How Did the Badlands Form? The Forces That Shaped Them

The Badlands of South Dakota took tens of millions of years to build and only a few hundred thousand years to carve into the dramatic spires, buttes, and striped cliffs visitors see today. The story begins with layer upon layer of volcanic ash, river sediment, and windblown dust settling across an ancient floodplain during the late Eocene and Oligocene epochs, roughly 37 to 28 million years ago. Once the climate dried out and rivers cut downward, those soft, loosely cemented layers were left exposed to rain, frost, and gravity, which have been sculpting them ever since at rates that make the Badlands one of the fastest-eroding landscapes on the continent.

The Ancient Floodplain That Built the Foundation

Before there were badlands, there was a vast, low-lying basin stretching across what is now western South Dakota and parts of Nebraska and Wyoming. Rivers flowing eastward from the young Black Hills and from volcanic ranges far to the west deposited sand, silt, and clay across this basin over millions of years. Massive volcanic eruptions in the Great Basin region contributed enormous quantities of ash that drifted east and settled across the floodplain, compacting into layers of pale, fine-grained rock. These deposits are collectively known as the White River Group, and they form the colorfully banded, fossil-rich strata that define the Badlands landscape.¹ The rock is soft by geological standards. Claystones, siltstones, and mudstones dominate, and none of them resist erosion the way granite or limestone would. That softness is not a minor detail. It is the single most important reason the Badlands look the way they do.

Fossil soils preserved within these layers tell us the environment was not always harsh. During the late Eocene, the region supported humid forests. By the early Oligocene, open woodlands had replaced much of the forest. The drying trend continued through the late Oligocene, shifting the landscape from savanna with gallery woodland along streams to open grassland with only scattered trees near water.² That progressive drying mattered enormously for what came next, because drier conditions meant less vegetation to anchor soil, more exposure to wind, and eventually a surface vulnerable to the kind of rapid erosion that creates badland terrain.

How Uplift and River Cutting Exposed the Layers

Sediment that sits buried underground cannot erode into spires and buttes. Something has to lift those layers up and expose them. In the case of the Badlands, two regional forces worked together. First, broad uplift of the northern Great Plains tilted the landscape and raised it above sea level. Second, rivers began cutting downward through the sedimentary stack, creating steep valley walls. When streams cut deeper, they lower what geomorphologists call “base level,” and that change ripples outward. Hillslopes steepen, side valleys form, and the entire drainage network adjusts.³ In softer rocks like those of the White River Group, this adjustment happens quickly. Once the main rivers had cut down far enough to expose the pale mudstones and claystones, the stage was set for the dramatic erosion that continues today.

The Black Hills, sitting just west of the Badlands, played their own supporting role. Uplift of the Black Hills created a sharp elevation gradient that increased stream power in the surrounding region. Rivers draining the eastern flanks of the Hills carved deeply into the plains, helping to expose the soft Tertiary sediments to surface erosion. The combination of regional uplift and local river incision meant that layers that had been buried for millions of years were suddenly at the mercy of rain and frost.

Water as the Primary Sculptor

Rain is the main force carving the Badlands, and it works in several ways simultaneously. The most widespread is sheet erosion, where a thin film of water flows across bare slopes, detaching and carrying away fine particles. In badland landscapes, sheet erosion rates can be startlingly high. Measurements at a badland site in Spain found average sheet erosion of roughly 7 mm per year across bare and sparsely vegetated slopes, with some spots losing nearly 19 mm per year.⁴ Those numbers put badlands among the most erosive landscapes on Earth. At Badlands National Park itself, the widely cited estimate for the soft White River strata is about 2.5 cm per year, though researchers have noted that rigorous long-term measurements to confirm that figure are scarce.⁵

Beyond sheet erosion, concentrated flow carves rills and gullies. A hard rainstorm channels water into small streams that deepen into gullies surprisingly fast. These gullies branch and extend headward, dissecting what was once a gently sloping surface into the maze of ridges and ravines characteristic of badland terrain. Rainfall intensity matters more than total rainfall. In semiarid southeastern Spain, researchers found that it was the high-intensity storms, not the cumulative annual rainfall, that drove the biggest surges in both runoff and sediment loss.⁶ The same principle applies in South Dakota. The Badlands receive only about 40 cm of rain per year, but much of it falls in brief, intense thunderstorms during summer. A single downpour can do more sculpting than weeks of light drizzle.

