8 September 2015
Portland: progressive failure at Weare Cliffs
Posted by Dave Petley
One of the most landslide prone areas of the UK is Dorset (which incidentally is also one of my favourite parts of the world). It has a series of very large, very well developed landslides, as well as numerous rockfall sites on steep cliffs. Most landslide fatalities in the UK in recent years have occurred in Dorset (for example this one and this one). The Isle of Portland is a geologically fascinating area of Dorset. The extensive coastline of Portland is prone to a range of complex landslide mechanisms. At present there is an interesting progressive failure developing on the Weare Cliffs section of the coastline. The Daily Echo has a nice article about it, including these two images by Mark Godden:
Geoff Kirby has a really nice webpage describing this area of Portland, including details of the extensive quarrying that has occurred in this area. Many of the historic buildings in London (including Buckingham Place for example) are built from Portland limestone, and of course this is also the area after which Portland cement is named. The key to these landslides though is that the hard, strong Portland limestone overlies much weaker marls, clays and shales. It is deformation in these weaker lower beds that permits the development of the failures.
The images above suggest that the landslide has extended back through the area of the coastal path, which has already been rerouted. This landslide is likely to become quite interesting in the months ahead as we move into the wetter winter period. I have seen no suggestion to date that any infrastructure is threatened by this landslide on Portland though, so in many ways it is of academic interest. It would be fascinating to see a new aerial view of the landslide.