While it is interesting to see the wildlife of Badlands National Park, the real star here is the geology. The soils that can be seen in the exposed cliffs were laid down for tens of millions of years or more. Both the colors and shapes of the badlands are evidence of the geological history of this part of South Dakota.
At the bottom of the deep ravines, a dark grey material is the oldest of the visible strata. This rock, says signs in the park, is known as Pierre Shale (described by the
U.S. Geological Survey as "dark-gray clay shale with calcareous and ferruginous concretions and sandy members") and is the result of sedimentary mud that once lay at the bottom of a sea that covered what is now Badlands National Park. Because of its origin, this layer contains fossils of sea life such as ammonites, early clams, and baculites.
The Pierre Shale was deposited over the Upper Cretaceous period, also known as the Late Cretaceous period which lasted from about 98 million years ago to about 65 million years ago. At the end of that time, the vast sea that covered this part of North America receded mainly due to the rising elevation of the area caused by tectonic activity (the shifting of the Earth's plates). The sea floor mud formed fertile ground and not long after the waters retreated, geologically speaking, it was covered by a thick jungle.
Yellow Mounds Paleosol and Interior Paleosol
Decaying plant material mixed with the top layers of the Pierre Shale over the next 28 million years or so, resulting in a layer of yellowish soil. This layer is called the Yellow Mounds Paleosol. At some point around 37 million years ago, the jungle was covered by new sediments carried from the west. The minerals contained in this layer of soil were different than those of the Pierre Shale, so when the jungle grew up again in this new layer, the action of the plants and the organic material they mixed with it resulted in a soil with a strong reddish tint. This layer is called the Interior Paleosol.
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The Colors of the Yellow Mounds Paleosol and the
reddish purple Interior Paleosol are visible on the steep sides
of deep ravines in Badlands National Park.
Photo by Brad Sylvester. Copyright 2011, all rights reserved. |
Both the Yellow Mounds and Interior Paleosols were later covered again, becoming packed and fossilized by the weight of later sediments. Their distinctive coloration, however, can be seen in the cliff-sides exposed by erosion over the millenia.
Popcorn Rock or Bentonite Clay
More hints about the geologic history of Badlands National Park can be found in the make-up of much of the top layers of soil within the park. Although the area gets little rainfall for most of the year, the soil through much of the roughest areas of the park looks like dried mud. Indeed, it is, but mud of a very interesting nature.
It is a clay called Bentonite by geologists, but known as ball clay or popcorn rock in the local vernacular because of its unique properties. When it does rain in this arid part of South Dakota, The South Dakota Badlands Bentonite, because it contains sodium instead of calcium as it does in some other regions,
says John P. Bluemle, absorbs the water and expands to many times its original size. The repeated expansion and contraction as it dries again causes it to break up so instead of a smooth surface, you end up with rough balls of hard packed clay that are an inch or two in diameter.
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Bentonite or Popcorn Rock covers many of the steep hillsides
in Badlands National Park.
Photo by Brad Sylvester. Copyright 2011. Do not copy. |
These clay balls will often break away from the hillside and roll down the into the ravine when they get wet. When this happens, their soft outer surface will often pick up pebbles as it rolls. If you look carefully at the bottoms of the gullies, you might find some of these clay balls that look like someone took hours to stud them with a variety of small rocks.
Bentonite is formed from the breakdown of volcanic rock and ash. With the super-sized volcanic caldera of Yellowstone National Park lying not too far to the northwest of Badlands National Park, it's pretty easy to determine the origin of the badlands bentonite.
Clastic Dikes
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Clastic dikes run down the center of many of the clay hills
of Badlands National Park.
Photo by Brad Sylvester. Copyright 2011. Do not copy. |
Another interesting geologic feature helps to give the badlands its rugged appearance. Clastic dikes are vertical wall-like structures of harder rock that erodes more slowly than the surrounding bentonite and paleosols. The result is that it often looks like the spine of the mountains, sticking up with vertical sides from the surrounding rock and soil. Clastic dikes are interesting formations that also tell us something about the history of this land.
According to
Harman D. Maher Jr. of the University of Nebraska at Omaha's Department of Geology and Geography, it is believed that clastic dikes are formed when tectonic activity causes the ground to move in such as way as to open up deep crevices. These open crevices are quickly filled in either from above or possibly from liquefied material being forced up from below. This filling material solidifies almost like cement leaving a vertical wall of harder rock encased in softer surrounding soils.
Because of the way it is formed, the material of the clastic dikes may be of a different geological age than much of the material surrounding it. Furthermore, sometimes these crevices open and fill more than once. So if a clastic dike already existed and the same crevice opens again, but wider, the original clastic dike may be encased in a another layer of material, showing several vertical layers of different types of rock when the clastic dike is eventually exposed by erosion.
Source note: Much of the geologic history of the Badlands is told by signage within the park itself or was related to me by park rangers, and it is these that serve as the primary resource for the geological history that I relate here. Where other sources are used, they are referenced within the text.
