From Ancient Inland Seas to Granite Peaks and Layered Badlands. Discover how uplift, erosion, and deep time shaped the dramatic landscapes of western South Dakota.
Black Hills and Badlands Geology is the story of two very different landscapes that sit side by side.
The Black Hills are a dome of very old, hard crystalline rock, ringed by tilted layers of younger rock. The Badlands are much younger, softer layers that still lie nearly flat, and rivers and rain are cutting them away fast. What looks rugged and dramatic today is the result of hundreds of millions of years of steady change.
If you've ever stood at an overlook and wondered why the Badlands look banded, how Mount Rushmore could be carved into solid rock, or why seashell fossils appear in the middle of the Great Plains, the answers are written in the layers beneath your feet.
The pale band is granite, in dark schist at Mount Rushmore.
Read the rocks
Choose a place, then a rock. The drawing and the notes change with your choice.
Highlighted: Harney Peak GraniteSchist, slate and quartziteDeadwood FormationMadison LimestoneSpearfish FormationHogback sandstonePhonolite at Devils Tower
WestEast
A simplified slice, not to scale. The dashed line shows layers that once covered the dome, now eroded away. The small sketch shows Devils Tower, off this slice.
Harney Peak Granite
About 1.7 billion years old, younger than the metamorphic rock it pushed into
Molten rock rose into the older metamorphic rocks and cooled while it was still underground. This is the Harney Peak Granite, which is still the rock's name in geology, and the mountain itself is Black Elk Peak today.
What to notice
Granite tends to form rounded domes, vertical spires, and exposed crystalline surfaces, and you may see visible mineral grains in the rock face.
Where to see it
Mount Rushmore, Needles Highway, Sylvan Lake and Cathedral Spires.
Crystalline core: This is the very old, hard rock at the center. The granite is younger than the rock it pushed into.
Schist, slate and quartzite
Older than the granite
Sediment and volcanic rock filled basins along active faults, and heat and pressure turned that pile into the schist, slate and quartzite of the central Hills.
What to notice
Schist is a dark, flaky rock, often sparkling with mica. It sits around the granite in the central Black Hills.
Where to see it
Around Keystone, and the Open Cut from the Sanford Lab Homestake Visitor Center in Lead.
Crystalline core: This is the very old, hard rock at the center. The granite is younger than the rock it pushed into.
Deadwood Formation
About 500 million years old
A sea crept in, and its shoreline sand became the Deadwood Formation. Erosion had worn the old mountains down, more than a billion years went by, and this rock sits straight on that worn surface.
What to notice
Look for sandstone and shale sitting straight on the older rock. Geologists call the gap between them the Great Unconformity.
Where to see it
Around the town of Deadwood.
Tilted layers around the dome: They generally get younger as you go outward, and intrusions (magma that pushed in later), faults and erosion complicate it.
Madison Limestone
About 350 million years old, and locally it's the Pahasapa Limestone
A warm, shallow sea built the thick limestone of Spearfish Canyon, Wind Cave and Jewel Cave. Limestone forms more vertical walls and defined ledges than the softer clay of the Badlands.
What to notice
Look for the horizontal bands within the cliff faces. Notice how some layers protrude slightly while others recede, because those differences reflect variations in hardness and composition.
Where to see it
Spearfish Canyon, Wind Cave and Jewel Cave.
Tilted layers around the dome: They generally get younger as you go outward, and intrusions (magma that pushed in later), faults and erosion complicate it.
Spearfish Formation
About 250 million years old
The coast turned hot and dry and left the red Spearfish Formation. This soft rock wears down into a valley and forms a ring of red rock around the Hills.
What to notice
Look for red siltstone with gypsum.
Where to see it
South of Rapid City, and the red cliffs at Devils Tower.
Tilted layers around the dome: They generally get younger as you go outward, and intrusions (magma that pushed in later), faults and erosion complicate it.
Hogback sandstone
About 145 to 100 million years old: the Lakota Formation and Fall River Sandstone
The outer ridge, which geologists call the hogback, is hard sandstone, and it's the last high ground before the plains. The rusty yellow sandstone of the Lakota Formation holds cycads, fossil wood and the footprints of meat-eating dinosaurs.
What to notice
Look for the rusty yellow sandstone on the ridge. Beyond it, gray sea shale lies under the prairie, and because shale is soft it doesn't hold up a ridge.
