1911 Encyclopædia Britannica/Eocene
EOCENE (Gr. ἠώς, dawn, καινός, recent), in geology, the name suggested by Sir C. Lyell in 1833 for the lower subdivision of the rocks of the Tertiary Era. The term was intended to convey the idea that this was the period which saw the dawn of the recent or existing forms of life, because it was estimated that among the fossils of this period only 31% of the species are still living. Since Lyell’s time much has been learned about the fauna and flora of the period, and many palaeontologists doubt if any of the Eocene species are still extant, unless it be some of the lowest forms of life. Nevertheless the name is a convenient one and is in general use. The Eocene as originally defined was not long left intact, for E. Beyrich in 1854 proposed the term “Oligocene” for the upper portion, and later, in 1874, K. Schimper suggested “Paleocene” as a separate appellation for the lower portion. The Oligocene division has been generally accepted as a distinct period, but “Paleocene” is not so widely used.
In north-western Europe the close of the Cretaceous period was marked by an extensive emergence of the land, accompanied, in many places, by considerable erosion of the Mesozoic rocks; a prolonged interval elapsed before a relative depression of the land set in and the first Eocene deposits were formed. The early Eocene formations of the London-Paris-Belgian basin were of fresh-water and brackish origin; towards the middle of the period they had become marine, while later they reverted to the original type. In southern and eastern Europe changes of sea-level were less pronounced in character; here the late Cretaceous seas were followed without much modification by those of the Eocene period, so rich in foraminiferal life. In many other regions, the great gap which separates the Tertiary from the Mesozoic rocks in the neighbourhood of London and Paris does not exist, and the boundary line is difficult to draw. Eocene strata succeed Cretaceous rocks without serious unconformity in the Libyan area, parts of Denmark, S.E. Alps, India, New Zealand and central N. America. The unconformity is marked in England, parts of Egypt, on the Atlantic coastal plain and in the eastern gulf region of N. America, as well as in the marine Eocene of western Oregon. The clastic Flysch formation of the Carpathians and northern Alps appears to be of Eocene age in the upper and Cretaceous in the lower part. The Eocene sea covered at various times a strip of the Atlantic coast from New Jersey southward and sent a great tongue or bay up the Mississippi valley; similar epicontinental seas spread over parts of the Pacific border, but the plains of the interior with the mountains on the west were meanwhile being filled with terrestrial and lacustrine deposits which attained an enormous development. This great extension of non-marine formations in the Eocene of different countries has introduced difficulties in the way of exact correlation; it is safer, therefore, in the present state of knowledge, to make no attempt to find in the Eocene strata of America and India, &c., the precise equivalent of subdivisions that have been determined with more or less exactitude in the London-Paris-Belgian area.
It is possible that in Eocene times there existed a greater continuity of the northern land masses than obtains to-day. Europe at that time was probably united with N. America through Iceland and Greenland; while on the other side, America may have joined Asia by the way of Alaska. On the other hand, the great central, mediterranean sea which stretched across the Eurasian continents sent an arm northward somewhere just east of the Ural mountains, and thus divided the northern land mass in that region. S. America, Australia and perhaps Africa may have been connected more or less directly with the Antarctic continent.
Associated, no doubt, with the crustal movements which closed the Cretaceous and inaugurated the Eocene period, there were local and intermittent manifestations of volcanic activity throughout the period. Diabases, gabbros, serpentines, soda-potash granites, &c., are found in the Eocene of the central and northern Apennines. Tuffs occur in the Veronese and Vicentin Alps—Ronca and Spelecco schists. Tuffs, basalts and other igneous rocks appear also in Montana, Wyoming, California, Oregon, Washington, Idaho, Colorado; also in Central America, the Antillean region and S. America.
