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

December 15. The Badlands of South Dakota

First today, I think I have been remiss in not pointing you to a great paleontology podcast, Palaeocast. All things fossils. The most recent episode is about ceratopsians, the group of dinosaurs that included Triceratops. Check it out. And speaking of ceratopsians, the oldest one known was just announced last week. It was a raven-sized little thing that lived in Montana about 105 million years ago. LINK • Link2

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Yesterday I promised that today we’d talk about some of the Oligocene debris that helped bury the Rocky Mountains. We’ll go to South Dakota to do that. 

You can find the sediments that were shed off the mountains up in the mountains, but because of the Miocene and later erosion, a lot of that stuff is gone, eroded and washed onto the plains or down the Missouri River. In South Dakota’s Badlands, a nice pile of the Oligocene sediment has been preserved.

It’s eroded, to be sure, into fanciful angular shapes – that’s why the place is called Badlands, the word for a region of exposed rock, often fairly soft rock, that has eroded into steep slopes separated by an intricate meshwork of canyons and ravines. They tend to exist in arid country today, where erosion rates are relatively low and the erosion that does happen is usually catastrophic, as in flash floods. The rocks eroded tend to be relatively young, because younger rocks are often more poorly consolidated than older rocks, but there’s nothing sacred about that. 

Exposure of Oligocene Brule Formation in South Dakota Badlands (National Park Service photo)

The White River Badlands of South Dakota preserve a thick wedge of the Oligocene sediments that eroded off the nearby Black Hills, a Laramide uplift. The sediment was carried to the east by rivers of various sizes, some that flowed strongly enough to carry gravels, and some that were parts of shallow braided streams carrying sand and silt. Flood plains saw extensive mud deposits. Over time, from the late Eocene into the early Oligocene, the deposits built up a pile that totaled more than 100 meters thick – 300 feet – in the Badlands National Park area. Across the eastern Rockies region, equivalents of these Oligocene deposits reach thicknesses of 700 feet or more.

The first package of rocks that makes up the South Dakota Badlands is called the Chadron Formation, gravels, sands, silts, muds. As the sediment was being laid down, this area was certainly not badland, but harbored abundant life. The Chadron is famous for its titanotheres or brontotheres. The nature of the cement and porosity in the Chadron formation is such that the rocks weather into smooth, rounded hillocks.

River and flood deposition wasn’t continuous over the 11 million years of the Oligocene, and in the Badlands of South Dakota, a lot of the reddish layers interbedded with the white rocks are paleosols – ancient soil horizons, red because iron was concentrated in them. There were also occasional lakes in which limestones formed, and one of them marks the break between the Chadron Formation and the overlying, younger Brule Formation. It’s really quite similar to the Chadron, but the cement in it includes considerable volcanic ash and calcite, so it holds together well and tends to form steep slopes and knife-edge ridges. The Brule is famous for turtle fossils and oreodonts. Oreodonts had teeth that were suited for eating oreo cookies – well, no, not really – oreodonts were fox-sized mammals that are not very closely related to any modern animals, but camels and pigs are not too far from oreodonts on the evolutionary tree. Technically they are called artiodactyls, even-toed herbivores. Lots of their fossils have been found in the Badlands rocks.

There’s a distinctive volcanic ash bed at the top of the Brule Formation, erupted about 30 million years ago, and it was followed by more deposition in the mid- to late Oligocene of the Arikaree formation, generally the highest unit in the Badlands National Park area.

Try to envision a vast sheet of these Oligocene strata laid out across the intermountain basins of the Rockies as well as much of the western Great Plains. There were rivers, certainly, and both erosion and deposition were both still going on, but at a rate that was nothing like it had been in Eocene and Oliogene time. The region was relatively level for millions of years – close to 25 million years. The modern badlands topography is a very recent development, within the past million years and more like just 500,000 years ago. Significant rainfall and snowmelt associated with the glacial period provided the water that cut aggressively into the Oligocene sediments, creating the modern badlands. They continue to erode and develop today.

—Richard I. Gibson

Links & References
Chadron, Brule, Arikaree
Arikaree 
Reference: Geology of National Parks, by Ann Harris and Esther Tuttle, Kendall/Hunt Publishing.