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 thou- 

 sands 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 sur- 

 vivors 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 insu- 

 perable 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 impas- 

 sable barriers to plant migration, and branches of the sea 

 effectually stopped the gaps between the mountain sys- 

 tems. 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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