Oligocene System. The British strata belonging to this system occur only in Hampshire, the Isle of Wight, and Devonshire. The series is as follows:
- 4. HEMPESTEAD BEDS: fresh-water marls and clays overlaid by marine septarian clays. About 260 feet.
- 3. BEMBRIDGE BEDS: marls and limestone; fresh-water below, estuarine above. About 110 feet.
- 2. OSBORNE BEDS: fresh-water clays, marls, sands, and limestone. About 100 feet.
- 1. HEADON BEDS: variable series of clays, marls, sands, and limestones. The lower division is of fresh- and brackish-water origin; the middle partly marine, partly fresh-water; the upper fresh-water. About 150 feet.
Usually included as Oligocene are the lacustrine beds of Bovey Tracey in Devonshire, consisting of sands and clays with lignites. Between the basalt-beds that compose the denuded plateaus of Antrim and the Inner Hebrides (Mull, &c.) occur thin layers of clay and lignite—the so-called leaf-beds—which are probably of the same age.
Foreign Equivalents.—Oligocene strata, chiefly of fresh- and brackish-water origin, but containing intercalations of marine beds, overlie the Eocene of the Paris basin and that of Belgium. They likewise appear in Germany, where they form the oldest Tertiary deposits—no Eocene having yet been detected in that region. The German Oligocene is mainly of fresh-water origin in its lower and upper portions, while marine deposits predominate in the middle of the series. It is noted for its beds of lignite or brown coal. In Switzerland the Oligocene attains a thickness of several thousand feet, chiefly conglomerates and sandstones, known as Molasse, and mostly of fresh-water origin; the basal portions, however, are marine and brackish-water. Other areas of fresh-water Oligocene more or less notable are met with in Alsace, Breisgau, and Wirtemberg. In Auvergne, central France, lacustrine deposits of the same age are well developed, and, like most of the Oligocene strata, have yielded great numbers of organic remains.
Life of the Period.—The flora of Oligocene times was abundant and varied. Palm-trees (Sabal, Flabellaria), both large and small, seem to have grown over all Europe. Amongst conifers were various American types (Libocedrus, Chamaecyparis, Sequoia, Taxodium) and other forms, such as Glyptostrobus, like G. heterophyllus of Japan and China, Widdringtonia, a genus now found only in South Africa and Madagascar. There were also proteaceous plants (Dryandra) of Australian affinities, and species of custard-apple, gum-tree, spindle-tree, maple, acacia, mimosa, lotus, aralia, camphor-tree, cinnamon-tree, evergreen oak, laurel, &c., besides such familiar forms as birch, horn-beam, elder, elm, poplar, walnut, &c. Evergreens were the prevalent forms. The invertebrate fauna needs but little notice. Amongst notable molluscs were volutes, cowries, olives, cones, spindle-shells, &c. Cerithium was particularly plentiful in the estuaries of the period; while lamellibranchs were well represented by modern types of marine and fresh-water habitats. Amongst the birds common in Europe were paroquets, trogons, marabouts, cranes, flamingoes, ibises, pelicans, eagles, secretary-birds, sand-grouse, &c. At the beginning of the period many mammals of extinct types lived in Europe, such as Paleotherium and Anchitherium, survivors from the Eocene; certain transitional forms of ungulates, such as Cainotherium (a small animal somewhat resembling the living chevrotains in outward appearance), Xiphodon (a slenderly built deer-like animal), and Anoplotherium (a long-tailed animal about the size of an ass, with two toes on each foot); various tapirid animals, small rhinoceroses, Hyænodon (a carnivore), also forms of squirrel, civet, martin, mole, musk-rat, &c.
Physical Conditions.—During Oligocene times a wide land surface appears to have extended over all the British area. In the region lying between what is now Antrim and the west coast of Scotland great fissure-eruptions took place, and sheet after sheet of basalt was poured out, so as eventually to form broad plateaus that extended northwards beyond Skye. In the intervals between successive eruptions these plateaus became clothed with vegetation, the debris of which has been here and there preserved in the deposits of shallow lakes that dotted the surface of the volcanic country. It is probable that at this time there was land-connection between Europe and North America by way of the Faroe Islands and Iceland, in both of which tracts similar basaltic plateaus occur, containing intercalated layers of lignite, &c. like those of Antrim and the Western Islands of Scotland. The Oligocene strata of the south of England and the Franco-Belgian area are evidence that the sea or estuarine waters which occupied that region in Eocene times (see EOCENE SYSTEM) were gradually silted up. In Germany there existed great fresh-water lakes, fringed by wide marsh-lands and by dense forests of a subtropical character. As the lakes became partially silted or dried up vegetation encroached upon their deserted beds, only to be buried under fresh accumulations of sand and mud when the water had again risen. That these lakes were now and again in direct communication with the sea is shown by the occurrence of thick layers of marine origin intercalated amongst the fresh-water beds. For some time, indeed, the lacustrine areas were entirely usurped by the sea, which may have entered them from submerged regions in the east of Europe. In Switzerland, in like manner, we have evidence of changing conditions. At first the sea covered a considerable portion of the country, but eventually it disappeared, and its place was taken by a series of brackish-water lagoons and fresh-water lakes. The deposits accumulated in those lakes now form considerable mountains at the base of the Alps (Rigi, Rossberg). In central France, as in Germany, lacustrine conditions were characteristic of the period, one or more great lakes having occupied a considerable area in Auvergne. In southern Europe the Mediterranean had withdrawn from wide regions which were deeply submerged by it in Eocene times, but it still covered a more extensive area than at present. The climate of the Oligocene period was uniformly genial, but hardly so tropical as that of the preceding period. See EOCENE SYSTEM.