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