182 Part III. Chapter 2. 
and still others remained on the mountain summits not covered by the great 
ice sheet and migrated from these northward into the arctic regions again, 
or went south along the Rocky Mountains to form an element of the high 
mountain flora. The consequence of this shifting and migration was to produce 
at the present an arctic flora much richer in types, which were moulded by 
the changes into more plastic and world conquering forms. 
We must believe that in the southwestern projection of the continent 
beginning even with the upper Cretaceous there was a flora practically distinct 
from the rest of the continent, because it was during this geologic period and 
the lower Tertiary that Mexico was under process of formation and these 
changes must have profoundly influenced the vegetation and its distribution. 
Such types as the various genera and species of the Cactaceae, Yucceae, and 
others which inhabit the Great Basin and arid southwest at the present day 
undoubtediy had their origin in the Mexican region and later during late 
Tertiary and post glacial times migrated northward into the present territory 
of the United States. . 
Chapter II. History and Development of North American Flora 
during the Glacial Periods. 
1. The Extension of the old Glaciers. 
The history of the present flora of North America begins with Glacial 
period, when by a change of level in the North American continent, by an 
increase of precipitation and decrease of temperature, a great continental ice 
sheet covered the land mass at the north. The southern limit of the great 
glaciers is indicated by a vast deposit of morainic material which stretches 
across North America from the Atlantic to the Pacific oceans reaching in one 
of its lobes the Ohio River at the present Cincinnati. Three great centers of 
glaciation are recognized by geologists and glaciologists, viz., the Labrador 
center of the northeast, the Keewatin center west of the present Hudson 
Bay and the Cordilleran center along the Pacific Ocean. These were 
elevated above their present level, and from them continental glaciers descended 
southward across the continent, sending forward several well marked lobes, 
such as the Erie Glacier, the Saginaw Glacier, the Traverse Glacier, the Michigan 
Glacier, the Green Bay Glacier, the Chippewa Glacier, the Superior Glacier 
and Minnesota Glacier with driftless areas sometimes between these lobes’)- 
The maximum period of glaciation with one, or two interglacial epochs lasted 
sufficiently long to profoundly modify the Tertiary flora and to introduce new 
factors into the distribution of plants. 
1) FENNEMAN, N. M.: On: the Lakes of southeastern Wisconsin. Wisconsin Geological and 
Natural History Survey, Bulletin No. VIII Educational Series No. 2 1902 see chart p. 2. 
