After Marsland times there was more erosion, in some places by rushing streams that cut down 91 meters (300 feet) through soft sediments, to the top of the Monroe Creek Formation. In these channels the Runningwater Formation was deposited because it filled in the stream valleys and wound around the high spots. This channel deposit is not found everywhere, but it does have an equivalent in southwestern South Dakota. Other deposits of similar age are found in many parts of the Great Plains, and they contain fossil animals like those found in the Runningwater Formation.
The turbulent streams which deposited the Runningwater Formation were flowing off newly uplifted land to the west. This was the beginning of the most recent major uplift of the Rocky Mountains, and it signalled a great change in the pattern of deposition on the Great Plains. No longer would broad blankets of sediments be deposited by sluggish streams originating in the low, broad warp of the Rockies.
This latest uplift is called the Rocky Mountain Revolution. It brought on a period of alternating cycles of deep channel cutting and stream deposition. Floodplain deposits were restricted to narrow ribbons in river-cut valleys. Even more important than the changes in deposition was the effect of this uplift on the climate. As the Rockies began to rise to their present height, the climate became increasingly arid and the tree-dotted savanna of the old Great Plains gave way to grasslands.
Several kilometers south of Agate, the Sheep Creek Formation was laid down during the Middle and Late Miocene. The appearance of the grazing horse Merychippus in these channel and floodplain deposits marked the establishment of the grasslands as the newly dominant ecosystem of the Great Plains. At that time the “modern” fauna began to replace the old, and new patterns of life were established.
Again rejuvenation of the stream system, probably reflecting further uplift in the west, started another erosional interval that began to wash away the beds just deposited. When deposition followed in the Late Miocene, a new series of channel and floodplain deposits, the Lower Snake Creek Beds, was laid down. On them was deposited the Upper Snake Creek Beds, and together they span most of the Late Miocene and the Early and Middle Pliocene epochs. Harold Cook collaborated with W. D. Matthew of the American Museum of Natural History, publishing important papers on the numerous finds from these fossiliferous deposits. Animals new to science are still being discovered in the Snake Creek Beds.
After Snake Creek times, the area immediately around Agate was left out of the mainstream of events on the Great Plains. The continuing uplifts of the Rocky Mountains were no longer recorded here in cycles of downcutting and channel deposition. If the cycles continued here, all traces have now been washed away—an unlikely possibility. The view from the high plains above the valley of the Niobrara River reveals only the rolling surface of the pre-Runningwater deposits.
A more complete record is found in the river terraces of major streams, the North Platte to the south and the White and Cheyenne Rivers to the north. These terraces tell the story of continuing uplifts. To the south, east, and northeast of the Platte the record is also written in fossil bones, but these are outside the scope of our story.
Northwestern Nebraska, northeastern Colorado, southeastern Wyoming, and southwestern South Dakota today remain a promised land for paleontologists studying mammal life in North America during the middle and later Cenozoic Era. The fossil deposits in the Agate area are surpassed in importance only by the Late Eocene and Oligocene deposits of the Big Badlands and Pine Ridge in South Dakota.
Ecology: Change and Adaptation
During the Age of Mammals (the Tertiary Period), three major environments dominated western Nebraska. The first of these occurred during the Paleocene, Eocene, and Oligocene epochs. This was a forest system where trees were the major component of the flora. Meadows were found only in scattered areas and can be considered a minor element. There is no geologic or paleontologic record of the Paleocene and Eocene in the Agate area, but when our present knowledge of the early Tertiary Rocky Mountain floras is projected eastward a bit, a predominantly forested landscape is indicated.