Jurassic Coast spectacular landslide
Image credit: Landslide near Seatown in Dorset, April 2021. © James Loveridge
With kind permission of The Jurassic Coast Trust.jurassiccoast.org
On Tuesday 15 April, a major landslide event occurred on the Jurassic Coast cliffs between Seatown and Eype. Around 300 metres of cliff, weighing an estimated 4000 tonnes, collapsed into the sea, the largest landslide in this area for 60 years.
Landslides along this section of coast are the natural result of coastal and weathering processes acting on the local geology. At sea level, the base of the cliff is cut into layers of Eype Clay. Further up the 130 metre high cliffs are layers of sandstone. During the wet winter weather of 2021, rainwater infiltrated into the permeable sandstone, and percolated downwards until it reached the impermeable clay. As it could not move any further, the water built up in the sandstone making it heavier. At the same time, the top of the clay became more and more slippery. In the long period of dry weather that followed in March and early April, the sandstone dried out and lost some of its strength. It gave way under gravity, and moved downwards and outwards towards the sea over the slippery clay. This movement is known as a rotational slide as the rocks near the sea move outwards, and those at the top of the cliff move downwards. The actual landslide was not unexpected as cracks close to the cliff top had been seen during the preceding days.
A virtual tour of the landslide and detailed explanation can be watched on the Jurassic Coast World Heritage site website. Seatown Landslide - Jurassic Coast World Heritage Site
This rotational slide is part of the natural regeneration of the Jurassic Coast. Wave erosion undermines the base of the cliffs and natural weathering and erosion processes further up make the rocks unstable. The debris from the landslide will be gradually broken up by the waves and transported to beaches along the coast, as part of the natural cycle of erosion and deposition.
The landslide affected part of the South West Coast path which has been slightly rerouted. Dorset Council Countryside Ranger Service has worked closely with the local landowner, the National Trust to fence off the unstable area and ensure the coast path was safe. The area will be closely monitored as further landslides are expected and there are dangerous active mudflows in the exposed clay at sea level.
Further information about the Jurassic Coast and its landforms can be found here. jurassiccoast.org
More detailed information about the geology of this section of Coast in the Bridport, Eype and Seatown area can be found on the comprehensive website produced by Ian West.
Geology of the Wessex Coast, Southern England by Ian West - Index and Home Page (soton.ac.uk)
The physical geography of the Jurassic Coast is dynamic and other sections are currently active. Cliff movements are being closely monitored between Bowleaze Cover and Redcliff Point near Weymouth, where permeable limestone rocks lie above clays. Large cracks have appeared at the cliff top suggesting a landslide is possible in the near future.