PLEISTOCENE  , in

geology, the epoch which succeeded the Pliocene; it is the last of the Tertiary periods, and hence the lower subdivision of the quaternary or modern era . The name was introduced by Sir C . Lyell in 1839 (from Gr . 7r)^Ei(TTov, most, and uatvbs, recent), the rocks of this period containing a higher percentage of living forms than the youngest of the Tertiary formations . By many writers " Pleistocene " has been regarded as synonymous with " Glacial Period " or the " Diluvium " of some geologists . In the northern hemisphere the protracted period of glaciation, with its predominating influence upon modern topography and faunal distribution, was undoubtedly the outstanding feature of the time . The phenomena of the Glacial period (q.v.), which was by no means strictly limited to the northern latitudes, are dealt with under that head, but there are certain other characteristics of the Pleistocene period which bear no direct relationship to glaciation, and these will be dealt with here . The gradual inception of colder conditions in the northern hemisphere which lead up to the more extreme conditions of glaciation clearly began in the latter part of the Pliocene period, and the effects of this cooling are seen not only in northern Europe and America but as far south as the Mediterranean . The result of this is that there is a certain indefiniteness as to the exact base line to be adopted for the Pleistocene formations; thus the Forest Bed of Cromer and certain beds in Sicily and Italy are by some authors placed in this period and by others in the Pliocene (q.v.) . Again it is clear that in parts of northern Europe, Siberia and North America, the conditions characteristic of a glacial period are still existent; even in Scotland and Norway the last traces of glacial action are remarkably fresh, and the last remnants of great glacial centres still linger in the Alps and other lofty southern mountains . Many of the formations of this period can be shown by their fossil contents to belong to early quaternary time, but since so many of these deposits are strictly local in character, and since the fauna and flora present in any one spot have been determined by local geographical conditions which have assisted or retarded the migration of certain forms, it is a matter of extreme difficulty—one may say impossibility—to reduce the Pleistocene formations to any generally applicable chronological order . For similar reasons it is impossible to define strictly the upper limit of the formations of this period, and to say where the Pleistocene ends and where the Recent or Holocene period begins .

The

composition and distribution of the Pleistocene fauna and flora present many points of extreme interest . The feature of greatest importance is that man existed somewhere and in some condition before and in this period; but no really satisfactory proof has so far been forthcoming which will set back his first appearance before the beginning of the glacial peripd (Pithecanthropus erectus found by E . Dubois in Java is regarded as of Pliocene age) . The presence of the remains o man or of his works might reason-ably be taken as a criterion of the Pleistocene age of a deposit—if we omit the remains of historical time . But here again it has to be borne in mind that historical time is continually being set back by archaeological research, and further, the difficulty of employing artefacs of stone as chronological indicators is shown by the fact that even at the present day implements of stone are still in use, and that different local races of early men must have been in diverse stages of development in Pleistocene as in later ages . It is, therefore, only with the utmost caution that chronological subdivisions of the period, such as those mentioned below, based' upon the form and degree of finish of stone implements, can be used in anything but local correlations unless the evidence is supported by satisfactory fossils . Next to the appearance of man the most striking characteristic of the land fauna was the existence of numerous large-bodied mammals; Elephas antiquus, for instance, attained a more excessive bulk than any other proboscidean either before or since, the woolly rhinoceros, the great hippopotamus, the cave bear, cave lion and giant deer were all larger than their living representatives . No less striking is the disappearance of these large forms together with highly specialized creatures such as Machaerodus within the same period, through the action of the same causes which had re-moved the bulky and specialized reptiles of an earlier geological period . The Pleistocene mammalia of Europe include Elephas antiquus, E. primogenius (mammoth), R. antiquitatis (tichorhinus) (the woolly rhinoceros), R. mercki (especially in Silesia), R. leptorhinus (south-east Europe), Elasmotherium (Silesia and south Russia), Hippopotamus major, Bos primigenius (aurochs, extinct in historical time), Bison priscus, Bison europaeus (still living in the Caucasus and Lithuania), Bos (Bubalus) pallasi (north Europe), camels in south Russia and Rumania, Equus fossilis and varieties, Cervus (Megaceros) giganteus (= hibernicus) (the great Irish " elk " and its varieties) ; Cervus elaphus, C. aleus, Rangifer tarandus and R. groenlandicus (reindeer), Capreolus caprea, Capra ibex, Saiga tatarica, Ovibos moschatus, Felis spelaeus, Hyaena spelaea, Ur-sus spelaeus, badger, weasel, glutton, hare, lemming (Myodes torquatus and M. lemmus), Spermophilus, Alactaga, Arctomys, Castor fiber, Lagomys, Trogontherium . In North America there were numerous mammals common to Europe and North Asia, including the musk-ox, mammoth and horse ; the mastodon held on into this period in America but not in Europe; there were also lamas, tapirs, camels (Camelus auchenia), Machaerodus, Mylodon, Procyon, Alces . In South America there was at first a very characteristic endemic fauna including Megatherium, Mylodon, Grypotherium, Lestodon, Toxodon, Typotherium, Glyptodon, Macrauchenia, Capybara, Rhea, to which were added later, Mastodon, Machaerodus, Lama and other North American forms . In Australia a very distinct assemblage of large marsupials and monotremes lived in the Pleistocene period; including Phascolus, Diprotodon, Thylacoleo, Nototherium and a large extinct Echidna; placental mammals were not then known in this region .

