THE TERTIARY ERA: THE EOCENE
Ages must have passed while the ocean flowed over this part of the
world, and the chalk mud, with its varied remains of living things,
gradually accumulated at the bottom. At last a change came. Slowly the
sea bed rose, till the chalk, now hardened by pressure, was raised
into land above the sea level. As soon as this happened, sea waves and
rain and rivers began to cut it down. There is evidence here of a wide
gap in the succession of the strata. Higher chalk strata, which
probably once existed, have been washed away, while the underlying
strata have been planed off to an even surface more or less oblique to
the bedding-planes. The highest zone of the chalk in the Island (that
of _Belemnitella macronata_) varies greatly in thickness, from 150 ft.
at the eastern end of the Island to 475 at the western. The latest
investigations give reason to conclude that this is due to gentle
synclines and anticlines, which have been planed smooth by the erosion
which preceded the deposition of the next strata,--the Eocene.[11] At
Alum Bay the eroded surface of the chalk may be seen with rolled
flints lying upon it, and rounded hollows or pot-holes, the appearance
being that of a foreshore worn in a horizontal ledge of rock, much
like the Horse Ledge at Shanklin.
The land sank again, but not to anything like the depth of the great
Chalk Sea. We now come to an era called the Tertiary. The whole
geological history is divided into four great eras. The first is the
Eozoic, or the age of the Archaean,--often called Pre-Cambrian--rocks;
rocks largely volcanic, or greatly altered since their formation,
showing only obscure traces of the life which no doubt existed. Then
follow the Primary era, or, as it is generally called, the Palaeozoic;
the Secondary or Mesozoic; and the Tertiary or Kainozoic. Palaeozoic is
used rather than Primary, as this word is ambiguous, being also used
for the crystalline rocks first formed by the solidification of the
molten surface of the earth. But Secondary and Tertiary are still in
constant use. These long ages, or eras, were of very unequal duration;
yet they mark such changes in the life of animal and plant upon the
earth that they form natural divisions. The Palaeozoic was an immense
period during which life abounded in the seas,--numberless species of
mollusca, crustaceans, corals, fish are found,--and there were great
forests, which have formed the coal measures, on land,--forests of
strange primeval vegetation, but in which beautiful ferns, large and
small, flourished in great numbers. The Secondary Era may be called
the age of reptiles. To this era all the rocks we have so far studied
belong. Now we come to the last era, the Tertiary, the age of the
mammals. Instead of reptiles on land, in sea and air, we find a
complete change. The earth is occupied by the mammalia; the air
belongs to the birds such as we see to-day. The strange birds of the
Oolitic and Cretaceous have passed away. Birds have taken their modern
form. In some parts of the world strata are found transitional between
the Secondary and Tertiary.
The Tertiary is divided into four divisions,--the Eocene, the
Oligocene (once called Upper Eocene), the Miocene, and the Pliocene;
which words signify,--Pliocene the more recent period, Miocene the
less recent, Eocene the dawn of the recent.
In the Eocene we shall find marine deposits of a comparatively shallow
sea, and beds deposited at the mouth of great rivers, where remains of
sea creatures are mingled with those washed down from the land by the
rivers. These strata run through the Isle of Wight from east to west,
and we may study them at either end of the Island, in Whitecliff and
Alum Bays. The strata are highly inclined, so that we can walk across
them in a short walk. Some beds contain many fossils, but many of the
shells are very brittle and crumbly; and we can only secure good
specimens by cutting out a piece of the clay or sand containing them,
and transferring them carefully to boxes, to be carried home with
equal care. Often much of the face of the cliff is covered with slip
or rainwash, and overgrown with vegetation. Sometimes a large slip
exposes a good hunting ground.
Now let us walk along the shore, and try to read the story these
Tertiary beds tell us. We will begin in Whitecliff Bay. Though easily
accessible, it remains still in its natural beauty. The sea washes in
on a fine stretch of smooth sand sheltered by the white chalk wall
which forms the south arm of the bay. North of the Culver downs the
cliffs are much lower, and consist of sands and clays of varying
colour, following each other in vertical bands. Looking along the line
of shore we notice a band of limestone, at first nearly vertical like
the preceding strata, then curving at a sharp angle as it slopes to
the shore, and running out to sea in a reef known as Bembridge Ledge.
