The Dorset and East Devon coast contains a wide range of landslides, both large and small.

The coast is unstable because there are numerous situations where porous strata, principally the Chalk and Upper Greensand, lie over impermeable clays.

Black Ven landslide

How Landslides work

Rain water sinks through the porous rocks but once it reaches the underlying clays it can sink no further. The water builds up along the junction between the rock layers and seeps out of the cliffs as a series of springs.

After periods of prolonged rainfall, the build up of water increases the weight of the cliff top. Increased pore pressure reduces the friction and allows large sections of the cliff top to break away. As the cliff top block subsides, it rotates along the slip plane within the cliff, resulting in the flat surface tipping back towards the cliff.

The displacement shunts thousands of tonnes of material into the undercliffs, the area between the cliff top and the beach, generating mudslides within the softer Jurassic clays that in turn slide towards and across the beach. The terraced nature of the undercliffs is due to hard bands of limestone within the clays.

The slip on Stonebarrow over Christmas 2000/2001 created a spectacular 'waterfall' of mud and rocks that crashed to the beach in a matter of hours.

Each landslide is an attempt by the slope to reach a stable angle. Along the coast marine erosion removes the slipped material, thereby constantly destabilising the slopes. In contrast, many of the slopes inland, (especially in West Dorset) were once unstable (particularly in the Ice Age) but the lack of erosion has allowed the slope to reach a stable angle.