The Geological Story of the Isle of Wight Part 4

THE TERTIARY ERA: THE EOCENE

Ages must have pa.s.sed while the ocean flowed over this part of the world, and the chalk mud, with its varied remains of living things, gradually acc.u.mulated 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 foresh.o.r.e 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 pa.s.sed 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 sh.e.l.ls 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 sh.o.r.e, 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 sh.o.r.e 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 sh.o.r.e, 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 sh.o.r.e, 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 Suss.e.x are two synclinal troughs known as the London and Hamps.h.i.+re 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 Hamps.h.i.+re basin, the higher strata only in the Isle of Wight.

[Ill.u.s.tration: 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 sh.e.l.ls of a marine worm, _Ditrupa plana_. The sandstone runs out on the sh.o.r.e. About 35 ft. above the bas.e.m.e.nt 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 sh.o.r.e, 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 l.u.s.tre, are beautiful fossils. A little higher is a zone with _Ditrupa_, and further on a band of _Cardita_. Other sh.e.l.ls 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 ma.s.s 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 sh.e.l.ls, and the remains of the plant and animal life of the neighbourhood of a great tropical river alternate in the deposits.

[Ill.u.s.tration: 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 Hamps.h.i.+re basin the strata are now cla.s.sified 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 sh.e.l.ls occur. The Bagshot Sands, in Whitecliff Bay, are about 138 feet thick, in Alum Bay, 76 feet, according to the latest cla.s.sification. 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.

[Ill.u.s.tration: 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 sh.o.r.e 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 sh.e.l.ls 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 sh.e.l.ls 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 sh.e.l.ls 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 sh.e.l.ls found at Barton on the Hamps.h.i.+re coast. At Whitecliff Bay the fossils are, unfortunately, very friable.

At Alum Bay the pathway to the sh.o.r.e 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 sh.e.l.ls 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 sh.e.l.ls are chiefly several species of _Pleurotoma_, _Rostellaria_, _Fusus_, _Voluta_, _Turritella_, _Natica_, a small bivalve _Corbula pisum_, a tubular sh.e.l.l of a sand-boring mollusc _Dentalium_, _Ostroea_, _Pecten_, _Cardium_, _Cra.s.satella_. The fauna is like a blending of Malayan and New Zealand forms of marine life. Throughout the Eocene from the London clay onward the sh.e.l.ls 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 lat.i.tudes. The general character of the flora as of the sh.e.l.ls was Oriental and Malayan; both being succeeded in later strata by a flora and fauna with greater a.n.a.logy 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 gla.s.s making. They are almost unfossiliferous.

The pa.s.sage 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.]

Chapter IX

THE OLIGOCENE

We pa.s.s on to strata which used to be called Upper Eocene, but are now generally cla.s.sified 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 ma.s.ses of limestone in Headon Hill belong to the Upper division. The limestones are full of freshwater sh.e.l.ls, 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 sh.o.r.e 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 sh.e.l.ls, 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 incra.s.sata_, from which the bed is known as the "Venus" bed, the sh.e.l.l formerly being called _Venus_, later _Cytherea_, at present _Meretrix_. The marine beds contain many drifted freshwater sh.e.l.ls 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-sh.e.l.ls. 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 sh.e.l.ls 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 sh.o.r.e 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 sh.o.r.e 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 sh.o.r.e, 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 sh.e.l.ls 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.