Online Encyclopedia
Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.
PLEISTOCENE
Online EncyclopediaEncyclopedia Home :: PIG-POL
del.icio.us it!
|
PLEISTOCENE , in See also:geology, the See also:epoch which succeeded the See also:Pliocene; it is the last of the See also:Tertiary periods, and hence the See also:lower subdivision of the See also:quaternary or See also:modern era. The name was introduced by See also:Sir C. See also:Lyell in 1839 (from Gr. 7r)^Ei(TTov, most, and uatvbs, See also:recent), the rocks of this See also: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 " See also:Diluvium " of some geologists. In the See also:northern hemisphere the protracted period of glaciation, with its predominating See also:influence upon modern See also:topography and faunal See also:distribution, was undoubtedly the outstanding feature of the See also: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 See also:head, but there are certain other characteristics of the Pleistocene period which See also:bear no See also:direct relationship to glaciation, and these will be dealt with here. The See also:gradual inception of colder conditions in the northern hemisphere which See also:lead up to the more extreme conditions of glaciation clearly began in the latter See also:part of the Pliocene period, and the effects of this cooling are seen not only in northern See also:Europe and See also:America but as far See also:south as the Mediterranean. The result of this is that there is a certain indefiniteness as to the exact See also:base See also:line to be adopted for the Pleistocene formations; thus the See also:Forest See also:Bed of See also:Cromer and certain beds in See also:Sicily and See also: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, See also:Siberia and See also:North America, the conditions characteristic of a glacial period are still existent; even in See also:Scotland and See also:Norway the last traces of glacial See also:action are remarkably fresh, and the last remnants of See also:great glacial centres still linger in the See also:Alps and other lofty See also:southern mountains. Many of the formations of this period can be shown by their fossil contents to belong to See also:early quaternary time, but since so many of these deposits are strictly See also:local in See also:character, and since the See also:fauna and See also:flora See also:present in any one spot have been determined by local See also:geographical conditions which have assisted or retarded the See also:migration of certain forms, it is a See also:matter of extreme difficulty—one may say impossibility—to reduce the Pleistocene formations to any generally applicable See also:chronological See also: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 See also:Holocene period begins. The See also:composition and distribution of the Pleistocene fauna and flora present many points of extreme See also:interest. The feature of greatest importance is that See also:man existed somewhere and in some See also:condition before and in this period; but no really satisfactory See also:proof has so far been forthcoming which will set back his first See also:appearance before the beginning of the glacial peripd (Pithecanthropus erectus found by E. See also:Dubois in See also:Java is regarded as of Pliocene See also:age). The presence of the remains o man or of his See also:works might See also:reason-ably be taken as a criterion of the Pleistocene age of a See also:deposit—if we omit the remains of See also:historical time. But here again it has to be See also:borne in mind that historical time is continually being set back by archaeological See also:research, and further, the difficulty of employing artefacs of See also: In See also:Madagascar the Aepiornis, Megaladapis, and certain extinct lemuroid creatures have See also:left their remains. The advance and See also:retreat of glacial conditions in northern latitudes had a marked influence upon See also:animal and plant See also:life, and was the means of determining the present distribution of many of the living mammalia and See also:plants; some were driven permanently southward, some northern forms still live isolated on the higher See also: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, See also:hyena, rhinoceros and elephants) with See also:cold-temperate or See also:arctic genera, presents no real difficulty, since their distribution was doubtless merely a matter of See also:food See also:supply; and some of these, like the woolly rhinoceros and mammcth, were provided with a thick hairy pelt. Although in the See also:main the arrangement of land and See also: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 See also:absence of the See also:Bering Straits permitted See also:free communication between Europe and North America, and the absence of the Straits of See also:Dover allowed a similar interchange between Great See also:Britain and See also:France; while an See also:extension of the sea in the See also:Caspian region and of the Arctic Sea in northern Russia acted as a See also: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 See also:limestone and dolomitic rocks, calcareous See also:sinter (travertine or tufa) formed by springs, See also:ancient See also:river and See also:lake alluvial and lacustrine terraces, elevated marine beaches, submerged forests, ancient lake deposits and See also:peat beds, See also:laterite, See also:loess and See also:sand See also:dunes. Some of the prevalent styles of classifying the deposits of the glacial formations of this period are mentioned in the See also:article GLACIAL PERIOD. The following subdivisions are often employed by See also:European geologists: a younger See also:division, Reindeer time =Magdal6nieni See also:stage; a See also: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 See also:place the Magdalenien in a See also: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 " See also:palaeolithic "they are of little value in geological See also:chronology unless they are supported by palaeontological evidence. See E. See also:Geinitz, Das Quartar von See also:nord See also:Europa (See also:Stuttgart, 1904), with very full references; T. C. Chamberlin and R. D. See also:Salisbury, Geology, vol. iii. (New See also:York, 1906), for references to American authorities. U. A. Additional information and CommentsThere are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML. Site content, images, and layout Copyright © 2006 - Net Industries, worldwide. |
|
|
[back] PLEIADES, ATLANTIDES |
[next] PLEONASM (Gr. srAeovaoµbs, from 1rXeova?'ecv, to a... |