Chapter VIII. The Tertiary Era: the Eocene

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. 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.

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.

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.

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.