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 Limnaea 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 debris of
strata fallen in confusion, in which are beautiful specimens of Limnaea
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 Limnaea and Cyrena. The
How Ledge limestone forms the top of the Lower Headon. It is full of
well-preserved Limnaea and Planorbis.
The Upper and Lower Headon are mainly fresh or brackish water
deposits. The purely freshwater beds contain _Limnaea_, _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 Limnaeae it
contains. The quarries at Quarr were noted in more ways than one. In
later times the remains of early mammalia,--Palaeotherium,
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, Limnaea 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 Algae, 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_, _Limnaea_,
_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 Palaeotherium
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.