The term “Badlands” describes a specific type of dry terrain characterized by soft sedimentary rocks, clay-rich soils, and extensive erosion that has carved the landscape into a labyrinth of steep slopes, gullies, buttes, and sharp spires. The most famous example is the White River Badlands, located primarily in Badlands National Park in southwestern South Dakota. These dramatic formations result from a long geologic history of layered sediment deposition followed by a relatively recent, rapid period of carving by water and wind.
Setting the Stage: The Ancient Environment
The foundation of the Badlands began millions of years ago when the region was vastly different than the semi-arid grasslands of today. Approximately 75 million years ago, the area was covered by the Western Interior Seaway, a shallow inland sea. Clay and mud settled on the seafloor, eventually hardening to form the deepest layer of the Badlands stratigraphy, the Pierre Shale.
As the Rocky Mountains uplifted, the seaway receded, exposing the shale and ushering in a long period of terrestrial deposition. Between 37 and 34 million years ago, during the Eocene epoch, the environment transformed into a subtropical floodplain and forest. Rivers flowing from the Black Hills and other western highlands spread massive amounts of sediment, creating the Chadron Formation.
The most significant layers were deposited during the Oligocene epoch (34 to 28 million years ago). The climate transitioned from humid and forested to a cooler, drier savanna. Rivers continued to deposit silts and clays, but volcanic activity far to the west in the Great Basin also contributed significant amounts of ash. These multicolored layers of soft sediment—including mudstone, siltstone, and volcanic ash—were stacked, providing the raw, easily carved material for the future Badlands.
The Mechanism of Water and Wind Erosion
The Badlands terrain only began to take its current form relatively recently, starting about 500,000 years ago, when regional rivers began cutting down into the soft sedimentary layers. The primary force shaping the landscape is water erosion, specifically rapid runoff from heavy, infrequent rainstorms. The clay-rich sediments are largely impermeable and, combined with the region’s sparse vegetation cover, this means that rainfall cannot soak into the ground quickly.
Instead, the water rushes across the surface, carving a dense network of gullies, ravines, and steep drainage channels. This destructive runoff rapidly dissects the soft rock, creating the distinctive, intricate topography. A phenomenon known as differential erosion is responsible for the unique shapes like buttes and hoodoos.
In this process, a harder, more resistant layer, such as a bed of sandstone or volcanic ash, acts as a caprock, protecting the softer clay and silt layers directly beneath it. As the unprotected rock around it washes away, the capped column remains, forming a spire or butte. Wind erosion plays a supporting role, primarily by smoothing and rounding the exposed surfaces and contributing to the transport of loose sediment.
A Window to the Past: The Fossil Record
The constant, fast erosion of the Badlands makes the area a globally significant site for paleontology. As the soft sedimentary rocks are continually washed away, they expose the fossilized remains preserved within the layers. The Oligocene epoch layers hold the world’s richest deposits of mammal fossils from that time, often referred to as the “Age of Mammals”.
These exposed fossils reveal a detailed picture of ancient life, including the remains of animals like the oreodonts, which were sheep-sized grazing mammals, and nimravids, which were saber-toothed cat-like predators. The erosion acts as a natural excavation process, constantly bringing new specimens to the surface before they can be completely destroyed by weathering. Paleontologists have recovered hundreds of species, offering researchers an unparalleled record of mammalian evolution during a period of significant climate change.
Ongoing Transformation and Climate Influence
The Badlands terrain is not a static formation; it is a dynamic landscape that is actively being reshaped every day. The rate of erosion is remarkably fast in geologic terms, with the landscape currently wearing away at an average rate of approximately one inch (25 millimeters) per year. This rate is significantly faster than the erosion of harder rock formations, such as the granite in the nearby Black Hills.
Modern climate factors continue to drive this transformation, particularly the extremes of temperature and precipitation. Severe weather events, like intense thunderstorms, cause the rapid runoff that cuts new channels and undercuts existing spires. Freeze-thaw cycles also contribute, as water seeps into cracks, freezes, expands, and breaks apart the rock structure. Geologists estimate that at the current rate of erosion, the Badlands wall may be completely flattened to a mixed-grass prairie in the next 500,000 years.