Erosion from Below

Not all of the carving happens at the surface. Water that infiltrates into soft, clay-rich sediments can dissolve soluble minerals and physically disperse clay particles, creating underground tunnels called pipes. Over time, these pipes enlarge until the roof collapses, forming gullies that seem to appear from nowhere. This process, called piping, is especially common in badlands developed on marl, a calcium-carbonate-rich mudstone. Field observations have confirmed that some of the largest gullies in marl badlands, including features with steep, abrupt headcuts, likely originated as collapsed subsurface pipes rather than as surface channels that deepened gradually.⁷

Piping is sneaky because you cannot see it until the damage is done. A slope that looks stable from the outside may be honeycombed with internal voids. When the surface finally collapses, it can produce dramatic vertical-walled gullies that look very different from the smooth, rounded channels formed by surface runoff. In the Badlands of South Dakota, certain clay-rich layers within the White River Group are particularly susceptible to dispersion when wet, making piping a contributor to the landscape’s ongoing transformation.

Freeze-Thaw and the Crumbling Surface

Water does not just erode by flowing. In cold climates, it also destroys rock by freezing. When water trapped in pores and cracks expands as it turns to ice, it pries grains apart. In clay-rich badland slopes, freeze-thaw cycles act as a primary weathering agent, causing surface swelling of one to five centimeters during winter months as ice lenses form in the soil.⁸ When temperatures rise again, the loosened material is ready to be washed away by the next rainstorm. This freeze-thaw preparation is especially important in the Badlands, where winters are cold enough for repeated cycling and the clay-rich sediments hold water readily.

Wetting and drying cycles compound the damage even without freezing. Clay minerals, especially the swelling types, expand when they absorb water and shrink when they dry, opening cracks that invite deeper infiltration during the next rain. High temperatures between wetting events reinforce physical breakdown by creating thermal stress in the rock surface.⁸ The net effect is that the surface of a badland slope is in a constant state of preparation and removal. Weathering loosens material, and water carries it away. The two processes feed each other in a cycle that keeps erosion rates high.

How Fast the Badlands Are Disappearing

The Badlands are not a static monument. They are actively eroding, and at rates fast enough to be visible within a human lifetime. The roughly 2.5 cm per year estimate for the soft strata at Badlands National Park means that a cliff face could retreat about a meter in 40 years.⁵ Some features erode much faster. Pinnacles formed in soft sediments have been measured narrowing and shrinking at rates of 3 to 27 centimeters per year, primarily driven by rain washing away the outer layers of the column until it becomes too thin to stand.⁹ At that pace, the slender spires visitors photograph today will look noticeably different within a decade or two.

The implication is sobering for anyone who thinks of national parks as permanent. The Badlands landscape as we know it is geologically temporary. The same forces that created the dramatic scenery are steadily wearing it down. Given enough time, the soft sediments will erode entirely, leaving only the harder layers and the underlying Pierre Shale. That process will take hundreds of thousands of years, but by geological standards, the Badlands are in the middle of a relatively brief performance.

Why So Little Grows There

Vegetation and erosion are locked in a feedback loop in badland landscapes. Plants slow erosion by shielding the soil from raindrop impact, binding particles with roots, and slowing surface runoff. But where erosion is already fast, plants cannot establish. The Badlands’ steepest, most actively eroding slopes are essentially bare, which keeps erosion rates high, which keeps them bare. In the Spanish badlands, small bare catchments had the highest runoff and erosion rates, while catchments with complete vegetation cover had the lowest.⁶ The same pattern plays out in South Dakota, where the upper prairie grasslands surrounding the park give way abruptly to the barren, eroding formations below.

Even thin biological crusts on the soil surface can make a difference. In arid and semiarid environments, communities of cyanobacteria, mosses, and lichens form a living skin over the ground. Experiments in Capitol Reef National Park in Utah showed that these crusts significantly reduced erosion compared to bare soil by stabilizing fine particles at the surface.¹⁰ Where foot traffic or grazing destroys the crusts, erosion accelerates. This finding has implications for visitor management at badland sites worldwide and helps explain why certain slopes with even a thin biological coating erode more slowly than adjacent bare surfaces.

