Dig through the soil almost anywhere and you hit bedrock. These are the 15 rocks you'll meet most often in cliff faces, road cuts, quarries and mountain sides, with how to recognise them and roughly where on Earth they show up.
Sedimentary
Made of bits of other rock, shells or mud, laid down in layers by water, wind or ice and then squeezed and cemented. Most cliff faces on the continents are sedimentary.
Cooled from molten rock. Slow cooling deep underground grows big crystals (plutonic); fast cooling at the surface gives fine-grained lava rock (volcanic).
Sand grains, mostly quartz, glued together by silica, calcite or iron oxide. It forms from old rivers, beaches and desert dunes; dunes leave big sweeping cross-beds. Sandstone resists erosion well, so it stands up as bold cliffs, mesas, arches and canyons.
How to spot it
Gritty, sandpaper feel; you can see the grains with a hand lens
Layers, often slanted cross-bedding
Buff, yellow, orange to brick red (iron stains)
Did you know? Petra in Jordan is carved straight into a sandstone cliff.
Stacked limestone beds at El Torcal, Andalusia, Spain
02Sedimentary
Limestone
Old seafloor
Calcium carbonate from shells, coral, algae and lime mud that piled up in warm, shallow seas. Rainwater slowly dissolves it, carving karst: caves, sinkholes, gorges, cracked pavements and tall tower hills. In cliffs it is usually pale grey and neatly bedded.
How to spot it
Pale grey, cream or white
Fossils: shell bits, coral, crinoid "stems"
Fizzes with dilute acid (try vinegar on powder); a knife scratches it
Caves, fluting and solution pits
Did you know? The Great Pyramids of Giza are built from limestone, and their blocks are full of fossil shells.
Seven Sisters chalk cliffs and Belle Tout lighthouse, Sussex, England
03Sedimentary
Chalk
Soft white limestone
A very pure, soft limestone made of coccoliths, microscopic chalky plates from plankton that rained down in Cretaceous seas about 100–66 million years ago. It weathers into rolling downs inland and sheer, brilliant white cliffs on the coast, usually striped with rows of black flint.
How to spot it
Bright white and soft; it marks your fingers
Rows of dark flint nodules along the beds
Fizzes strongly with acid
Sheer white sea cliffs, sea stacks and arches
Did you know? The Cretaceous period is named after it: creta is Latin for chalk.
Mancos Shale badlands under a sandstone cap, Capitol Reef, Utah
04Sedimentary
Shale & mudstone
Hardened mud
Compacted mud and clay that settled in quiet water: lake beds, lagoons, deltas and the deep sea floor. Mudrocks make up around two-thirds of all sedimentary rock. Shale splits into thin flakes and is weak, so it makes crumbly slopes, badlands and soft bands between harder sandstone or limestone. Black shales hold oil, gas and some of the best fossils on Earth.
How to spot it
No visible grains, smooth rather than gritty
Splits into thin, flaky sheets (shale) or blocky chunks (mudstone)
Grey, black, red or green
Ribbed, gullied slopes beneath harder caprock
Did you know? The Burgess Shale in the Canadian Rockies preserves soft-bodied animals from 508 million years ago.
Conglomerate sea cliff at Ganavan, near Oban, Scotland
05Sedimentary
Conglomerate
Natural concrete
Rounded pebbles and cobbles set in a sandy cement, like natural concrete. It forms where fast rivers, flash floods and storm beaches dump coarse gravel, often at the foot of rising mountains. It tends to wear evenly, so it builds odd rounded domes, towers and knobbly cliffs. With sharp, angular stones instead of rounded ones it is called breccia.
How to spot it
Rounded pebbles you can pick out with a finger
A jumble of sizes and rock types
Pebbles sticking out of the cliff, some leaving empty sockets
Bulbous domes and stone pillars
Did you know? The monasteries of Meteora in Greece sit on top of conglomerate pillars up to 400 m tall.
Silica-rich magma that cooled slowly, kilometres underground, growing crystals big enough to see: glassy quartz, blocky white or pink feldspar and black mica. Huge granite bodies (batholiths) sit in the roots of mountain ranges. Once erosion strips the cover off, granite stands as smooth domes, tors, boulder piles and enormous sheer walls.
How to spot it
Interlocking crystals you can see, often a few mm or more
Salt-and-pepper, pink or grey speckle with no layers
Very hard; a knife will not scratch the quartz
Rounded domes, tors and big blocky joints
Did you know? El Capitan in Yosemite is a single wall of granite about 900 m (3,000 ft) tall.
Columnar basalt at the Giant's Causeway, Northern Ireland
07Igneous
Basalt
The ocean floor’s lava
The most common volcanic rock on Earth: dark, runny lava rich in iron and magnesium. It floors every ocean and builds shield volcanoes like Hawaii and Iceland. On land, giant eruptions stacked flood basalts kilometres thick. As a thick flow cools and shrinks it cracks into the famous six-sided columns.
How to spot it
Black to dark grey; crystals too small to see
Heavy for its size
Gas-bubble holes (vesicles), sometimes filled with white minerals
Columns, or stacked flows that weather into giant steps
Did you know? The Deccan Traps in India once covered roughly 1.5 million km² and erupted around the time the dinosaurs died out.
