HISTORICAL GEOLOGY AND PALEONTOLOGY (continued)
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.
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.