A badland is a dry landscape where soft sedimentary rock and clay-rich soil have been so heavily eroded by water that the ground is cut into a maze of steep gullies, sharp ridges, buttes and hoodoos, with little vegetation and almost no soil. Badlands form where three things meet: rock such as mudstone, shale or volcanic ash that is easy to erode but lets little water soak in; sparse plant cover; and rain that comes rarely but heavily. The type example is Badlands National Park in South Dakota, which protects 982 km² of eroded buttes and pinnacles that are wearing away by up to about 25 mm a year. Others include the Red Deer River valley of Alberta, the Painted Desert of Arizona and the Bardenas Reales of northern Spain. Badlands occur on every continent except Antarctica.
What sets badlands apart from an ordinary eroded hillside is the density of their channels. Geomorphologists measure drainage density, the total length of stream channels per square kilometre of land, and in badlands it runs from about 48 to 464 km per km², far higher than in vegetated country. Where a bed of harder sandstone lies on top of the soft rock, it shields what is beneath and leaves flat-topped buttes or slender hoodoos standing above the gullies. Some badlands also drain underground through "pipes", tunnels and cavities washed out of the clay, though not all do. Overgrazing or pollution that strips plant cover can create man-made badlands too. The six places below show the variations.
Badlands National Park, South Dakota: the original makȟóšiča
The word comes from here. The Lakota called the area makȟóšiča, from makȟá (land) and šíča (bad), because of its extreme temperatures, lack of water and rough terrain. French-Canadian fur trappers described it in similar terms as les mauvaises terres pour traverser, "bad lands to travel through". Geographers later borrowed the name for every landscape of the same kind.
The park's rocks tell a story of a sea turning into dry land. The oldest layer, the Pierre Shale, is dark marine mud laid down about 75 million years ago in the Western Interior Seaway, which reached from the Gulf of Mexico to the Arctic Ocean. Above it lie the river and floodplain sediments of the White River Group, deposited between about 37 and 23 million years ago: the Chadron Formation, the Brule Formation (about 32 to 26 million years ago) and the Sharps Formation, which caps the highest peaks. Ash from volcanic eruptions far to the west is mixed through the upper beds.
Erosion has worked on these layers for roughly the past two million years as the land slowly rose. The White River sediments soak up little rain, so brief summer thunderstorms strip bare slopes by as much as 2.5 cm a year, which makes these badlands geologically young and still changing. The White River that drains the park is named for its white-grey water, coloured by the sand, clay and volcanic ash it carries away, and the bentonite clay in these beds makes the slopes even easier to erode. The park, authorised in 1929 and established as a national monument in 1939 before becoming a national park in 1978, also protects the largest undisturbed mixed-grass prairie in the United States. Its highest point, Red Shirt Table, reaches 1,020 m (3,340 ft). Our Badlands National Park guide covers its trails and wildlife.
Dinosaur Provincial Park, Alberta: badlands as a fossil window
Badlands are poor land for farming but excellent land for fossil hunting, because erosion keeps exposing fresh rock and nothing grows to hide it. Dinosaur Provincial Park, about 220 km east of Calgary in the Red Deer River valley, is the clearest proof. The badlands there are carved from three Late Cretaceous formations, laid down over about 1.5 million years. The Dinosaur Park Formation, deposited between about 76.5 and 74.8 million years ago by rivers flowing towards the Western Interior Seaway, holds most of the articulated skeletons.
Fifty-eight dinosaur species have been found in the park, and more than 500 specimens have gone to museums around the world. That fossil record, together with the badlands and the cottonwood woodland along the river, earned it UNESCO World Heritage status on 26 October 1979. The park was established on 27 June 1955. It shows that badlands are not only desert: here the eroded slopes sit inside rolling prairie, with prickly pear and pincushion cacti flowering in late June among some of the most northerly cacti in North America.
The Painted Desert, Arizona: colour from the Chinle Formation
The Painted Desert is a badland on a much larger scale, running about 190 km from near the east end of Grand Canyon National Park southeast into Petrified Forest National Park, roughly 19,420 km² in all. Members of Francisco Vázquez de Coronado's expedition named it El Desierto Pintado in 1540 while looking for the Colorado River.
