The Cliffs

The cliffs at Charmouth are made up of layer upon layer of soft mud, silt and clay built up over many millions of years. These are classified as ‘Sedimentary Rocks’, as they are made up of billions of tiny particles of sediment that have been turned to stone by millions of years of pressure from the layers above.

Layers of sediment build up in lakes, rivers and the sea bed or even in deserts usually taking hundreds of thousands or even millions of years. If the layers of sediment stay buried they have a chance of being turned to stone. Minerals can fill up the gaps between tiny particles like sand and mud forming sandstones and mudstones. Alternatively the layers could become so squashed that all the grains of sediment are packed hard together and stay that way.

The layers in the Charmouth cliffs were laid down at the bottom of a calm sea about 185 million years ago during the early part of the Jurassic period and are mostly made of mud and clay. However, the mud did not mix with minerals and turn into hard mudstone but stayed crumbly and squashy, making it very week when it comes into contact with water from rain or the sea. This means the cliffs are very unstable and there are mudflows and landslides all the time. Because of their instability it is important to act responsibly if you go fossil collecting. DO NOT go near the cliff face, DO NOT climb on landslides or mudflows and DO NOT hammer or try to dig into the cliff face, landslides or mudflows (visitors fossil collecting code).

Long after the dark Jurassic mudstones and siltstones formed, the layers were tilted slightly to the East and then eroded, the tilt is easy to see if you look carefully at the cliffs from a distance. The erosion erased around 85 million years worth of the rock record from above the Jurassic mudstone destined to become the Charmouth cliffs.

Near the top of the cliff the rocks change from a dark grey colour to a golden yellow colour. This top layer sits over an UNCONFORMITY, a gap in time where the rock record has been worn away. The Cretaceous rocks that formed after this are the yellow ones at the top of the cliff. They are mostly sandstone although like the mud and clay rocks below them they are very weak and are another cause of the landslides at Charmouth .