Where to see it
Dinosaur Park in Rapid City.
Tilted layers around the dome: They generally get younger as you go outward, and intrusions (magma that pushed in later), faults and erosion complicate it.
Phonolite at Devils Tower
Around 50 million years old
Not every igneous rock here is ancient. In the northern Hills and just beyond them, magma pushed into the sedimentary layers during or shortly after the uplift. Geologists agree Devils Tower began as magma, and they still can't agree on exactly how it cooled.
What to notice
Devils Tower is the largest example of columnar jointing in the world. Those columns formed as the rock cooled and shrank.
Where to see it
Devils Tower and Terry Peak.
Younger igneous rock: This rock cuts through the older layers.
The small sketch shows Devils Tower, off this slice.
Oldest at the bottom, not to scale. The heavy dashed line marks the very long gap above the Yellow Mounds. The pale patches at right are loose surface deposits on the low ground.
Surface deposits
Much younger than the bedrock stack
The Park Service lists stream alluvium, landslide deposits, and wind-blown sand and silt lying on the surface in the park.
What to notice
Look for loose stream, landslide and wind-blown material lying over the bedrock. The bedrock stack and that loose material are two different records.
Where to see it
Creek bottoms, and the Mammoth Site in Hot Springs for ice age deposits.
Above the gap: The bedrock here starts about 37 million years ago.
Sharps Formation
30 to 28 million years old
The Sharps Formation is the youngest major bedrock formation in this sequence, with sandstones from old river channels.
What to notice
Look above the pale ash band for siltstone and sandstone from old river channels.
Where to see it
Cedar Pass.
Above the gap: The bedrock here starts about 37 million years ago.
Rockyford Ash
About 30 million years old
The Rockyford Ash is an ash unit at the base of the Sharps Formation, and the Park Service calls it the only layer of pure volcanic ash in the park.
What to notice
Look for a pale band of volcanic ash.
Above the gap: The bedrock here starts about 37 million years ago.
Brule Formation
34 to 30 million years old
The Brule Formation is the banded layer most people picture, laid down as the climate turned cooler and drier and the floodplains gave way to open savanna.
What to notice
Some of those bands are ancient soils, so a thin stripe of color can be a piece of old ground. The sandy bands nearby are where a river ran.
Where to see it
Cedar Pass and the Conata Picnic Area.
Above the gap: The bedrock here starts about 37 million years ago.
Chadron Formation
37 to 34 million years old
The Chadron Formation is the light gray claystone above the gap, laid down on a hot, wet floodplain the Park Service compares to the Everglades. It holds ancient alligators and brontotheres, the largest fossil mammals in the Badlands.
What to notice
Look for rounded gray slopes above the yellow.
Where to see it
Yellow Mounds Overlook.
Above the gap: The bedrock here starts about 37 million years ago.
Yellow Mounds
Formed after the sea drained away
The Yellow Mounds sit on top of the Pierre Shale, and they're a paleosol, the same shale after it weathered into soil once the sea drained away, with a mineral called goethite giving them their mustard color.
What to notice
Look for the mustard-colored mounds below the gray slopes. The next rock in the stack doesn't start until about 37 million years ago, so this boundary holds a very long gap.
Where to see it
Yellow Mounds Overlook.
Below the gap: These are the sea mud and the soil it weathered into.
Pierre Shale
75 to 69 million years old
The Pierre Shale is the base, dark gray sea mud laid down between 75 and 69 million years ago. It records a sea, so it holds marine life such as ammonites, baculites and mosasaurs, which were giant marine reptiles.
What to notice
Look for dark gray shale at the lowest exposures. It's the base of the Badlands and the soft ground under the prairie.
Where to see it
Low exposures in Badlands National Park, and the plains around the Hills.
Below the gap: These are the sea mud and the soil it weathered into.
The pale patches at right are loose surface deposits on the low ground.
Why the Black Hills and the Badlands Look So Different
Two places this close can look nothing alike, and it comes down to age, hardness and shape. The granite at the center of the Hills is more than twenty times older than the oldest rock in the Badlands. Granite formed from molten rock deep underground, and its crystals lock together, while the Badlands are soft mudstone, claystone and ash, so the Hills keep their peaks and the Badlands get cut into fine ridges.
The Black Hills are also a dome, with layers that tilt away from the center, while the Badlands layers still lie nearly flat, which is why the stripes run level across the walls for miles.
