formation and partly of clay beds which
once rested on the Chalk. In places it is 20 feet thick. Some of the
best timber in the county grows upon it.
Gravel covers a good deal of the surface in Berkshire. It is found
both on the high ground and in the valleys. The high-level gravels
are often over 10 feet thick and the valley gravels are more than 20
feet thick in several places. Windsor, Bray, Maidenhead, Cookham,
Twyford, Wokingham, Reading, Theale, Pangbourne, and Newbury stand
partly or wholly upon gravel.
Alluvium, the modern deposit of the rivers, covers a good deal of
ground in some places, more especially in the valley of the Kennet.
Sarsens are blocks of sandstone which are found on or near the
surface of the ground or in the beds of gravel. They were probably
derived in part from the Reading Beds and in part from the Bagshot
Beds. The illustration on page 40 shows three sarsen stones lying
at the bottom of a thick bed of gravel in a gravel pit on Chobham
Ridges. The locality is in Surrey, but not far from the Berkshire
border, and similar examples occur in Berkshire.