The Folkestone Warren landslide (Figure 1) is a very large, deep-seated coastal landslide that is well known to geologists and engineers (see references). It is about 3 km wide, and up to 350 m in length. There are nine Surveys in the National Landslide Database IDs 1774/1 to 1774/9.
The village of Capel-le-Ferne is situated behind the landslide at the cliff top, and the wide undercliff is given over to a country park.
The Dover to London mainline railway (via Cheriton) passes through the entire length of the Warren, with the Abbotscliff Tunnel at the eastern end and the Martello Tunnel at the western end. Since its construction the railway has been regularly disrupted by landslide activity, and considerable expenditure has been, and continues to be, involved in remediation and monitoring. The old Dover road is situated close to the cliff edge. This has also been disrupted, and re-routed in parts, for the same reason.
The geology of the platform at beach level varies along the length of the Warren. At the eastern end the platform changes from Gault Formation to Lower Chalk (see below). The change occurs approximately in line with the eastern end of the concrete toe-weight apron (constructed in 1953), near Horsehead Point. Rock revetment has been placed to the east of the concrete apron by Network Rail. It is notable that westward of Horsehead Point the platform is displaced seaward by about 50 m due to movements of the various slide masses (Trenter & Warren, 1996).
The upper part of the chalk sequence here is the White Chalk Subgroup comprising from top to bottom the New Pit Chalk Formation resting on the Holywell Nodular Chalk Formation. The lower part of the chalk sequence is the Grey Chalk Subgroup formed by the Zig Zag Chalk Formation resting on the West Melbury Marly Chalk Formation. Superficial deposits forming the Clay-with-Flints overlie the chalk.
Beneath the chalk sequence lies the Gault Formation, with the thin Chloritic Marl bed at their junction. The Gault Formation consists of between 44 and 48 m of over-consolidated clay. Beneath the Gault Formation is the Folkestone Formation (Lower Greensand Group) with the Sulphur Band at their junction. The Folkestone Formation consists of calcareous and glauconitic sands and sandstones with some argillaceous horizons (Trenter & Warren, 1996).
The Folkestone Warren landslide is one of the largest on the English coast and is a classical example of a deep-seated (approx. 40 m below sea level) multiple retrogressive, compound mechanism, having translational, rotational, and graben features.
The Folkestone Warren cliffs are divided into the chalk 'High Cliff', essentially the overall landslide backscarp, and the sea 'cliff' consisting largely of landslipped material. The 'High Cliff' as such is not directly influenced by the sea, unlike the Folkestone Warren landslide as a whole.
The landslide has been extensively studied since 1915 (Figure 2), when a major re-activation occurred throughout the complex seriously disrupting the railway constructed in 1844 (Hutchinson, 1969; Hutchinson, et al., 1980; Trenter & Warren, 1996; Warren & Palmer, 2000). Included in this event were chalk slides from the High Cliff, in particular the 'Great Fall' landslide. Hutchinson et al. (1980) describe the three dominant sub-vertical joint sets within the Chalk and their influence on the form of the High Cliff and on landslides within it. In recent decades a large amount of instrumentation and remedial works has been installed and maintained by Network Rail within the Warren.
The age of the original landslide event is not known. Most landslide movements in recent times have been confined to the undercliff in the form of re-activations of existing deep-seated landslide slide masses. However, numerous significant slope failures of the High Cliff are believed to have occurred since the 1800s.
As part of a programme of work monitoring coastal erosion and landsliding at several sites around the coast of Great Britain, BGS has surveyed the Folkestone Warren landslide complex — see Terrestrial LiDAR Survey Techniques. Results from this work will be published soon.
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