Pleistocene (Gr., 'most recent') or GLACIAL SYSTEM. This system comprises the older accumulations belonging to the Quaternary or Post-Tertiary division. Almost all the molluscs met with in beds of this age are existing species. The system is termed Pleistocene or Glacial according as we have reference to the character of its organic remains or to the physical conditions under which the greater portion of its deposits were accumulated. These deposits are of very diverse nature and origin, and are subject to endless modifications, but nevertheless they show certain well-marked phases which are persistent over wide areas. Thus, throughout all northern Europe and the hilly and mountainous districts of the central and southern regions of the continent they exhibit the same general character and succession. The deposits of these regions consist for the most part of glacial and fluvio-glacial detritus, which betoken the former presence of a great ice-sheet in northern Europe, and of extensive snowfields and glaciers in the mountain districts farther south. For an account of these deposits and glaciation generally, see GLACIAL PERIOD. In the regions outside of the glaciated areas the Pleistocene system is represented principally by fluviatile accumulations, calcareous tufas, peat, and deposits in caves. The old river-gravels, &c. are well seen in the valleys of southern England, France, Belgium, central Europe, Spain and Portugal, Italy, &c., where they occur at the surface. But when they are followed into regions in which glacial and fluvio-glacial accumulations are well developed they disappear underneath these or are dovetailed with them. Cave-deposits are of course met with even in glaciated regions, but in such countries no Pleistocene accumulations overlie the glacial and fluvio-glacial detritus of the latest cold stage of the glacial period.
Thus, in general terms, the Pleistocene deposits of northern Europe and the mountainous regions of the central and southern parts of the continent are of glacial origin, while the accumulations outside of those areas are chiefly fluviatile. The latter were for some time believed to be upon the whole younger than the former, but the two series are now generally recognised to be contemporaneous. The occurrence of fossiliferous beds intercalated between sheets of morainic matter (boulder-clay, &c.) proves that the so-called glacial period was interrupted more than once by epochs of milder climatic conditions, during which the inland ice of the north retired from all the low grounds, while the great glaciers of the Alps, &c. shrank back to the inner recesses of the mountains. The organic remains obtained from fresh-water interglacial deposits have been correlated with those which occur in the river-accumulations of the non-glaciated tracts, and the result is that these accumulations are now admitted to be for the most part of interglacial age also. In short, the peat, river-deposits, tufas, and cave-accumulations are the equivalents in time of the glacial and interglacial deposits.
Life of the Period.—The plants and animals of the Pleistocene betoken great changes of climate—one series indicating an extremely cold or Arctic climate, while the other could only have flourished under extremely clement and uniform conditions. While a cold climate prevailed, such plants as Dryas octopetala, Betula nana, Salix polaris, &c. flourished in the plains of Germany, and similar northern and Arctic forms clothed the low grounds of Switzerland. In northern France grew birch, alder, larch, spruce, juniper, and yew, while in Northern Italy the Cembran pine flourished in the neighbourhood of Ivrea, and the Scotch fir on the shores of Lake Varese, positions in which it is needless to say they could not live now. Contemporaneous with this flora we meet with land and fresh-water shells which are equally indicative of cold and ungenial conditions. And the same tale is told by the boreal and Arctic species of molluscs which occur more or less abundantly in the shelly clays of north Germany, Scandinavia, and the British Islands, and by the presence of northern forms in the Pleistocene marine beds of the Mediterranean area. The character of the land animals is quite in keeping with this evidence. Living in the low grounds of central and southern Europe at this time were reindeer, glutton, musk-sheep, Arctic fox, Alpine hare, marmot, snowy vole, mammoth, woolly rhinoceros, &c. The remains of these northern and Arctic plants and animals are met with both in glaciated countries and in the caves and fluviatile deposits that occur in regions that never were covered with glacier-ice. The relics and remains of man himself also accompany the same flora and fauna. In strong contrast with such an assemblage of plants and animals is that of which we find abundant traces in interglacial beds and cave- and river-deposits. In northern France grew willows, hazels, ash, dwarf elder, sycamore, spindle-trees, perfumed cherry-tree, box, Clematis, common ivy, judas-tree, Canary laurel, &c. The presence of that laurel, which flowers in winter, proves that the winters must have been very clement, and the other plants are indicative of a genial humid climate. The summers were not so hot and dry as they now are in France, and the winters were not so cold; and similar conditions obtained in Germany and the Mediterranean region—although the floras of those different zones were distinguished from each other by the presence of certain forms and the absence of others. The land and fresh-water shells associated with this flora are equally indicative of genial conditions, and similar evidence is supplied by the mammalia. Thus, we find a strange commingling of southern and temperate forms which is quite in keeping with the similar association in one and the same place of various plants which no longer live together in Europe. Amongst the animals were hippopotamus, African elephant, hyæna, serval, lion, leopard, various extinct species of elephant and rhinoceros, an extinct dwarf hippopotamus and machairodus. Contemporaneous with these were urus, bison, horse, stag, roe, saiga, beaver, hare, rabbit, otter, weasel, wild-cat, fox, wild-boar, brown bear, grizzly bear, cave-bear, Irish deer, &c. The relics and remains of Palæolithic man likewise accompany this flora and fauna.
