Extension of the old Glaciers. 183 
With the coming of the cold, which closed the Tertiary and inaugurated 
the Glacial period, the consequent accumulation of ice on the northern con- 
tinental areas destroyed the ancient plant formations and associations. The 
forests were broken down by the ice storms of winter and their remains were 
ground to pieces, by.the moving glaciers. The semitropic species were killed 
by the increasing cold and such genera as Ficus, Cinnamomum, Eucalyptus, 
Laurus, Artocarpus, Myrtus, Piper, were exterminated, and extensive swaths 
were cut into the Tertiary forest by the great ice sheet, and only those species 
survived in their old haunts south of the glaciers which were adapted, as species 
of Liriodendron, Quercus, Fraxinus, Tsuga and Abies, etc., to grow under the 
influence of the increasing cold. The ground they had covered south of and 
along the great terminal moraine afforded new areas for the occupancy of two 
classes of plants, arctic plants and glacial bog plants. By the reversal of the 
drainage lines and consequent destruction of low ground vegetation, new 
habitats suited to these plants arose in advance of the ice invasion, spreading 
away from the centers of ice accumulation. Where this migration moved to 
the west, the plants were later destroyed, but their southward extension brought 
them into areas which were not within reach of the subsequent ice invasion. 
But the fact, says TRANSEAU’'), that the plants have survived the ice advances 
proves that they were easily able to establish themselves in new areas, as 
rapidly as the climate changed. Not more than five such geographic migra- 
tions of more or less latitude, corresponding with the five glacial extensions 
must have occurred. For during the intervals, when, as shown by the animal 
and plant remains found in interglacial deposits, the temperature was nearly 
as high, as at the present time. - 
The following tentative classification is proposed by CHAMBERLAIN °) for 
the drift” deposits of the Mississippi Valley, which probably represent the 
several stages of advance and recession of the great ice un during the 
Pleistocene. 
Glacial or Pleistocene Series. 
(see Fig. 4 at the head of Chapter III.) p Löls 
ı. Albertan drift sheet. 5. Illinois till-sheet. 
2. Interglacial deposits(Aftonian). 6. Interglacial deposits. 
3. Kansan till-sheet. 7. Iowan till-sheet. 
4. Interglacial deposits (Bucha- 8. Interglacial deposits(Toronto) °). 
nan). 9. Wisconsin till-sheets (earlier andlater). 
The Albertan stage is displayed in the RER province of Alberta, 
where the first formation of it was due to the extension of the glaciers east- 
ward from the Rocky Mountains. Farther east, the till-sheet passes into the 
1) TRANSEAU, E.N.: Botanical Gazette, loc. eit. p. 410. 
2) Scort, W. B.: An Introduction to Geology, 1897: 515. Ä 
3) CorLeman, A. P.: Glacial and interglacial Beds near Toronto. Journal of Geology IX: 
310. May. June 1901. 
