An extensive tract of ancient landslips affects the Lower Greensand Group (Atherfield Clay Formation to Folkestone Formation) along the Greensand escarpment between Aldington and Folkestone, a distance of about 11 miles. You can often see evidence of fresh movement on the slope at “The Roughs” west of Hythe in wet winters. You can also see the longer-term effect of this landslip on a concrete Sound Mirror, a First World War aircraft early-warning device (although this one dates from between the Wars).
The original slip probably dates from the “Big Thaw” at the end of the last Ice Age, about 11,000 to 12,000 years ago. The antiquity of this landslip is evidenced by its effect on Portus Lemanis, a Roman fort built on the landslip below Lympne, later known as Stutfall Castle by the Early Medieval English (a name that implies the ruins were used as a sheepfold).
You can see the remains of some of its walls, which have been displaced by the landslip (see Photo. 1). This slope was clearly unstable in Roman times, as noted in the Geological Survey Memoir (Smart et al., 1966). This may be one of the few examples of Roman engineers getting it wrong. They seem not to have recognised the characteristic undulations indicative of unstable, landslipped ground, but then they didn’t have experience of reading the post-glacial landscape of these northern climes.
This tract of landslips includes shallow slides, as at The Roughs, and deep-seated “rotational slips”.
A rotational slip occurs along a slip surface that is concave down the slope and elongated along it. The upper part of the slip surface may be nearly vertical, in which case, after the slip has occurred, the remaining in-situ ground upslope may be visible as a “back-scar” cliff. The slip surface becomes less steep downslope as it follows the curved trajectory and, towards the toe of the slope, it curves upwards, so the angle of the slip surface is reversed.
This reversed slope is significant. The proportion of the slipped ground on the steep, upslope part of the slip surface decreases as it moves downslope over a surface with an ever-decreasing slope angle. This reduces the downslope sliding force (as happens when you approach the bottom of a playground slide). Meanwhile, the ground accumulating at the toe of the slip increases the proportion of the slipped ground over the landward-sloping part of the slip surface. This increases the upslope sliding force. Eventually, the two opposing sliding forces balance each other, and the slip stops moving (see Fig. 1).
Large-scale rotational slips comprise a succession of complex, overlapping slip surfaces of different ages. The slip shown in Fig. 1 is oversimplified. Secondary slips occur in the slipped ground above the main slip surface. These often form during the main slipping episode but can also occur at various intervals afterwards. The oldest, deepest slips can be recognised by the back-scar furthest up-slope. These are visible along the slopes above Hythe and Sandgate and can be recognised by their steep slopes, sometimes inland cliffs, and often by their curved outline when viewed on plan.
A slip surface, once formed, remains slippery and offers less resistance to ground movement along it than intact ground. Therefore, it only needs a relatively small out-of-balance force to reactivate an old slide. So, you can see that it is not a good idea to add weight to the top of a landslip or remove weight from the toe, as this will upset the balance and is likely to reactivate the slide. But this does happen.
There were two large landslips in Sandgate in the 19th Century, in 1827 in east Sandgate and in 1893 in west Sandgate, both of which involved the reactivation of existing slip surfaces. Drainage was installed in the ground at the site of the 1827 slip in the mid-19th Century to increase the stability of the ground. The 1893 slip occurred immediately to the west of the 1827 slip and to the east of the (former) Military Hospital, which also included ground drainage to stabilise the ground. The 1893 slip was described by Topley (1893), and the following is a précis of his account:
The slip was about 920 yds (841m) wide, extending from the eastern end of the grounds of the old Encombe House westwards towards the eastern end of the Military Hospital. It stretched seaward about 233 yds (213m) from the back of Encombe Grounds to the high-water mark, which itself had moved about 100 yds (91m) seaward. The greatest downward movement at the western part of Encombe Grounds was about 10 ft (3m), but at the eastern end of the grounds, where there were numerous small slips, the total movement may have been almost as great.
Topley included the following observations in his account. These must have been made shortly after the slip, as evidenced by his observation of freshly slipped clay on the foreshore, which wouldn’t have survived many high tides.
“The rainfall in February was unusually heavy.
“The slipped area is not drained, whereas the districts immediately to east (the 1827 slip) and west of it (the Military Hospital) are drained…….
“In consequence of extensive groyning to the west of Sandgate, the eastward travel of shingle was stopped and the seafront of Sandgate became almost bare of shingle…….
“The loss of shingle has no doubt rendered the land more insecure, there being less permanent weight on the foreshore.
“The recent slip commenced at about low spring tide on the evening of 4th March; the movement diminished as the tide rose …. and at low tide next morning a second slip took place.
“The band of clay opposite Gloucester Terrace and Wellington Terrace was forced up; the movement continuing during the Monday and perhaps later.”
(Topley 1893)
These observations describe the conditions necessary to reactivate an earlier landslip. High groundwater levels, caused by heavy rainfall and exacerbated by poor drainage, reduce friction on the ancient slip surfaces, lowering the frictional resistance to sliding. The loss of shingle on the beach removed weight from the toe of the landslip, reducing the upslope stabilising force. The account also describes the characteristic upward movement at the toe of the slip, indicative of a rotational slip, and the continued slow movements occurring over the next couple of days, which are typical of slips in clayey soils. Minor slips and slow “creep” movements were occurring as recently as the 1990s. Major civil engineering works to stabilise the slip were undertaken along the Sandgate seafront in the 1990s.
Further descriptions of the landslip (and much else) are available in the Digital Archives of The Sandgate Society – https://sandgatesociety.com/archive-photographs/
If you were to take a deviation off the RSW and walk west along the Sandgate seafront during a low-water spring tide, you should see the toe of the ancient landslip described by Topley. This is a series of crescent-shaped rock outcrops on the foreshore, with strata showing a marked landward dip of about 40°, much greater than their in-situ dip (Photo 2).
These rocks are limestone, part of the Hythe Formation, and have been carried down the slope by landslipping. They originate from an outcrop that was originally a seaward extension of the current outcrop on the slope behind the seafront, several metres above ground level there. The rocks visible on the shore are equivalent to those circled in Fig. 1. They lie at the seaward extremity of the slip and on that part of the slip surface that tilts landward.
Andrew Coleman
Rev. 13/10/2026
References:
Smart, J. G. O., Bisson, G., & Worssam, B. C. (1966). Geology of the Country around Canterbury and Folkestone. Her Majesty’s Stationery Office.
Topley, W. (1893). The landslip at Sandgate. Proceedings of the Geologists’ Association, 13(2), 40–47. https://doi.org/10.1016/S0016-7878(93)80025-7
