Sedimentary rocks

There are three main types of rock: sedimentary, igneous and metamorphic.

Formation of sedimentary rocks

A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.

The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form.

The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation.

These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.

What are they like?

Sedimentary rocks contain rounded grains in layers. Examples of sedimentary rock are:

• chalk
• limestone
• sandstone
• shale

The oldest layers are at the bottom and the youngest layers are at the top. Sedimentary rocks may contain fossils of animals and plants trapped in the sediments as the rock was formed.