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 poor 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 main type of landslide that occurs at the coast is called slumping .
Slumping (also landslips) Slumps occur in weaker rocks and involve some rotational movement. Slumping can occur after heavy rainfall or earthquakes Back tilted slopes Large blocks break away Sliding surface is concave Soft boulder clay cliffs can be undercut by the sea and slumps are common.