They're part of the same regional geological system, though. The uplift raised the Hills, erosion carried material off them, and the Park Service says most of the ash in the Badlands washed in with sediment eroded from the Black Hills.
The Hills keep their peaks.
The Badlands get cut into fine ridges.
So a good part of the Badlands is the Black Hills, moved east and laid down again.
Daniel Milks, owner and guide at My XO Adventures.
Reading the Landscape as a Visitor
When we guide guests through western South Dakota, we often pause at overlooks and ask a simple question: what do you notice first? Color, texture, layering, sharp ridges and rounded hills each connect directly to geological processes.
The Mount Rushmore and the Big Badlands tour crosses from Black Hills granite to Badlands sediment in a single day, which is the whole story in one drive.
When we guide guests through this region, one of the first things we explain is that the Black Hills and the Badlands are chapters in the same geologic history. The granite peaks, the limestone canyons, the striped clay hills and the fossils beneath the surface all connect back to one long sequence of events.
Dates this old are estimates, so I've rounded them.
Billions of years ago
More than 2.5 billion years agoThe oldest rocks in this story formed, and they're among the oldest exposed rocks in the Black Hills.
Roughly 2.2 to 1.9 billion years agoMuch of the sediment and volcanic rock of the central Hills piled up in basins, and heat and pressure later turned it into schist, slate and quartzite.
About 1.7 billion years agoThe Harney Peak Granite cooled deep underground, after pushing into those older rocks.
The next billion years and moreErosion wore the old mountains down, and no rock from this long span survives here.
Hundreds of millions of years ago
About 500 million years agoA sea crept in, and its shoreline sand became the Deadwood Formation.
About 350 million years agoA warm, shallow sea built the thick limestone of Spearfish Canyon, Wind Cave and Jewel Cave.
About 320 to 250 million years agoThe first cave passages formed, the Minnelusa Formation buried them, and the coast later turned hot and dry and left the red Spearfish Formation.
About 200 to 100 million years agoRivers, floodplains and shorelines left the rock that holds this region's dinosaur fossils.
The last 75 million years
About 75 to 69 million years agoThe Western Interior Seaway laid down the Pierre Shale, the rock at the base of Badlands National Park.
Roughly 70 to 60 million years agoThe Black Hills uplift began, and sources differ on the exact start.
About 60 to 50 million years agoMost of the dome developed, and magma made Devils Tower around 50 million years ago.
About 37 to 28 million years agoRivers and volcanic ash built the Chadron, Brule and Sharps Formations of the Badlands.
The last few million yearsRenewed regional uplift and downcutting went on shaping the land, and about 500,000 years ago the Cheyenne and White Rivers began carving the Badlands.
How the Black Hills Were Formed
The Black Hills are a dome. Deep tectonic forces pushed ancient rock upward, the younger sedimentary layers above it tilted outward around the rise, and erosion stripped the top away to expose the core. What appears today as forested mountains rising from the plains is a geological dome created by pressure from below and shaped by millions of years of erosion.
The Oldest Rocks and the Granite Core
To understand the geologic history of the Black Hills, we have to go much farther back than the uplift. Near Little Elk Creek, north of Nemo, there's a banded rock more than 2.5 billion years old that started out as granite, and it's among the oldest exposed rocks in the Black Hills.
About 1.7 billion years ago, during an ancient continental collision, molten rock rose into older metamorphic rocks and cooled while it was still underground. That's the Park Service's regional figure, and you may see 1.6 billion on one of its other pages, because sources round this date differently. The Forest Service describes the granite's outcrops as complex igneous rocks made of hundreds of separate intrusions, and some of the melt cooled into pegmatite, a very coarse rock with large crystals of feldspar, mica and quartz.
After the granite, the record here stops. Geologists call a gap like this an unconformity, and this one is known as the Great Unconformity.
Tilted mica schist.
The Black Hills Uplift
The uplift came far later, and it was a long process, so different sources date different stages of it. It belongs to the Laramide orogeny, the mountain building the Park Service dates across this region to about 80 to 50 million years ago. For the Black Hills themselves, one Park Service page says the uplift began about 70 million years ago, a 2009 Park Service report puts the start closer to 58 million years ago, and the agency's geology atlas says most of the development happened about 60 to 50 million years ago. Instead of forming a long mountain chain, the pressure here pushed the ancient core upward in a dome-like structure.
