Cliff Fall in Dorset

8 Nov 2010

Anyone holidaying in Dorset might be aware that you can walk the undercliff from Lyme Regis to Axmouth in what is now a quite pleasant wooded environment stretching some 10 miles or so that is maintained as a National Nature Reserve. It is in fact part of the South West Coastal Footpath, the section taking in the Jurassic Shore of Dorset and Devon. Most of the time there is no access to the beach because of the danger of falling rubble. What you are actually walking along is the cliff face that has collapsed over the last couple of centuries, a sort of no man's land between shoreline and cliff-top, a kind of half way house, neither up nor down. In Lost Cities and Sunken Lands, Nigel Pennick (Capall Bann:1997) describes a collapse of one section of these cliffs in 1839, after a very wet summer. By December fissures had opened up on a farm and a few days later two coastguards heard a great tearing sound and quickly skedaddled. On the shore other eyewitnesses said the beach heaved and the sea was violently agitated - a dark ridge rose up from the sea bed. All around them the cliffs started to fall accompanied by flashes of fire and a strong smell of sulphur. What was going on?

Might this have been an earthquake and might electro-magnetic forces have been at work as in two days 150 million cubic feet of material collapsed, an estimated 8 million tons of disrupted rock and riding it some 20 acres of prime farm land.

In one of those quirks of fate two famous geologists of the day were nearby, as it was Christmas time. The reverend WD Coneybeare lived in Axminster and his more famous contemporary, William Buckland, was staying with him for the festivities. They immediately paid a visit to the site and preserved for geological posterity, a description of the event - as they saw the aftermath. A deep chasm, or ravine, was formed, three quarters of a mile in length and 50 yards deep and 80 yards wide. An account in the Bath Journal of January 20th repeated the flashing lights theme ... and an intolerable stench. Later on in 1840 it was said a reef of rocks from  the bottom of the sea rose up at the same time as the collapse of the cliffs, forming an enclosed bay suggesting a rotational movement of rock strata. This was taken up by geologist WA MacFadyen in the 20th century, who suggested shear slippage played a controlling part in the mechanism of the movements, the reefs forming the uprising toe of the shear.

Richard Fortey, in The Hidden Landscape, says the Jurassic rocks of Dorset are overlain by the Gault Clay, a well known geological feature laid down in the Cretaceous era. This in turn is overlain by chalk and it is primarily the ability of the chalk to fracture easily and to slide along the plane of the Gault Clay layer that is responsible for such landslips - but this ignores the earthquake possibility, the flashing lights and the smell of sulphur. Donald Campbell in Exploring the Undercliffs, Coastal Publishing Co., Wareham: 2006, likewise does not mention an earthquake - or anything out of the ordinary. Over the last 160 years the event that triggered the landslip has been sanitised into a neat geological explanation.

Note ... Amazingly, fields of grain and turnips growing through the winter of 1839  were later harvested in the early summer of 1840 as the fields had plopped down on top of the fallen rocks - which had collapsed beneath the fields. Not all the fields were preserved of course, but some of them came to rest at what is now known as Goat Island.