It is presumed that in the Glacial epoch the genus was exterminated except in the areas in western North America where it still persists.
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.
The time since the last glacial epoch is but a fraction of the time since the first probably no more than a fifteenth or a twentieth.
The incoming of the Glacial epoch does not appear to have been accompanied by any acclimatization of the plants - the species belonging to temperate Europe were locally exterminated, and Arctic forms took their places.
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.
The great terminal moraine of the glacial epoch crosses the N.E.
The effect of the Glacial epoch in Europe is shown in northern Africa by the moraines of the higher Atlas, and the wider extension of the glaciers on Kilimanjaro, Kenya and Ruwenzori, and by the extensive accumulations of gravel over the Sahara.
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,.
At the close of the glacial epoch the north Asiatic flora spread westwards into Europe and intermingled with the surviving vegetation.
There are still enormous glaciers about the head of the Brahmaputra, but the glacial epoch of the Chang-t'ang highlands has passed away, though comparatively recently.