The bizarre landforms called badlands are, despite the uninviting name, a masterpiece of water and wind sculpture. They are near deserts of a special kind, where rain is infrequent, the bare rocks are poorly consolidated and relatively uniform in their resistance to erosion, and runoff water washes away large amounts of sediment. On average, the White River Badlands of South Dakota erode one inch per year. They are formidable redoubts of stark beauty where the delicate balance between creation and decay, that distinguishes so much geologic art, is manifested in improbable landscapes - near moonscapes - whose individual elements seem to defy gravity. Erosion is so rapid that the landforms can change perceptibly overnight as a result of a single thunderstorm.
At Badlands National Park, weird shapes are etched into a plateau of soft sediments and volcanic ash, revealing colorful bands of flat-lying strata. The stratification adds immeasurably to the beauty of each scene, binding together all of its diverse parts. Viewed horizontally, individual beds are traceable from pinnacle to pinnacle, mound to mound, ridge to ridge, across the intervening ravines. Viewed from above, the bands curve in and out of the valley like contour lines on a topographic map. A geologic story is written in the rocks of Badlands National Park, every bit as fascinating and colorful as their outward appearance. It is an account of 75 million years of accumulation with intermittent periods of erosion that began when the Rocky Mountains reared up in the West and spread sediments over vast expanses of the plains. The sand, silt, and clay, mixed and interbedded with volcanic ash, stacked up, layer upon flat-lying layer, until the pile was thousands of feet deep. In a final phase of volcanism as the uplift ended, white ash rained from the sky to frost the cake, completing the building stage.
During the Oligocene epoch 40 to 25 million years ago, the region that is now the White River Badlands supported many kinds of animals. The land was then lush, well watered, and much warmer than now. The animals, mostly mammals, roamed the floodplains; many died in floods and were quickly buried in river sediments. Conditions for preservation were excellent; the Oligocene beds are one of the world's richest vertebrate fossil sites, though they represent only a short segment of Earth history.
Broad regional uplift raised the land about 5 million years ago and initiated the erosion that created the Badlands. The White River, which now flows west to east five or ten miles south of the park, eroded a scarp, the beginning of what is now called the Wall. Numerous small streams and rills furrowed the scarp face and eventually intersected to create the Badlands topography. Each rainstorm over the next 5 million years chewed away at the Wall, making its crest recede northward away from the river as its base followed suit. This is an old story in the arid and semi-arid regions of the West. It always happens in rocks that are relatively non-resistant erosion and it always starts with a scarp.
At Badlands National Park, the White River produced the scarp. The physical character of the Badlands varies considerably according to the nature of the materials. For example, the frosting of volcanic ash at Badlands National Park succumbs quickly as the Wall advances northward into the upper plain - the original land surface - exposing the more durable underlying beds of the Brule formation. Nature's answer to great resistance is to carve steeper slopes, resulting here in incredibly slender spires above knife-sharp ridges and intricately creased slopes. Deeper into the layer cake, the more rounded ridges and spurs, and gentler slopes, reflect the softer mudstone of the Chadron formation. Numerous islands of Chadron mudstone dot the plain in front of the Wall. They look like nothing more than mud mounds, except for their striking color-banding which matches perfectly that of the base of the Wall. They are remnants, soon to be gone, of the earlier, ever changing Badlands.