The Geomorphic Processes of Toadstool Geologic Park
and Badlands in Nebraska and South Dakota

Justin Abel, Marla Bethke, Dusty Gutierrez, and Bryan Longwell

ES 546

Field Geomorphology

Table of Contents
Introduction Badlands Toadstool Geologic Park Stratigraphy Geomorphic Processes Erosional Processes Images


Toadstool Geologic Park is located 16 miles northwest of Crawford, Nebraska in Oglala National Grassland. Toadstool park derives its names from the unique erosional features found throughout the exposed cliffs where layered sandstone overlies a softer clay-rich layer. Wind and water erosion sculpt the sandstone blocks and remove the underlying clay. The result is a rounded sandstone block on a pedestal of clay-rich siltstone. The toadstools vary greatly in size from a few feet tall to large well developed blocks many meters high. Toadstool Geologic Park is miniscule in size compared to the Badlands of South Dakota, but has proved invaluable to the study of the White River Group. One Geologist Dr. Benton states that “Toadstool Geologic Park is a rosetta stone for the White River sequence in southwest South Dakota to those in east central Wyoming (Benton et al., 2015).” In the Fall of 2015 Students from the Field Geomorphology class from Emporia State examined the site, taking pictures and analyzing the complex stratigraphy of channel sandstone of the Brule Formation that lead to these intricate formations.

The Badlands

While the word badlands has many meanings, it originally referred to the difficulty of traversing the terrain. When the word is capitalized, it denotes the formal name of the national park within the area. In a geological sense, the term badlands describes a highly eroded landscape with little vegetative cover in arid to semiarid climates (Benton et al., 2015). The badlands of southwestern South Dakota and northwestern Nebraska are some of the most iconic landscapes of this geomorphic topography. The badlands formed into the scenic landscape they are known for today over millions of years. The stratigraphic layers that make up this area were deposited in several environments varying from humid forested floodplains to semi-arid desert conditions. “The Badlands as we see it today are the result of a shift from deposition to erosion that began about 660,000 years ago (Stamm et al., 2013 cited in Benton et al., 2015).” In modern times, the badlands annually receive 17 inches of precipitation on average, 77% of which comes in late spring and summer in the form of quick, heavy rainstorms (Stoffer, 2003). These hard rain events add to the erosion of the area through flash flooding. The sudden influx of water from the heavy rain and the low permeability of the soil cause the water to flow over the land surface as runoff (Stoffer, 2003). A large portion of the land in the badlands is barren or has scarce plant life. Vegetation that would typically control both flooding and erosion rates has a difficult time growing in the badlands environment (Stoffer, 2003). The stratigraphic layers that make up this area consist of the White River Group and part of the Arikaree Group. The White River Group is made up of three separate formations; the Chamberlain Pass Formation, the Chadron Formation, and the Brule Formation. The only formation in the Arikaree that makes up the Badlands is the Sharps Formation (Stoffer, 2003).

Toadstool Geologic Park

Toadstool Geologic Park is located in the Oglala National Grasslands in northwestern Nebraska. Due to the varying degrees of lithification of the rock found in the park, the area is characteristic of unevenly eroded terrain. The more resistant channel sandstone has become balanced atop pedestals of softer rock, which causes the structures to resemble huge toadstools. The sandstone channel deposits contain a fossilized history of the mammals that lived in this area millions of years ago. The center of Toadstool Geologic park is located at the intersection of two normal faults; the Toadstool Fault and a smaller fault that has not been named that cut through the channel sandstone just south of the larger fault (Maher et al., 2003). The stratigraphy of Toadstool Geologic Park begins with the oldest Eocene age Chadron Formation, then the Oligocene Brule Formation, followed by the Miocene age rocks of the Arikaree Group (Stoffer, 2003).


