South Dakota’s landforms divide along the Missouri River. East of it lie glacial landforms: the Coteau des Prairies plateau, the flat James River lowland, and thousands of small lakes left by ice sheets. West of it lie unglaciated plains of Cretaceous shale, broken by buttes, the eroded pinnacles of the Badlands, and the Black Hills, a granite-cored dome that rises to Black Elk Peak, the state’s highest point at 2,208 m (7,244 ft). The lowest point is the shore of Big Stone Lake on the Minnesota border, at about 294 m (965 ft). Across the state’s 199,730 km² (77,116 sq mi), each of these surfaces records a different chapter of the past 2.5 billion years, and reading them in order, oldest first, explains why they sit where they do.
| When | What happened | Where you see it now |
|---|---|---|
| Over 2.5 billion years ago | Archean granites form the crystalline basement | Core of the Black Hills |
| About 1.71 billion years ago | Harney Peak granite intrudes | Black Elk Peak and the central Black Hills |
| Late Cambrian to Mississippian | Shallow seas lay down sandstone and limestone | Spearfish Canyon walls, Wind Cave, Jewel Cave |
| Late Cretaceous, about 75 million years ago | Western Interior Seaway deposits the Pierre Shale | Plains of West River, base of the Badlands |
| Early Cenozoic | Black Hills uplifted, with volcanic activity in the north | The Black Hills dome, Bear Butte |
| About 37 to 23 million years ago | Rivers and ash falls build the White River Group | Badlands National Park |
| Pleistocene | Ice sheets cover the east and divert the Missouri | Coteau des Prairies, James River lowland, Big Stone Lake |
| 1946 onward | Missouri River mainstem dams | Lake Oahe and three other reservoirs |
Precambrian Granite and Paleozoic Seas in the Black Hills
The oldest rocks in South Dakota, more than 2.5 billion years old, are the Little Elk and Bear Mountain granites of the Archean eon, exposed today in the heart of the Black Hills. Around 1.71 billion years ago a new body of magma cooled underground to form the Harney Peak granite, named after the summit it forms. Black Elk Peak, renamed from Harney Peak by the U.S. Board on Geographic Names on 11 August 2016 to honour the Lakota medicine man Black Elk, is its summit. Nearby, granite has weathered into the Needles of Custer State Park, pillars that the sculptor Gutzon Borglum first proposed carving before choosing Mount Rushmore, 6 km east-northeast of the peak, where he worked from 1927 to 1941.
At the other end of the state, the Proterozoic left a very different rock: the pink Sioux Quartzite, about 3,000 m thick, which crops out in the southeast and was quarried for many prominent buildings in Sioux Falls. Its fine red layers also yielded the pipestone that Plains peoples carved into ceremonial pipes.
After a long gap of erosion, seas spread across the region in the Late Cambrian. Their beach sands and gravels became the Deadwood Formation, which stands about 120 m (400 ft) thick in the walls of Spearfish Canyon in the northern Black Hills. Later Paleozoic seas deposited limestone, and when sea level fell at the end of the Mississippian the Pahasapa Limestone was dissolved into fissures, sinkholes and caves. The Laramide uplift later enlarged those voids into two of the longest caves on Earth, both formed mainly in the Pahasapa. Jewel Cave, protected as a national monument since 1908, has 354.6 km of mapped passages; Wind Cave, the first cave anywhere to become a national park in 1903, has 270.4 km, and holds about 95 per cent of the world’s known boxwork, thin blades of calcite projecting from its walls.
The Western Interior Seaway and the Pierre Shale Plains
In the Late Cretaceous a shallow sea stretched from the Gulf of Mexico to the Arctic, splitting North America in two. Its dark mud became the Pierre Shale, the thickest and most widespread sedimentary unit in South Dakota, originally about 760 m (2,500 ft) thick near the Wyoming line and only tens of feet in the east. The formation takes its name from Fort Pierre on the Missouri, and after the sea withdrew, the shale east of the Black Hills was eroded into a broad undulating plain. West of the Missouri the sea also laid down Niobrara chalk holding mosasaur and plesiosaur bones.
The low permeability of the shale left a poorly drained landscape cut by streams, dotted with small conical tepee buttes where lenses of harder limestone resist erosion. As the seaway retreated it left the beach sands of the Fox Hills Formation and then up to 150 m of river sediment in the Hell Creek Formation, with Triceratops and Tyrannosaurus bones. An inch-thick boundary clay above it records the asteroid impact of 66 million years ago.
