Hell's Mouth, Cornwall

Gwithian Towans. Looking NNE. Folding in Mylor Slates. A north-westerly overturned F2 anticline with a near-horizontal hinge in silty banded slates. The cliff is capped by head overlain by blown sand. P006047

A large landslide occured on the North Cornwall coast on the afternoon of Friday 23 September.

Richard Hocking and Brian O'Connell, engineers at Cornwall Council, filmed the event, which was a large rock slide/fall near Hell's Mouth on between Portreath and St Ives.

A landslide on this scale is unusual in the South West of England, and the opportunity to witness it extremely rare.

Rapid failure

The rock slide/fall appears to have developed progressively from the foot of the cliff and taken out a significant section of cliff-top over about 20 seconds.

However, tension cracks had been noted in the cliff-top three days previously by council engineers.

Cliff geology

Geologically the cliff consists of mudstones and sandstones of the Porthtowan Formation and slates/siltstones of the Mylor Slate Formation, both of Devonian age.

These rock materials are weathered and have variable strength.

There is a complex interrelationship of regional faulting and local jointing which allows the rock mass to divide into blocks which with time become unstable.

The rock material that failed, at least on the evidence of the video, appeared to be in a dry state.

Five days warning

On Monday 19 September a jogger had reported that a large crack had developed in the coastal path. Later that day Cornwall Council decided to divert the path, working with the landowners The National Trust.

Further surveys by Cornwall Council on Thursday 22 September observed that the failure had progressed and that large tension cracks were opening up.

On Friday 23 September, reports of material falling from the cliff were noted throughout the day. The cliff collapsed by about 4.50 p.m.

Read the full Cornwall Council story: Amazing cliff collapse caught on camera

Contact the Landslide Response Team

British Geological Survey
Keyworth
Nottingham
NG12 5GG
E-mail: Landslides team
Telephone: 0115 936 3143
Fax: 0115 936 3276