Chamberlain Pass Formation:

The Chamberlain Pass Formation was deposited in the mid to late Eocene and it is found in between the Yellow Mounds paleosol on the bottom and the Interior paleosol on the top (Stoffer, 2003). Because of the red coloration that the Interior paleosol has overprinted on the upper part of the Chamberlain Pass, the two are difficult to distinguish. It has an average thickness of about 16 meters (Evans 1999) but in some areas it only has a thickness of 4 m (Stoffer, 2003). The formation is made up of a conglomerate bed consisting of flood-plain deposits. A fossilized tooth recovered from the gravel within the formation established the age of the deposit as Eocene (Stoffer, 2003). It should be noted that the Chamberlain Pass formation is only exposed on the South Dakota side of the Badlands. In Nebraska the formation is overlain by the Peanut Peak Member of the Chadron Formation (Benton et al., 2015).

Chadron Formation:

The Chadron Formation is made up of three members listed oldest to youngest; the Ahearn, the Crazy Johnson, and the Peanut Peak (Stoffer, 2003). It is named after the town of Chadron, Nebraska where outcrops of this formation can be found. The formation was deposited in the late Eocene epoch and is made up of poorly consolidated mudstones that consist of a sandy clay and is gray to olive gray in coloration (Stoffer, 2003). The clays of the Chadron Formation represent floodplain muds that were deposited when rivers overflowed their banks. The rivers that deposited these muds were similar to the meandering rivers seen today in the eastern half of the United states(Benton et al., 2015). They are characterized by having an abundance of water along with large amounts of fine sediments in suspension and coarser sand and gravel at the bottom of the channel (Benton et al., 2015). The large amounts of clay called “smectite” that is found in this formation is derived from chemical weathering of the volcanic ash that was prevalent (Benton et al., 2015). This particular form of chemical weathering is called hydrolysis and turns the volcanic ash first to Smectite and then to Kaolinite. Kaolinite can be found in abundance in the chamberlain Pass Formation at the base of the badlands (Benton et al., 2015). The process of hydrolysis occurs under humid climatic conditions; therefore, as the climate turned colder and drier at the end of the Eocene the proportion of clay to ash starts to decrease (Benton et al., 2015). This helps identify it compared to the red of the paleosol found beneath it as well as the Brule Formation above it. The paleosols of the Chadron Formation are similar to modern soils that would be found in forests today (Benton et al., 2015). There are lenses of a white limestone throughout the lower formation (Stoffer, 2003).

Brule Formation:

Overlying the Chadron, the Brule is the uppermost formation in the White River Group. It was deposited during the Oligocene (Stoffer, 2003). Within the Brule there are two members known as the Orella (lower) and the Whitney (upper) in Nebraska and the Scenic (lower) and the Poleslide (upper) in South Dakota (Evans, 1999). This difference in the naming is most likely due to political reasons between the two states. The lower member consists of an alternating banding of rusty-red paleosols and grayish-white sediment deposits while the upper member is made up of massive light gray sandstones (Stoffer, 2003). Paleontological surveys of the badlands document high concentrations of bone in distinct stratigraphic layers and regions throughout the park (Benton et al., 2015). It is surmised that that these bone beds, as they are called, represent both watering holes and fluvial channel deposits within the Scenic Member. The Poleslide Member tends to have fossils spread throughout the member and is associated with slower rates of deposition (Benton et al., 2015). These distinctions between the two members help in the identification of the formation. The Brule formation has an average thickness of 140 meters and is made up of both fluvial and aeolian deposits (Evans, 1999). The paleosols differ dramatically from the Chadron Formation to the Brule Formation. Fossils roots at the bottom of the Brule Formation are sparser and smaller than what can be found in the Chadron Formation and suggest an open rangeland environment. In the upper part of the Brule Formation the size of the fossils roots are pencil thin or smaller suggesting an environment found in modern prairies. The Rockyford Ash bed separates the Brule Formation from the Sharps Formation that was deposited during volcanic events from what is now the Nevada region during the late Oligocene (Stoffer, 2003).

