At the close of the glacial epoch the north Asiatic flora spread westwards into Europe and intermingled with the surviving vegetation.
The most interesting feature of the glacial epoch is the extinct Lake Agassiz, which the receding ice of the later glacial period left in the Red River Valley of Minnesota,.
It is presumed that in the Glacial epoch the genus was exterminated except in the areas in western North America where it still persists.
When the ice of the last glacial epoch had retreated so far that Its front lay on a northward slope, belonging to the drainage area of the Great Lakes, bodies of water accumulated in front of the ice margin, forming glacio-marginal lakes.
Some of the estimates make the lapse of time since the first glacial epoch more than a million years, while others make it no more than one-third as long.
