Vegetation south of the Land Ice. 199 
The Forest Flora in the Mountains. The influence of the glaciers on the 
distribution of trees in the Rocky Mountains and the Sierra Nevada Mountains 
of California was most marked. Let us trace the history of the vegetation of 
this region before considering this point. We have already alluded to the 
extension of the sea which in the upper Cretaceous separated the western part 
of North America from the eastern. The Great Basin during the Lias was an 
enclosed natural lake, which was gradually filled by the action of aerial agents. 
Further during the early Cretaceous, the region of Missouri was covered by a 
sea and in Nebraska, there are evidences of a long continuance of a fresh- 
water lake. It is also probable, that the prairie region in the Miocene period 
was covered water. With the drying up of these seas and inland lakes, the 
prairie region was formed and the plant associations following the retreat of 
the water culminated in a prairie grass formation, which still further acted as 
an influence in the separation of the eastern and western floras. The factors 
mentioned above were sufficient to introduce a differentiation into the floras of 
the two widely separated regions and the separation of the tree vegetation of 
North America into eastern and western types may be said to have be 
That this separation was not fully accomplished until, after the Glacial period 
is proved by the presence of remains of Seguoia trees in the eastern United 
States of a later date. The climate of the eastern slopes of the Rocky 
Mountains during the upper Cretaceous period and subsequent must have been 
more humid than the climate of today, which is comparatively arid. We have 
proof of this climate in the petrified forests of Arizona, where the trunks of 
gigantic trees have been preserved unlike the flora of that region at the present 
day. In the soft shale rock at Florissant, Colorado are found fossil leaves, 
fruits and twigs of trees clearly allied to the living redwoods or sequoias of 
California, to oaks, hornbeams, alders, walnuts, chestnuts, elms, ashes, sumachs, 
hollies and other trees and shrubs arguing for a different climate in the far past. 
The oncoming of the glacial period and consequent refrigeration produced 
even a more marked change in the distribution of tree vegetation than the in- 
land sea of upper Cretaceous times. With the development of the continental 
glacier, as the ice sheets spread from the two great centers of accumulation, 
they united in the region north of lakes Superior and Huron. With their near 
approach to the lakes the area of conifers was divided into an eastern and 
western section. The trees of the western section were submitted to the action 
of the local mountain glaciers and their areas of distribution were thus broken 
up and many of the species of eastern affinity were destroyed and others were 
restricted within narrow limits. This has not been worked out for all the 
species of western trees, but the principles may be illustrated by a discussion 
of the glacial and post glacial distribution. Of the giant trees of California 
Sequoia gigantea (= 5. Washingtoniana) JOHN MUIR ‘) observed that the location 
1) Muir, Joun: On the post -glacial en of Sequoia gigantea. Proceedings American 
Association for Advancemient Science 1876: 242—253. 
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