Sharps Formation:

The Sharps Formation makes up the upper layers of the Badlands. It is the first member of the Arikaree Group and formed during the late Oligocene (Stoffer, 2003). It is made up of a tuffaceous sandstone, sand, and floodplain paleosols. There are not many deposits of younger age than the Sharps Formation found in the badlands. This is due to the erosional rate being faster than the deposition rate which in turn has led to large amounts of sediment being lost (Stoffer, 2003).

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Geomorphic Processes

Depositional Environments:

The depositional environment for the badlands and Toadstool Geologic Park has changed dramatically over time. When the first sediment layers were being deposited, the area was made up of a broad floodplain, the elevation was still near sea level, and the climate was hot and humid, forming a subtropical environment (Stoffer, 2003). During this time, the erosional rate was low which allowed sediment to build up and then the Laramide Orogeny began to uplift the land. The Laramide Orogeny was unique in that the shallow angle of subduction did not produce volcanoes but instead compressed and shortened the overlying continental crust (Benton et al., 2015). This compression caused the Rocky Mountains to form as well as elevate the Black Hills. The Black hills rose during the Paleocene and early Eocene (Curry, 1971; Flores and Ethridge, 1985 cited in Benton et al., 2015). The land became drier and more like a savannah or steppe environment as sediment was carried down from the Black Hills and began to fill in the valleys as rivers cut through the area (Stoffer, 2003). This sediment was deposited at the base of the Black Hills and over the top of the previous layers in the Badlands. Other loads of sediment were deposited by the rivers that flowed through the area at the time. At the end of the Oligocene, the land transformed into semi-arid desert like conditions that are still present today where aeolian processes now play a prominent role in how the land is shaped (Stoffer, 2003). Erosional rates are now happening at a faster rate than the depositional rates and any sediment that is deposited in the area is soon eroded and carried away.

Rivers of the badlands:

Three rivers that now cut through the area are the Cheyenne, White, and Bad Rivers. These rivers help erode away and form much of the land as they meandered back and forth across the area. Before the last ice age, these rivers flowed northward towards the Hudson Bay with a gentle gradient (Stoffer, 2003). The glaciation that occurred during the these last ice ages had an indirect effect on these rivers. When the continental ice sheets formed, the rivers’ paths were blocked which caused large lakes to form at the front of the ice sheets (Stoffer, 2003). These lakes and the blockage of the rivers helped result in the rivers changing their course. The lakes would then overflow and form modern rivers such as the Missouri which cut through the land forming deep gorges which has slowly been translated upstream over the years (Stoffer, 2003).

Depositional Environments:

The depositional environment for the Badlands and Toadstool has changed dramatically over time. When the first layers were being deposited, the area was made up of a broad floodplain. It was forested and the elevation was still near sea level (Stoffer, 2003). The climate was hot and humid, forming a subtropical environment. Then, when the Laramide orogeny began to uplift the land, the land became drier and more like a savannah or steppe. Sediment carried down from the Black Hills started to fill in the valleys as rivers began to cut through the area. At the end of the Oligocene, the land became like the semi-arid desert like conditions that are still present today. Aeolian deposition also began to take place during this time (Stoffer, 2003).

Evolution of a Toadstool:

The sandstone capped, siltstone pedestals, colloquially known as “toadstools”, are formed from the escarpment of the Brule Formation. The sandstone is more resistant than that of the less resistant underlying siltstone, which means the siltstone is eroding at a faster rate than the sandstone. The sandstone itself erodes due to joints and other cracks within the rock segments. These joints add surface area that allow other processes such as chemical weathering to get a better foot hold and break down the rock. The siltstone beneath is eroded away mostly by water and wind processes. This results in relatively thin columns of siltstone that hold up massive blocks of the more resistant sandstone which produce the famous formations that are seen today. These sandstone blocks balance atop the columns until gravity’s consistent force and continued erosion finally cause them to collapse. When the sandstone blocks fall from their perch, they act as a mass wasting process and carry with them more of the underlying sediment. This also leaves the siltstone beneath the sandstone more exposed to the elements such as the heavy rains that occur in the area.

