CENOZOIC

With the opening of the Cenozoic the condition of interior North America was somewhat as at present. The Rocky Mountains, which had been formed at the close of time Cretaceous, constituted an extensive highland to the west which was undergoing rapid erosion. Within this mountainous region were several large basins similar to the present "parks" of the Colorado Rockies, and in which were deposited much of the debris from time erosion of the surrounding mountains during the early part of the Cenozoic. Eastward from the Rocky Mountain front lay an extensive lowland or plain sloping gently eastward from the foothills and across which flowed the streams draining the eastern slope of the Rockies. During the early part of the Cenozoic this lowland plain did not receive any extensive deposits. It may have been that the intermountain basins trapped most of the erosional debris so that very little reached the plains region. (Schuchert 1933, p. 391) A more plausible explanation is that the plains were high enough so that the streams crossing them had sufficient gradient to carry not only their entire load of sediments but were able to abrade their channels, and strip off parts of the Cretaceous deposits.

Beginning with the Oligocene Epoch the major sites of deposition were changed. Almost no Oligocene sediments are known from the Rocky Mountain region but in the Great Plains one of the most complete records of this time is furnished by substantial thicknesses of sands, gravels and clays. What is true for the Oligocene may also be said for the Miocene and Pliocene. Many rivers and streams originating in the Rocky Mountains and flowing through deep and rugged channels with swift currants carrying abundant erosional debris, rather suddenly emerged up on the gently sloping surface of this plain. As a result of a loss of gradient, the currents no longer were able to carry their loads and began building great alluvial aprons. As the stream channels became clogged the streams migrated slowly back and forth spreading sands and gravels over the plain. Torrential downpours of rain at times caused sheets of water to spread far and wide over the lowland between the stream channels, each flood leaving an additional thin sheet of silt. At times there seems to have been abundant vegetation along the stream courses and on the higher ground. The plains evidently were grassy and gave support to herds of roaming wild beasts which were preyed upon by predacious animals. Local lakes existed during the time of greatest humidity and were inhabited by flourishing algal growths, fresh water invertebrates, and fish. Primitive alligators and fresh water turtles inhabited the streams while land tortoises must have been numerous. There were changes in climate; at times aridity caused a drying up of the lakes, and encrustations of salt and gypsum were formed ever the dessicating muds; while at other times the climate seems to have been mild and moderately humid. There were short times when erosion predominated over deposition, resulting in short breaks in the record; however, many hundreds of feet of deposits were laid down during the Cenozoic, fortunately enclosing the remains of many animals and plants. We therefore have in this region a record of events, the completeness of which, for all except the earliest Cenozoic, is second to no other known. We can now proceed to take up a more detailed examination of this record, attempting to portray as completely as possible a picture of each of the important epochs of geologic time represented here.

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