The Geologic History of Agate
Although the whole story of Agate Fossil Beds dates from the formation of the Earth four and one half billion years ago, only the last 600 million years is known in detail. It was about 600 million years ago that many plants and animals began to have hard parts—parts likely to be preserved as fossils. The few fossils contained in older rocks are often folded, twisted, squeezed, and distorted so that their original character is all but erased. That isn’t always the case, of course. Some of these old rocks, the Belt Series in Montana, look as though they were deposited only a few million years ago; they contain traces of algal colonies indicative of the generally simple life forms on the primitive Earth. The old rocks, deposited during the first four billion years of Earth’s history, record the Precambrian Eons, spanning eight-ninths of geologic time.
The evolutionary development of skeletal remains has aided in the study of geologic history. The last 600 million years have been divided into units for ease of discussion and comparison. The three largest divisions are the eras—Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life). We are viewing all this from our vantage point at (what is now) the most recent episode of the Cenozoic.
To begin at the known beginning, we only need go back to the start of the Paleozoic Era some 600 million years ago. No Precambrian, Paleozoic, or Mesozoic rocks can be seen around Agate, but we know from rocks found in comparable areas about what to expect under the surface at Agate: thousands of meters of sedimentary rocks, most of them laid down in an ocean. During the Paleozoic and most of the Mesozoic eras, up until about 70 million years ago, the west-central United States was covered by seas. The area now occupied by the Rocky Mountains was then a long north-south trough in which thick sediments collected. To the east, the present Great Plains area was the floor of a shallower sea. Sediments collected in the trough and on the sea bottom. Gradually, over a period of 530 million years, the sediments accumulated to a thickness of over 2,100 meters (6,900 feet). A dynamic “give-and-take” process, this sedimentation was the result of periods when the sea rose and fell several times.
If the Paleozoic sounds a little dull, it’s because we haven’t told the whole story. During the Paleozoic Era, all the major groups of organisms evolved. The seas swarmed with trilobites and shellfish of all kinds, some weird and fantastic and some very like those alive today. With them coexisted fish and sea-lilies, seaweeds and giant swimming “scorpions.” For the first time, plants and then animals came out of the sea to live on the land. These events and creatures are preserved in Paleozoic rocks. The Mesozoic Era saw the development of mammals from reptiles, the rule of giant dinosaurs, and the beginnings of flight. Strange reptiles evolved and returned to the sea, or glided through the air on motionless wings. The Sundance Formation, deposited in the northern Rocky Mountains about the middle of the Mesozoic Era, is noted for the masses of bullet-shaped squid shells found in it. Water, land, and air were full of life.
In the Middle Mesozoic, the trough drained and much of it became an area of swamp and tropical forest extending from Montana to southern Utah. This was the domain of our largest dinosaurs, giant reptiles whose bones were preserved in impressive numbers. Como Bluff and Bone Cabin, Wyoming; Morrison, Colorado; and Dinosaur National Monument and Cleveland, Utah, are the sites of quarries where many fine specimens of Apatosaurus, Diplodocus, Stegosaurus, and Allosaurus have been collected. These caches of bones have made our large museums showplaces known throughout the world.
During the last 65 million years of the Mesozoic, the trough fluctuated up and down. During much of that time a broad, shallow sea covered central North America from the Gulf of Mexico to the Arctic Ocean. The sea was filled with fish like the giant Portheus (3.5 meters/11.5 feet long), with squid-like animals floating about in elaborate chambered shells, and with reptiles which had gone back to the water. Where there was a broad coastal plain, it swarmed with dinosaurs, the ones that make Lance Creek, Wyoming and Hell Creek, Montana famous collecting grounds for museum field parties. Yet at the end of the era the trough was a seaway again, and in the Agate area a final blanket of black mud, the Pierre Shale, was deposited in the sea.
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.
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.