Erosional Processes:

Flash Flooding

This event happens whenever large amounts of rainwater or meltwater is not able to soak into the ground, whether that is due to oversaturation or impermeable material, and is forced to flow over the land surface. When flash floods occur, the water carries off mass quantities of sediment along with vegetation and other pieces of debris. Once the waters recede, the land is often left more exposed to weathering and erosion as there is less coverage. In the Badlands and Toadstool, these floods often occur annually with the heavy sudden rains during the summer.

Acid Rain

This process occurs when rain droplets mix with carbon dioxide in the atmosphere to form carbonic acid and bicarbonate. Then this slightly acidic rain falls onto the land and dissolves some of the carbonate cement of the sandstone. This cement is normally made up of calcite or aragonite. This chemical process accelerates the weathering of the sandstone. In performing an acid test on a sample of the channel sandstone taken from the Toadstool Geologic Park. We found that the acid intensely fizzed on contact with the sample of sandstone. This suggest that the cement holding a grains of sand together is composed of high amount of carbonate instead of silica. This means that rainfall with have an accelerated rate of erosion on the sandstone than if it was composed of silica dioxide.

Wind Abrasion

This process is the lifting and movement through aeolian processes and subsequent collision with rocks faces. The collision causes pieces of the rock to break off. This process is also called sand blasting. In the badlands and Toadstool area, there are not many trees or other obstacles to stop the wind and prevent it from further breaking down the outcrops.

Clay Swelling

Clay swelling occurs when the clay expands through water absorption due to large amounts of water in clay such as smectite. When there is little water in the clay, it dries out and forms cracks on the surface.

Frost Wedging

Frost wedging occurs when water fills cracks in rocks then freezes. As it freezes, it pushes the crack further apart. Through multiple cycles of melting and refreezing, this process can force large blocks to be separated from the rest of the rock layer. For the study area, and Toadstool especially, this process helps result in the massive blocks of sandstone and other rocks that are scattered throughout the land.

Mass Wasting

As stated earlier, one way that mass wasting occurs in the badlands is in Toadstool when the sandstone blocks collapse and slide down their siltstone columns. The definition of mass wasting is just the movement of large pieces of sediment, rocks, and/or other debris being pulled down a slope due to gravity. Water helps to this movement by adding lubrication to the affected area. In other areas of the badlands, sediment accumulates along the steep slopes until the angle of repose is exceeded. This sediment then slides down the rest of the slope, picking up and carrying more along the way.


Living things also help to change and form the landscape. The roots of plants penetrate the rock as they grow. This helps to both break the rock apart but it also helps keep the rocks from completely eroding. The plants the roots belong to help protect the rock and sediment from other erosional forces like wind. Since there is not that much in the way of plant life in the badlands or Toadstool Geologic Park, the outcrops are more exposed to the elements. Other organisms add to the erosion of the landscape. Small animals burrow into the rocks to make dens which leave tunnels that could possibly be filled with water or other material later, or, once the rock has eroded back enough to expose the tunnel, allow more surface area to be available that can be eroded by other processes.

Water Penetration

The smectic clay that is found in the White River Group causes little water penetration on the exposed ground. The clay swells and forms a slick surface known locally as “Jumbo” (Benton et al., 2015). Once the clay surface becomes supersaturated the surface water is unable to seep into the ground. This results in an increase in surface runoff and sheet flow. When the clay dries out it forms a hard abrasive crust like sandpaper called popcorn texture (Benton et al., 2015).