It has been very generally assumed by geologists, mainly upon the evidence of plant remains, that the Eocene period opened with a temperate climate in northern latitudes; later, as indicated by the London Clay, Alum Bay and Bournemouth beds, &c., the temperature appears to have been at least subtropical. But it should be observed that the frequent admixture of temperate forms with what are now tropical species makes it difficult to speak with certainty as to the degree of warmth experienced. The occurrence of lignites in the Eocene of the Paris basin, Tirol and N. America is worthy of consideration in this connexion. On the other hand, the coarse boulder beds in the lower Flysch have been regarded as evidence of local glaciation; this would not be inconsistent with a period of widespread geniality of climate, as is indicated by the large size of the nummulites and the dispersion of the marine Mollusca, but the evidence for glaciation is not yet conclusive.
Eocene Stratigraphy.—In Britain, with the exception of the Bovey beds (q.v.) and the leaf-bearing beds of Antrim and Mull, Eocene rocks are confined to the south-eastern portion of England. They lie in the two well-marked synclinal basins of London and Hampshire which are conterminous in the western area (Hampshire, Berkshire), but are separated towards the east by the denuded anticline of the Weald. The strata in these two basins have been grouped in the following manner:—
|London Basin.||Hampshire Basin.|
|Upper||Upper Bagshot Sands.||Headon Hill and Barton Sands.|
|Middle||Middle Bagshot Beds and part of Lower Bagshot Beds.||Bracklesham Beds and leaf beds of Bournemouth and Alum Bay.|
|Lower||Part of Lower Bagshot Beds, London Clay, Blackheath and Oldhaven
Beds, Woolwich and Reading Beds, Thanet Sands.
|London Clay and the equivalent Bognor Beds, Woolwich and Reading Beds.|
The Thanet sands have not been recognized in the Hampshire basin; they are usually pale yellow and greenish sands with streaks of clay and at the base; resting on an evenly denuded surface of chalk is a very constant layer of green-coated, well-rounded chalk flint pebbles. It is a marine formation, but fossils are scarce except in E. Kent, where it attains its most complete development. The Woolwich and Reading beds (see Reading Beds) contain both marine and estuarine fossils. In western Kent, between the Woolwich beds and the London Clay are the Oldhaven beds or Blackheath pebbles, 20 to 40 ft., made up almost entirely of well-rounded flint pebbles set in sand; the fossils are marine and estuarine. The London Clay, 500 ft. thick, is a marine deposit consisting of blue or brown clay with sandy layers and septarian nodules; its equivalent in the Hampshire area is sometimes called the Bognor Clay, well exposed on the coast of Sussex. The Bagshot, Bracklesham and Barton beds will be found briefly described under those heads.
Crossing the English Channel, we find in northern France and Belgium a series of deposits identified in their general characters with those of England. The anticlinal ridge of the English Weald is prolonged south-eastwards on to the continent, and separates the Belgian from the French Eocene areas much as it separates the areas of London and Hampshire; and it is clear that at the time of deposition all four regions were intimately related and subject to similar variations of marine and estuarine conditions. With a series of strata so variable from point to point it is natural that many purely local phases should have received distinctive names; in the Upper Eocene of the Paris basin the more important formations are the highly fossiliferous marine sands known as the “Sands of Beauchamp” and the local fresh-water limestone, the “Calcaire de St Ouen.” The Middle Eocene is represented by the well-known “Calcaire grossier,” about 90 ft. thick. The beds in this series vary a good deal lithologically, some being sandy, others marly or glauconitic; fossils are abundant. The Upper Calcaire grossier or “Caillasses” is a fresh-water formation; the middle division is marine; while the lower one is partly marine, partly of fresh-water origin. The numerous quarries and mines for building stone in the neighbourhood of Paris have made it possible to acquire a very precise knowledge of this division, and many of the beds have received trade names, such as “Rochette,” “Roche,” “Banc franc,” “Banc vert,” “Cliquart,” “Saint Nom;” the two last named are dolomitic. Below these limestones are the nummulitic sands of Cuise and Soissons. The Lower Eocene contains the lignitic plastic clay (argile plastique) of Soissons and elsewhere; the limestones of Rilly and Sézanne and the greenish glauconitic sands of Bracheux. The relative position of the above formations with respect to those of Belgium and England will be seen from the table of Eocene strata. The Eocene deposits of southern Europe differ in a marked manner from those of the Anglo-Parisian basin. The most important feature is the great development of nummulitic limestone with thin marls and nummulitic sandstones. The sea in which the nummulitic limestones were formed occupied the site of an enlarged Mediterranean communicating with similar waters right round the world, for these rocks are found not only in southern Europe, including all the Alpine tracts, Greece and Turkey and southern Russia, but they are well developed in northern Africa, Asia Minor, Palestine, and they may be followed through Persia, Baluchistan, India, into China, Tibet, Japan, Sumatra, Borneo and the Philippines. The nummulitic limestones are frequently hard and crystalline, especially where they have been subjected to elevation and compression as in the Alpine region, 10,000 ft. above the sea, or from 16,000, to 20,000 ft., in the central Asian plateau. Besides being a widespread formation the nummulitic limestone is locally several thousand feet thick.