This is the Bembridge limestone; and the beginning of the reef marks
the northern boundary of Whitecliff Bay, the shore, however,
continuing in nearly the same line to Bembridge Foreland, and showing
a continuous succession of Eocene and Oligocene strata. The strata
north of the limestone are nearly horizontal, dipping slightly to the
north. In the Bembridge limestone we see the end of the Sandown
anticline, and the beginning of the succeeding syncline. The strata
now dip under the Solent, and rise into another anticline in the
Portsdown Hills. North and south of the great anticline of the Weald
of Kent and Sussex are two synclinal troughs known as the London and
Hampshire basins. Nearly the whole of our English Eocene strata lies
in these two basins, having been denuded away from the anticlinal
arches. The Oligocene only occur in the Hampshire basin, the higher
strata only in the Isle of Wight.
[Illustration: FIG. 3.]
COAST SECTION, WHITECLIFF BAY.
BM _Bembridge Marls._
BL _Bembridge Limestone._
O _Osborne Beds._
H _Headon Beds._
BS _Barton Sand._
B _Barton Clay._
Br _Bracklesham Beds._
Bg _Bagshot Beds._
L _London Clay._
R _Reading Beds._
Ch _Chalk._
P _Pebble Beds._
S _Sandstone Band._
Above the Chalk we come first to a thick red clay called Plastic clay.
It is much slipped, and the slip is overgrown. The only fossils found
in the Island are fragments of plants; larger plant remains on the
mainland show a temperate climate. This clay was formerly worked at
Newport for pottery. The clay is probably a freshwater deposit formed
in fairly deep water. On the mainland we find on the border shallow
water deposits called the Woolwich and Reading beds. (The clay is 150
to 160 ft. thick at Whitecliff Bay, less than 90 ft. at the Alum Bay.)
We come next to a considerable thickness of dark clay with sand, at
the surface turned brown by weathering. This is the London clay, so
called because it underlies the area on which London is built. At the
base is a band of rounded flint pebbles, which extends at the base of
the clay from here to Suffolk. In it, as well as in a hard sandstone
18 inches higher up, are tubular shells of a marine worm, _Ditrupa
plana_. The sandstone runs out on the shore. About 35 ft. above the
basement bed is a zone of _Panopaea intermedia_ and _Pholadomya
margaritacea_, at 50 ft. another band of _Ditrupa_, and at about 80
ft. a band with a small _Cardita_. In the higher part of the clay are
large septaria,--rounded blocks of a calcareous clay-ironstone, with
cracks running through them filled with spar. _Pinna affinis_ is found
in the septaria. The thickness of the clay in Whitecliff Bay is 322
feet. It can be seen on the shore, when the tide happens to have swept
the sand away. Otherwise the lower beds are hardly visible, there
being no cliff here, but a slope overgrown with vegetation.
In Alum Bay the London clay, about 400 ft. in thickness, consists of
clays, chiefly dark blue, with sands, and lines of septaria. In the
lower part is a dark clay with _Pholadomya margaritacea_, still
preserving the pearly nacre. There are also _Panopaea intermedia_, and
in septaria _Pinna affinis_. All these with their pearly lustre, are
beautiful fossils. A little higher is a zone with _Ditrupa_, and
further on a band of _Cardita_. Other shells also are found in the
clay, especially in the lower part. They are all marine, and indicate
a sub-tropical climate. Lines of pebbles show that we are near a
beach. In other parts of the south of England remains from the land
are found, borne down an ancient river in the way we found before in
the Wealden deposits.
But times have changed since the Wealden days, and the life of the
Tertiary times has a much more modern appearance. From leaves and
fruits borne down from the forest we can learn clearly the nature of
the early Eocene land and climate. Leaves are found at Newhaven, and
numerous fossil fruits at Sheppey. The character of the vegetation
most resembled that now to be seen in India, South Eastern Asia, and
Australia. Palms grew luxuriantly, the most abundant fruit being that
of one called Nipadites, from its resemblance to the Nipa palm, which
grows on the banks of rivers in India and the Philippines. The forests
also included plants allied to cypresses, banksia, maples, poplars,
mimosa, custard apples, gourds, and melons. The rivers abounded in
turtle--large numbers of remains of which are found in the London clay
at the mouth of the Thames--crocodiles and alligators. With the
exception of the south east of England, all the British Isles formed
part of a continental mass of land covered with a tropical vegetation.