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Toadstool Geologic Park

Joints and cracks first occur at the surface of the sandstone that could possibly be caused by bioturbation of burrowing animals and rooted vegetation. Toadstool Geologic Park, Nebraska. Photo by Justin Abel.
Erosional processes, primarily of rainwater, widen the cracks and joints and begin separation of the sandstone slab. Toadstool Geologic Park, Nebraska. Photo by Justin Abel.
The joints and cracks are widened to the extent that individual pedestals begin to form and separate from the formation. Toadstool Geologic Park, Nebraska. Photo by Justin Abel.
The sandstone escarpment with apparent horizontal striations, indicating the separate depositional layers, and a sod house in the foreground. Toadstool Geologic Park, Nebraska. Photo by Bryan Longwell.
A large sandstone block sits on top of a mound where it once stood on a clay pedestal, but collapsed due to continual erosional processes and gravity. Toadstool Geologic Park, Nebraska. Photo by Justin Abel.
Two small “toadstools” remain standing on their clay pedestals. Toadstool Geologic Park, Nebraska. Photo by Justin Abel.
A view from on top of the escarpment toward the northeast. Notice the students standing on top and at the base of the escarpment for scale. Toadstool Geologic Park, Nebraska. Photo by Justin Abel.
The face of the escarpment portrays a dendritic drainage pattern, indicating a uniform resistance to erosion. Large sandstone blocks reside within the main channel possibly due to the washout of smaller pedestals by heavy rain events. Toadstool Geologic Park, Nebraska. Photo by Justin Abel.
Differential weathering is shown here between the rock layers where sandstone protrudes as a more erosional resistant ledge and the clay as a less resistant slope. Notice the deep vertical cracks in the sandstone layers, indicating an increased rate of weathering and erosion along them. Toadstool Geologic Park, Nebraska. Photo by Bryan Longwell.
More differential weathering is depicted, though between sandstone ledges, with layers having rounded edges and others more square edges. In the right-center of the image, a particular layer is characterized by an almost spherical, cracked edge resembling cauliflower. Toadstool Geologic Park, Nebraska. Photo by Bryan Longwell.
Clay-rich sediment of the Chadron Formation showing the popcorn texture. Notice the chalcedony quartz vein running from the top of the photo to the bottom These veins can be found throughout slopes of the Chadron Formation throughout Toadstool Park. Toadstool Geologic Park, Nebraska. Photo by Justin Abel.
Channelized sandstone boulder from the Brule Formation Toadstool Geologic Park, Nebraska. Photo by Marla Bethke.
A high-oblique aerial view of the escarpment towards the northwest. In the lower-left corner of the image a kite can be seen from another photo rig that was used as a lifting mechanism for the camera. Toadstool Geologic Park, Nebraska. Photo taken by the Field Geomorphology class, Fall 2015.
A low-oblique aerial view showing the sharp peaks and drainage pattern of the sandstone formations as well as their scarce vegetation cover. A shallow dry stream bed is visible at the bottom and right side of the image, bordered by grass cover, most likely carved by intense rainfall. Toadstool Geologic Park, Nebraska. Photo taken by the Field Geomorphology class, Fall 2015.
Another low-oblique aerial view showing several dry stream beds meandering down a gentle slope, bordered by grass cover, and converging into a basin. Toadstool Geologic Park, Nebraska. Photo taken by the Field Geomorphology class, Fall 2015.

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Benton, Rachel., Terry, Dennis., Evanoff, and Emmett., McDonald, H., 2015.The White River Badlands: Geology and Paleontology. Indiana University Press. p.13-18, 310-31, 39,177-187.

Evans, J.E., 1999. Recognition and implications of Eocene tufas and travertines in the Chadron Formation, White River Group, Badlands of South Dakota. Academic Search Premier, EBSCOhost: Sedimentology, vol. 46 no. 5, p. 771-789.Accessed online. Nov. 2015 772-774.

Maher, Harmon D. Jr.; Engelmann, George Felix; and Shuster, Robert Duncan, 2003.Roadside Geology of Nebraska, Book 51. (2003). Faculty Books and Monographs, 240 p.

Stoffer, Philip W. 2003.Geology of Badlands National Park: A Preliminary Report. U.S. Geological Survey, Open-File Report 03-35, 65 p.Accessed online Nov. 2015. 6-8, 46-55, 57-58.