The colours come from the rock itself. The desert is cut into layers of siltstone, mudstone and shale of the Triassic Chinle Formation, rich in iron and manganese compounds that stain the hills red, lavender and grey. Thin beds of resistant limestone and volcanic rock cap the mesas, the same caprock principle that leaves buttes standing in South Dakota. Volcanic ash in the Chinle supplied the silica that turned buried logs into the petrified wood of the national park. The Mogollon Rim to the south casts a strong rain shadow, giving the area a cold desert climate with some of the lowest precipitation in northern Arizona. The Painted Desert guide follows its colours in more detail.
Bardenas Reales and Tabernas: the badlands of Spain
Bardenas Reales, Navarre: clay, gypsum and cabezos
Europe has badlands too. The Bardenas Reales in southeast Navarre cover about 418 km² of semi-desert in the Ebro valley, 45 km from north to south and 24 km east to west, at 280 to 659 m above sea level. The ground is made of clay, chalk and sandstone deposited in an inland basin after the rise of the Pyrenees, then eroded by water and wind into canyons, plateaus and isolated hills called cabezos. Its streams flow only seasonally and stay dry most of the year.
The Bardenas shows how rock chemistry changes a badland's look. The central White Bardena takes its name from salt crusts left by gypsum in the soil, and it is the most desert-like part, with broad plains and deep gullies. The Black Bardena, on the border with Aragon, is a set of plateaus at different heights cut by rivers and partly covered in vegetation. Between them a central plateau stands about 100 m above the surrounding ground. Across the whole area, gravel and sandstone sit on top, with fine sand, clay and limestone below, so the hard layers form the caps and the soft ones wash out beneath.
Tabernas, Almería: desert badlands of marl and sandstone
About 30 km north of the city of Almería, the Tabernas Desert covers a 280 km² nature reserve between the Sierra de los Filabres, the Sierra Alhamilla and the Sierra Nevada. The mountains block moist winds from the Mediterranean, and annual rainfall is only about 150 to 220 mm, most of it outside the hot months. It is often described as the only true desert in Europe.
The ingredients are textbook. Millions of years ago the corridor between the two sierras lay under the Mediterranean, and when the sea withdrew it left a basin of marl and sandstone. Rain here is rare but torrential, and the bare marl cannot absorb it, so storms cut gullies and arroyos instead, and the scarce plants cluster along those watercourses. Because the scenery resembles the American Southwest, film crews have used Tabernas since the 1950s, and spaghetti westerns were shot at purpose-built frontier towns such as Fort Bravo, one of three that still operate.
Valle dei Calanchi, Italy: the badlands under Civita di Bagnoregio
Not every badland lies in a desert. In central Italy, about 120 km north of Rome between Lake Bolsena and the Tiber valley, the Valle dei Calanchi ("valley of the badlands") shows the same landform in the green hills of northern Lazio. Two gullied valleys, the Fossato del Rio Torbido and the Fossato del Rio Chiaro, are cut into a base layer of soft marine clay. Above the clay lie layers of tuff and lava from old volcanoes. Streams, weather and past deforestation drive the erosion, and landslides take away the clay from below.
The result is a village on a shrinking pedestal. Civita di Bagnoregio sits on a block of tuff whose clay foundations keep eroding, which is why it is known as "the city that dies". It is home to only about 16 people, and since 1965 its only link to the outside world has been a reinforced-concrete footbridge from neighbouring Bagnoregio. Here the caprock that protects a butte in South Dakota protects a hilltop village, and the gullies of the calanchi are the same process eating away at its edges.
From South Dakota to Lazio, the recipe stays the same: soft, impermeable rock, too little vegetation to hold it, and rain that arrives in bursts. Change the rock and you change the colours and shapes, from grey Cretaceous mudstone in Alberta to Triassic reds in Arizona and gypsum-white plains in Navarre. To see what happens when the harder layer wins out, read our explainer on the hoodoo, the capped rock pillar that badlands leave behind.