In

Madagascar the Aepiornis, Megaladapis, and certain extinct lemuroid creatures have left their remains . The advance and retreat of glacial conditions in northern latitudes had a marked influence upon animal and plant life, and was the means of determining the present distribution of many of the living mammalia and plants; some were driven permanently southward, some northern forms still live isolated on the higher mountain regions, others like the reindeer and musk-ox returned northward as soon as the conditions permitted . The apparently curious admixture of what are now often regarded as tropical or sub-tropicalforms (lion, hyena, rhinoceros and elephants) with cold-temperate or arctic genera, presents no real difficulty, since their distribution was doubtless merely a matter of food supply; and some of these, like the woolly rhinoceros and mammcth, were provided with a thick hairy pelt . Although in the main the arrangement of land and sea was little different from that which obtains at the present time, one or two features existed in the Pleistocene period which had a considerable influence on faunal migration . For instance, the absence of the Bering Straits permitted free communication between Europe and North America, and the absence of the Straits of Dover allowed a similar interchange between Great Britain and France; while an extension of the sea in the Caspian region and of the Arctic Sea in northern Russia acted as a bar to free passage between Europe and Asia in those regions . The formations of Pleistocene age, other than those of direct glacial origin, include deposits on the floors of caves in limestone and dolomitic rocks, calcareous sinter (travertine or tufa) formed by springs, ancient river and lake alluvial and lacustrine terraces, elevated marine beaches, submerged forests, ancient lake deposits and peat beds, laterite, loess and sand dunes . Some of the prevalent styles of classifying the deposits of the glacial formations of this period are mentioned in the article GLACIAL PERIOD . The following subdivisions are often employed by European geologists: a younger division, Reindeer time =Magdal6nieni stage; a middle division, Mammoth time=Salutreen2 stage; and an older division, Elephas antiquus time=Chell6en3 stage . While some authors include all the above in the " glacial period," others would place the Magdalenien in a post-glacial division . The. terms Magdal6nien, &c., are really archaeological, based upon the characters of the implements found in the deposits, and like the similar terms " eolithic " and " palaeolithic "they are of little value in geological chronology unless they are supported by palaeontological evidence . See E . Geinitz, Das Quartar von nord Europa (Stuttgart, 1904), with very full references; T .

C . Chamberlin and R . D .

Salisbury, Geology, vol. iii . (New York, 1906), for references to American authorities . U . A .