Rock at the surface breaks into pieces, called sediment: gravel, sand, silt and clay. Water, wind and ice carry the pieces away and drop them, often in flat layers. Over time the layers become sedimentary rock, made from bits of older rock or from minerals that come out of water.
- Weathering breaks rock into sediment.
- Erosion carries it away.
- Deposition drops it in layers, often in lakes and seas.
- Compaction: layers piled on top squeeze out water and press the grains together.
- Cementation: minerals left by water (often calcite or silica) glue the grains together.
| Grain size | Sediment | Rock |
|---|---|---|
| Over 2 mm, rounded | Gravel | Conglomerate (breccia if the pieces are sharp-edged) |
| 1/16 to 2 mm | Sand | Sandstone |
| 1/256 to 1/16 mm | Silt | Siltstone |
| Under 1/256 mm | Clay | Shale (splits into thin sheets) |
Key idea
Sediment becomes rock when it is buried, squeezed (compaction) and glued by natural cement (cementation). Rocks made of grains are named by their grain size.
Why it works
Under 1 km of sediment, the weight on each grain is huge. It pushes grains closer and squeezes out water. Water still flowing through the pores carries dissolved minerals, which crystallize between the grains like glue. Drying alone does neither: a pile of dry sand on a beach is still loose sand.
Worked example: Name it by size
A rock is made of rounded grains about 0.5 mm across, cemented together. What is it?
- 0.5 mm is between 1/16 mm (about 0.06 mm) and 2 mm.
- That is sand-sized, so the rock is sandstone.
Answer: Sandstone
Worked example: Rounded or sharp?
Two rocks are both made of pebbles 1 to 3 cm across. In one the pebbles are rounded; in the other they are sharp-edged. Name both.
- Pebbles are gravel-sized (over 2 mm).
- Rounded gravel: conglomerate. Its pebbles were tumbled a long way by water.
- Angular gravel: breccia. Its pieces did not travel far.
Answer: Conglomerate (rounded) and breccia (angular)
Common mistake
Thinking sediment turns into rock just by drying out. It takes burial under more layers (compaction) and natural cement deposited by water (cementation). Check: mud left in the sun cracks and crumbles; it does not become shale.