Agate Fossil Beds National Monument - Geology

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 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 agates that give the park its name are found in a thin band along ash deposits just above the Miocene bone beds, and range 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.