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_ (_Sphaera_) _corrugata_, a flatter
bivalve _Astarte_,--and a smaller oblong shell _Panopaea_,--also a
peculiar shell of triangular form, _Trigonia_,--one species _T.
caudata_ has raised ribs running across it, another _T. daedalea_ has
bands of raised spots. A pretty little coral, looking like a
collection of little stars, _Holocystis elegans_, one of the Astraeidae,
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 _Terebratulae_, and others with fragments of sea
urchins. The Terebratulae and Rhynchonellae 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 Lingulae, 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 Cycadaceae 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 Gryphaea Beds._
Sc _Scaphite. "_
Lc _Lower Crioceras "_
W _Walpen Clay._
Uc _Upper Crioceras Beds._
WS _Walpen and Ladder Sands._
Ug _Upper Gryphaea 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._