Why does Badlands National Park look the way it looks? What created those crumbly, fantastic, rock formations, some of them very colorfully layered? Two words sum it all up: deposition and erosion (water, wind, ice wedging/heaving).

An afternoon view at Panorama Point, Badlands National Park / Rebecca Latson

Badlands National Park staff explain the park’s geology:

Deposition

This is the process of rocks gradually building up. Over the course of millions of years, the layered rocks of the Badlands were slowly stacked on top of each other like a layer cake. These rocks were deposited by a number of natural forces which range from shallow inland seas to rivers to wind. Deposition began about 75 million years ago with the formation of the Pierre Shale, the base of the geologic formations in the park. Deposition ended about 28 million years ago with the Sharps Formation, the uppermost unit of Badlands stratigraphy.

Erosion

This is the process of rocks gradually wearing away. The Badlands began eroding about 500,000 years ago as the Cheyenne and White Rivers carved their way through the landscape. They are the reason for the narrow channels, canyons, and rugged peaks of the Badlands which we see today. And the Badlands are still eroding – it is estimated that the Badlands erode at the rate of one inch per year, which is a rapid rate for rocks. In contrast, the granite of the Black Hills, to the west of Badlands National Park, erodes at the rate of one inch per 10,000 years. Scientists estimate that in the next 500,000 years, the Badlands will have eroded completely!

Badlands’ Rock Layers

The layers of the Badlands correspond with different moments in geologic time. They start with the oldest layer at the bottom, then move upwards in space and time towards the youngest layer, which sits on top of the formations. To unlock the stories of the park’s different layers, it is often easiest to start where most good stories do: at the beginning.

The Pierre Shale, which forms the bottommost layer of the park’s geology, was deposited 75-69 million years ago. It was laid down by a shallow inland sea known as the Western Interior Seaway. This seaway stretched from the Gulf of Mexico to the Arctic Ocean, covering most of the modern Great Plains in shallow, warm water. This sea contained ancient marine life which would have existed at the same time as dinosaurs, but because dinosaurs couldn’t swim, none of them are found in the Pierre Shale or in the Badlands as a whole. Instead, the creatures that we find from this time include shelled cephalopods like ammonites and baculites alongside mosasaur, a giant marine reptile that could measure over 50 feet long.

On top of the Pierre Shale sit the Yellow Mounds, which are just an altered version of the Pierre Shale despite their striking differences in appearance. After the Western Interior Seaway drained North into the Arctic Ocean, the leftover shales weathered into soils. Those soils are now preserved as the Yellow Mounds, which are what geologists call a paleosol. Paleosols are ancient fossilized soils preserved in the rock record, and they often appear as brightly colored layers like the Yellow Mounds, which gets its mustardy color from a mineral called Goethite.

The Chadron Formation, consisting largely of light gray claystone beds, was deposited about 37-34 million years ago across an ancient floodplain. The environment of the Chadron Formation would have been hot and wet, like Everglades National Park is today. It was home to creatures that we associate with these modern environments like ancient alligators, as well as some animals that no longer exist, like the massive Brontothere.

The Brule Formation, deposited 34-30 million years ago, represents a cooler and drier time in geologic history. The hot, wet vegetated floodplains of the Chadron Formation now transformed into an open savannah, where occasional river channels would cut through the plains. Many grazers, like the oreodonts commonly found in Badlands National Park, made good use of eating the grasses and plants which grew here. Consequently, there were also predators who made good use of the grazers, like nimravid, a cat-like animal with saber teeth.

Above the Brule lies the Sharps Formation, the youngest geologic formation of the park at 30-28 million years old. The base of the Sharps Formation is the Rockyford Ash, a volcanic tuff formed from ash that came from eruptions in the Great Basin, where Utah and Nevada are today. Much of the Sharps Formation is characterized by sandstone river channels as the climate continued to cool and dry.

Badlands Fossils

Badlands National Park deposits contain one of the world’s richest fossil beds, with finds of more than 250 fossil vertebrate species, including both herbivores and carnivores. Ancient horses and rhinos once roamed here, as did cat-like mammals and tiny, hornless deer. Even gigantic marine reptiles called mosasaurs once swam the shallow inland sea located in what is now arid landscape in and around Badlands.

This national park has undergone several environmental changes, from warm, shallow inland sea to lush forest to sub-tropical open woodlands and grasslands. Mammals, vegetation, and even insects left behind evidence of their lives during the late Eocene (56-33.9 million years) and early Oligocene (33.9-23 million years) epochs. Wander anywhere in Badlands and you might just discover a fossil hackberry seed, dung beetle ball, or something perhaps even larger. If you do find something, remember to leave it where you found it and report your find to a park ranger.

Up close to a fossil dung beetle ball about 1/2 inch (1.27 cm) in diameter, Badlands National Park / Rebecca Latson

You can read about Traveler contributing photographer and writer Rebecca Latson’s morning spent fossil hunting with two of the park’s rangers.

Note: It's illegal to collect fossils in Badlands National Park, or any other park, for that matter, without a permit. If you happen to find a fossil in the park, here’s what you should do:

1. Leave the fossil exactly where it was found
2. Take photos and get GPS coordinates
3. Report it in person or email your finds to the park

Why is this important? Why can’t you just take that one little fossil with you? Every fossil discovered provides new information about Badlands geology and paleontology. To take a fossil removes that information from the record. To even move the fossil from where you found it also does the same thing and might cause a misinterpretation about the environment in which that creature once lived. Do the right thing if you find a fossil out there.

Here are a couple of few fun facts about this park’s geology with which you can impress family, friends, and colleagues.

The Badlands formations have a lifespan of approximately one million years. They erode at about one inch a year (so come and visit while they are still there).

While exploring Badlands National Park, you may see isolated columns of rocks and soil topped with grass and other vegetation. Those are sod tables. According to park staff:

The soil and grass of sod tables protect the rock below from erosion by soaking up rain during intense storms, while the exposed rock around sod tables cannot absorb water and quickly washes away. The different rates of erosion make it appear like sod tables are springing up out of the Badlands, when they are really wearing away more slowly than the rock around them.

A sod table at Badlands National Park / Rebecca Latson

For more detailed information about Badlands National Park’s geology and paleontology, you find more resources here and can download a PDF of Badlands National Park’s Geologic Resource Evaluation Report.

The colorful rock layers of Badlands National Park / Rebecca Latson