POST-TERTIARY PERIODS, the last main division of the Geological Record, and which includes all the formations accumulated from the close of the Tertiary Periods down to the present day. No sharp line can be drawn at the top of the Tertiary groups of strata. On the contrary, it is often difficult, or indeed im possible, satisfactorily to decide whether a par ticular deposit should be classed among the younger Tertiary or among the Post-Tertiary groups. In the latter all the mollusks are be lieved to belong to still living species, and the mammals, although mostly of existing species, include some which have become ex tinct. These extinct forms are numerous in proportion to the antiquity of the deposits in which they have been preserved. Accordingly a classification of the Post-Tertiary strata has been adopted, in which the older portions, con taining a good many extinct mammals, have been formed into what is termed the Pleisto cene, Post-Pliocene, or .Glacial group, while the younger deposits, containing few or no extinct mammals, are termed Recent. The Post-Ter tiary period§ are now generally termed Pleisto cene and designate the units of geological time from the end of the Tertiary to the beginning of the Historic. This term was introduced by Sir Charles Lye11.
The gradual refrigeration of climate was prolonged and intensified in Post-Tertiary time. Ultimately the northern portion of the Northern hemisphere was covered with snow and ice, which extended into the heart of Europe, and descended far southward in North America. The previous denizens of land and sea were in large measure driven out, or even in many cases wholly extirpated by the cold, while northern forms advanced southward to take their places. The reindeer, for instance, roamed in great numbers across southern France, and Arctic vegetation spread all over northern and central Europe, and what is now the temperate zone of America. After the cold had reached its climax, the ice-fields began to retreat, and the northern flora and fauna to retire before the advance of the plants and animals which had been banished by the increasingly severe tem perature.
The insensible gradation of what is termed the Pleistocene into the Recent series of de posits affords a good illustration of the true relations of the successive geological formations to each other. We can trace the gradual pas sage because it is so recent that there has not yet been time for those geological revolutions which in the past have so often removed or con cealed the evidence that would otherwise have been available to show that one period or group of formations merged insensibly into that which followed it. The Recent formations are those which have been accumulated since the present general arrangement of land and sea, and the present floras and faunas of the globe were established. They are particularly distinguished by traces of the existence of man. Hence the geological age to which these belong has been spoken of as the Human Period. But there is good evidence that man had already appeared on earth during Pleistocene time, so that the discovery of human relics does not afford cer tain evidence that the deposit containing them belongs to the Recent series. Nevertheless it is in this series of the Post-Tertiary deposits that vestiges of man become abundant, and that proofs of his advancing civilization are con tained.
During the Recent period the same agencies have been and are at work as those which have been in progress during the vast succession of previous periods. No trace can anywhere be detected of a break in the continuity of the evolution through which our globe has passed. See of Geologic Time) and consult the bibliography under that article. Consult also Fairchild, H. L., Geol ogy of New York State) (in Bull. Geol. Soc. America, Vol. XXIV, Washington 1913).