Author: Katie Burden

Mapping the Undercliffs foreshore

The Axmouth to Lyme Regis Undercliffs National Nature Reserve is probably the most wild and remote part of the Jurassic Coast World Heritage Site. It is the product of massive landslide activity that dates back tens of thousands of years into the last Ice Age. The famous Bindon landslide of 1839 created Goat Island, a 16 acre block of Chalk that broke away from the inland cliff, isolated by a vast chasm that subsided behind it. Seaward, the sea bed was uplifted by over 40 feet and a pool, considered for use as a harbour, was created inshore of it. The Victorians flocked to view the scene and even Queen Victoria visited from the Royal Yacht.

The Reverend Conybeare, vicar of Axmouth, and the geologist William Buckland, provided the first ever scientific account of the landslide while Buckland’s wife, Mary and Daniel Dunster created fabulous water colours of the scene, invaluable in the days before photography. Conybeare and Buckland speculated that the landslide had moved over the Jurassic rocks, foundering into the soft Cretaceous Foxmould Sands. In the 1980’s a PhD student John Pitts spent years hacking his way through the Undercliffs to map every break in slope created by the landslides over time. He and Professor Denys Brunsden suggested that Bindon slip had moved along a weak layer of shale in the highest of the Triassic rocks.

Just to the east of Goat Island lies the Dowlands landslide and beyond that, the Plateau which is created from a block of slipped Chalk that is about 1km long. All three are massive landslides developed in the same geology but they are also different; why? The foreshore in front of the Plateau and Dowlands landslides is dominated by massive boulder fields but on closer examination, those boulders sit on blocks of solid geology that have slipped with the massive landslides. The sea has cut right through them and the foreshore is a cross section through the massive landslides; map the geology and we get a better understanding of just how the landslides moved.

Natural England liked the idea and provided a grant to fly a UAV survey – an Unmanned Aerial Vehicle – a remote controlled helicopter with a camera. The resulting photomoazaic covers about 2km of the foreshore down to a resolution of just 2cm, making it possible to accurately map rocks down to the size of a golf ball! Careful mapping over several weeks has uncovered all sorts of observations about the rocks, how they were before the slips moved and how the slips themselves have failed. Under the Plateau, the slip or shear surfaces are curved and cut up through the Jurassic rocks. This is the classic feature of a rotational landslide and the Plateau block is indeed gently rotated or back tilted. Dowlands is similar but here a whole series of narrow slices of rock march down to the sea like a giant staircase. These observations are leading to a new interpretation of how Goat Island and the Chasm formed but at this time they remain to be challenged and tested, so we are not going to put them forward just yet; watch this space!

Check out our youtube video to see how things were done in the field

https://youtu.be/izntZKpSyz4

Richard Edmonds

Back to Blog