Banc Dolhelfa landslide

The Banc Dolhelfa landslide with BGS-mapped landslide polygon.

The Banc Dolhelfa landslide.

The Mid Wales Geology Club (project leader, Colin Humphrey) in association with the Central Wales RIGS Group (chairman, Professor Bill Fitches), carried out a walk over survey of the landslide in Banc Dolhelfa in April 2010. 

The summary information for this landslide case study has been taken entirely from Welsh RIGS report 578 and is not the work of the BGS.

The landslide

The Banc Dolhefa landslide (BGS National Landslide Database ID 11853/1) is about 5 km south of Llangurig in Mid Wales and is one of the largest bedrock slides in Mid Wales.

The landslide is 650 m long and 150–200 m wide, calculated to have displaced around one third of a million cubic metres of rock. It is currently inactive, but a falling block was reported in 1850.

This is a complex landslide, predominantly translational in nature, with evidence of planar rock and debris sliding on the south side of the depletion zone and block failure by falling on the north side.

Failed rock on both sides broke into a debris flow in the accumulation zone.  Evidently the south side slid on bedding planes partly concordant with the topographic slope, and the back and north side failed on subvertical joints, leaving a steep escarpment.

Subsequent progressive failure and retreat of the escarpment has occurred by slumping and rock fall.

Superficial deposits landslides

The Banc Dolhelfa Landslide

Most landslides in the area are in superficial deposits, but here the Silurian bedrock has failed. The trigger for the original failure is thought to be post-glacial movement on the visible fault parallel to the upper south edge. Subsequently there has been some further collapse and retreat of parts of the northern escarpment. Scree deposits show that weathering and rock displacement has continued from the upper part of the slide in recent times.

Geology

Geology map of Band Dolhelfa Landslide

The geology associated with this landslide is the Glanyrafon Formation, which comprises interbedded mudstones and sandstones that are approximately 430 million years old. The lower slopes of the landslide are covered with Devensian Till. There is a steeply dipping east–north–east-striking fault very close to the southern flank of the landslide, passing beneath it at depth. The Pystgotwr Grit Formation is to the north and not incorporated into the landslide.

Note on the mapped landslide extent

The red hashed area is the landslide deposit as published by BGS althought the more recent detailed work in the RIGS report shows the deposit to be larger and more rectangular, with the south flank running parallel to the fault and the slide mass extending to the River Wye at the toe.

As with all BGS landslide maps, only the deposit is shown and the backscarp of this landslide can be seen above the deposit where the underlying Ordnance Survey map highlights a scarp slope where the Pystgotwr Grit Formation crops out.

Visiting the site

The RIGS is an excellent, clearly visible example of a complex landslide in bedrock and is of amateur and undergraduate geological and geomorphological interest. On-site there are features which offer an opportunity for wide-ranging technical discussion of failure mechanisms.

An alternative to visiting the site itself is for small groups to view it from across the Wye valley, especially from SN 914 748, although parking is unsuitable for large vehicles. No rights of way exist over or near the landslide; many parts are dangerously steep and there is also a risk of rock fall.

Reference

Humphrey, C.  2012.  The Landslide at Banc Dolhelfa, near Llangurig, Powys. Open University Geological Society Journal, volume 33:2, p33–40.

Contact the Landslide Response Team

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