Quaternary

QUATERNARY, in geology, the time-division which corn prises all the time which has elapsed from the end of the Pliocene to the present day. The term was proposed by J. Desnoyers in 1829. The Quaternary is thus the fourth of the great time-divi sions in the geological scale—the Primary or Palaeozoic, the Secondary or Mesozoic and the Tertiary or Cainozoic being the first three—but it represents relatively such a small space of time that some geologists hesitate to give it equal rank, and regard it merely as a subdivision of the Tertiary. Broadly, as the Tertiary may be called the Age of Mammals, the Quaternary may be called the Age of Man. Although man or his ancestors were evolved dur ing the Tertiary, it is in the Quaternary that man becomes the dominant animal. Two divisions of the Quaternary period are usually recognized : 1 Pleistocene 2 Recent or Holocene In England many extinct species of animals and mollusca occur in the lower but not in the upper division.

General History of the Quaternary Period.

The faunas of the later Pliocene indicate a definite lowering of temperature and with the opening of the Pleistocene both fauna and flora begin to assume a definitely arctic character. A period of intense cold then ensued—usually known as the Glacial period (q.v.) or the Great Ice age. The greater part of the British Isles north of a line from South Wales to the mouth of the Thames came under the influence of ice action. As a belt circling the North Pole, there were several ice-caps, similar to the one that covers most of Greenland at the present day, and the various ice-caps or ice-centres waxed and waned in relative importance in such a way that the same district was sometimes affected by ice from one centre, sometimes by ice from another centre. Finally cli matic conditions were ameliorated and the ice retreated. It left in its wake not only great stretches of purely glacial deposits such as boulder clay, but the waters derived from the melting of the ice gave rise to torrents spreading over the still frozen ground and depositing great fans of gravel and sand. After the retreat of the ice a broad tract of cold desert or tundra encircled the Northern Hemisphere. This seems to have given place to a broad belt of wind-swept steppeland where great clouds of fine dust filled the air and were deposited as a mantle of loess. With the

increase of humidity, forests sprang up and clothed the greater part of Europe north of the Mediterranean region, and it is in this Forest period that we are still living. This general sequence of events was interrupted by minor cycles which are adequately discussed by C. E. P. Brooks in his book The Evolution of Climate. It will be obvious that the ice-sheets were the control ling factors in Pleistocene times. Very few animals or plants, if any, could live in the regions actually covered by ice; in con sequence the glacial deposits are devoid of fossils. Round the margins of the ice extensive floras and faunas existed ; they mi grated under the influence of fluctuations in the intensity of the cold. One finds thus at least three sets of Pleistocene deposits: (a) glacial deposits without fossils; (b) contemporaneous extra glacial deposits such as river gravels and cave deposits with mam malian and other land or fresh-water fossils, and indications of the presence of man (artefacts or implements, etc.) ; (c) con temporaneous marine deposits with fossils. The great problems of Quaternary geology centre around the correlation of the dif ferent sets of deposits and of the sequence of events they indicate.

At least five chronological scales have been instituted for Quaternary deposits: (a) that based on a study of marine faunas where, unfortunately, each province has its own history, and those best known are the Baltic, North sea and Mediterranean; (b) that based on a study of mammalian faunas; (c) that based on a study of human cultures, specially as exemplified by arte facts; (d) that based on a study of the sequence of plant forma tion in peat mosses; (e) that based on the history of continental deposits—glacial, fluvio-glacial, lacustrine and fluviatile, with their fossils, if any. In addition, a most important method of study of the Quaternary sequence is that of land forms—the rising or fall ing of the level of the sea relative to the land; the cutting of valleys by rivers and the formation of successive terraces; the formation of successive shore lines by seas and lakes.