THE STORY TOLD BY FOSSIL PLANTS
ducing numerous lakes, ponds, and bogs. The freshness of the deposits it left—its moraines, its bowlder till, and its sand plains, all scarcely modified in the relatively few thousands of years that have elapsed since the last ice sheets disappeared—emphasize the nearness of the great glaciers to the period of human history.
At the beginning of Pleistocene glaciation the flora of all three of the continents of the Northern Hemisphere was essentially similar. The retreat of the last ice sheet left an impoverished flora in Europe and two great asylums of survivors in eastern North America and eastern Asia. The explanation of this difference is, broadly speaking, very simple. In America and Asia, with their extensive coastal plains and north-south mountain chains, there were no insuperable barriers to the dispersal of plants southward, away from the frozen lands, but in Europe the mountain ranges (the Pyrenees, Alps, Carpathians, Balkans, Caucasus), which trend east and west, and many of which were themselves lofty enough to be local centers of glaciation, formed impassable barriers to plant migration, and branches of the sea effectually stopped the gaps between the mountain systems. Hence many of the plants of the Pliocene forests of Europe were unable to escape extinction.
Great sheets of ice accumulated over the land during at least four separate epochs. Each of these epochs lasted 10,000 to 20,000 years, and they were separated by long epochs of genial climate, known as interglacial epochs, each lasting for thousands of years, during which the floras spread northward, even to points beyond their present range. Many such interglacial floras are represented in deposits in Europe and have been diligently investigated in connection with the economic study of peat bogs. The best known interglacial flora of North America, where the extensive peat resources
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