HISTORICAL GEOLOGY AND PALEONTOLOGY (continued)
CENOZOIC (continued)
Pliocene
The Pliocene is represented in the great plains region by the
Ogalalla formation which extends from Kansas and Colorado far into
Nebraska, but which does not now extend into the immediate region of
Scotts Bluff. It may have occurred here at one time but since has been
removed by erosion. It is extensively developed in the western part of
Nebraska in the region of Lodgepole Creek and south. The extent of this
formation in the northern part of Nebraska has not been ascertained;
however, Pliocene faunas are known from this region.
In its typical development (Darton 1903, p. 1) the Ogalalla formation
is a calcareous grit or soft limestone containing a greater or less
amount of interbedded and intermixed clay and sand, with pebbles of
various kinds sprinkled through it locally, and sometimes with a basal
red conglomerate.
Faunas from the Pliocene are not very abundant, or at least are not
well known. However, in deposits of this epoch are found the first
appearance of the more modern single toes horse, Pliohippus, its high,
crowned teeth attesting to a diet of hard grasses from the plains.
Rhinoceroses were still abundant and roamed the plains, apparently in
herds, while camels of larger and more modern types than those of the
Miocene still flourished.
Pleistocene
During the Pleistocene the Great Plains as well as the Pocky
Mountains experienced extensive uplifting which resulted in their
present elevations. This increase in elevation caused a much greater
precipitation and the streams began a period of renewed activity sinking
their courses deeper in the mountain region and entrenching themselves
below the surface of the plain. The eastern portion of the plain has
been completely removed, perhaps due to extensive glacial floods. With
the rising of the Black Hills there was deeper erosion around it and the
High Plains in that region have been largely removed, their present
northern edge being represented by Pine Ridge.
Erosion is still in progress, especially in the smaller streams where
the water has sufficient declivity to carry its load. In the larger
streams, the valleys are beginning to be built up, as in the Middle
Cenozoic, because the volume of water is not adequate to carry away the
waste from the adjoining slope.
The great diastrophic movements which occurred at the close of
Pliocene time and continued during the Pleistocene might be considered
as ushering in the present. The Pleistocene is principally distinguished
from the recent by its great ice sheets which spread over nearly
one-sixth of the existing lands. So recent is this last great episode of
geologic time that the ice sheets have not yet completely disappeared,
being still very much in evidence in Antarctica and Greenland. In North
America, glaciation centered in Canada, and during its maximum stages is
believed to have covered eastern Nebraska. Numerous local centers of
glaciation also existed in the Rocky Mountains.
The great changes in relief and climate caused the extinction of many
older groups of animals, and during the Pleistocene modern groups,
including man, make their appearance.
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