Wikipedia Deep Dive

Sedimentary rock

13 min read

The Earth's Thin Skin

Here's a strange fact: if you could walk across all the land on Earth, you'd spend nearly three-quarters of your journey walking on sedimentary rock. It covers seventy-three percent of our planet's land surface. And yet, this vast blanket of stone makes up only eight percent of the Earth's crust by volume.

Sedimentary rock is a veneer. A geological disguise.

Beneath this thin covering lies the real bulk of our planet's outer shell—igneous rock forged in ancient volcanic fires, and metamorphic rock twisted by unimaginable pressure and heat. But we live our lives on the sedimentary layer, building our cities on it, drilling through it for water and oil, reading the history of life itself in its stacked pages.

We've even found sedimentary rocks on Mars. The Curiosity rover has photographed them, layered and patient, waiting four billion years for someone to notice.

What Makes a Rock Sedimentary?

The process begins with destruction.

Somewhere, an existing rock is being torn apart. Wind grinds it down. Water dissolves it. Ice cracks it open. Temperature swings shatter it. The fragments—called sediments—range from boulders to particles so fine they float in air. This geological debris travels: carried by rivers, blown by wind, dragged by glaciers, or simply tumbling downhill under gravity's patient pull.

Eventually, the journey ends. The sediment settles. Layer accumulates upon layer, century after century, millennium after millennium. The weight of newer deposits presses down on older ones. Water carrying dissolved minerals seeps through the gaps between particles. Those minerals precipitate out, crystallizing in the spaces like mortar between bricks.

This is cementation. This is how loose sand becomes sandstone. How mud becomes shale. How the temporary becomes permanent.

But not all sedimentary rocks come from broken-down older rocks. Some form from the bodies of living things. Others precipitate directly from seawater when the chemistry is right. The category is broader than most people realize.

The Four Families

Geologists divide sedimentary rocks into four groups based on how they form. Each tells a different story about the Earth's past.

Clastic Rocks: The Broken Pieces

Clastic sedimentary rocks are made of fragments—the word comes from the Greek "klastos," meaning broken. These are the rocks born from the destruction of other rocks. The fragments, called clasts, get cemented together into something new.

What matters most with clastic rocks is size. Geologists obsess over particle diameter, and for good reason: the size of the grains tells you about the energy of the environment that deposited them.

Think about it. A raging mountain river can tumble boulders downstream. A lazy lowland stream can only nudge sand along its bed. A calm lake lets mud drift slowly to the bottom. The particle size is a fossil record of ancient water velocity.

The official size categories are gravel, sand, and mud. Gravel means anything larger than two millimeters in diameter—roughly the size of a small pea. Sand ranges from two millimeters down to sixty micrometers, which is about the thickness of a human hair. Mud is everything smaller: first silt, then clay, particles so fine you can't see them individually without a microscope.

This gives us our main clastic rock types. Conglomerates and breccias are made of gravel—the difference being that conglomerates contain rounded pebbles (tumbled smooth by transport) while breccias contain angular fragments (deposited close to their source, before water could round them off). Sandstones are made of sand. Mudrocks are made of mud.

The naming gets more specific from there. A sandstone rich in feldspar minerals is called arkose. One with abundant rock fragments embedded in a muddy matrix is a lithic wacke. A sandstone made almost entirely of quartz grains with virtually no mud between them is a quartz arenite—some of the purest, most beautiful stone you'll ever see.

Biochemical Rocks: Built by Life

Some rocks are essentially concentrated death.

Most limestone forms this way. Coral reefs grow for thousands of years, each generation of tiny animals building their calcium carbonate skeletons atop the skeletons of their ancestors. When the reef finally becomes rock, it's made almost entirely of fossilized coral. Shells of mollusks and single-celled organisms called foraminifera contribute to the pile. The White Cliffs of Dover—those iconic chalk faces that greeted returning British soldiers during both World Wars—are made primarily of the microscopic shells of ancient algae called coccolithophores.

Coal tells an even more dramatic story. It's concentrated ancient forest. During the Carboniferous Period, roughly three hundred million years ago, vast swamps covered much of what would become North America and Europe. Trees and ferns grew, died, and fell into oxygen-poor water where they couldn't decay properly. Layer upon layer of plant material accumulated, was buried, compressed, and chemically transformed. The carbon that those ancient plants pulled from the atmosphere is still there, locked in the coal, waiting to be released when we burn it.

