Sedimentary Rocks

Sedimentary rocks form from compressed layers of sediment: tiny bits of broken-down rock, dead plants, sea shells, and other small material that has settled at the bottom of rivers, lakes and oceans. Over thousands or millions of years, the layers are squeezed by the weight of more sediment piling up on top, plus chemically glued together by minerals carried in groundwater. The result is layered rocks like sandstone, limestone, chalk, shale and coal. Sedimentary rocks cover most of Earth's land surface, and they contain almost every fossil ever found.

  • % of Earth's land surfaceApproximately 75%Even though only 8% of crust by volume
  • Three main typesClastic, chemical, organicDifferent formation processes
  • Famous chalk cliffThe White Cliffs of DoverPure chalk laid down 65 to 100 million years ago
  • Coal formationApproximately 300 million yearsMostly during the Carboniferous period
  • Time to form 1 metreTens of thousands of yearsDepending on sediment supply
  • Famous fossil rockBurgess Shale, Canada508 million-year-old marine life

The three types

  • Clastic: made of physical bits and pieces of older rocks. Sandstone (from sand), shale and mudstone (from clay and mud), conglomerate (from gravel and pebbles).
  • Chemical: minerals dissolved in water settle out and accumulate as rock. Rock salt (sodium chloride) forms when sea water evaporates. Some limestones form chemically from dissolved calcium carbonate.
  • Organic: made from the remains of dead living things. Coal (from buried ancient plants), most limestone and chalk (from microscopic sea shells), some oil-shale (from buried algae).

How sedimentary rocks form

  1. Weathering: wind, rain, frost and rivers slowly break older rocks into small pieces (called sediment).
  2. Transport: rivers, glaciers, wind and waves carry the sediment to a new place, usually a sea, lake or river floodplain.
  3. Deposition: the sediment settles to the bottom and builds up layer by layer.
  4. Compaction: more layers pile up, squeezing the lower layers under their weight.
  5. Cementation: minerals dissolved in groundwater (especially calcite, silica and iron oxides) precipitate between the grains, gluing them together.

Together steps 4 and 5 are called lithification: the process of loose sediment becoming solid rock. It can take anywhere from thousands to millions of years.

Famous sedimentary rocks

  • Sandstone: cemented sand grains. Often red, yellow or grey. Common in deserts, riverbeds and beaches. Famous examples: Petra in Jordan, Bryce Canyon in Utah.
  • Limestone: mostly calcium carbonate. Often built from sea shells. Used for buildings, cement and even toothpaste. Famous: Cliffs of Moher in Ireland, the Cotswolds in England.
  • Chalk: a special soft white limestone made of microscopic plankton shells. The White Cliffs of Dover are pure chalk.
  • Shale: hardened mud. Splits into thin layers. Often contains fossils.
  • Coal: hardened plant remains, mostly from the Carboniferous period 360 to 300 million years ago when giant swamp forests covered much of the world.
  • Conglomerate: cemented pebbles and gravel. Looks like natural concrete.
Fact The famous White Cliffs of Dover are made of pure chalk, formed approximately 65 to 100 million years ago from the shells of microscopic sea creatures called coccolithophores. A single cubic centimetre of chalk contains the remains of millions of these tiny plankton shells. The chalk layers underneath the cliffs extend across the English Channel into France: the entire south of England is sitting on the floor of an ancient Cretaceous sea.

Why sedimentary rocks are important for fossils

Almost every fossil in every museum was found in a sedimentary rock. The reason is simple: sedimentary rocks are the only rocks that form gently enough to preserve dead things without destroying them. Igneous rocks are too hot, and metamorphic rocks have been squashed and cooked.

Sedimentary rock layers form a kind of natural time chart. Older layers sit underneath younger ones, so by walking down through a sequence of layers you walk back through time. This is how palaeontologists can put a date on dinosaur fossils, fossil fish, ancient trees, and the first humans.

Did you know? Most of the world's oil and natural gas are found in sedimentary rocks. They formed from huge amounts of microscopic sea life (mostly algae and plankton) that died, settled to the seabed, and were slowly buried, cooked and squeezed over hundreds of millions of years. The North Sea oilfields, the Texas oilfields and the giant Middle Eastern reserves are all in sedimentary rocks.
Deeper dive: how coal was made by the dragonfly-sized insects of the Carboniferous

Most of the world's coal was made during the Carboniferous period, approximately 360 to 300 million years ago. The name "Carboniferous" literally means "carbon-bearing". During this period, much of Earth's land was covered in huge swamp forests of giant ferns, club mosses and primitive trees. The plants grew, died, fell into the swamp, and (because there were not yet any bacteria or fungi capable of efficiently breaking down their wood) piled up in vast layers without rotting away.

Over millions of years, the layers of plant material were buried deeper and deeper under more sediment. Heat and pressure slowly drove off water and gases, leaving a concentrated dark substance behind. Different amounts of cooking produced different grades of coal: peat (least cooked, soft and crumbly), lignite, bituminous coal, and finally anthracite (most cooked, hardest and richest in carbon).

The Carboniferous was a strange world. Oxygen levels in the air were around 35% (compared with 21% today), which allowed insects to grow to enormous sizes. Dragonflies had wingspans of 70 cm: bigger than a seagull. Giant millipede-like creatures called Arthropleura reached 2 metres long. Tree-sized clubmosses called Lepidodendron reached 30 metres tall. Almost every chunk of coal we burn today is the buried remains of one of these strange tropical forests.

This is also why coal is a finite resource. The conditions that made it (high oxygen, no decomposing fungi, huge swamp forests) lasted only briefly in geological terms and may never come again. Once we burn it, that ancient stored carbon is back in the atmosphere as CO2, contributing to climate change. The Earth made coal slowly over hundreds of millions of years; we are burning it in just a few hundred.

For more, see igneous rocks and the rock cycle.