The dome kept changing after that. Streams stripped the layers from its top, groundwater enlarged the caves in the limestone, and that 2009 report describes later regional uplift that renewed the downcutting of the streams.
Imagine pressing upward beneath a stack of layered blankets. The center rises first, and the outer layers tilt away from the middle. That's what happened here, and it explains why the Black Hills look so different from the surrounding plains and why so many South Dakota rock formations sit within a relatively small area.
The Rings Around the Hills
Today, if you travel outward from the center of the Black Hills, you can see those tilted layers forming rings around the core, a pattern geologists sometimes call the "bullseye" structure of the Hills. The sedimentary formations generally get younger as you go outward from the ancient crystalline core.
Just outside the old crystalline rock you come to the sandstone of the Deadwood Formation and then the limestone that makes the canyons and holds the caves. Past the limestone come red sandstones and shales, and then the soft red Spearfish Formation. The table further down this page names each of these rocks and where you can see them.
The red Spearfish Formation.
Harney Peak Granite
Schist, slate and quartzite
Deadwood Formation
Madison Limestone
Spearfish Formation
Hogback sandstone
Phonolite at Devils Tower
A simplified slice, not to scale. The dashed line shows layers that once covered the dome, now eroded away.
The Granite Core and Mount Rushmore
Mount Rushmore is carved into Harney Peak Granite, part of the granite core of the Black Hills that the uplift and erosion brought to the surface. Granite is composed primarily of quartz, feldspar, and mica, and its interlocking mineral structure makes it strong and resistant to erosion compared to the surrounding sedimentary rock. That durability is what allowed sculptors to carve Mount Rushmore with confidence that it would endure, because softer rock such as shale or claystone would have eroded too quickly. Without the exposure of this ancient granite, the monument could not have been carved where it stands today.
Even so, granite isn't immune to change. Water seeps into fractures, freeze and thaw cycles widen cracks, and weathering keeps reshaping the surface. The monument stands because the rock erodes slowly.
Granite Formations Beyond the Monument
Cathedral Spires and the formations along the Needles Highway are striking examples of granite shaped by long-term weathering. Vertical cracks, known as joints, formed as the granite cooled and later expanded when overlying rock was removed. Water worked into those fractures, freeze-thaw cycles widened them, and the surrounding material eroded away. What remains are the tall, narrow spires that define the skyline in parts of the central Black Hills.
The Cathedral Spires.Granite meeting a meadow in the Black Hills.
Younger Igneous Rock in the Northern Hills
Geologists agree Devils Tower began as magma, and they still can't agree on exactly how it cooled, because the erosion that exposed it erased the evidence.
Devils Tower and Its Red Rock
12
The numbers on the photo match the numbered notes.
Devils Tower
The Park Service says the Tower formed around 50 million years ago, one to two miles below the surface, and erosion began to expose it somewhere between 5 and 10 million years ago.
Its rock is a rare one called phonolite porphyry, and it's the largest example of columnar jointing in the world. Those columns formed as the rock cooled and shrank.
Spearfish Formation
The red rock in the photo is the soft Spearfish Formation. It wears down into a valley and forms a ring of red rock around the Hills.
Look for red siltstone with gypsum.
The Ancient Inland Sea of South Dakota
Long before the Black Hills uplift reshaped the region, seas came and went here again and again. The National Park Service says a warm, shallow sea covered this area about 350 million years ago, and seas kept advancing and retreating for 240 million years after that. The one most people mean by the ancient inland sea came near the end. The Western Interior Seaway stretched from the Gulf of Mexico to the Arctic Ocean during the Cretaceous period, covering what is now the Great Plains, and it laid down the Pierre Shale at the base of Badlands National Park.
Fine mud, sand and calcium-rich material settled to the sea floor in horizontal sheets, stacking one atop another like pages in a book. In deeper water the mud hardened into shale, and in shallower water shifting currents left sand that became sandstone. The limestone cliffs of places like Spearfish Canyon began on the floor of those much older seas, as shell fragments, microscopic organisms and calcium carbonate compacted into thick rock layers.
The seas weren't the whole story, because between them this land was dry coast, river plain and shoreline. That's where this region's dinosaur rock comes from, and it sits around the outside of the Hills. The Morrison Formation has given up Allosaurus, Barosaurus and Camarasaurus.
