372 Story of the Mississippi-Missouri. — Claypole. 
from the sea and a process of crumpling and crushing set in 
corresponding to that which the Atlantic states had under- 
gone. The massive Cretaceous and early Eocene beds be- 
tween ten and twenty thousand feet in thickness were up- 
heaved by an earth-thrust from the Pacific to form the back- 
bone of the Rocky mountains, which, buttressed and rein- 
forced by other ranges of somewhat later date, elevated at the 
end of the Eocene and during the Miocene, have raised the 
high western rampart of the continent from Mt. St. Elias to 
Mexico. 
During the same period also occurred that elevation of the 
Cretaceous and Tertiary rocks of the Atlantic border which 
has placed some of them six and eight hundred feet above the 
sea and not improbably the peninsula of Florida dates its 
origin to about the same epoch. 
The reader can now realize the progress made by the con- 
tinent during the secondary and Tertiary eras. The whole 
western area was then added and the land out of which the 
wide western states are made was for the first time laid dry. 
Not until this had occurred was the Missouri river possible- 
But when the Pacific revolution had exposed the large western 
region and raised the Rocky mountains to a hight far exceed- 
ing that of the Appalachians and had added to North America 
a tract of land as great as its whole previous area a new sys- 
tem of drainage ensued and the magnificent rivers of the 
Northwest were developed. From the slopes of newly risen 
mountains, from the wide, flat prairies lying between them 
and from the plateaus and isolated groups of hills scattered 
over the land came the new streams all making their way into 
the great central valley. During the elevation and for a long 
series of years this western country was a lake region. Broad 
sheets of fresh water lay here and there over its surface 
dammed in by the unequal rising of the land. One of the most 
striking features of the geology of these western states is this 
great development of lakes. They held the waters brought 
down by the rivers from the upper lands and were swelled by 
the melting snows on the mountains. These torrents brought 
in vast masses of sand and mud which filled the beds of the 
lakes while the outflowing streams cut down their outlets so 
that in process of time this " lake-age " of the west passed 
away. It did not pass however without leaving an ample 
