geologists subdivide, beginning with the earliest, into Pleistocene, Glacial, and Recent.
The most important features of the Post-Pliocene or Quaternary period are: First, the advent of man and contemporaneous flora and fauna. Second, the great Glacial period—the period when the glaciers extended over the greater part of Europe and North America, as attested by the drift formation with its immense bowlders torn from mountain sides and carried a hundred miles or more; glacial scratches—grooves made by the rocks carried by the glaciers on the surfaces upon which they moved; terminal and lateral moraines, heaps of rocks left by the melting ice marking the limits of the glaciers. This Glacial epoch separates the Post-Pliocene into the three divisions mentioned before, which may be called Preglacial, Glacial, and Post-glacial, instead of Pleistocene, Glacial, and Recent.
It will, perhaps, be of interest to briefly indicate some of the hypotheses that have been advanced to account for this Glacial epoch. In the first place, any hypothesis, in order to
satisfy the necessities of the proposition, must include two seemingly opposed conditions and explain their interaction. To form a glacier, both heat and cold are necessary; to form an ice mass, there must be something to be frozen; therefore there must exist sufficient heat to vaporize water, charging the atmosphere with aqueous vapor, which when carried to higher altitudes or subjected to cold is condensed and precipitated in the form of snow, the accumulation of which forms the glaciers. Hence, in a glacial region, if the mean temperature is comparatively high, the snowfall must be great, otherwise the heat would melt the snow faster