The term “Badlands” describes a specific type of arid terrain marked by soft sedimentary rocks, steep slopes, and minimal plant life. The striking landscape of buttes, ravines, and gullies is the result of a geological process spanning millions of years, involving both the slow accumulation of material and its subsequent intense erosion. This cycle of creation and destruction has shaped one of the world’s most visually dramatic landforms.
The Geological Foundation: Sedimentation and Layering
The Badlands began as a massive basin for collecting fine-grained sediments, a process that started around 75 million years ago. Initially, the region was covered by the Western Interior Seaway, a shallow inland sea that stretched from the Gulf of Mexico to the Arctic Ocean, depositing marine mudstones and shales. As the seaway retreated, the landscape was exposed, and a new era of terrestrial deposition began, largely from ancient river systems and floodplains.
These succeeding layers consist of fine-grained clays, silts, and sands, along with deposits of volcanic ash. The ash originated from eruptions as far away as the Great Basin, hundreds of miles to the west. These ash falls mixed with the river sediments, forming layers of bentonite clay, which are highly susceptible to erosion because they swell when wet. This “layer cake” of sedimentary rock, built up over millions of years, provided the soft, easily carved material necessary for the Badlands to form.
Exposure to the Elements: Uplift and Drainage
The deep layers of sediment remained buried until large-scale tectonic forces began to reshape the region. The Laramide Orogeny, the mountain-building event that created the Rocky Mountains and the nearby Black Hills, caused the land surface to gradually rise beginning about 70 million years ago. This regional uplift tilted the sedimentary strata, exposing the ancient deposits that were once deep underground.
The slow, continuous rise of the land increased the gradient of major rivers flowing through the area, such as the Cheyenne and White Rivers. This increased gradient gave the rivers greater cutting power, causing them to deeply incise the landscape and drain away the protective surface layers of younger sediment. By approximately 500,000 years ago, this river cutting had fully exposed the softer, older sedimentary layers, initiating the intense erosion that defines the Badlands today.
The Sculpting Process: Differential Erosion
The dramatic shapes of the Badlands are a direct result of a process called differential erosion, where softer and harder rock layers erode at different rates. The soft claystones and mudstones wear away quickly, while more resistant layers, often composed of ancient river channel sandstones, act as caprocks. These caprocks shield the material directly beneath them from precipitation, creating the signature features like buttes, tables, and isolated spires.
The primary force driving this sculpting is intense water runoff from infrequent but heavy rainfall events. Because the clay-rich soil is largely impermeable and lacks vegetation, rainwater cannot soak in and instead flows rapidly across the surface. This sheet wash collects into numerous streamlets, carving a dense, intricate network of deep gullies and steep-sided ravines. Freeze-thaw cycles also contribute to the breakdown, as water seeps into cracks in the porous rock, expands as it freezes, and causes rock fragments to break off.
A Dynamic Landscape: Ongoing Weathering and Change
The Badlands are a landscape in constant, rapid transformation. The softness of the exposed sedimentary rock, combined with the lack of vegetation to hold the slopes in place, results in some of the highest measured erosion rates in the world. Estimates indicate that the landscape is eroding at an average rate of about one inch (2.54 centimeters) every year.
This rapid rate means that features like razor-sharp ridges and pinnacles can collapse following a single major thunderstorm. Geologists estimate that at this pace of erosion, the dramatic topography of the Badlands will be completely worn down and leveled within the next 500,000 years. The forces of deposition and erosion are locked in an ongoing cycle, where the landscape is continually being destroyed and reformed.