If you look closely at certain sedimentary exposures in western South Dakota, you may even see marine fossils embedded within the rock, and if you do, leave them in place. Seashell fossils in this region are proof that the landscape we now associate with pine forests and prairie was once underwater. Getting those sea floors back to the surface took two things, uplift and erosion, because without them these layers would have stayed buried.
Spearfish Canyon Geology
Spearfish Canyon geology offers one of the clearest surface examples of Black Hills and Badlands Geology in action. Towering limestone walls rise above the canyon floor, exposing rock that began on the floor of a shallow sea hundreds of millions of years ago. Those layers accumulated in horizontal sheets long before the uplift, and when tectonic forces pushed the region upward, they were lifted and tilted.
Spearfish Creek then cut down through the uplifted rock, deepening and widening the canyon over time. Water exploits natural fractures in limestone, freeze-thaw cycles expand cracks, and gravity pulls loosened rock downward.
Limestone cliffs in Spearfish Canyon.
Caves in the Black Hills: The Underground Side of This Story
The geology continues underground in the limestone of the Black Hills. Most of the region's well known caves developed in thick limestone that was laid down beneath a warm, shallow sea about 350 million years ago. The National Park Service says the first passages in Wind Cave formed about 320 million years ago, that the Minnelusa Formation was then laid down above the limestone, and that the Black Hills uplift later opened more fractures. Over time, acidic water dissolved the limestone, slowly carving passages, chambers, and underground networks.
This process, known as dissolution, explains why many Black Hills caves feel smooth and sculpted. In other areas, ceilings collapsed or fractured later, creating jagged formations. Both styles are part of the same geologic history of the Black Hills, shaped by uplift, water movement, and time.
If you're planning to add a cave stop to your trip, I put together a practical guide to the best options, what each cave experience feels like, and how to choose the right one for your group. You can use the Black Hills cave tours guide to compare tours, timing, accessibility, and what you'll see once you're underground.
The sedimentary layers of the Badlands in low light.
How the Badlands Were Formed
The Badlands were formed when thick layers of sediment were deposited across western South Dakota and then slowly carved by erosion into ridges, buttes, gullies, and sharp spires. They did not rise up like a mountain range. The land was built from layered deposits and then sculpted by water, wind, and freeze-thaw cycles, so the formations you see today are the result of material being removed.
From Floodplains to Layered Formations
After marine conditions ended, the region shifted into broad floodplains fed by ancient rivers. Mud, sand, and silt spread across the landscape in repeated cycles, and volcanic ash from eruptions in the Great Basin, where Utah and Nevada are today, washed in. Over millions of years, these layers hardened into the claystone, siltstone, and sandstone you see across hillsides and canyon walls. About 500,000 years ago the Cheyenne and White Rivers began carving through them, and that carving made the Badlands.
Sedimentary Rock Layers in Badlands National Park
Badlands rock holds three different kinds of layer, and it helps to keep them apart. A formation is a body of rock that geologists can map, a paleosol is an ancient soil preserved in the rock, and an ash unit is a layer made of volcanic ash.
The Yellow Mounds from Yellow Mounds Overlook.
Why the Badlands Look Striped
You do not need a geology degree to read the landscape. Each band of color records a different time of deposition, shaped by shifting rivers, floodplains, climate patterns, and even distant volcanic eruptions. When a floodplain sat still long enough, plants grew and the ground weathered, and then the next flood buried that soil.
Start by identifying the horizontal bands, and notice which layers appear thicker and which are thin. Thicker, more resistant layers often form ledges or flat-topped ridges, while softer layers form rounded or crumbling slopes. In some areas you may even spot fossil-bearing layers within the sediment, and any fossil you see stays where it is.
Red and tan bands in the buttes.
Surface deposits
Sharps Formation
Rockyford Ash
Brule Formation
Chadron Formation
Yellow Mounds
Pierre Shale
Oldest at the bottom, not to scale. The heavy dashed line marks the very long gap above the Yellow Mounds.
The Badlands Erosion Process
The dramatic landforms of the Badlands exist because the material is soft enough to erode quickly. Rainfall is the most powerful driver. When storms move across western South Dakota, water runs quickly over exposed slopes instead of soaking deeply into the ground, and that runoff cuts narrow channels into the soft rock. Each heavy rain deepens gullies and sharpens ridgelines, and over time small channels grow into larger drainage systems. When you stand at an overlook and see parallel ridges stretching across the landscape, you're looking at patterns created by repeated water flow.
