Agate Fossil Beds National Monument

Agate Fossil Beds National Monument

Geology

Rivers and Streams

Among the natural communities of plants and animals existing in the high plains ecosystem, none is as lush or rich in animal life as the riparian community. Riparian zones are the lush belts of vegetation found along rivers and wetlands. The Niobrara flows through the four mile length of Agate, meandering and curving to create 11 miles of river bank. These river banks play a vital role in the plant and animal communities as well as the water quality of the river.

The reach of the Niobrara river within the park is unconfined, meaning it meanders or bends throughout a wide flood plain and changes course relatively often. The flood plain of the Niobrara is a quarter-mile wide in places. This creates an interesting landscape of river twists and turns and oxbow ponds and sloughs filled with cattails, irises, reeds and water loving plants and a great environment for a diverse variety of wildlife. Oxbow ponds are the horseshoe shaped ponds that are the result of a very sharp bend being cut off from the river. Along the river banks, reeds and cattails grow tall and hang over the river providing shade to keep the water cool and reduce the amount of evaporation during hot days.

Though the Niobrara River is the only continuously flowing water in the park there are several ephemeral tributaries to the river. Tributaries are streams that run into and contribute water to a river or larger stream. Ephemeral streams are streams that only flow after a major rain event and can be identified by dry channels in depressions between hills. These are the types of areas in which flash floods can occur that cause death and destruction of property. Though the streams rarely flow and do not flow for very long, they are erosive, sometimes carrying large amounts of sediment to the river. Sediment, soil and sand material that is suspended in the flow of the water deposits itself when the flow slows down, shrinks in volume, or spreads over a greater area.

A major source of water for the Niobrara in and around the park is ground water, water that is stored in and released from underground aquifers and reservoirs. These large reservoirs can be refilled by rainfall if water can infiltrate that far into the ground. Groundwater naturally comes to the surface through seeps and springs but is also brought up by wells. A spring is a place where groundwater flows naturally from the soil or rock formation onto the land surface or into a body of surface water. Seeps are similar but are usually less defined and do not flow as springs do; here they are characterized by creating a marshy area near the river. There is little specific information known about Agate’s groundwater but park staff are currently involved in projects to learn more to be able to better manage groundwater use.

The river running through the park creates a special prairie habitat that is not seen in drier areas. The meandering river creates about 200 acres of riparian area which is the greener, wetter areas near a stream where specialized plants grow. Plants such as willows, reeds, sedges and wild licorice thrive in the riparian areas. Willows and other water-loving shrubs and trees provide browse for white-tail deer. The riparian area also provides home for salamanders and frogs that need more moisture than the dry uplands provide. Park staff carefully monitor and manage the riparian area to restore it to its natural condition by controlling non-native plant species such as Canada Thistle.
 
Quality of water is enhanced by riparian areas as the two are interdependent. Trees and shrubs shade the water, reducing evaporation and keeping water cooler, which in turn is beneficial to the aquatic life. Overall, water quality at Agate is good, having low levels of nitrates and phosphates that are monitored through yearly sampling. Agate also monitors water quality by observing macroinvertebrates (insects and other arthropods) that live in the water. Macroinvertebrates are sensitive to pollution and extreme flow fluctuation, so a decrease in species diversity can indicate a problem with the water quality. The Niobrara river is not extremely large at Agate, generally only about 2.5 meters (8.25 feet) wide and flows at around eleven cubic feet per second.

Geologic Formations

Agate Fossil Beds National Monument is a small park in the northwest corner of Nebraska, with only 2,700 acres of federally managed land included in the 3050 acres within the park boundary. The park takes its name from thin lenses of agate (White River Silicate Group) in the area, which range in color from amber to light gray. Miocene-age rocks are exposed in the park in the bluffs above the Niobrara River wetlands and contain an excellent fossil record.
 
The Rocky Mountains were uplifted in many pulses of deformation between 70 to 40 million years ago. Sediments from the uplifting mountains were initially deposited near the mountains and then later transported by rivers eastward onto what eventually became the Great Plains. This river-borne silt was accompanied by wind-borne volcanic ash from eastern Nevada and western Utah, and the fine grained ash rich sediments were deposited in vast sheets called the White River beds. The earliest documented bedrock at Agate dates to the Oligocene era, 34 million years ago, but most of Agate’s Oligocene deposits are well buried beneath later Miocene deposits. Oligocene-era beds are well exposed at Badlands National Park, 130 miles northeast of Agate.
 
During the early Miocene era, beginning about 25 million years ago, streams in the area that now includes Agate Fossil Beds National Monument shifted and cut down to produce valleys. These valleys were later filled in with sediments as the Great Plains continued to build up or aggrade. Aggradation resulted in the formation of wide savannas during the Miocene, those savannas being dotted with small water holes and the whole landscape populated with herds of animals (e.g., chalicotheres, rhinoceroses, entelodonts, beardogs, land beavers, camels, horses, pocket gophers). Ongoing research is documenting the grass species present on the ancient savanna. A major drought occurred in the Agate area during the Early Miocene. It is believed that when many of the drought-stricken and exhausted animals came to the remaining water holes in an effort to survive, the animals collapsed and died in and around the water. As the muddy water dried, the fossil beds were formed. Agate’s older fossil layer is about 21 million years old and covered by a layer of ash, and its younger bed is 20 million years old. These layers are in what are now called the Harrison, Anderson Ranch, and Marsland Formations.

In the last five million years the High Plains have continued to uplift to their current elevation of about 4,400 feet a.m.s.l. and the savannas have changed to the grasslands of today. During the uplifting process rivers and streams have meandered across the plains and eroded the older deposits, forming the bluffs and valleys that we see today.
 
The modern Niobrara valley at Agate is a complex array of Late Pleistocene and Holocene geomorphology, stratigraphy, and paleosols reflecting significant climate variations over the past 12,000-15,000 years. Current research in the park is providing radiocarbon dates for the middle to late Holocene materials, documenting thousand year or less fluctuations between warm and cooler climates and varying amounts of annual moisture.
 
The agate that gives the park its name is found in a thin band along ash deposits just above the Miocene bone beds, and ranges in color from amber to light gray. This stone is a variety of quartz (silicon dioxide) called chalcedony. Iron, manganese, and/or aluminum inclusions in the original silica deposits give the agate different colors in various locations, and often form dendritic “moss” patterns in the material.
 

Natural Features & Ecosystems

Agate Fossil Beds National Monument is a small park in the northwest corner of Nebraska, with only 2,700 acres of federally managed land included in the 3050 acres within the park boundary. The park takes its name from thin lenses of agate (White River Silicate Group) in the area, which range in color from amber to light gray. Miocene-age rocks are exposed in the park in the bluffs above the Niobrara River wetlands and contain an excellent fossil record.

Much of the geologic history is recorded in the natural features found at the park today.