The mountain chains of England, Scotland, and Wales rose as now, but
higher. Long denudation has worn them down since. In the south-east of
England the coast line fluctuated; and sea shells, and the remains of
the plant and animal life of the neighbourhood of a great tropical
river alternate in the deposits.
[Illustration: FIG. 4]
SECTION THROUGH HEADON HILL AND HIGH DOWN.
SHOWING STRATA SEEN AT ALUM BAY.
G _Gravel Cap._
Bm _Bembridge Limestone._
O _Osborne Beds._
UH _Upper Headon._
MH _Middle " ._
LH _Lower Headon._
BS _Barton Sand._
B _Barton Clay._
Br _Bracklesham Beds._
Bg _Bagshot Sands._
L _London Clay._
R _Reading Beds._
Ch _Chalk._
The London clay is succeeded by a great thickness of sands and clays
which form the Bagshot series. These are divided in the London basin
into Lower, Middle, and Upper Bagshot. In the Hampshire basin the
strata are now classified as Bagshot Sands, Bracklesham Beds, Barton
Beds, the last comprising the Barton Clay and the Barton Sand,
formerly termed Headon Hill Sands. There is some uncertainty as to the
manner in which these correspond to the beds of the Bagshot district,
as the Tertiary strata have been divided by denudation into two
groups, and differ in character in the two areas. It is possible that
the Barton Sand represents a later deposit than any in the London
area.
Almost the only fossil remains in the Bagshot Sands are those of
plants, but these are of great interest. In Whitecliff Bay the beds
consist for the most part of yellow sands, above which is a band of
flint pebbles, which has been taken as the base of the Bracklesham
series, for in the clay immediately above marine shells occur. The
Bagshot Sands, in Whitecliff Bay, are about 138 feet thick, in Alum
Bay, 76 feet, according to the latest classification. In Alum Bay the
strata consist of sands, yellow, grey, white, and crimson, with clays,
and bands of pipe clay. This is remarkably white and pure, as though
derived from white felspar, like the China clay in Cornwall. The pipe
clay contains leaves of trees, sometimes beautifully preserved.
Specimens are not very easy to obtain, as only the edges of the leaves
appear at the surface of the cliff. They have been found chiefly in a
pocket, or thickening of the seam of pipe clay, which for forty years
yielded specimens abundantly, afterwards thinning out, when the leaves
became rare. The leaves lie flat, as they drifted and settled down in
a pool. With them are the twigs of a conifer, occasionally a fruit or
flower, or the wing case of a beetle. The leaves show a tropical
climate. The flora is a local one, differing considerably from those
of Eocene deposits elsewhere. The plants are nearly all dicotyledons.
Of palms there are only a few fragments, while the London clay of
Sheppey is rich in palm fruits, and many large palms are found in the
Bournemouth leaf beds, corresponding in date to the Bracklesham. The
differences may be largely due to conditions of locality and
deposition. The Alum Bay flora is characterised by a wealth of
leguminous plants, and large leaves of species of fig (_Ficus_);
simple laurel and willow-like leaves are common, of which it is
difficult to determine the species, and there is abundance of a
species of _Aralia_. The character of the flora resembles most those
of Central America and the Malay Archipelago.
