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.
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.
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.
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:
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