Coastal Processes Landslides Beaches Bays & Headlands		Landslides 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.
ANIMATION SEQUENCE Rain water soaks into the cliffs		Black Ven between Lyme Regis and Charmouth. Upper Greensand lies over Lower Jurassic clays. The largest coastal mudflow inEurope took place here in the winter of 1958/9
After periods of prolonged rainfall, the cliff tops break away The sliding blocks send sand and mud tumbling through the undercliff and onto the beach The landslides reach a stable angle, leaving a stepped or terraced cliff...until the next time it rains		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 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. Click here to download a small animation (1,132Kb) Click here to download larger animation (3,1600Kb) Attention!......These are large files and unless you have fast internet access, will take a very long time to download. You will also require Windows Media Player, (version 6.4.07.1112 definitely works) The cliff top at Black Ven in August 1994. A large block of the cliff top slid into the undercliff, rotating backwards in the process
		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. Stonebarrow, just east of Charmouth. During Christmas 2000/01, one of the largest landslides seen for many years took place. Hundreds of thousands of tonnes of sand and mud poored over the cliff and onto the beach
		Inland 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.
		Examples of landslides
		The Hooken Landslide at Beer Head occurred in 1789/90 and involves the Chalk and Upper Greensand sliding over Triassic clays.
		The Undercliffs between Axmouth and Lyme Regis form the largest landslide complex on the site. Several slips are prehistoric while others such as the Dowlands Chasm of 1839 were widely recorded at the time.
		The landslides of West Dorset involve the Upper Greensand sliding over Lower Jurassic clays. Initial slips are rotational in nature but the movement of sand displaces vast volumes of the Lower Jurassic clays that slide towards the sea over a number of benches created by the harder bands of limestone.
		The Isle of Portland is surrounded by spectacular landslides that are, in part controlled by the dip of the strata and the presence of large north-south trending joints through the massive Portland Limestone. The strata dip to the south and the east and consist of Portland Limestone overlying Portland Sand and the Kimmeridge Clay. As a result, on the West Weares, the landslides take the form of topples while on the east, movement is down dip and therefore much larger. The Great Southwell Landslide which helped form Church Ope Cove, is the second largest historical landslide in the UK
		Active landslides are at work between Bowlease Cove and Red Cliff Point. Here it is porous limestone, the Osmington Oolite, sliding over the Oxford Clay.
		Landslides in the Osmington area have closed the coast path and threaten the car park
		White Nothe is a prehistoric landslide involving the Chalk and Upper Greensand slipping over the Kimmeridge Clay