Sedimentary rocks are made when sand, mud and pebbles get laid down in layers. Over time, these layers are squashed under more and more layers. Eventually, the layers are lithified – turned to rock. Sedimentary rocks can be formed in deserts , lakes, rivers and seas .
The sediment (the sand, mud and pebbles) that make up Sedimentary rocks come from other rocks that have been worn down by wind, rain and snow. When we find sedimentary rocks we can work out what the environment was like when those rocks were formed – old sand dunes and river channels are preserved in the rocks.
Much of southern England is made up of sedimentary rocks, shown here as long blue and purple areas.