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.