It is obvious, therefore, that the Pleistocene period was distinguished by great climatic oscillations. At one time the whole of northern and north-western Europe, down to the 50th parallel N. lat., was covered with a vast mer de glace, while from the Alps and all the considerable mountain-ranges of middle and southern Europe great glaciers descended to the low grounds. From ice-sheet and glaciers mighty rivers flowed all the year round, but in summer they rose in flood and inundated wide tracts, which in time became overspread with sand and loam. It was under such conditions that a boreal and Arctic vegetation clothed the low grounds of middle Europe. Considerable tracts of that region, during the last cold stage of the glacial period, appear to have resembled steppes, and to have been inhabited by jerboas (jumping hare), spermophiles, &c. The same lands, which in some places were clothed with pine-forests, were roamed over by great herds of reindeer, mammoths, &c.—the bones of which sometimes occur together in such large numbers as to lead to the belief that the animals may have perished in snowstorms or ‘blizzards.’ It was then, too, that the reindeer and its associates flourished in the low grounds of southern France, where they were hunted by Palæolithic man. With the advent of interglacial times such ungenial conditions of climate passed gradually away—the ice-sheet vanishing from the low grounds of north-western Europe, while the mountain-glaciers of central and southern regions dwindled to insignificance. Great migrations of plants and animals accompanied these changing conditions, the Arctic-alpine flora and northern and alpine fauna retreating northwards and retiring to mountain elevations. At the climax of interglacial times an extremely mild and genial climate, recalling that of the Pliocene, prevailed in Europe. The Canary laurel, the fig-tree, the judas-tree, and many others flourished then as far north as Paris, in which region frost in winter was rarely or never experienced. Elephants, hippopotamuses, rhinoceroses, &c., and vast herds of bovine and cervine animals then wandered over all temperate Europe—the British area included. How often such changes of climate were repeated has not yet been ascertained, but interglacial beds occur on at least two horizons—as in France and the alpine lands of central Europe. Hence there would appear to have been at least three glacial epochs separated by two intervening epochs of genial climatic conditions. In northern Europe only one well-marked interglacial epoch is generally admitted by geologists. But the evidence is not conclusive. It is obvious, indeed, that the preservation of interglacial accumulations must have been exceptional within regions which have been severely glaciated. With the return of ice-sheet and glaciers, fluviatile and other deposits which had been laid down during interglacial times would be ploughed up and commingled with other morainic material. It is only here and there, therefore, that patches of such deposits have escaped destruction. The relics of interglacial times are most abundantly met with in countries which were beyond the reach of the ice. The closing stage of the Pleistocene was a glacial one; so that in the valleys of central and western Europe the ossiferous river-gravels of the last interglacial period are more or less buried under the fluvioglacial gravels and loams of the latest glacial epoch. The latest Pleistocene deposits in the British area are marine clays containing Arctic and boreal shells. These deposits go up to 100 feet or so in Scotland.
Considerable geographical changes supervened during Pleistocene times. The proofs are seen in certain raised beaches in the maritime districts of north-western Europe, in the marine clays with their Arctic and boreal shells, and the marine sands, &c. of glacial and interglacial age, which are well developed in the British area. Again, the distribution of the mammalian fauna of the Pleistocene points in like manner to considerable changes in the relative level of land and sea. Thus it would appear that in interglacial times Europe was connected, across the Mediterranean, by one or more land passages with north Africa; while at the same time the British area was continental. Indeed, certain evidence leads to the belief that the European lands stretched out into the Atlantic as far as what is now the line of 100 fathoms. Towards the close of the last interglacial epoch, however, a considerable submergence of the British area supervened—for undisturbed interglacial shell-beds have been met with up to heights of several hundred feet. It is noteworthy also that the low grounds of north Germany were likewise submerged just before the invasion of that region by the last great mer de glace. The cause of such changes of level has been much canvassed by geologists. From the fact that evidence of submergence so frequently accompanies proofs of severe glaciation, it has been inferred that the subsidence may have been due to the presence of the ice. It has been suggested, for example, that the weight of the great ice-sheets which covered such vast regions in our hemisphere during glacial times may have displaced the earth's centre of gravity, and thus caused a rise of the sea in the north. Others, again, think it probable that under the pressure of a great ice-sheet the earth's crust may have yielded and sunk down more or less gradually. Some, again, have thought that a thick ice-sheet would exercise sufficient attraction upon the sea to cause it to rise upon the land. It must be admitted, however, that some of the oscillations of level which took place in Pleistocene times were on much too considerable a scale to be explained by any of the hypotheses referred to. If, for example, the considerable submergence which happened just before the advent in the British area of the last mer de glace was due to the pressure of an ice-sheet covering some region farther north, it is hard to understand how great confluent glaciers afterwards succeeded in covering an area lying several hundred feet below the surface of the sea. Again, if the crust of the earth were so readily deformed under the weight of an ice-sheet, how shall we explain the absence of any traces of contemporaneous marine action in those low-lying regions of south England and the Continent which were invaded by the great northern ice-sheet? Whatever influence the ice-sheets of the glacial period may have had upon the sea-level, it seems most probable that the greater oscillations were the result of considerable earth-movements, such as have taken place at many different stages of the world's history.