While the foraminiferal limestones were being formed over most of southern Europe, a series of clastic beds were in course of formation in the Carpathians and the northern Alpine region, viz. the Flysch and the Vienna sandstone. Some portions of this Alpine Eocene are coarsely conglomeratic, and in places there are boulders of non-local rocks of enormous dimensions included in the argillaceous or sandy matrix. The occurrence of these large boulders together with the scarceness of fossils has suggested a glacial origin for the formation; but the evidence hitherto collected is not conclusive. C. W. von Gümbel has classified the Eocene of the northern Alps (Bavaria, &c.) as follows:—
|Upper Eocene||Flysch and Vienna sandstone, with younger nummulitic beds and Häring group.|
|Middle Eocene||Kressenberg Beds, with older nummulitic beds.|
|Lower Eocene||Burberg Beds, Greensands with small nummulites.|
The Häring group of northern Tirol contains lignite beds of some importance. In the southern and S.E. Alps the following divisions are recognized.
|Upper Eocene||Macigno or Tassello—Vienna Sandstone, conglomerates, marls and shales.|
|Middle Eocene||Nummulitic limestones, three subdivisions.|
|Lower Eocene||Liburnian stage (or Proteocene), foraminiferal limestones with|
fresh-water intercalations at the top and bottom, the Cosina beds, fresh-water
In the central and northern Apennines the Eocene strata have been subdivided by Prof. F. Sacco into an upper Bartonian, a middle Parisian and a lower Suessonian series. In the middle member are the representatives of the Flysch and the Macigno. These Eocene strata are upwards of 5500 ft. thick. In northern Africa the nummulitic limestones and sandstones are widely spread; the lower portions comprise the Libyan group and the shales of Esneh on the Nile (Flandrien), the Alveolina beds of Sokotra and others; the Mokattam stage of Egypt is a representative of the later Eocene. Much of the N. African Eocene contains phosphatic beds. In India strata of Eocene age are extensively developed; in Sind the marine Ranikot beds, 1500 to 2000 ft., consisting of clays with gypsum and lignite, shales and sandstones; these beds have, side by side with Eocene nummulites, a few fossils of Cretaceous affinities. Above the Ranikot beds are the massive nummulitic limestones and sandstones of the Kirthar group; these are succeeded by the nummulitic limestones and shales at the base of the Nari group. In the southern Himalayan region the nummulitic phase of Eocene deposit is well developed, but there are difficulties in fixing the line of demarcation between this and the younger formations. The lower part of the Sirmur series of the Simla district may belong to this period; it is subdivided into the Kasauli group and the Dagshai group with the Subáthu group at the base. Beneath the thick nummulitic Eocene limestone of the Salt Range are shales and marls with a few coal seams. The marine Eocene rocks of N. America are most extensively developed round the coast of the Gulf of Mexico, whence they spread into the valley of the Mississippi and, as a comparatively narrow strip, along the Atlantic coastal plain to New Jersey.
