7 October 2011
Watch this – surely the very best ever cliff collapse video
Posted by Dave Petley
Update: more information is now available about this rockfall
There have been some amazing rockfall videos in the past, but this one I think tops them all. This video was apparently recorded at Hells Mount, North cliffs near to Portreath in Cornwall, SW England. It is genuinely astonishing:
There is a newspaper article about it here.
A few things to note. First these cliffs are about 100 m high and are formed from Devonian slates, greywakes and siltstones. Thus, they erode very slowly. Second watch, the video for the extraordinary precursory activity – prior to final collapse the whole rock mass is clearly deforming. So although this looks at first sight to be instantaneous, it is probably actually the final stage of a progressive process. Finally, note the weather and the state of the ground. It is sunny – certainly no sign of any rain, and the ground appears to be dry. The amount of dust that the landslide generates suggests that the lower levels are dry too. Indeed the video was posted on 3rd October, which was during the warmest spell of October weather on record in the UK> The sea is calm. Thus, there is no obvious trigger – this apparently is a genuine progressive failure.
Update: more information is now available about this rockfall
Comments?
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
Any comment from the videographer as to what tipped him/her off to start filming?
Yeah. How did he figure it out?
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New description added to YouTube video: “The failure occurred at about 5pm on Friday 23rd September 2011. It took place on the North Coast of Cornwall. There were only a few people watching, with about the same number of inquisitive seals in the water! There were lots of ‘small’ failures just before the main event. This appears to be a progressive failure, the frequency and size of the smaller failures increased until the inevitable happened. The sound was incredible, those cliffs are about 150-200ft high. I’d been chatting to the couple mentioned in the clip only a few minutes before the footage was taken. They can just be seen walking away in the distance; the path had been fenced off and diverted, so they were quite safe. An amazing sight, and credit to R who took the video, I still can’t stop revisiting it! Thanks again for all the comments, who says Geology isn’t an exciting subject?!. Oh, & finally…. I’m happy to report that no seals or humans were hurt in the filming of this clip! ACSM(93)”
The trigger may have been long-term undercutting by waves. You can see the first major rockfalls at the very bottom of the cliff and these are directly into the surf. The cliff appears to have been undercut there. Hence, the lack of correlation with weather conditions.