Pleistocene in other Continents.—In North America deposits of the same character and showing the same general succession as those of Europe are encountered—the glacial and interglacial conditions that characterised the latter continent having been equally characteristic of the former. The Pleistocene fauna of North America embraced Mastodon, a true elephant, species of horse, bison, beaver, peccary, bear, &c., and gigantic extinct forms of sloth, such as Megatherium, Mylodon, and Megalonyx. In South America the Pampa deposits have yielded a large number of remains of the great sloths and armadillos (Glyptodon), besides other mammals. There is abundant evidence also to show that snow-fields and glaciers had in Quaternary times a considerable development in the Cordilleras, while in Fuegia ice seems to have overflowed much of the low grounds. In South Africa former snow-fields and glaciers have left their traces in the Kaga and Krome mountains. So, again, New Zealand and Australia seem to have had a glacial period. Little is known of the mountains of central Asia, but old moraines and erratics have frequently been observed in the Chinese ranges, while, as is well known, the glaciers of the Himalayas had formerly a very great development.
Cause of the Pleistocene Climatic Changes.—Many speculations as to the cause of the climatic changes of Pleistocene times have been indulged in. It must be admitted, however, that none of these explanations is without its difficulties. But the theory which best accounts for the facts and has gained the widest acceptance is that advanced by the late Dr Croll. According to him, the strongly contrasted climates of the Pleistocene period were the indirect result of the increased eccentricity of the earth's orbit, combined with the precession of the equinox. During a period of high eccentricity that hemisphere which had its winter in aphelion would be subject to severe cold, and its snows would not disappear during the succeeding short summer, the temperature of which would be lowered by its presence. Thus in time that hemisphere would be subjected to severe glacial conditions. In the opposite hemisphere the state of things would be very different, for the winter would be short and mild and snow would not accumulate. Under such widely contrasted conditions between the northern and southern hemispheres, the trade-winds, and through them the great equatorial ocean-currents, would be powerfully affected. The winds blowing from the glaciated hemisphere would be much stronger than those coming from the other, and the equatorial oceanic currents would thus be impelled across the equator into that hemisphere whose summer happened in aphelion. This influx of warm water would necessarily increase the temperature of that hemisphere, while the other would have its temperature correspondingly lowered. Owing to the precession of the equinox, however, the conditions of the two hemispheres would tend to be reversed every 11,000 years or so; so that during a cycle of great eccentricity each hemisphere would experience an alternation of extremely cold and very genial climatic conditions. The last period of high eccentricity commenced some 240,000 years ago and lasted for about 160,000 years. Some geologists have objected that the glacial period cannot be carried so far back in time. Thus, from an examination of the rate of erosion in the postglacial gorges of some rivers in North America (Falls of Niagara, Falls of St Anthony), some observers conclude that only 7000 or 8000 years have elapsed since the close of the ice age. But in all such measurements and estimates there are elements of uncertainty which render the conclusions based upon them of little value.
Antiquity of Man in Europe.—All the human relics met with in Pleistocene deposits belong to what is known as the Palæolithic stage. Hitherto no indubitable evidence is forthcoming to show that man was an occupant of Europe before the glacial period. It is quite possible he may have been, but we lack evidence sufficient to prove this. He certainly lived, however, throughout the glacial and interglacial conditions described above. It is remarkable that no trace of his occupation has been met with in beds of later date than those pertaining to the close of the last interglacial epoch. If we were to judge from negative evidence (which it is always unsafe to do) we should infer that he vanished from Europe during the last glacial epoch. The oldest human relics hitherto discovered in postglacial beds are Neolithic.
See Croll's Climate and Time (1875). Various objections which have been urged against this theory have been answered by its author in his Climate and Cosmology (1886). See also G. F. Wright, The Ice Age in North America (1889), and the articles in this work on GEOLOGY, ANTHROPOLOGY, MAN, EUROPE, STONE AGE.