The Spittles Landslip, Lyme Regis
Richard Edmonds (UK)
On the evening of 6 May 2008, a 300m section of the cliffs east of Lyme slid towards the sea creating one of the most spectacular landslides in recent years. Members of the local fire brigade were training along Gun Cliff, the easterly promenade of the town, and they, along with local residents, witnessed the event. Apparently, a flock of seagulls were suddenly spooked into flight amid much squawking, which drew people’s attention to what was happening. A huge block of dark, Lower Jurassic shale was sliding over the sea cliff and onto the beach accompanied by a deep rumbling noise and, shortly after, a strong smell of sulphur.
Luckily, it was high tide and no one was on the beach at the time. I suspect that if anyone had been, they would have seen numerous small falls, cracks and movements taking place before the main failure and would have had time to get away. However, the risk to the public was obvious and the Lyme Regis coastguard team were called as dusk descended.
The following morning revealed the extent of the landslide, especially from the air. There are a few perks in my job with the County Council and as a member of the local coastguard rescue team, but someone had to go up in the new Maritime Coastguard Agency helicopter and I was picked as ‘the man on the spot’. The helicopter had been requested by the County and District Councils amid concerns for both the old rubbish tip that had been exposed (again) and public safety. Strapped in, with the main door open, the pilot flew in ever expanding circles round the slip, allowing photographs to be taken from every angle.
Anatomy of a landslide
The cliffs along this part of the coast consist of 200 million-year-old, Lower Jurassic Blue Lias overlain by Shales with Beef, Black Ven Marl and Belemnite Marks (the Charmouth Mudstone Formation). These rocks are capped by Upper Greensand from the Cretaceous Period that lies on the eroded surface of the Jurassic rocks. It is the combination of porous sandstone overlying impermeable clay that gives rise to landslides. Rainwater sinks through the cliff top, ponds up along the clay surface and seeps out at the cliff face. After prolonged periods of rain, the water pressure builds up and lubricates the junction between the two rocks, allowing huge sections of the cliff top to slide away. The biggest slips often happen in the spring as it takes time for the water to soak through the Upper Greensand. It is odd to think that last summer’s floods and the wet winter are only just been expressed as a landslide this spring. Lower down, the Jurassic clays contain layers of hard limestone that form benches in the cliff with similar slip surfaces on top of them. Material gathers on these benches and is shunted forward as the cliff tops slide down, rather like a gigantic penny falls in an amusement arcade.
The 2008 landslide
The latest slip has only involved the Shales with Beef. The lowest sheer surface is at the top of the Blue Lias and can be seen where the original sea cliff has not been buried. Before the slip, there were three terraces in the cliff, formed along harder bands of limestone.
We think that the sequence of events was as follows. In 2002 and 2006, small slips or rock falls took place in the centre of the area that was affected by the recent landslide. Cracks opened up in the slope above. The wet summer of 2007 and winter of 2007/2008 allowed water to build up along the lowest slip surface above the Blue Lias and a failure took place along this line of weakness. A second weak surface just above Table Ledge also slipped, allowing the second terrance to slide towards the sea. This left the third terrace unsupported and it slumped down to fill the space, pushing the second terrance forward and upward into a raised block with a chasm behind it. Over the following week the main block continued to move and spill over the edge of Table Ledge where it broke up and collapsed onto the beach, pushing the toe of mud further across the beach.
The landslides of Black Ven and the Spittles
Black Ven and the Spittles, the cliffs between Lyme Regis and Charmouth, are famous for landslides and feature strongly in the Geological Conservation Review volume on Mass Movement published by the Joint Nature Conservation Committee. In 1924, the coast road was destroyed by movement in what had been, up till then, a scrubby cliff on top of which were potato fields on the seaward of the road. In 1958/1959, the largest coastal landslide in Europe took place in the middle of Black Ven. In May 1986, a large slip, still known as ‘The New Slip’, took place immediately east of the 2008 landslide and, in 2000/2001, the cliff top broke away and a long fissure opened up in the woods, at the back of the landslide complex. In addition, numerous smaller events have taken place on a yearly basis.
The entire cliff and coastal slope consists of a series of ancient landslides that formed during the last interglacial episode of the Pleistocene, some 125,000 years ago. At that time, sea levels were higher than today and the cliffs were eroding by means of huge landslides. Then, the ice sheets returned and sea levels dropped by a staggering 140m. Without erosion by the sea, the landslides simply spilled out across the dried-up sea floor until they reached a stable angle. The old Charmouth Road ran through this ancient landslide complex, which had become vegetated, until renewed erosion reactivated or ‘unzipped’ the landslides below it. The wooded slope above the Spittles is the back of the sea cliff that was last active 125,000 years ago. The underlying pattern of movement is from the east to the west and has taken place in just the last 80 years. Geologists are used to working in unimaginable lengths of geological time and it is quite amazing to see a process of reactivating landslides taking place over the span of a single lifetime!
Parts of Lyme Regis have been built on this landslide complex, including the seafront where West Dorset District Council has just spent £24 million pounds on stabilisation measures. The concern now is with East Cliff and parts of the town that are built on the ancient landslide that abuts this latest event. The council, with their consultants (High Pint Rendel), has spent the last two years mapping the landslides and drawing up a coastal defence scheme to protect the town. However, this is a difficult challenge as the reason why the coast is beautiful, interesting, internationally important and famous for fossils is precisely because the cliffs are eroding. Currently, the World Heritage Site Team, Natural England and the District Council are in discussions to find an engineering solution that is acceptable to the environmental interests, is viable in engineering terms and cost effective; the three main tests for government funding.
Visiting the site
The landslide is still moving and consists of huge blocks of shale, soft, treacherous mud and quicksand. It also contains the old town tip, which has been breached, spilling glass and metal onto the beach. Furthermore, the slip has completely blocked the beach between Lyme Regis and Charmouth, even at low tide. The layers of rock involved are not the most fossiliferous, although there will be plenty of fossils to be found. However, these will not become exposed until rough weather washes away the soft mud to leave them scattered on the beach.
As with any other part of the coast, the beach is the safest and best place to search for fossils. Please stay clear of the landslide.
Richard Edmonds was the Earth Science Manger on the Jurassic Coast World Heritage Team. I would like to thank Rameus Gallois and Denys Brunsden for their help in interpreting this event