Winter adds another layer to the process. Water seeps into cracks and pores within the rock, and when it freezes it expands and widens them, so loosened material breaks free when the ice melts. That's one reason certain slopes appear crumbly or unstable underfoot. Wind removes fine particles from exposed surfaces, especially during dry periods, and gravity pulls loosened sediment downward. The contrast between soft and slightly harder layers creates the ridges and spires that give the Badlands their distinct appearance.
The National Park Service estimates the Badlands erode about one inch a year, which it calls a rapid rate for rocks, and it says the granite of the Black Hills erodes about one inch in 10,000 years. It also says scientists estimate the Badlands will have eroded completely in the next 500,000 years. Those are broad, round estimates. Real erosion rates vary with the material, the location and the conditions. The ridges you photograph today will look slightly different decades from now.
Fossils in Badlands National Park
Black Hills and Badlands Geology also preserves evidence of ancient life, particularly in the fossil bearing layers of Badlands National Park. Preserved within the sedimentary rock layers are the remains of animals that lived here roughly 37 to 28 million years ago, and these fossils provide a detailed record of ancient ecosystems that once covered western South Dakota.
When most visitors think about fossils, they imagine dinosaurs. Badlands National Park tells a different story. The park's mammal-bearing formations developed after the non-avian dinosaurs had gone extinct. Instead, this region supported early mammals in a warm, subtropical environment filled with river systems and vegetation very different from today's prairie.
What Types of Fossils Are Found Here
The fossil beds of the Badlands contain:
Early horses about the size of small dogs
Rhinoceros-like mammals
Saber-toothed predators
Small camels
Tortoises
Rodent species
Ancient plants
The layers don't all hold the same animals, because the land kept changing. The Pierre Shale holds sea life. The Chadron Formation holds ancient alligators and brontotheres, the largest fossil mammals in the Badlands, from the hot, wet years. The Brule Formation holds plant-eating mammals such as the oreodonts, which are one of the most common fossils found in the Badlands, along with the cat-like nimravids with saber teeth.
Why the Badlands Preserve Fossils So Well
Three things make this region so good at preserving fossils. Flood events covered remains quickly, which protected them from scavengers and decay. Clay and silt are fine enough to preserve delicate structures. And the same erosion that sculpts the landscape gradually exposes fossils at the surface, which allows scientists to discover them.
Fossils add an entirely different dimension to Black Hills and Badlands Geology, because they connect rock layers to living ecosystems. When you stand in the Badlands today, you're standing in a place that once supported subtropical forests and early mammals, and when we guide guests through this region, fossils often spark the deepest curiosity.
If you want to see them, the best place is the Ben Reifel Visitor Center. The Fossil Preparation Lab there is typically open from the second week in June through the third week of September, and you can watch park paleontologists prepare real fossils and ask them questions. Every fossil you see in the park stays where it is, and the law protects them all.
Where to See Geology Come to Life
You don't have to be a geologist to recognize the forces that shaped the Black Hills and Badlands. Some places make the story easy to read just by standing still and looking around. In the Black Hills, granite peaks like the Cathedral Spires reveal the uplifted core of the region, and the surrounding limestone cliffs show younger layers that tilted outward as the dome rose.
The Badlands tell their story in color. Soft clay and ash layers form steep slopes that expose millions of years of ancient environments, and places like Pinnacles Overlook, Big Badlands Overlook, and the Notch Trail offer clear views of sediment bands shaped by floods, droughts, and long-vanished wildlife. When you visit these areas after reviewing the Unique Landforms, Ecology, or Weather pages, the landscape feels alive with meaning.
Pinnacles Overlook.
The Rocks at a Glance
This table runs roughly from the center of the Black Hills outward, and then from the bottom of the Badlands to the top. Ages are rounded, and the first three rows are the crystalline core, so their order isn't a simple outward sequence.
