The courses followed by the major streams of the interior of the North American continent during Paleozoic and Mesozoic times, cannot now be pointed out with any high degree of certainty. The numerous upwarps and twistings to which the continental ridge was subjected, caused large portions of what is now North America to be at times above the sea, at other times below, and the drainage was shifted from area to area, from one direction to another. During a period as recent as the late Cretaceous the drainage of the middle Northwest flowed westward into a great sea that overlapped the western part of Iowa and extended away to the Sierras of California. There were no Rocky mountains such as we now know, for it was not until the close of the Cretaceous that the ridges of the Rockies, as a whole, rose above the waters; not until this event took place, could there be a Mississippi valley with extensive surfaces sloping from mountain chains on the east and west toward a central axis; not till then could the master stream of the continent enter upon its permanent career.
Proceedings of the Iowa Academy of Science
©1907 Iowa Academy of Science, Inc.
"Some Features of the Channel of the Mississippi River between Lansing and Dubuque, and Their Probable History,"
Proceedings of the Iowa Academy of Science, 14(1), 213-220.
Available at: https://scholarworks.uni.edu/pias/vol14/iss1/16