The series in Alabama, which may be taken as typical of the Gulf coast Eocene, is as follows:—
|Upper Jacksonian||White limestone of Alabama (and Vicksburg?).|
|Middle Claibornian||Claiborne series. |
|Lower||Chickasawan Sands and lignites.|
Midwayan or Clayton formation, limestones.
The above succession is not fully represented in the Atlantic coast states.
On the Pacific coast marine formations are found in California and Oregon; such are the Tejon series with lignite and oil; the Escondido series of S. California (7000 ft.), part of the Pascadero series of the Santa Cruz Mountains; the Pulaski, Tyee, Arago and Coaledo beds—with coals—in Oregon. In the Puget formation of Washington we have a great series of sediments, largely of brackish water origin, and in parts coal-bearing. The total thickness of this formation has been estimated at 20,000 ft. (it may prove to be less than this), but it is probable that only the lower portion is of Eocene age. The most interesting of the N. American Eocene deposits are those of the Rocky Mountains and the adjacent western plains, in Wyoming, Nevada, Nebraska, Colorado, &c.; they are of terrestrial, lacustrine or aeolian origin, and on this account and because they were not strictly synchronous, there is considerable difficulty in placing them in their true position in the time-scale. The main divisions or groups are generally recognized as follows:—
|Upper||1 Uinta Group, 800 ft. (? = Jacksonian)||Diplacodon.|
|Middle||2 Bridger Group, 2000 ft. (? = Claibornian)||Uintatherium.|
|Lower||3 Wind River Group, 800 ft.||Bathyopsis.|
|4 Wasatch Group, 2000 ft. (? = Chickasawan)||Coryphodon.|
|Basal||5 Torrejon Group, 300 ft.||Pantolambda.|
|6 Puerco Group, 500 to 1000 ft.||Polymastodon.|
|1 South of the Uinta Mts. in Utah.
2 Fort Bridger Basin.
3 Wind river in Wyoming.
|4 Wasatch Mts. in Utah.|
5 Torrejon in New Mexico.
6 Puerco river, New Mexico.
The Fort Union beds of Canada and parts of Montana and N. Dakota are probably the oldest Eocene strata of the Western Interior; they are some 2000 ft. thick and possibly are equivalent to the Midwayan group. But in these beds, as in those known as Arapahoe, Livingston, Denver, Ohio and Ruby, which are now often classed as belonging to the upper Laramie formation, it is safer to regard them as a transitional series between the Mesozoic and Tertiary systems. There is, however, a marked unconformity between the Eocene Telluride or San Miguel and Poison Canyon formations of Colorado and the underlying Laramie rocks.
Many local aspects of Eocene rocks have received special names, but too little is known about them to enable them to be correctly placed in the Eocene series. Such are the Clarno formation (late Eocene) of the John Day basin, Oregon, the Pinyon conglomerate of Yellowstone Park, the Sphinx conglomerate of Montana, the Whitetail conglomerate of Arizona, the Manti shales of Utah, the Mojave formation of S. California and the Amyzon formation of Nevada.
Of the Eocene of other countries little is known in detail. Strata of this age occur in Central and S. America (Patagonia-Megellanian series—Brazil, Chile, Argentina), in S. Australia (and in the Great Australian Bight), New Zealand, in Seymour Island near Graham Land in the Antarctic Regions, Japan, Java, Borneo, New Guinea, Moluccas, Philippines, New Caledonia, also in Greenland, Bear Island, Spitzbergen and Siberia.