Black Hills rocks
Black Hills rocks
Age
Rock
What it does in the landscape
Where you can see it
More than 2.5 billion years
Banded gneiss near Little Elk Creek, called the Little Elk Granite
Among the oldest exposed rocks in the Black Hills
The Little Elk Creek area north of Nemo
Older than the granite
Schist, slate and quartzite
The darker, often sparkling rock around the granite
Around Keystone, and the Open Cut from the Sanford Lab Homestake Visitor Center in Lead
About 1.7 billion years
Harney Peak Granite and pegmatite
The peaks, spires and carving surface of the central Hills
Mount Rushmore, Needles Highway, Sylvan Lake and Cathedral Spires
About 500 million years
Deadwood Formation, sandstone and shale
The first layer above the Great Unconformity
Around the town of Deadwood
About 350 million years
Madison Limestone, also called Pahasapa Limestone
The cliffs of the canyons and the rock that holds the big caves
Spearfish Canyon, Wind Cave and Jewel Cave
Around 300 million years
Minnelusa Formation, sandstone, shale and limestone
The layer that buried the early caves
West Rapid City
Permian, within about 300 to 250 million years
Opeche Shale and Minnekahta Limestone
Red siltstone and a thin pink to gray limestone
The hills along US Highway 16 south of Rapid City
About 250 million years
Spearfish Formation, red siltstone with gypsum
The soft red ring around the Hills
South of Rapid City, and the red cliffs at Devils Tower
About 200 to 145 million years
Sundance Formation and neighboring sandstones and shales
Yellow cliffs and ledges that change from place to place
The yellow cliffs around Devils Tower, and pink to maroon sandstone at Hot Springs
About 145 to 100 million years
Lakota Formation and Fall River Sandstone
The hard sandstone of the hogback ridge
Dinosaur Park in Rapid City
Around 50 million years for Devils Tower
Phonolite and other igneous rock of the northern Hills
Peaks and towers that resisted erosion
Devils Tower and Terry Peak
Badlands rocks
Badlands rocks
Age
Rock
What it does in the landscape
Where you can see it
75 to 69 million years
Pierre Shale, a formation of dark marine shale
The base of the Badlands and the soft ground under the prairie
Low exposures in Badlands National Park, and the plains around the Hills
After the sea drained away
Yellow Mounds, a paleosol
The mustard-colored mounds
Yellow Mounds Overlook
37 to 34 million years
Chadron Formation, light gray claystone
Rounded gray slopes above the yellow
Yellow Mounds Overlook
34 to 30 million years
Brule Formation, banded mudstone and sandstone
The stripes most people picture
Cedar Pass and the Conata Picnic Area
About 30 million years
Rockyford Ash, an ash unit at the base of the Sharps
A pale band of volcanic ash
Where the base of the Sharps is exposed in the park
30 to 28 million years
Sharps Formation, siltstone and river sandstone
The top of the major bedrock sequence
Cedar Pass
Much younger
Stream, landslide and wind-blown deposits on the surface
Loose material above the bedrock
Creek bottoms, and the Mammoth Site in Hot Springs for ice age deposits
Rapid City to Mount Rushmore
If you drive US Highway 16 from Rapid City toward Mount Rushmore, you're heading generally toward the older crystalline core of the Hills. You leave the red rocks south of town, climb through sandstone and limestone, and reach schist around Keystone. Not every rock along the way is older than the last, because faults and folds move things around, and the granite at the end of the drive is younger than the schist you pass before it.
Badlands Loop Road
Badlands Loop Road gives you the layers as you go. Yellow Mounds Overlook shows the yellow paleosol with the gray Chadron Formation above it, and around Cedar Pass you're looking at the banded Brule Formation and the Sharps Formation above that.
Which of Our Tours Shows What
Our Private Mount Rushmore and Black Hills Scenic Tour has Iron Mountain Road, Needles Highway, Sylvan Lake and Cathedral Spires on its route, so that's the one for the granite. The state closes Needles Highway to vehicles with the first snow until April 1 or later.
Our Badlands National Park Tour spends its time on places like Badlands Loop Road and the Ben Reifel Visitor Center, which adds context that helps the landscape make sense.
Exact stops can shift with weather and road conditions.
Seeing Time Instead of Just Scenery
Understanding Black Hills and Badlands Geology isn't about memorizing dates or rock names. It's about learning to recognize patterns in the land.
In the central Black Hills, pay attention to the character of the rock. As you travel outward, watch for limestone cliffs, red shale valleys, and sandstone ridges, because those are the tilted sedimentary layers around the dome.
In the Badlands, layering is the key. Notice the horizontal bands across slopes, the differences in color between layers, and the places where erosion has carved narrow ridges and where slopes appear smoother.