Roundtop Andesite lava flow forming the Palisades, Crater Lake, Oregon
08Igneous
Andesite
Ring of Fire lava
The lava of subduction-zone volcanoes, named after the Andes. It has more silica than basalt, so it is stickier and builds steep, explosive stratovolcanoes rather than gentle shields. Most andesite has a fine grey background dotted with small white feldspar and black hornblende or pyroxene crystals.
How to spot it
Medium grey, sometimes greenish, reddish or purple
Fine background with scattered small crystals (porphyritic)
Lighter than basalt, darker than granite
Blocky lava flows and columns on the flanks of tall cone volcanoes
Did you know? Andesite is roughly what the average continental crust is made of, chemically speaking.
Eroded tuff valleys and fairy chimneys, Cappadocia, Türkiye
09Igneous
Tuff
Rock made of ash
Volcanic ash, pumice and rock fragments blasted out by explosive eruptions, then compacted, or welded while still hot. Soft tuff is easy to carve, which is why people dug homes and churches into it in Cappadocia and built ancient Rome with it. Welded tuff is hard and forms pale vertical cliffs, often with cooling columns.
How to spot it
Light-coloured (cream, pink, grey) and surprisingly lightweight
Gritty matrix with pumice lumps and rock chips
Ash layers draped over the landscape
Caves, fairy chimneys and pale cliffs under a harder caprock
Did you know? The Toba eruption in Sumatra, about 74,000 years ago, left tuff hundreds of metres thick around today’s Lake Toba.
The gabbro peaks of the Black Cuillin from Elgol, Isle of Skye
10Igneous
Gabbro
Basalt’s deep twin
Same chemistry as basalt, but it cooled slowly underground, so it is coarse-grained: dark pyroxene with grey-white feldspar. It makes up the lower ocean crust. On land you mostly find it where slabs of old ocean floor were pushed up onto the continents (ophiolites), or in huge layered intrusions. It weathers rough and grippy, which climbers love.
How to spot it
Dark grey, black or greenish with visible crystals
Little or no quartz, unlike granite
Heavy and tough
Rough, rusty-brown weathered crags
Did you know? The Black Cuillin on Skye are gabbro, while the Red Cuillin next door are granite.
Shale or mudstone squeezed and gently heated during mountain-building. Microscopic mica flakes line up, so the rock splits into thin, flat, smooth sheets: slaty cleavage. That cleavage often cuts across the original bedding at an angle. Slate has roofed houses for centuries and makes blackboards and billiard tables.
How to spot it
Dark grey, blue-black, green or purple with a dull sheen
Splits into dead-flat, thin slabs
Rings when tapped, unlike shale
Quarry faces and big heaps of discarded slabs
Did you know? Spain now quarries most of the world’s roofing slate, mainly in Galicia and León.
Mudrock cooked further than slate, until the mica crystals grow big enough to glitter. The flakes line up into wavy, crinkled layers, and it is often studded with red garnets. Schist forms in the cores of mountain belts and weathers into jagged, flaky crags.
How to spot it
Glittery silver, gold or green sheen (mica, chlorite)
Wavy, crinkled layers that peel into flakes
Red garnet "studs" are common
Ragged crags and slabby outcrops
Did you know? Manhattan's skyscrapers stand on the Manhattan Schist; it comes to the surface in Central Park.
Tightly folded gneiss, Gjelsvik Mountains, Antarctica (hiker for scale)
13Metamorphic
Gneiss
Striped and ancient
Rock heated and squeezed almost to melting, until its minerals separate into light bands (quartz and feldspar) and dark bands (biotite, hornblende), often folded into swirls. Gneiss and granite together make up the ancient shields at the heart of every continent. The oldest known rocks on Earth, about 4 billion years old, are gneisses.
How to spot it
Light and dark stripes, often wavy or folded
Coarse grains like granite, but banded
Hard; does not split into sheets
Bare, ice-scraped knobs in lake-dotted shield country
Did you know? The Acasta Gneiss in northern Canada is about 4.0 billion years old.
Carrara marble quarries in the Apuan Alps, Tuscany
14Metamorphic
Marble
Recrystallised limestone
Limestone or dolomite baked until its calcite regrows into a mosaic of interlocking, sugary crystals, wiping out the fossils. Pure marble is snowy white; traces of clay, iron or graphite add grey veins, pink, green and swirls. Water polishes it smooth in river gorges.
How to spot it
Sugary, sparkling crystal faces on a fresh break
White, grey, pink or green, often veined or swirled
Steel scratches it; it fizzes with acid
Smooth, water-sculpted gorge walls
Did you know? The Taj Mahal is clad in Makrana marble, and Michelangelo’s David is carved from Carrara marble.
Quartzite boulder field under the bluffs at Devil's Lake, Wisconsin
15Metamorphic
Quartzite
Toughest of the lot
Quartz sandstone recrystallised so tightly that the grains fuse together, so it breaks straight through the grains, not around them. It is one of the hardest, most weather-proof common rocks, so it holds up long ridges, bold bluffs, table mountains and fields of sharp boulders long after softer rock has worn away.
How to spot it
Glassy, sugary look; white, pink, purple or rusty red
Scratches glass; steel will not scratch it
Breaks with smooth, shell-like fractures
May still show sandstone ripple marks and cross-beds
Did you know? Devil's Lake in Wisconsin sits between bluffs of 1.7-billion-year-old Baraboo Quartzite.