Organic Life of the Eocene Period.—As it has been observed above, the name Eocene was given to this period on the ground that in its fauna only a small percentage of living species were present; this estimation was founded upon the assemblage of invertebrate remains in which, from the commencement of this period until the present day, there has been comparatively little change. The real biological interest of the period centres around the higher vertebrate types. In the marine mollusca the most noteworthy change is the entire absence of ammonoids, the group which throughout the Mesozoic era had taken so prominent a place, but disappeared completely with the close of the Cretaceous. Nautiloids were more abundant than they are at present, but as a whole the Cephalopods took a more subordinate part than they had done in previous periods. On the other hand, Gasteropods and Pelecypods found in the numerous shallow seas a very suitable environment and flourished exceedingly, and their shells are often preserved in a state of great perfection and in enormous numbers. Of the Gasteropod genera Cerithium with its estuarine and lagoonal forms Potamides, Potamidopsis, &c., is very characteristic; Rostellaria, Voluta, Fusus, Pleurotoma, Conus, Typhis, may also be cited. Cardium, Venericardia, Crassatella, Corbulomya, Cytherea, Lucina, Anomia, Ostrea are a few of the many Pelecypod genera. Echinoderms were represented by abundant sea-urchins, Echinolampas, Linthia, Conoclypeus, &c. Corals flourished on the numerous reefs and approximated to modern forms (Trochosmilia, Dendrophyllia). But by far the most abundant marine organisms were the foraminifera which flourished in the warm seas in countless myriads. Foremost among these are the Nummulites, which by their extraordinary numerical development and great size, as well as by their wide distribution, demand special recognition. Many other genera of almost equal importance as rock builders, lived at the same time: Orthophragma, Operculina, Assilina, Orbitolites, Miliola, Alveolina. Crustacea were fairly abundant (Xanthopsis, Portunus), and most of the orders and many families of modern insects were represented.
When we turn to the higher forms of life, the reptiles and mammals, we find a remarkable contrast between the fauna of the Eocene and those periods which preceded and succeeded it. The great group of Saurian reptiles, whose members had held dominion on land and sea during most of the Mesozoic time, had completely disappeared by the beginning of the Eocene; in their place placental mammals made their appearance and rapidly became the dominant group. Among the early Eocene mammals no trace can be found of the numerous and clearly-marked orders with which we are familiar to-day; instead we find obscurely differentiated forms, which cannot be fitted without violence into any of the modern orders. The early placental mammals were generalized types (with certain non-placental characters) with potentialities for rapid divergence and development in the direction of the more specialized modern orders. Thus, the Creodonta foreshadowed the Carnivora, the Condylarthra presaged the herbivorous groups; but before the close of this period, so favourable were the conditions of life to a rapid evolution of types, that most of the great orders had been clearly defined, though none of the Eocene genera are still extant. Among the early carnivores were Arctocyon, Palaeonictis, Amblyctonus, Hyaenodon, Cynodon, Provivera, Patriofelis. The primitive dog-like forms did not appear until late in the period, in Europe; and true cats did not arrive until later, though they were represented by Eusmilus in the Upper Eocene of France. The primitive ungulates (Condylarths) were generalized forms with five effective toes, exemplified in Phenacodus. The gross Amblypoda, with five-toed stumpy feet (Coryphodon), were prominent in the early Eocene; particularly striking forms were the Dinoceratidae, Dinoceras, with three pairs of horns or protuberances on its massive skull and a pair of huge canine teeth projecting downwards; Tinoceras, Uintatherium, Loxophodon, &c.; these elephantine creatures, whose remains are so abundant in the Eocene deposits of western America, died out before the close of the period. The divergence of the hoofed mammals into the two prominent divisions, the odd-toed and even-toed, began in this period, but the former did not get beyond the three-toed stage. The least differentiated of the odd-toed group were the Lophiodonts: tapirs were foreshadowed by Systemodon and similar forms (Palaeotherium, Paloplotherium); the peccary-like Hyracotherium was a forerunner of the horse, Hyrochinus was a primitive rhinoceros. The evolution of the horse through such forms as Hyracotherium, Pachynolophus, Eohippus, &c., appears to have proceeded along parallel lines in Eurasia and America, but the true horse did not arrive until later. Ancestral deer were represented by Dichobune, Amphitragulus and others, while many small hog-like forms existed (Diplopus, Eohyus, Hyopotamus, Homacodon). The primitive stock of the camel group developed in N. America in late Eocene time and sent branches into S. America and Eurasia. The edentates were very generalized forms at this period (Ganodonta); the rodents (Tillodontia) attained a large size for members of this group, e.g. Tillotherium. The Insectivores had Eocene forerunners, and the Lemuroids—probable ancestors of the apes—were forms of great interest, Anaptomorphus, Microsyops, Heterohyus, Microchaerus, Coenopithecus; even the Cetaceans were well represented by Zeuglodon and others.