The greatest shift happens when visitors realize they're looking at time made visible. The granite, the fossil-bearing layers and the erosion happening today are all parts of one record, and you can see each of them in a single trip. As guides, our responsibility is to explain how the land formed and to help visitors feel the scale of that history. Awe deepens because it's grounded in understanding.
Glossary of Black Hills and Badlands Geology Terms
These definitions make the landscape easier to read.
Columnar Jointing
The pattern of long columns that forms when cooling molten rock shrinks and cracks. Devils Tower is the largest example in the world.
Crystalline Rock
Hard rock made of interlocking mineral crystals, formed from cooled molten material or changed by heat and pressure deep in the Earth. Granite and schist are examples from the core of the Black Hills.
Formation
A body of rock that geologists can recognize and map as one unit.
Fossil Beds
Places where preserved remains or impressions of ancient plants and animals are found. The Badlands contain some of the most significant fossil beds in North America.
Paleosol
An ancient soil preserved in the rock. The Yellow Mounds in Badlands National Park are one.
Pegmatite
A very coarse igneous rock with unusually large crystals. The Black Hills are known for it around the Harney Peak Granite.
Phonolite Porphyry
The rare igneous rock that Devils Tower is made of. It began as magma, molten rock deep underground.
Schist
A dark, flaky rock made when heat and pressure change older rock, often sparkling with mica. It sits around the granite in the central Black Hills.
Unconformity
A gap in the rock record, where one layer sits on a much older one because the rock in between was never laid down or was worn away.
Uplift
A geological force that pushes rock layers upward from below the Earth's surface. The Black Hills were formed through this slow dome-shaped uplift.
Explore More Through the Visitor Learning Center
If you’d like to keep building your understanding of the region, the Visitor Learning Center offers guides that connect directly to the geology you’ve learned here. Each topic adds a new layer of insight, helping you see how the land, wildlife, weather, and human story all tie back to the forces that shaped the Black Hills and Badlands.
If you’d like to explore further, these resources offer clear, science-based explanations from the agencies and organizations that study and protect the land every day.
Below are common questions visitors ask about Black Hills and Badlands Geology and how the region was formed.
How old are the Black Hills?
That depends on whether you mean the rock or the mountains. The granite at the center is about 1.7 billion years old, and some nearby rock is more than 2.5 billion years old. The dome is far younger, because it rose within roughly the last 70 million years.
What kind of rock is Mount Rushmore carved from?
Mount Rushmore is carved from the Harney Peak Granite, and it’s the same granite you see at the Needles and Black Elk Peak.
Are there dinosaur fossils in Badlands National Park?
No, the Park Service says none are found in the park. The Pierre Shale, its oldest layer, records a sea, and the mammal-bearing formations above it developed after the non-avian dinosaurs had gone extinct. This answer is about Badlands National Park, because other places called badlands can hold dinosaur rock, and dinosaur rock also sits around the outside of the Black Hills.
Why are there seashell fossils in South Dakota?
Millions of years ago, a shallow inland sea covered much of this region. Marine life lived and died here, leaving behind fossil evidence that is now embedded in sedimentary rock layers.
Did glaciers carve the Black Hills or the Badlands?
No, they didn’t. The glaciers that covered eastern South Dakota never reached the Black Hills or the Badlands, so water, frost and wind did the carving out here.
What is boxwork and why is it rare?
Boxwork is a honeycomb-like mineral formation found in Wind Cave. Cracks in the rock filled with minerals that later turned to calcite, and when slow water dissolved the limestone around them, those fillings were left standing as thin fins. The National Park Service says Wind Cave holds more boxwork than all other caves on Earth put together.
Are the Black Hills still geologically active?
Major mountain-building activity has long ended, but erosion continues constantly. The landscape you see today is still slowly changing through weather and natural processes.
This page was written by Daniel Milks, owner and guide at My XO Adventures. Years of exploring the Black Hills and Badlands have shaped his understanding of the region’s geology, from the quiet power of granite peaks to the delicate layers of the Badlands. He shares these stories as a guide who sees how the forces of uplift, erosion, and deep time shape the places visitors love most.
Daniel’s approach to guiding blends curiosity, connection, and respect for the land’s long history. His experiences in the field help visitors read these landscapes as chapters in a much older story. You can learn more about his background, philosophy, and approach to guiding on the Author Page, created to offer a deeper look at the person behind My XO Adventures.
