Badlands Geology

Every butte in the park tells the same story, one layer at a time.

Swamp forests, about 60 million years ago

With the dinosaurs gone and the Rocky Mountains rising to the west, rivers carried sand and mud east into a low, wet country of channels, lakes and swamps. That sediment became the yellowish Bullion Creek Formation and the gray-brown Sentinel Butte Formation above it. Conifers, probably relatives of today's bald cypress and redwoods, grew with their roots in standing water. Floods buried their stumps, which slowly turned to stone, and fallen plants piled up as peat that became lignite coal.

Ash from distant volcanoes

Eruptions more than 250 miles away dusted the region with ash. Washed into a lake, it became the blue-gray bentonite clay, with a white core of ash, that bands the North Unit. Bentonite swells when wet, cracks into a crunchy "popcorn" crust when dry, and turns slick after rain.

Glaciers turn the river

The Little Missouri once flowed north toward Hudson Bay. During the Ice Age, continental glaciers blocked that route, at least 640,000 years ago, and forced the river east onto a shorter, steeper path. A steeper river cuts faster, and it began slicing down through the soft layers. Its old floors survive as gravel benches 160 to 270 feet above the water, and granite boulders the ice left behind still sit near Sperati Point in the North Unit.

Fire in the ground

As erosion exposed coal seams to the air, some caught fire and smoldered for years. The heat baked the clay above into hard red clinker, called scoria locally though it is not volcanic. Because clinker resists erosion, it caps ridges across the park. The most recent large burn, near Buck Hill in the South Unit, smoldered from 1951 to 1977; Coal Vein Trail walks through it.

Still being made

The badlands are not finished. Hillsides slump after wet springs, some North Unit blocks having dropped about 120 feet in one piece, and the sand on today's valley floors is less than 500 years old.

These pullouts and trails put the geology front and center. Each link opens the stop on the geology map.

South Unit (Medora)

• Skyline Vista: a gravel bench left by the Little Missouri during the Ice Age, about 260 feet above the river.
• Scoria Point Overlook: yellowish Bullion Creek layers below, a red clinker band, gray-brown Sentinel Butte layers above.
• Coal Vein Trail: ground that slumped as a coal seam burned beneath it from 1951 to 1977.
• Painted Canyon Visitor Center, off I-94 about seven miles east of Medora: the classic panorama of gray-brown layers, black coal seams and red clinker, with petrified stumps in the slopes nearby.
• Petrified Forest Plateau: upright petrified stumps up to about six feet tall. The National Park Service says the park has the third-highest concentration of petrified wood in the United States. A long hike on the park's west side.

North Unit (near Watford City)

• Slump Block Pullout: giant blocks of layered rock tilted back into the hillside.
• Cannonball Concretions Pullout: round sandstone concretions weathering out of softer rock.
• Oxbow Overlook: the blue bentonite band and a huge landslide below the river bend the glaciers helped create.

Collecting rocks, fossils or petrified wood is illegal in the park. For road and trail status, check park conditions. Plan the rest of your day with the national park guide, hiking near Medora and Wildlife Watch.