Discover Nebraska and Explore the Geodiversity and fossils of the Agate Fossil Beds National Monument
- Wayne Munday
- Apr 2
- 5 min read
Sip back and discover the Mid Western State of Nebraska and explore the geodiversity and fossils of the Agate Fossil Beds National Monument. Covering over 13 Km2 these fossils beds preserve a rich geological history and fossil record. The Monument is located in the High Plains along the Niobrara River and features the fossiliferous Harrison and Anderson Ranch Formations from the Arikaree Group dated to the Miocene Epoch some 23.03 - 5.33 million years ago. Made of fine-grained sandstone and siltstone with fluvial, aeolian, and volcanic sediments they tell a story of an environment that experienced severe drought where waterholes dried up and herds of animals perished en masse. These events preserved one of the world’s most significant Miocene-era fossil deposits discovered in the early 1900s at Carnegie Hill and University Hill collectively called Fossil Hills. These fossil sites have exposed some of the most complete skeletons of Miocene mammals that can be seen at the monument's visitor centre and museum. They include the pony-sized rhinoceros Menoceras, the large clawed herbivore Moropus, and the scavenging entelodont known as the Daeodon which crushed bones with its powerful jaws. Fossils of early camels like Stenomylus and the small beaver called Paleocastor that which dug corkscrew-shaped burrows known as Daemonelix add to the Monument's fossil record. The discovery of carnivore dens at Beardog Hill containing the remains of Daphoenodon and other predatory species represents the earliest known evidence of denning behaviour in large mammal carnivores. Just as the Alcoota Station Fossil Site in the Northern Territory in Australia the Agate Fossil Beds National Monument of Nebraska is also an invaluable site for understanding Miocene mammals and the impact of climate change on ancient ecosystems.
The Agate Fossil Beds National Monument is roughly a 4 hour drive north on the I-25 from Denver or a 3 hour journey south from Rapid City on the Heartland Expressway. Visiting the Agate Fossil Beds National Monument is a matter of planning ahead and checking out the handpicked AAA Diamond recommendations and reviews for places to stay.
The Agate Fossil Beds National Monument is situated on an elevated tableland formed by the Arikaree Group which extends from the Pine Ridge escarpment to the North Platte River. The geology of the Agate Fossil Beds belongs to the Great Plains Physiographic Province and shares its history with a broader region beyond park boundaries covering over 725,000Km2 of flat high plains stretching eastward from the Rocky Mountains. The underlying the region are horizontal layers of sandstone, shale, limestone, conglomerate, and lignite. The High Plains in the central Great Plains are overlaid with alluvial deposits transported from the Rockies while the northern lands contains glacial till above Mesozoic bedrock.
In addition to its impressive fossil record the Agate Fossil Beds National Monument is also known for its agates that occur in a thin band along ash deposits just above the Miocene bonebeds. These semi-precious gemstones range from green to amber and are protected under federal law prohibiting their collection within the Monument.
Agate is a cryptocrystalline form of quartz known for its colourful banding formed from both primarily as secondary deposits of volcanic rocks where silica-rich groundwater over time infills minerals into the cavities or vesicles of rocks and organic matter creating the characteristic banded patterns.
The local agates are primarily known as Moss Agate a type of chalcedony characterised by its distinctive green moss-like inclusions within a milky white or translucent background. The colour hues within Moss agates is determined by trace elements such as iron, manganese, chrome and aluminium.
The state fossil of Nebraska is the Columbian mammoth (Mammuthus columbi) adopted in 1967 after the discovery in 1922 of “Archie” now on display at the University of Nebraska State Museum. Archie roamed Nebraska during the ice age between 2 million - 10,000 years ago along with rhinoceros, camels, bison and sabre-toothed cats.
The Miocene Epoch lasted between 23.03 - 5.33 million years ago and was characterised by warmer climates with periods of global cooling and ice sheet growth that led to the global expansion of grasslands and the evolution of many modern mammal species. The Agate Fossil Beds are primarily within the Harrison Formation and the informally named Anderson Ranch Formation that were unaffected by the escaped Pleistocene glaciation therefore preserving the sediments.
Between 19 - 20 million years ago a severe drought struck the western Nebraska plains resulting in mass mortality among animals dependent on shrinking water sources. The fossilised remains of these animals were preserved by being covered in wind and stream derived sediments. Before the drought the region's climate was moderate and supported vast grasslands that resembled an African savanna.
As the climate became dryer the Rocky Mountain uplift further restricted moisture laden air from reaching the Nebraska plains increasing the frequency of drought. As water became scarce animals such as the horse like Moropus (meaning "slow foot") a large perissodactyl ("odd-toed") mammal with claws instead of hoofs and the small twin horned rhino Menoceras (meaning "Crescent Horns") would to congregate at dwindling waterholes where they would eventually perish.
The primary bonebeds of Carnegie Hill and University Hill are collectively known as Fossil Hills were the first major concentrated deposits of Tertiary Age mammals discovered in North America. This area has yielded the preserved skeletons of small gazelle like camelids called Stenomylus as well as formidably large entelodonts called Daeodon sometimes referred to as Hell Pigs with bone-crushing teeth.
Another notable mammal was the Palaeocastor one of the earliest known beavers but rather being aquatic and building dams they were terrestrial and burrowed extensive underground networks similar to prairie dog towns. These small mammals would excavate and live in 2.5 m long corkscrew shaped burrows called Daemonelix meaning “Devil's Helix” or “Devil's Corkscrew". The Daemonelix would offer protection from predators and the environment. Fossil birds found at the Agate Fossil Beds National Monument include the discovery of the first recorded Miocene crane-like species known as Gruiformes.
Carnivore fossils at the Agate Fossil Beds National Monument are relatively rare so the discovery of Daphoenodon and Temnocyon a large wolf-like beardog that preyed on Moropus, Menoceras and Stenomylus provides an insight in to the ecosystem at the time. Beardogs have also revealed the earliest known evidence of denning behaviour in large mammalian carnivores.