The Laramide Uplift of the Black Hills Dome
With the sea gone, the same Laramide mountain-building that raised the Rocky Mountains pushed up the Black Hills in the early Cenozoic, accompanied by volcanic activity in the northern hills. Erosion then stripped the arched sedimentary layers from the top of the dome, exposing the Precambrian core and leaving the younger rocks as rings dipping outward, a pattern often compared to a target. The finished range covers about 16,000 km² (6,000 sq mi) of South Dakota and eastern Wyoming, with peaks rising 600 to 1,200 m above their bases, and it is generally treated as an outlier of the Rocky Mountain system.
The uplift also drained the caves. As the land rose the water table fell, and in Wind Cave it now sits about 150 m below the surface. Streams cutting down through the rising rings carved gorges such as Spearfish Canyon, between roughly 60 and 30 million years ago. On the northeastern edge of the hills near Sturgis, Bear Butte is a laccolith, a blister of Eocene magma that pushed up the overlying rock from below, though some of it may once have broken through as a long-vanished volcano; it is sacred to the Lakota as Matȟó Pahá and to the Cheyenne, and became a state park in 1961.
White River Ash and the Carving of the Badlands
Through the Eocene the Black Hills shed debris eastward, and iron minerals in the exposed shale weathered into bright red and yellow soils, now seen as the Yellow Mounds of Badlands National Park. Between about 37 and 23 million years ago, rivers and floodplains spread the sediments of the White River Group across the plains, burying the hills to within about 600 m (2,000 ft) of their crests. From oldest to youngest, its Chadron, Brule and Sharps formations include volcanic ash blown in from eruptions far to the west; the white Rocky Ford ash that caps the Brule came from rhyolite eruptions in Oregon’s Western Cascades. The Brule alone has yielded about 150 genera of fossil mammals, from rodents to rhinoceroses, horses and camels.
Deposition stopped in the early Miocene. For roughly the past two million years, as the land slowly rose, rivers and storms have been cutting into these soft beds. The loose sediments absorb little rain, so summer thunderstorms strip them away by up to about 2.5 cm a year on bare slopes, and the Brule breaks into the jagged pinnacles and knife-edge walls that define the Badlands. The landform is geologically young and still disappearing. For the park itself, see this Badlands National Park guide, and for the flat-topped remnants that dot West River, this explainer on what a butte is.
Ice Sheets, Big Stone Lake and the Missouri River Dams
The youngest natural layer covers the east. During the Pleistocene, beginning about two million years ago, several ice sheets advanced into South Dakota and spread till, a mix of clay, sand and boulders, over most of the land east of the Missouri. The Coteau des Prairies, a flatiron-shaped plateau about 320 km long and 160 km wide, holds glacial deposits up to about 275 m (900 ft) thick on a core of resistant Cretaceous shale. Late in the last glaciation the ice split around it, sending the James lobe down the lowland to the west and the Des Moines lobe to the east, which deepened both valleys. That is why the James River lowland is so flat and the coteau so pocked with small glacial lakes, and why the Big Sioux River rises on the plateau and flows south past Watertown and Sioux Falls.
The ice also shaped the state’s rivers. Before the Ice Age, parts of the Missouri drained north toward Hudson Bay; advancing glaciers diverted it southeastward and joined its segments into a single river, which now splits the state into East River and West River. In the northeastern corner, glacial Lake Agassiz drained south through the Traverse Gap into Glacial River Warren at the end of the last ice age, and the valley it cut now holds Big Stone Lake, the source of the Minnesota River and South Dakota’s lowest ground.
The newest landforms in South Dakota are artificial. Under the Flood Control Act of 1944, the U.S. Army Corps of Engineers began building the Oahe Dam just north of Pierre in 1948 and opened it in 1962. Its reservoir, Lake Oahe, stretches about 372 km up the Missouri toward Bismarck, covers about 1,500 km² and has 3,620 km of shoreline. With Lake Sharpe, Lake Francis Case and Lewis and Clark Lake downstream, it turns the river through the centre of the state into a chain of lakes. How that river connects the Rockies to the Mississippi is traced in this complete guide to the Missouri River.