FERNS, FOSSILS AND FUEL 
When the glaciers retreated, for what may or may 
not be the last time, the arctic plants retreated with them 
toward the north, and the alpine plants moved up into 
the higher altitudes which had become free from eternal 
ice. The plains of Europe received back some of the 
exiled plants from the south. But as has been explained 
before, conditions for a return were different in Europe 
with its east-west mountain barriers and the Mediter¬ 
ranean Sea cutting it off from Africa and Asia Minor 
than they were in North America where there is no 
Mediterranean and the mountain chains run north and 
south. Thus, as we have already seen, though the floras 
of Europe and North America were identical in the 
Tertiary period, they became differentiated after the 
glacial epoch. 
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