Rocks are not as permanent as they seem. Rain, wind and freezing weather can cause them to break up over a period of time, into tiny pieces which are washed away by rain and rivers. This process is called ‘erosion’. Where rivers slow down or meet the sea, the rock fragments are left behind forming ‘sediment’, such as sand or mud. Layers of sediment gradually thicken and the tiny pieces of rock become stuck together to form larger rocks. These are called ‘sedimentary’ rocks.

Erosion and sedimentation are slow processes. They may take thousands of years to have a noticeable effect on the landscape about us.

 

 

 

Sedimentary rocks

‘Sedimentary’ rocks are made of layers of small particles. For example, chalk cliffs are made from layers of very small shells and skeletons of sea animals.

 

 

 

 

Metamorphic rocks

Sedimentary rocks are changed when they are subjected to heat and pressure near a volcano. This is how ‘metamorphic’ rocks, like marble are formed.

 

 

 

 

Igneous rocks

‘Igneous’ rocks are formed from magma which has cooled, either slowly inside the Earth, or quickly on the surface. Pumice or pummy stone and granite are formed like this.

 

 

 

 

The Delicate Arch, Utah, USA. Rock shapes like these are formed by erosion.