Geology at Agate Fossil Beds

 

An ash deposit exposed in a white cliff north of the road, 1 mile west of the Visitor Center.

NPS photo

Types of Rock Found at Agate

The kinds of rocks that you’ll find here at Agate Fossil Beds National Monument are sandstone and limestone with thin lenses of agate. There are two formations exposed at the monument. A rock formation is a group of rocks that share lithology (physical characteristics such as composition, grain size, fossil content, and color) and share a particular place in the stratigraphic column.

The principle of superposition in geology states that in a sequence of horizontal sedimentary strata, the oldest layers are at the bottom, and the youngest layers are at the top. This means that the upper Anderson Ranch formation is younger than the underlying Harrison formation. These two formations are separated by an “unconformity,” or an ancient, buried erosional surface that represents a break between periods of deposition. This period of net erosion over deposition lasted approximately two million years. There is no angular difference between the sediment layers of the younger Anderson Ranch Formation and the older Harrison Formation, so we call this kind of unconformity a “disconformity.”

The Environment in the Miocene

The sediments that are exposed in these formations are remarkably unaltered, which points to a dry/arid depositional paleoclimate. During the lifetimes of the early-Miocene mammal fossils that were uncovered here, the climate in this region became drier and a savannah of mixed trees, shrubs, and grass became a grassland. Streams of the Anderson Ranch Formation cut channels in the underlying Harrison Formation that became waterholes. These waterholes attracted animals, and many died during periods of drought. The rapid deposition of ash and sand provided a good environment for fossil preservation; the remains of animals were covered quickly protecting them from scavengers and decomposers, and the ash produced an alkaline chemical environment which stabilized the bones.

 

Black arrow points to the bone bed layer at the bottom of the Fossil Hills at Carnegie Hill.

The Bonebed Location

The level of the bonebed is near the base of the Fossil Hills. The top of the former waterhole is marked by white, limey mudstone and thinly laminated brownish sediments. Above these, evidence of paleosol (fossilized soil) is visible through silica-cemented root structures.

Determining the Age of the Mammals

The volcanic ash present in the sandstones at Agate Fossil Beds provides a reliable way to date the rocks. Zircon crystal dating measures the ratio of uranium-238 to lead; uranium-238 decays to lead at a constant rate. Using this method, scientists have determined that the sandstone directly beneath the bonebed is 22.9 million years old.

 

Exposed fossilized sand dunes visible from the Daemonelix Trail.

NPS photo

The Harrison Formation

Gray “salt-and-pepper" colored ash-rich sandstone with fossilized features such as sand dunes and spiral-shaped burrows called Daemonelices or “devil’s corkscrews,” which are displayed on our Daemonelix Trail and in the Visitor Center.

 

A diagram showing how slip-face surfaces form, with sediment accumulated on the windward “stoss” side, falling in layers down the downstream “lee” side.

The sand dune deposits formed from wind-blown sediments. The sloping lines visible in the rock face pictured below are “slip-face” surfaces, formed when grains accumulate on the windward side of a dune until they reach the angle of repose and cascade down the downstream side of the dune. The angle of repose is the steepest angle relative to the horizontal plane on which a granular material can be piled without slumping. The direction of the slip-face surfaces tells us that the prevailing wind direction was from the northwest at the time the dunes were formed.

 

Mixed grass covers slopes of tan sandstone buttes topped with a white limestone caprock which is partially covered in orange lichen.

The Anderson Ranch Formation

It's also called the Upper Harrison Formation. Tan or light beige colored stream deposits and paleosol (fossilized ancient soil) layers, topped with a silcrete layer and limestone lake deposits. The silcrete layer underneath the limestone caprock is a paleosol surface hardened by dissolved silica that formed a mineral matrix binding the paleosol sediments together. Due to the impervious nature of silcrete and therefor lack of drainage, the lithification of this layer allowed for the development of a broad, shallow fresh-water lake that is evidenced by the limestone layer that caps many of the buttes in this region.

 

The local variety of the chalcedony mineral, moss agate. It is a milky to translucent mineral with waxy luster and dark, fern-like inclusions.

Where Did the Sediments Come From?

Seasonal streams carried sand and silt from the uplifting Rocky Mountains to the west. Wind-blown ash was deposited at Agate, primarily from repeated eruptions of volcanoes in southern Utah and Nevada. Volcanic ash in the rocks is gradually broken down, and groundwater concentrates the silica into air pockets within the sedimentary rocks which are then filled with cryptocrystalline quartz, resulting in thin lenses of agate within these formations.

Groundwater moving through the sediments carried the dissolved mineral calcite (calcium carbonate), some of which precipitated and bound the grains of sand, silt, and ash together. Most of the rock in the Harrison and Anderson Ranch Formations is not well cemented and erodes easily, but in some places groundwater flow has concentrated the calcite cement, resulting in hard concretions that jut out of the hillsides. The hard white caprock of limestone topping many buttes and cliffs along the Niobrara is composed mostly of calcite.

The Type of Agate Found Here

The type of agate that was once found here is moss agate, a variety of chalcedony which lacks the characteristic banding of other agates, containing iron and manganese oxides which result in dark “dendritic” or fern-like inclusions in the transparent to milky mineral. We have some samples at the front desk in the Visitor Center, but you’re unlikely to find any along our trails these days, until erosion exposes more. If you are lucky enough to find some agate within the monument grounds, we ask that you leave it there so that other visitors may enjoy the natural beauty that Agate Fossil Beds offers.

 

A “hoodoo,” or thin sandstone spire of Anderson Ranch sandstone.

Erosional Features

By the late 1800s, erosion by the Niobrara River and its tributaries had exposed Miocene aged mammal fossils from the bonebed within the Fossil Hills, which paved the way for the excavations that occurred here in the decades to follow. Fed by a wetter climate, the Niobrara was a much larger stream thousands of years ago when it began cutting through the Harrison Formation. As the valley eroded, the river carved buttes such as Carnegie and University Hills. In other places, only thin spires, or “hoodos,” remain.

 

A circular window found at the east boundary of the park.

At the east end of the park, a circular window is visible in the north cliffs. Windows such as this form when groundwater weakens the cement matrix binding grains together within a thin “fin” of rock. Windblown sediments erode the face of the fin, eventually boring a hole through the rock.