Landslides on the Isle of Wight

St Catherine's Down and Appuldurcombe

Isle of Wight location

St Catherine's Down and Appuldurcombe are located in the southernmost part of the Isle of Wight. The landslides in this part of the island are ancient and relatively degraded, having formed at the end of the last ice age, some 10 000 years ago. At this time, periglacial conditions favoured heightened erosion and subsequent movement of the deposits. This is a similar scenario found throughout the UK.

Geology

Landsliding in the St Catherine's Down–Appuldurcombe area is associated with the Upper Greensand and Gault Formations (Figure 1).

These Formations are Upper Cretaceous in age, and have a gentle dip towards the south–south-east. The Upper Greensand Formation is a sequence of glauconitic sandstones, clayey sand and chert-rich layers, and is approximately 30–40 metres in thickness. The upper part of the Formation is characterised by a semi-continuous horizon of hard chert nodules. These form the more erosion-resistant, prominent cliff-type features in the scarps surrounding the Downs. Figure 2, Figure 3 and Figure 4 illustrate the landsliding in the area.

The Upper Greensand Formation is underlain by the Gault Formation, typically a stiff blue-grey clay, approximately 30 metres thick. The upper part of the formation in this area can be a relatively sandy clay and therefore slip surfaces may not always correspond with the top of the Gault Formation as might be expected. Due to the sandy nature of the deposit, the plane of movement may be deeper seated within the Gault Formation itself.

Landslides

Planes of weakness between underlying, more impermeable clay and the glauconitic permeable sands have been exploited by water seepage causing erosion and instability. These planes act as slip surfaces and a catalyst for the movement of material downslope. Figure 5 shows typical hummocky terrain caused by multiple rotational landslides within the Gault Formation. Figure 6 is a view of a landslide at Appuldurcombe.

Landslide triggers

Solifluction-type processes during the periglacial era (approximately 10,000 years ago) would have resulted in the downslope movement of saturated debris. At the time of movement, the deposits would have been susceptible due to high water contents, causing expansion of clays, which are prone to slow creep and plastic deformation.

Solifluction in the active layer of periglaciated terrain would have been widespread on slopes, particularly those formed of clays, mudstones and chalks, on slopes >4 degrees. Postglacial melt of permafrost permitted drainage and marginal stabilisation leaving shear surfaces with much lower residual strengths.

Why does BGS need to know about these landslides?

Although these landslides are ancient and no activity has been observed, it is important to know their location, mechanisms for development and extent. From an engineering perspective, these moved materials have a lower residual strength therefore:

  • there is a higher potential for compaction, which may cause differential settlement under load
  • Loose sandstone creates increased weathering potential causing weakening of the cement which weathers to loose sands
  • Landslides can be reactivated if the topography or drainage of an area is altered e.g. by deforestation, addition of material above the slide adding weight, alterations due to construction, excavation of the toe of the landslide etc.

Photo gallery


Further reading

Barber, K.E. 1987. Wessex and the Isle of Wight Field Guide. Quaternary Research Association. Cambridge

Chandler M.P. 1984. The Coastal Landslides Forming the Undercliff of the Isle of Wight. Unpublished PhD Thesis, Imperial College, University of London.

Geomorphological Services Ltd. 1989. Landsliding in and around the village of Luccombe. Report to the DoE.

Geomorphological Services Ltd. 1991. Coastal Landslip Potential Assessment: Isle of Wight Undercliff, Ventnor. Published by Geomorphological Services Limited.

Hutchinson, J.N. and Bromhead, E.N. 2002. Isle of Wight Landslides. In McInnes, R.G. and Jakeways, J. Instability Planning and Management. Proceedings of the International Conference. Ventnor, Isle of Wight, UK. 20-23rd May 2002. Thomas Telford.

Gillarduzzi, A., Clark, A.R., Fort, D.S., and Houghton, J.E.M. 2007. Monitoring coastal slope instability within the western Undercliff landslide, Isle of Wight, UK. In McInnes, R., Jakeways, J., Fairbanks, H. and Mathie, E (eds). Landslides and Climate Change. Challenges and solutions. Proceedings of the International Conference on Landslides and Climate Change. Ventnor, Isle of Wight, 21-24th May 2007. Taylor and Francis

Hutchinson J.N. 1965. A reconnaissance of coastal landslides on the Isle of Wight. Building Research Note No EN 11/65. Building Research Station.

Hutchinson, J.N. 1991. The landslides forming the South Wight Undercliff. In Chandler, R. J.(eds) Slope Stabillity Engineering. Proceedings of the International Conference on Slope Stability. 15-18th April 1991. Thomas Telford.

Insole, A., Daley, B. and Gale, A. 1998. The Isle of Wight. Geologists Association Guide No.60. The Geologists Association.

Rendel Geotechnics. 1995. The Undercliff of the Isle of Wight: A review of ground behaviour. Published by South Wight Borough Council.

White H.J.O. 1921. A short account of the geology of the Isle of Wight. Memoir of the geological survey of England and Wales. HMSO.

Useful Links

SCOPAC (Standing Conference on Problems Associated with the Coastline) web page SCOPAC report

Report on Bonchurch landslide, Ventnor Undercliff, Isle of Wight, UK

Bibliography from Southampton University

 

Contact the Landslide Response Team

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