At the end of the Mesozoic Era a profound change came over the land. The trough, with hundreds of meters of sediments accumulated on its bottom, was drained and folded by pressures built up in the Earth’s crust. To the south, the Colorado Plateau rose slowly and smoothly; to the north folding and faulting built complex mountain ranges. This uplift is called the Laramide Revolution because of the magnitude of the change it made on the face of the continent.
At the beginning of the Cenozoic Era, about 65 million years ago, the Rockies and the Great Plains were slowly rising. Rains fell and rivers carried the water, wearing away the old sediments. Some sediments were carried west into the Pacific Ocean, or deposited on the land and covered over by new sediments. Some sediments remained within the Rockies, settling into basins during the early Cenozoic. Other sediments were carried east by the rivers, beyond the Great Plains and into the Mississippi Embayment. Mountain building and erosion tended to cancel out one another in the Rockies, preventing the mountains from reaching great heights. The mountain bases were perhaps no more than 300 meters (1,000 feet) above sea level, and the crests perhaps 600 meters (2,000 feet) above that.
Before long, the basins within the Rockies filled with sediments. The basins overflowed, and at last sediments began to cover the Great Plains. This was near the end of the Eocene Epoch, about 35 million years ago. Subtropical rivers, flowing sluggishly, rolled out over their banks and left their loads of silt and clay on the featureless plain.
The oldest part of the blanket of sediments, the White River Beds, extended from Saskatchewan to Texas. Nearly 200 meters (650 feet) of muds, clays, silts, and river gravels were laid down during the 11 million years of the Late Eocene and Oligocene Epochs. Magnificent exposures of these beds can be seen at Scotts Bluff National Monument in the valley of the North Platte, in Toadstool Park north of Crawford, Nebraska, and particularly in the Big Badlands of southwestern South Dakota.
The kind of floodplain deposition characteristic of the Oligocene on the Great Plains ended about the beginning of the next epoch, the Miocene. In Nebraska the process continued, but eventually erosion began wearing away the accumulated sediments. The land to the west was uplifted a little, and the streams flowed off the high ground fast enough to cut down into what they had just deposited. On this eroded surface, the layers of tan silts, fine sands, and clays known as the Gering Formation were deposited.
On top of the Gering Formation, in the Late Oligocene, is the Monroe Creek Formation, the oldest formation actually exposed in the Agate area. This formation is named for exposures in Monroe Creek Canyon north of Harrison, Nebraska, and may be up to 75 meters (245 feet) thick. You can see a little of the Monroe Creek Formation’s pinkish silts and volcanic glass shards exposed in the valley of the Niobrara River. Where it is more exposed by erosion than at Agate, the Monroe Creek Formation forms magnificent cliffs along the Pine Ridge and similar areas of high ground. The best local examples are at Fort Robinson State Park between Harrison and Crawford, Nebraska. A close look can be obtained in Smiley Canyon just west of the fort, where old U.S. Highway 20 is maintained as a scenic drive.
After the Monroe Creek interval, near the end of the Oligocene, deposition of the famous Agate Fossil Beds began. Geologists have named this sequence of grayish silts and sands the Harrison Formation for its occurrence near Harrison, Nebraska. A short interval of erosion separates it in some areas from the Monroe Creek Formation below, and its coarser sands indicate increasing uplift of lands to the west. Wind played a smaller role in deposition than in Monroe Creek times, though that is certainly not true at the Stenomylus quarry. The Harrison Formation was the last of the truly widespread deposits seen in the Miocene of the Great Plains. Many rivers flowing eastward from the Rockies contributed sands to Wyoming, South Dakota, and Nebraska.
Generally the Harrison Formation and the overlying Marsland Formation are only moderately fossiliferous, but along the Niobrara River here at Agate, Nebraska, the accumulation of rhinoceros and camel skeletons is one of the wonders of the fossil world. Here, thousands of animals perished in two droughts which coincided with conditions perfect for preservation. About two kilometers (1.2 miles) east of the Monument headquarters the dried-out, mummified bodies of perhaps a hundred or more little camels, Stenomylus hitchcocki, were buried under windblown sand during the first drought.
Shortly after the camels were buried there was a brief period of erosion and then the Marsland Formation began to be deposited. Named for a little village east of Agate, the Marsland Formation consists of basal river channel deposits followed by about 45 meters (150 feet) of wind-blown tan-and-gray sands. It is in one of these river channel deposits that the Agate rhinoceros quarries are located.
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.