The second drought occurred early in Marsland times and literally hundreds of the little rhino, _Menoceras_, were preserved when their carcasses were broken up by a reborn river and buried like a mat of jackstraws in a river lake.
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.