[Illustration: PL. IV]
Nummulites Laevigatus
Turritella Limnaea
Imbricataria Longiscata
Cardita Planicosta
(Fusus) Planorbis
Leiostama Pyrus Euomphalus
Cyrena Semistriata
EOCENE AND OLIGOCENE
The Bracklesham Beds in Alum Bay (570 ft. thick) consist of clays,
with lignite forming bands 6 in. to 2 ft. thick; white, yellow, and
crimson sands; and in the upper part dark sandy clays, with bands
showing impressions of marine fossils. Alum Bay takes its name from
the alum formerly manufactured from the Tertiary clays. The coloured
sands have made the bay famous. The colours of the sands when freshly
exposed, and of the cliffs when wet with rain, are very rich and
beautiful,--deep purple, crimson, yellow, white, and grey. Some of the
beds are finely striped in different shades by current bedding. The
contrast of these coloured cliffs with the White Chalk, weathered to a
soft grey, of the other half of the bay is very striking and
beautiful. About 45 ft. from the top is a conglomerate of flint
pebbles, some of large size, cemented by iron oxide. In Whitecliff Bay
the Bracklesham Beds (585 ft.) consist of clays, sands, and sandy
clays, mostly dark, greenish and blue in colour, containing marine
fossils and lignite. Sir Richard Worsley, in his History of the Isle
of Wight, tells that in February, 1773, a bed of coal was laid bare in
Whitecliff Bay, causing great excitement in the neighbourhood. People
flocked to the shore for coal, but it proved worthless as fuel. It
has, however, been worked to some extent in later years. In some of
the beds are many fossils. Numbers have lately been visible where a
large founder has taken place. There are large shells of _Cardita
planicosta_ and _Turritella imbricataria_. They are, however, very
fragile. In a stratum just above these are numbers of a large
Nummulite (_Nummulites laevigatus_). These are round flat shells like
coins,--hence the name (Lat. _nummus_, a coin). They are a large
species of foraminifera. We may split them with a penknife; and then
we see a pretty spiral of tiny chambers. A smaller variety, _N.
variolarius_, occurs a little further on, and a tiny kind, _N.
elegans_, in the Barton clay. One of the most striking features of the
later Eocene is the immense development of Nummulite limestones--vast
beds built up of the delicate chambered shells of Nummulites,--which
extend from the Alps and Carpathians into Thibet, and from Morocco,
Algeria, and Egypt, through Afghanistan and the Himalaya to China. The
pyramids of Egypt are built of this limestone.
The Bracklesham beds are followed by the Barton clay, famous for the
number of beautiful fossil shells found at Barton on the Hampshire
coast. At Whitecliff Bay the fossils are, unfortunately, very friable.
At Alum Bay the pathway to the shore is in a gully in the upper part
of the Barton clay. The strata consist of clays, sands, and sandy
clays. The base of the beds is marked by the zone of _Nummulites
elegans_. Numerous very pretty shells of the smaller Barton types may
be found, with fragments of larger ones; or a whole one may be found.
Owing to the cliff section cutting straight across the strata, which
are nearly vertical, there is far less of the beds open to observation
than at Barton, which probably accounts for the list of fossils being
much smaller. The shells are chiefly several species of _Pleurotoma_,
_Rostellaria_, _Fusus_, _Voluta_, _Turritella_, _Natica_, a small
bivalve _Corbula pisum_, a tubular shell of a sand-boring mollusc
_Dentalium_, _Ostroea_, _Pecten_, _Cardium_, _Crassatella_. The
fauna is like a blending of Malayan and New Zealand forms of marine
life. Throughout the Eocene from the London clay onward the shells are
such as abound in the warm sea south east of Asia. Similarly the plant
remains take us into a tropic land, where fan palms and feather palms
overshadowed the country, trees of the tropics mingling with trees we
still find in more Northern latitudes. The general character of the
flora as of the shells was Oriental and Malayan; both being succeeded
in later strata by a flora and fauna with greater analogy to that now
existing in Western North America.
In Alum Bay the Barton clay is suddenly succeeded by the very fine
yellow and white sands which run along the western base of Headon
Hill, the curve of the syncline bringing them round from a nearly
vertical to an almost horizontal position. These are now known as the
Barton Sand. They are 90 ft. thick, the whole of the Barton beds being
338 ft. in Alum Bay, 368 ft. in Whitecliff. The sands were formerly
extensively used for glass making. They are almost unfossiliferous.
The passage from Barton clay to the sands in Whitecliff Bay is more
gradual. The sands here show some fine colouring which reminds us of
the more celebrated sands of Alum Bay.
[Footnote 11: See Memoir of Geological Survey of I. W. by H. J.
Osborne White, F.G.S. 1921, p. 90.]