Deep canyons, towering spires, and flat-topped tables can all be found among Badlands buttes. Yet, despite their complex appearance, they are largely a result of two basic geologic processes: deposition and erosion. Deep canyons, towering spires, and flat-topped tables can all be found among Badlands buttes. Yet, despite their complex appearance, they are largely a result of two basic geologic processes: deposition and erosion.
The serrated Badlands terrain did not begin eroding until about 500,000 years ago when water began to cut down through the rock layers, carving fantastic shapes into what had been a flat floodplain. The ancient fossil soils, buried for millions of years, became exposed once again. Many of the layers are gently warped and faulted due to mountain building activities that formed the Black Hills, 70 miles to the west.
Erosion is ongoing. Every time it rains, more sediment is washed from the buttes. One day, a peak may tower above the land; the next, a storm may weaken it just enough for it to crash to the ground. While the Badlands are long lasting in human terms, they are short lived in terms of geologic time. Evidence suggests that they will erode completely away in another 500,000 years, giving them a life of one million years. Compare that to the age of the earth, which is 4.6 billion years old. Even the Rocky Mountains, considered young, started to rise only 70 million years ago. On average, Badlands buttes erode one inch each year. However, change can occur much slower or faster.
As the Badlands buttes erode, some of the sediment is washed onto the prairie below, building up its level while the rest is carried by small streams to the White, Bad, and Cheyenne Rivers. These tributaries flow into the Missouri River, which drains into the Mississippi River. Eventually, some Badlands sediments will travel as far as the Gulf of Mexico.
The Loop Road hugs the Badlands wall, a long, narrow spine of buttes that stretches 60 miles from Kadoka west towards the town of Scenic. Wind, rain, and freeze/thaw action have gradually worn down the badlands sediments, leaving the Badlands Wall behind. As erosion has continued, the wall has retreated from the three major drainages. The town of Wall, South Dakota takes its name from this feature that dominates the horizon.
A quick look at the buttes will show that the Badlands were deposited in layers. These layers formed soft, sedimentary rocks, composed of minute grains of sand, silt, and clay that have been cemented into solid form. Geologists study sedimentary rocks to determine what type of environment caused the material to accumulate. Layers similar in character are grouped into units called formations with the oldest layers at the bottom.
The lighter colored Sharps Formation was primarily deposited from 28 to 30 million years ago by wind and water as the climate continued to dry and cool. Volcanic eruptions to the west continued to supply ash during this time. Today, the Brule and Sharps form the more rugged peaks and canyons of the Badlands.
As the Oligocene Epoch continued, a thick layer of volcanic ash was deposited, forming the bottom layer of the Sharps Formation. This Rockyford Ash serves as a boundary between the Brule and Sharps.
During the Oligocene Epoch, between 30 to 34 million years ago, the tannish brown Brule Formation was deposited. As the climate began to dry and cool after the Eocene the forests gave way to open savannah. New mammals such as oreodonts (sheep-like, herd mammals) began to dominate. Bands of sandstone interspersed among the layers were deposited in channels and mark the course of ancient rivers that flowed from the Black Hills. Red layers found within the Brule Formation are fossil soils called paleosols.
The greyish Chadron Formation was deposited between 34 to 37 million years ago as a river flood plain that replaced the sea. Each time the rivers flooded, they deposited a new layer on the plain. Alligator fossils indicate that a lush, subtropical forest covered the land. However, mammal fossils dominate. The Chadron is known for large, rhinoceros-like mammals called titanotheres. The formation can be recognized because it erodes into low, minimally vegetated grey mounds.
In this photo you can see the Yellow Mounds capped with Chadron Formation gray.
The sea drained away with the uplift of the Black Hills and Rocky Mountains, exposing the black ocean mud to the air. Upper layers were weathered into a yellow soil, called Yellow Mounds. The mounds are an example of a fossil soil, or paleosol.
The oldest formation is the Pierre Shale, these black layers were deposited between 69 and 75 million years ago during the Cretaceous Period when a shallow, inland sea stretched across what is now the Great Plains. Sediment filtered through the seawater, forming a black mud on the sea floor that has since hardened into shale. Fossil clams, ammonites, and sea reptiles confirm the sea environment.