PLEISTOCENE EPOCH, in geology, includes the time that elapsed be tween the close of the Tertiary and the dawn of the historical or recent period. Measured in years, it is the shortest of the epochs, but geologically it is one of the most interesting. It includes the great Ice Age during which glaciers advanced and retreated across vast areas in North America and northwestern Europe. It is also the epoch, during which, so far as can be determined by the available evi dence, man first appeared on the earth., For these two reasons it has been more discussed. Being so recent it left much evidence, not yet obliterated, of changes of land and water areas, and of variations in climate; but the very abundance of the evidence increases the task of deciphering and summarizing it, since for re mote epochs only the great changes are known through the evidence of the rocks.
It is fairly' certain that at the close of the Tertiary great areas in northern North Amer ica and Europe were more elevated than now. North America and Asia were perhaps united across Bering Strait; similarly the British Is lands were part of a European continent that stretched out from Norway toward Iceland and north of Russia and Sibera over areas now covered by the Arctic Ocean. Regarding the equatorial land areas less is known, but possibly North and South America were united and it is possible that the Desert of Sahara was in part at least covered by an arm of the ocean. Of the distribution of land and water in the Ant arctic regions the evidence found is not conclu sive, though it is probable that the Antarctic continent was of greater extent than now, with a land bridge from Australia to South America. How much higher the land areas toward the north in Europe and North America were is un certain, but it is probable that the regions about Hudson Bay and northern Sweden and Norway were much higher. The climate of these areas at the time was generally drier and cooler than it had been during the late Tertiary. During the early Pleistocene river valleys were deeply carved and the topography was strongly ac centuated. Gradually the climate in the North ern hemisphere became colder, and possibly moister. What produced this change is one of the most baffling problems in geology. In the
course of years perennial snow stood on the higher ground and gradually accumulating formed vast ice sheets or glaciers which slowly advanced from these higher regions over the regions southward, and in the case of such mountains as the Alps, to northward. Thus began the great Ice Age which lasted thousands of years. The ice sheets advanced, retreated and advanced again. Some of the retreats must have been of long duration. In fact the evidence from rock decay and the formation of soils leads some geologists to believe that the time that has elapsed since the final retreat and disappearance of the great continental glaciers is not as long as that of some of the interglacial intervals. The probable number and succession of the glacial advances and retreats and the work done by the ice in modifying the topog raphy of the glaciated regions are discussed elsewhere. (See GLACIAL PERIOD). At the end of the Ice Age the regions in the North that had been the center of glaciation were much lower than at its beginning and the climate was moister. A subsequent elevation and a drier climate make the conditions governing life what they are to-day. Outside the glaciated areas the changes of level and climate have left less trace.
The advances and retreats of the ice neces sarily meant great changes in climate. During the advances Arctic types of plants and animals were forced southward, during the retreats other types migrated northward, the Arctic types retreating or finding refuge on mountain ranges. One result of these migrations was the extinction of types that could not migrate readily and in particular of the great mammals. Man when he appeared waged war upon the large land animals that had withstood climatic changes. Thus it is that with the dawn of the historical period the fauna of Europe and America was impoverished as compared with periods immediately preceding, and the huge pachyderms, which, through Tertiary time, had been so striking a feature of the animal popu lation of what are now the temperate regions of the world, disappeared, leaving only their gi gantic remains to tell modern man the kind of monsters that have been the contemporaries of his prehistoric progenitors. See also QUATERNARY.