Dorset Council is warning beachgoers to keep away from a major rockfall on Hive Beach, near Burton Bradstock, on Thursday 17 December 2020. Access to both the beach and footpaths on the cliffs above have been closed off while the situation is assessed, but it is thought likely that the cliffs will remain unstable for some time. A rockfall on the same section of cliffs killed a woman bathing on the beach below in July 2012.
The cliffs at Hive Beach are made up of two main layers, the Bridport Sands at the base, and the Inferior Oolite (limestone made up of egg-shaped particles) above. These cliffs are usually very stable, but in very wet conditions, such as those experienced by the UK this week, the sandstones can become waterlogged and lose their cohesion, causing collapses which undermine the limestones above. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall.
The rocks of the Inferior Oolite are highly fossiliferous, and form part of the Jurassic Coast UNESCO World Heritage Site, making the area popular with both professional and amateur geologists and palaeontologists, as well as tourists visiting the area. However this weeks rockfall is reported to have affected only the much less fossiliferous Bridport Sands formation, and is unlikely to have produced much interesting material.
The Bridport Sands formation is the reservoir rock for the Wytch Farm Oil Field, and in other areas is a blue grey colour. However, on the beaches around Burton Bradstock it is an orange yellow colour, caused by the oxidation of the sands and the oil being washed out of the rock by rainfall. This is more aesthetically pleasing to the Human eye, but weakens the cliffs by lowering the cohesion of the sandstone, leading to frequent large rockfalls.
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