THE OZARKIAN AND ITS SIGNIFICANCE 529 
cutting of the inner gorge, which has continued to the present 
time. The elevation during the Tertiary was regional, the eleva- 
tion which commenced with the Ozarkian was continental or even 
more widespread. In the east there was a sharp demarcation 
of the Ozarkian from the present erosion by the Champlain 
depression and sedimentation ; which depression, as I suppose, 
was determined by the weight of the ice sheet. But in the 
plateau region there was no ice sheet—it did not extend so 
far —and therefore there was no depression and therefore no 
interruption of the erosion to the present time ; for the rising is 
still progressing. The whole series of phenomena in this region 
may be well explained by a local rise continuous from the end 
of the Cretaceous till now, except that it was interrupted at the 
end of the Pliocene by the Lafayette depression of McGee and 
afterwards greatly enhanced by the general continental elevation 
of the Ozarkian. 
3. Serra region. — But it is in the Sierra region alone that we 
find the Ozarkian erosion-work sharply marked off both from the 
Tertiary on the one hand and from the present on the other. It 
is here therefore that the distinctive Ozarkian work can be best 
studied. 
Brief history of the Sierra-—Yhe Sierra Nevada, as is well 
known, was formed at the end of the Jurassic by lateral pressure 
and strata-folding in the usual way. What kind of a mountain 
it was at that time, how high, and what its configuration we 
know not ; forthe continuous erosion of the Cretaceous and Ter- 
tiary times had nearly swept it clean away. The cycle of its 
mountain life had reached its last stages. By continuous erosion 
it had been reduced to a peneplain, with its wide-sweeping 
curves of broad shallow channels and low-rounded divides. 
The rivers had reached their base levels and rested. This was 
the work of the Cretaceous and Tertiary. 
Then came the post-Tertiary rejuvenation of the mountain 
‘As the term Ozarkzan had already been used by Broadhead for a Lower Silurian 
series in the Ozark region (Am. Geol., XI, p. 260, 1893) there may be an admissi- 
bility of its use in this connection. If so, I would propose the name Sverran as far 
moye appropriate. 