|Stages.||Paris Basin.||England.||Belgian Basin.||Mediterranean |
|Bartonien.||Limestone of Saint-Ouen.
Sands of Mortefontaine.
Sands of Beauchamp.
Sands of Auvers.
Upper Bagshot sands.
|Sands of Lede.|| Nummulitic
|Upper part of the
and Vienna and
Macigno of the
|Unita Group and|
|Lutétien.||Calcaire grossier.||Bracklesham and
Lower Bagshot sands.
|Bridger Group and|
|Yprésien.||Nummulitic sands of
Soissons and Sands of
Cuise and Aizy.
|Alum Bay leaf beds.||Sands of Mons en
|Wind River Group.|
|Plastic Clay and lignite
Woolwich and Reading beds.
Sands of Ostricourt.
|Limestones of Rilly and
Sands of Rilly and
Marls of Gelinden.
The non-placental mammals although abundant were taking a secondary place; Didelphys, the primitive opossum, is noteworthy on account of its wide geographical range.
Among the birds, the large flightless forms, Eupterornis, Gastornis, were prominent, and many others were present, such as the ancestral forms of our modern gulls, albatrosses, herons, buzzards, eagles, owls, quails, plovers. Reptiles were poorly represented, with the exception of crocodilians, tortoises, turtles and some large snakes.
The flora of the Eocene period, although full of interest, does not convey the impression of newness that is afforded by the fauna of the period. The reason for this difference is this: the newer flora had been introduced and had developed to a considerable extent in the Cretaceous period, and there is no sharp break between the flora of the earlier and that of the later period; in both we find a mixed assemblage—what we should now regard as tropical palms, growing side by side with mild-temperate trees. Early Eocene plants in N. Europe, oaks, willows, chestnuts (Castanea), laurels, indicate a more temperate climate than existed in Middle Eocene when in the Isle of Wight, Hampshire and the adjacent portions of the continent, palms, figs, cinnamon flourished along with the cactus, magnolia, sequoia, cypress and ferns. The late Eocene flora of Europe was very similar to its descendant in modern Australasia.
See A. de Lapparent, Traité de géologie, vol. iii. (5th ed., 1906), which contains a good general account of the period, with numerous references to original papers. Also R. B. Newton, Systematic List of the Frederick E. Edwards Collection of British Oligocene and Eocene Mollusca in the British Museum (Natural History) (1891), pp. 299-325; G. D. Harris, “A Revision of our Lower Eocenes,” Proc. Geologists’ Assoc. x., 1887–1888; W. B. Clark, “Correlation Papers: Eocene” (1891), U.S. Geol. Survey Bull. No. 83. For more recent literature consult Geological Literature added to the Geological Society’s Library, published annually by the society. (J. A. H.)
Bartonien from Barton, England. Lutétien ” Lutetia = Paris. Yprésien ” Ypres, Flanders. Landénien ” Landen, Belgium. Thanetien ” The Isle of Thanet. Sparnacien ” Sparnacum = Épernay. Laekenien ” Laeken, Belgium. Bruxellien ” Brussels. Panisélien ” Mont Panisel, near Mons.
Other names that have been applied to subdivisions of the Eocene not included in the table are Parisien and Suessonien (Soissons); Ludien (Ludes in the Paris basin) and Priabonien (Priabona in the Vicentine Alps); Heersien (Heer near Maastricht) and Wemmelien (Wemmel, Belgium); very many more might be mentioned.