icy mass, estimated to have exceeded a mile in thickness. Great glaciers also arose in the Alps, Pyrenees, and Caucasus and descended from these mountains far into the plains. The Ice Age, despite its name, was not one of uninterrupted cold. There seem to have been four advances and retreats of the ice, resulting in as many more or less warm intervals. The accompanying map represents Europe in the second glacial stage,
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the period of the greatest extension of ice fields and glaciers. Guesses about the duration of the Ice Age vary considerably; one estimate makes it begin about 500,000 years ago. Our own postglacial stage may have begun about 25,000 years ago.
The geography of Europe in the Ice Age was unlike what it is today. Considerable areas now submerged beneath the Atlantic Ocean were, then dry land. Great Britain and Ireland formed part of the Continent, and no North Sea separated them from Scandinavia. The