Chert, a rock made of microscopic silica, often forms from the accumulated skeletons of tiny marine organisms called radiolaria and diatoms. These creatures build their intricate glass-like shells from dissolved silica in seawater. When trillions of them die and sink to the ocean floor over millions of years, their remains can lithify into rock.

Chemical Rocks: Precipitated from Solution

Sometimes rocks form without any biological help at all. When water becomes supersaturated with dissolved minerals, those minerals precipitate out—crystallizing directly from solution.

This is how evaporites form. Picture an inland sea in a hot, arid climate. Water evaporates faster than it's replenished. The remaining water grows saltier and saltier. Eventually, minerals start crystallizing out: first calcite, then gypsum, finally halite—common rock salt. The ancient Zechstein Sea, which covered much of northern Europe about 250 million years ago, left behind salt deposits thousands of feet thick in some places.

Some limestones form chemically too. In shallow tropical seas, calcium carbonate sometimes precipitates directly from warm, carbon dioxide-rich water, forming tiny spherical grains called ooids. These accumulate on the seafloor like sand, eventually cementing into oolitic limestone. The Bahamas are actively forming this kind of limestone today.

The Miscellaneous Category

A fourth group catches everything else. Volcanic tuff forms when ash and small lava fragments ejected by eruptions settle and cement together. Impact breccias form when asteroid strikes shatter and redistribute local rock. These are sedimentary in the sense that material settled out of the air, but their origin stories are more violent than most.

The Transformation Underground

Deposition is only the beginning. What happens next—a process called diagenesis—transforms loose sediment into solid rock.

Early on, while sediments are still shallow, biological processes dominate. Worms and other burrowing creatures churn through the material, mixing layers, aerating mud. Certain minerals dissolve and reprecipitate. This shallow stage, called eogenesis, occurs within the first few tens of meters of burial.

But sediment keeps accumulating above. The weight grows. Temperatures rise—roughly twenty-five degrees Celsius for every kilometer of depth. Pressures climb. This is mesogenesis, the main act of diagenesis, where most of the real transformation happens.

Compaction squeezes grains closer together. Imagine taking a jar of marbles and pressing down hard—the marbles can't get smaller, but they can rearrange into a more efficient packing. Soft, ductile minerals like mica actually deform under the pressure, bending and flowing into available spaces.

Water gets expelled. Sediments are saturated when first deposited—think of wet beach sand. As pore space shrinks, that water has to go somewhere. These "connate fluids," as geologists call them, migrate upward through the rock column, sometimes traveling for kilometers before reaching the surface or becoming trapped.

Then there's pressure solution. Where grains press against each other, the contact points experience the highest stress. Stressed minerals are more soluble than unstressed ones. So the contact points slowly dissolve away, and the dissolved material reprecipitates in nearby pore spaces where stress is lower. The grains literally melt into each other, cementing themselves together.

Organic material transforms too. At sufficient temperature and pressure, plant debris becomes lignite, then coal. At even higher temperatures, it can become oil or natural gas—though that process requires specific conditions that most buried organic material never experiences.

If burial continues deep enough and temperatures climb high enough, diagenesis gives way to metamorphism. The sedimentary rock ceases to be sedimentary at all. Shale becomes slate, then schist. Limestone becomes marble. The original identity is overwritten.

Reading the Colors

Sedimentary rocks come in a remarkable palette, and the colors tell stories.

Iron is the master painter. It exists in two common forms: iron(II) oxide and iron(III) oxide. The Roman numerals refer to iron's oxidation state—essentially how many electrons it has given away to oxygen atoms.

Iron(II) oxide forms only when oxygen is scarce. It colors rock grey or greenish-grey. If you see these colors in a sedimentary rock, you're looking at material deposited in an anoxic environment—perhaps the bottom of a stagnant lake or a poorly oxygenated seafloor.

Iron(III) oxide is the opposite: it forms when oxygen is abundant. The mineral hematite is iron(III) oxide, and it's responsible for the red, orange, and brown colors that dominate so many landscapes. Think of the painted deserts of the American Southwest, or the red cliffs of Utah's canyon country. These rocks formed in environments with good access to atmospheric oxygen—typically continental settings in arid climates.

Thick sequences of red sedimentary rock are called red beds. They're common in the geological record and usually indicate ancient deserts or semi-arid floodplains. But color alone isn't definitive—some red beds formed in tropical environments. The chemistry of iron oxidation depends on more than just climate.

