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Field meeting on the coastal landslides of Kent
Field Meeting on the Coastal Landslides of Kent 1-3 July 1966 Report by the Director: J. N. HUTCHINSON Received 3 April 1967 FIFTEEN MEMBERS and friends assembled at the County Hotel, Canterbury, on the evening of Friday, 1 July, when the Director gave an introductory talk on the geology and landslides of the Kent coast and outlined the field programme. This was designed primarily to demonstrate the diverse types of coastal landslide occurring and to indicate the main factors determining the character of each type of failure . The terms used to describe the various types of landslide examined are defined and described elsewhere (Hutchinson, 1965; and in press).
Saturday, 2 July This day was spent entirely on the London Clay cliffs of north Kent. Attention was drawn particularly to the strong influence of the intensity of marine erosion on the character of the coastal landslides, which is well revealed in these relatively homogeneous cliffs. The cliffs visited were accordingly categorised as those suffering strong marine erosion , those exposed only to moderate erosion , and the abandoned cliffs, from the foot of which no debris is removed. The party travelled by coach first to Warden Point, Isle of Sheppey (TR 018725). This is the eastern, most exposed, part of the London Clay cliffs of north Sheppey and affords an excellent example of the behaviour of this formation under conditions of strong marine erosion . According to Steers (1964), the average rate of recession of the cliffs at Warden Point is as high as 9.7 ft./year (3 m.). The behaviour has been described by Hutchinson (1965). It is characterised by the occurrence of deep-seated rotational landslips, bench-shaped in plan. They usually extend along the coast for distances of between four and eight times the cliff height, here about 130 ft. (40 m.). The London Clay extends well below the foot of the cliffs, and at Warden Point its base is estimated from Whitaker (1908) and regional evidence to lie over 300 ft. (91 m.) below sea-level. Most of the landslips involve base failure. No subsurface investigations have been carried out , but the back-tilted and little-broken nature ofthe slipped masses suggests strongly that the failures take place on surfaces which are approximately circular. 227
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With time, the steep rear scarp of such a landslip is broken down by shallow slides and mudflows. The debris from these slowly fills the hollow left at the rear of the slipped mass and eventually spills past each of its extremities into the sea in the form of secondary mudflows. The continuing marine erosion removes the tongues of these mudflows and gradually forms a fresh wave-cut platform in the slipped mass. This effects a progressive overall steepening of the cliff, which eventually completes the cycle of degradation by precipitating a further, deep-seated landslip. Various lines of inquiry indicate that the length of the cycle at Warden Point lies between thirty and forty years (Hutchinson, 1965). The party gathered at a vantage point on the cliff-top, above a wellmarked arete of London Clay (TR 020725). The Director drew attention to the deep-seated rotationallandslip immediately to the south-east [K 40].1 It took place in 1950,and illustrates many of the features mentioned above. Its slipped mass has a back-tilt of about 40° and is now partly buried by mudflow deposits deriving from the rear scarp. The regional dip here is less than 1° northward. The adjacent landslip to the north-west [K 39] is of similar type, but is larger and in a more advanced stage of the degradation cycle. As the tide was fairly low, it was possible to see the interesting structures in the wave-cut platform off the present coast. They consist of bands of cementstone in London Clay, running approximately parallel with the coast and back-tilted at angles of up to 40° or 50°. The exposures represent the planed-off remnants of earlier landslips, and reveal their deep-seated, rotational character. The coastwise extent of the cementstone bands corresponds closely, in general, with that of the present-day landslips, a relationship which is particularly clearly illustrated at the junction between landslips [K 38] and [K 39]. Thus, once formed, the pattern of landslips and dividing aretes tends to maintain the same lateral position through successive cycles of degradation. The habit probably derives largely from the lateral drainage pattern in the cliff away from the aretes. Before leaving the cliff-top, the Director indicated the site of the former St. James's Church at about the position of the present cliff-foot immediately to the north of where the party was standing. The church was dismantled in 1875shortly before the site slipped down the cliff (Hull, 1967). Landslides just seaward of the church in 1856 and 1859 have been described by Redman (1859a and b). The party then descended the cliffs to the beach by way of the dried-up mudflow on the north-west side of the 1950 slip. Both this route and that used for ascent become impracticable, and even dangerous, in the winter months. The small cliff cut by the sea in the slipped mass of that failure 'The symbols in square brackets are the site references used in the survey of coastal landslides in Kent (Hutchinson, in preparation).
