The geography of the present NiobraraRiver valley is closely tied to the Sandhills of Nebraska. Geologists estimate that deposition began about 98 million years ago, when marine seas once covered the area. The seas were followed by alluvial stream deposits, and finally windborne (Aeolian) deposits and volcanic periods. The Sandhills likely formed during multiple episodes during the last 10,000 years, with the most active period forming the largest dunes some 5-8,000 years ago. The ancientNiobraraRiver was likely half as deep and two to three times wider than today’s. As the river flowed, it cut into the earth, narrowing and deepening, and eventually creating the high bluffs that provide so much beauty along the river today.
As you float down the Niobrara, look closely and you may see four primary geological layers on your journey.
Starting with the topmost—and therefore youngest—layer beneath the sands, you may see the Ash Hollow formation. This sandstone layer is most easily seen high on the bluffs of the river in the section flowing through the Fort Niobrara National Wildlife Refuge. Look for rocky outcrops on the north (river left) side. Made primarily of volcanic ash and sand, this layer holds fossilized tortoise, horse, rhinoceros, and other animal bones.
Beneath the Ash Hollow is the Valentine Formation. This loosely-consolidated sandstone crumbles easily, but holds the primary source of the Niobrara River in this area: the Ogallala, or High Plains, aquifer. About 70% of the water in the river comes directly from groundwater. You can see this through seeps and waterfalls, which occur where the bottom of the Valentine meets the top of the Rosebud Formation. Look for water trickling down the bluff walls, particularly on the south (river right) side of the river. The Niobrara NSR is known for the fossils found in the river valley, and the Valentine holds an incredible diversity of wildlife. Alligators, hedgehogs, mastodons, and odd-looking mammals called "oreodonts" are just a few.
A finer-grained siltstone, the reddish-hued Rosebud forms the "floor" of the Ogallala aquifer. Water cannot seep into the rock as easily as it can in the Valentine, and a stream cannot cut down through it as quickly. There are more than 200 spring-fed waterfalls found along the scenic portion of the river, and most are "held up" by the Rosebud formation. You may have to walk up a short creek to find one, but look for waterfalls on the south (river right) side of the river. Fossils are somewhat rare within the Rosebud.
The "base" of the Niobrara NSR is Pierre Shale, a gray to black rock that is difficult to see in the western half of the river. Those who visit the eastern half of the river, which is much more difficult to float, may see dark bluffs of the rock. It is also visible if you drive to the intersection of Highway 7 and the river. Pierre Shale holds the fossils of marine reptiles, sharks, and bivalves.
Please note that the collection of fossils within the Fort Niobrara National Wildlife Refuge, Smith Falls State Park, and on private land without permission of the landowner is prohibited. Please enjoy the fossils at visitor centers and museums, rather than collecting them yourself.