Definition · Plain-language
Sedimentary rock
Sedimentary rock forms when fragments of weathered rock, minerals or organic matter settle in layers and are pressed and cemented into solid rock.
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Built up in layers
Sedimentary rock is made from sediment: loose material such as sand, mud, pebbles, shell fragments or dissolved minerals. Weathering and erosion break down existing rock and carry the pieces to rivers, lakes, seas and floodplains, where they settle. Over long periods, layer upon layer accumulates; the weight of overlying material squeezes the grains together (compaction) and natural mineral “glue” binds them (cementation), turning loose sediment into solid rock. The result is often visibly layered, with each band, or stratum, recording a period of deposition.
Three ways it can form
Geologists group sedimentary rock by origin. Clastic rock is made of fragments of older rock cemented together — sandstone from sand, shale from mud, conglomerate from rounded pebbles. Chemical rock forms when minerals dissolved in water crystallise out, as when evaporating seawater leaves rock salt or gypsum. Organic (or biochemical) rock forms from the accumulated remains of living things, such as limestone built from shells and coral, or coal from compressed plant matter. The same surface processes underlie all three, but the raw material differs.
A record of Earth’s history
Because sedimentary rock forms quietly at the surface, layer by layer, it preserves an unrivalled archive of the past. Its strata can be read like pages: lower layers are generally older than those above. Crucially, the gentle conditions allow the remains of plants and animals to be buried and turned into fossils, which is why almost all fossils are found in sedimentary rock. Ripple marks, mud cracks and grain sizes also reveal ancient environments — telling geologists whether a layer formed in a desert, a river or a sea.
Key facts
At a glance
- Definition: rock formed from compacted, cemented layers of sediment
- Process: weathering, erosion, deposition, compaction and cementation
- Texture: usually shows layers, called strata
- Three types: clastic, chemical and organic (biochemical)
- Examples: sandstone, limestone, shale, rock salt, coal
- Special role: the main rock type that preserves fossils
Common misconceptions
What people often get wrong
Often heard: Sedimentary rock forms from heat and pressure like other rocks.
Actually: Sedimentary rock forms at the surface from sediment being deposited, compacted and cemented — not from melting or intense heat. It is metamorphic rock that requires high heat and pressure.
Often heard: Fossils can be found in any kind of rock.
Actually: Almost all fossils occur in sedimentary rock, because its gentle, layered formation can bury and preserve remains. The heat and pressure that form igneous and metamorphic rock usually destroy fossils.
Often heard: The layers in sedimentary rock have no particular order.
Actually: In undisturbed rock, lower layers are older and upper layers younger — the principle of superposition. This ordering lets geologists read strata as a timeline of past events.