Interestingly, in temperate climates where rainfall is more abundant, vegetation can actually reclaim badland terrain if left undisturbed. Research on European badlands has shown that without ongoing disturbance from grazing or fire, plants gradually encroach and can eventually stabilize the slopes, effectively ending the badland phase of the landscape.¹¹ Where reforestation has occurred in Mediterranean badlands, erosion rates dropped by at least half in areas covered by pine-needle litter compared to bare surfaces.⁴ This means that some badlands exist only because human land use, whether overgrazing, deforestation, or burning, keeps vegetation from recovering.

Badlands Are Not Unique to South Dakota

The word “badlands” describes a type of terrain, not just one place. Similar landscapes exist on every continent where soft, poorly consolidated sediments meet semiarid or seasonally dry climates. Italy’s “calanchi” are classic badland terrain carved into clay-rich marine sediments. Spain’s Tabernas Desert, famous as a filming location for old westerns, contains extensive badlands in marl and gypsum. Drumheller in Alberta, Canada, and the Bisti Wilderness in New Mexico both qualify. The unifying feature is always the same: soft rock, sparse vegetation, and aggressive erosion by water.

Researchers studying badlands across different climates have found that even the shape of the hills can differ depending on sun exposure. In Spain’s Tabernas Desert, only about a quarter of the catchments studied were symmetrical. Roughly 40 percent had the shadier slope at a lower gradient than the sun-facing slope, while 35 percent showed the opposite pattern.¹² The difference arises because sun-facing slopes dry faster, support less vegetation and fewer biological crusts, and erode differently from shadier slopes that retain more moisture. Solar radiation, surface area, and the gradient of the channel at the valley bottom were the main factors controlling which direction the asymmetry went. The takeaway is that even something as simple as which way a hillside faces can permanently alter the shape of a badland ridge.

Human Activities and the Creation of New Badlands

People do not just observe badlands. We sometimes create them. Overgrazing, deforestation, and poor agricultural practices strip vegetation and compact soil, setting off the same erosion cycle that produces natural badlands. In temperate climates especially, badlands either formed originally under more arid conditions or owe their existence to some form of land mismanagement.¹³ Parts of the Mediterranean basin, sub-Saharan Africa, and central Asia have expanding badland-like terrain tied directly to centuries of overuse.

Climate change adds another layer of complexity. Shifts in rainfall patterns, particularly increases in the frequency of high-intensity storms, can accelerate erosion in existing badlands and trigger it in previously stable terrain. Conversely, some climate models predict that certain semiarid regions will become wetter, potentially allowing vegetation to stabilize currently active badlands. The future of any given badland landscape depends on the balance between the rate at which rain and frost tear it apart and the rate at which plants can colonize and protect the surface. Understanding the runoff processes that drive erosion is considered critical for predicting how these landscapes will respond to changing climate and land use in the coming decades.¹⁴

Badland-Like Landscapes on Mars

The processes that form badlands on Earth have left similar marks on Mars. Alluvial fans sourced from erosional alcoves carved into crater rims show that liquid water once flowed across the Martian surface with enough force to erode bedrock and transport sediment. Models estimate that the cumulative wet period responsible for these features lasted anywhere from about a hundred years to a million years, excluding dry intervals.¹⁵ The channel shapes and spatial patterns suggest an arid climate where surface runoff came mainly from snowmelt on steep slopes, possibly driven by shifts in the planet’s orbital tilt. The parallel is striking: soft sediments, intermittent water, sparse stabilization by anything resembling vegetation, and a landscape deeply incised as a result. Mars will never develop the richly layered, fossil-bearing badlands of South Dakota, but the erosion physics that carved both worlds are fundamentally the same.

The Fossil Record Locked Inside the Layers

One reason the Badlands of South Dakota are so scientifically valuable is that the same rapid erosion that creates the scenery also continuously exposes new fossils. The White River Group sediments are among the richest Tertiary fossil deposits in North America, containing the remains of ancient horses, rhinoceroses, saber-toothed cats, oreodonts, and many other species that inhabited the floodplain when it was still accumulating sediment.¹ The fossil soils themselves record the environmental changes described earlier, from humid late Eocene forests to the dry Oligocene grasslands that replaced them.² Each color band in the Badlands walls represents a different chapter of that environmental story, with the reddish and yellowish layers often marking ancient soil horizons and the paler bands representing ash falls or flood deposits. Erosion destroys the landscape but it also opens a window into a world 30 million years gone, making the Badlands a rare place where the same force that threatens the scenery is also the reason we can read its history.