Black or dark grey rocks often indicate organic matter. When plants or algae die in oxygen-poor water, they can't decay completely. The carbon-rich remains accumulate, coloring the sediment dark. Black shales—fine-grained mudrocks rich in organic material—typically formed at the bottom of deep, stagnant water bodies where oxygen never reached.

This is one reason why black shales are often source rocks for oil and natural gas. The organic material preserved in them, given enough time and the right burial conditions, can transform into hydrocarbons.

The Texture of Time

Beyond color, geologists study texture—the size, shape, and arrangement of the particles that make up a rock.

Grain size reveals energy. Coarse gravel means fast-moving water or steep slopes. Fine clay means calm conditions—perhaps a deep lake bottom or a quiet ocean floor far from shore. Medium sand might indicate a beach, a river channel, or a wind-blown dune.

Sorting tells another part of the story. A well-sorted sediment has grains of uniform size; a poorly sorted one contains a jumble of different sizes. Beach sand is usually well sorted—waves act as a natural sieve, separating grains by size. Glacial deposits are often poorly sorted, because ice doesn't discriminate; it carries and drops everything together.

Grain shape matters too. Angular fragments haven't traveled far from their source—they broke off and were deposited nearby, before water or wind could round their edges. Rounded grains have journeyed, tumbled, polished by countless collisions with other grains.

Surface texture provides even finer detail. Some sand grains have a frosted appearance, covered with tiny pockmarks. This frosting is characteristic of eolian sandstones—rocks formed from wind-blown dunes. The pockmarks come from high-velocity impacts between airborne grains, impacts that don't happen in water because water cushions the collisions.

Libraries of Stone

Sedimentary rocks are archives. They preserve records that would otherwise be lost.

Fossils, obviously. The vast majority of fossils occur in sedimentary rocks because the depositional process is what allows preservation. An organism that dies and is rapidly buried by sediment has a chance of becoming a fossil. One that dies and lies exposed on the surface will simply decay.

But sedimentary rocks record more than individual organisms. They record environments. A sequence of strata might show a river delta gradually building out into a sea, or a desert advancing over what was once grassland. The rocks remember climate shifts, sea level changes, tectonic uplifts. Each layer is a moment frozen in time, stacked in order.

This is why sedimentology matters so much to understanding Earth's history. The fossil record tells us what lived. The sedimentary record tells us how they lived—what the world looked like around them.

And sometimes the records reveal catastrophes. Thin layers of unusual sediment mark asteroid impacts, volcanic super-eruptions, rapid climate shifts. The famous iridium layer at the end of the Cretaceous—evidence of the asteroid that killed the dinosaurs—is preserved in sedimentary sequences around the world.

Resources and Risk

We rely on sedimentary rocks more than most people realize.

Drinking water often comes from aquifers—porous sandstones and limestones that store and transmit groundwater. Oil and natural gas accumulate in sedimentary formations, trapped in pore spaces or fractures. Coal is sedimentary. So are many ore deposits, including banded iron formations that provide much of the world's iron.

Construction depends on sedimentary rocks too. We build on them, cut through them for tunnels and road cuts, quarry them for building stone. Engineers need to understand the strength, porosity, and bedding of these rocks to build safely. A sandstone formation that looks solid might be dangerously weak along its bedding planes. A shale might swell when it gets wet, destabilizing foundations.

The practical importance of sedimentology explains why so many geologists specialize in it. Understanding sedimentary rocks isn't just academic curiosity—it's essential for finding water, extracting energy, building infrastructure, and predicting natural hazards.

A Thin Veneer with Deep Stories

The next time you pick up a piece of sandstone or shale, remember what you're holding. It's not just rock. It's a fragment of an ancient landscape, transformed by time and chemistry into something durable enough to survive until you found it.

The grains in that sandstone might have eroded from a mountain range that no longer exists, been carried by a river that dried up millions of years ago, deposited in a sea that has since retreated or advanced. The clay in that shale might preserve the last traces of organisms that lived and died when your ancestors were still fish.

Sedimentary rocks cover most of the land we walk on. They're the ground beneath our feet, the cliffs on our coastlines, the canyon walls in our national parks. They're simultaneously commonplace and extraordinary—the Earth's thin skin, recording everything that has happened on this planet for the past four billion years.

Eight percent of the crust by volume. Seventy-three percent of the surface we see. That mismatch tells you something important: the Earth is always reshaping itself, always burying the new under the newer, always turning the temporary into the permanent.

And we get to read the results.