Episode 380 Makoshika State Park, Montana

The badlands at Makoshika State Park are in Montana, and so am I, but Montana’s big. The park is just outside of Glendive, almost at the eastern border of the state. It took me and my friend about seven hours to get there from Butte, but it was worth it.

Eastern Montana is quite different from western Montana, geologically. The west is broken, thrusted, pulled apart, and intruded, while the east is – with exceptions – largely more or less flat-lying, relatively undisturbed sedimentary rocks piled upon each other, with the oldest at considerable depth and the younger rocks at or near the surface.

At Makoshika, the rocks are from the last part of the Cretaceous Period and the early part of the Cenozoic Era, what we used to call the Tertiary. So the famous K-T boundary, the extinction point for the dinosaurs and a lot more, is in these rocks. Nowadays we call the earliest period of the Cenozoic the Paleogene, and if you’re really interested in the story of the names of the Cenozoic periods, check the podcast episodes from early December 2014.

Erosional features in the Fort Union Formation at Makoshika.
Photo by Dick Gibson.

There’s actually a coal bed near the K-T boundary at Makoshika, but the coolest thing about the rocks in my opinion is their weird and fanciful erosional shapes. The younger Tertiary rocks, the Fort Union Formation, are alternating sandstones and siltstones and shales, with a wide range of cementation, so the rocks respond to erosion very differently. Some erode easily while others stand out in rugged relief. Erosion creates pedestals and balanced rocks, natural bridges and sharp ridges, as well as softer slopes incised by rivulets large and small when it rains.

The sediments that came to make up the Fort Union Formation were deposited in rivers, lakes, and swamps around 65 to 60 million years ago, Paleocene time. There’s a bit more about the Fort Union Formation in the episode for December 4, 2014, which you can find on the blog, history of the earth calendar.blogspot.com, together with transcripts for this and older episodes. The Fort Union Formation’s rocks included many highly vegetated swamps which over time dehydrated and were compressed into thick coal beds, among the most important coal producers in the United States today. U.S. demand for coal is decreasing, as cheaper, more abundant fuels for generating electricity, most notably natural gas, are used more, but in 2017 burning coal was still the source for about 30% of US electricity. Just ten years ago, that proportion was close to 50%. Coal mines in the Fort Union Formation in Wyoming, south of Makoshika, produce about 40% of all US coal.

The Cretaceous rocks underlying the Fort Union Formation are similar kinds of rocks, but they contain dinosaur bones, especially further west, and they’re called the Hell Creek Formation. Triceratops and Tyrannosaur fossils have been found in the Cretaceous rocks at Makoshika, along with a nearly complete Thescelosaurus, a small, 10-foot-long probable herbivore discovered in 1997.

The name Makoshika is from the Lakota words maco sica, meaning 'bad land' or 'land of bad spirits,' but despite that I find the land remarkably beautiful. One of the reasons the landscape displays such spectacular erosional features is the fact that this area is gently uplifted. The Cedar Creek Anticline, a long, linear fold above a deep-seated fault trending northwest through this part of southeastern Montana, reaches its northern end near Makoshika.

You’d never think of this as a mountain uplift, but older, more erodible rocks of the Hell Creek Formation have been warped to the surface here, where wind and water have sculpted them for many thousands of years. Features like natural bridges and balanced rocks and delicately carved monuments are ephemeral in geologic terms, usually surviving for hundreds or a few thousand years at most. But they are continually replaced by other features, until the rocks with their variable resistance are all gone, washed down the rivers. That happens eventually, even in semi-arid country like eastern Montana.

—Richard I. Gibson