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was first examined and a probable back-tilt of 33° measured (TR 02107245) . The back-tilted bands of cementstone exposed in the foreshore were also inspected. In the cliffs to the north-west the Director pointed out that three distinct categories of material were being removed from the cliffs by mar ine erosion; London Clay in situ beneath the aretes, disturbed London Clay from the landslip masses, and mudflow material from each side of the slipped masses. A short halt was made by the tongue of the secondary mudflow issuing round the south-east end of the large landslip [K 39] immediately northwest of the 1950 failure (TR 020726). The characteristic texture of the mudflow deposit, consisting of discrete fragments of stiff to hard material in a matrix of structureless, softened clay, was well displayed , as was the tendency of the tongue to drain and stiffen and be overridden by the softer material from behind. Just to the north-west of this point , Dr. K. M. Clayton noticed a sharp, fairly level discontinuity, 15 to 20 ft. (4.5 to 6 m.) above the cliff-base. This proved to be the slip surface of a shallow mudflow which was overtopping the degraded, slipped mass, there of brown London Clay. The flow, which was about 2 ft. (0.6 m.) thick and fairly stiff, had clearly been moving as a block on one discrete and highly polished slip surface, well exposed beneath small, overhanging portions of the mudflow. The party returned to the coach by ascending the cliffs at the east end (TR 017727) of the next landslip to the west [K 38), another deep-seated rotational failure, which occurred in about 1945. The slopes were thinly spread with a pale-coloured sand which appeared to have been washed out of a sandy gravel capping the cliff. It was mentioned that similar examples of seepage erosion may be seen in the Bagshot and Claygate Beds capping the cliffs a few miles to the west. A short coach journey carried the party to the Ferry House Inn on the Isle of Harty, where lunch was taken. Harty was visited in order to see the very different types of landslide which take place on freely degrading slopes of London Clay; that is, slopes from which the removal of debris at the foot has ceased. Before lunch a short examination was made of the abandoned cliff of London Clay just west of the inn [KA 49]. The cliff is fronted by saltings and no longer suffers any appreciable erosion at its foot. The party walked to the highest point of the cliff (90 ft. (27.5 m.) O.D.) whence (TR 011661) it could be seen that most of the slope was affected by shallow landsliding, predominantly of successive rotational type. The average inclination of the slope varies between about t ItOand 12to. Recent studies on the abandoned London Clay cliffs of Kent and Essex were outlined: they indicate that shallow landsliding will continue until the cliff has degraded to an inclination of about 8° (Hutchinson, 1967, and in preparation).
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After lunch, the party travelled to the north-west part of Harty (TR 016676), to an abandoned cliff of London Clay [KA 50] in a more advanced state of degradation. There, in a grassy slope of 8io average inclination, a subdued, cross-slope undulation could be discerned with an amplitude of about 25 yds. (23 m.) and a pitch of roughly 1 ft. (0.3 m.). This represents the degraded remains of an old landslip, whose present smooth contours may well have been produced in part by ploughing. The last length of London Clay cliffs to be visited was that lying to the east of Herne Bay. With a present average recession rate of generally less than 5 ft.jyear (1.5 rn.), the cliffs provide an example of the behaviour of the London Clay under more moderate marine erosion. Their average height is about 100 ft. (30.5 m.), The party alighted from the coach on the top of the East Cliff (TR 188683) [KD 70], which is defended at its foot by a concrete sea-wall. The Director remarked that its main interest lay in the stabilisation works carried out on the cliff slopes, namely the trimming back of the slopes to inclinations of between about 12° and 19°, and the installation of deep counterfort drains with secondary feeders. The main stabilisation works are described by Duvivier (1940). The location of an old, deep-seated rotational landslip was pointed out (TR 18756840 to 18906840). This was probably initiated before about 1907, when, it is believed, the first seadefences were built. Subsequent renewals of movement in this landslip, involving upheaval of blue London Clay on the foreshore and damage to the sea-wall, occurred in the winter of 1935-6 (Duvivier, 1940) and during the period 1956-7 (Wise, 1957). Moving to the east along the cliff-top, the party next examined the length of coast known as The Lees [K 71) (TR 192684 to 194684). This is virtually unprotected: an intermittent line of old timber piles offshore probably represents the remains of the 1907 breastwork. It is the best remaining example (shortly due to be destroyed by an extension of the sea-defences to the east) of the behaviour of London Clay cliffs under moderate marine erosion. The Director pointed out that under these conditions, the effects of weathering on the intensely fissured, stiff clay is to produce a pattern of cliff degradation dominated by the shallower forms of mass movement. This is characterised by the formation of a succession of carries in the cliff slopes, each occupied by a mudflow and bounded by aretes of dried London Clay. At The Lees, five such mudflow carries were seen, with widths varying between about 0.8 and 1.8 times the cliff height. It was emphasised that the essential difference between this site and Warden Point is that here virtually the only material eroded by the sea is from the tongues of the mudflows. The mudflow activity is thus stimulated and perpetuated and the sea prevented from attacking London Clay in situ. In general, therefore, this length
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of cliff is never sufficiently steepened for a deep-seated rotational landslip to be initiated. A further 300 to 400 yds. (274 to 365 m.) to the east, above Beltinge Cliff, the party halted on the edge of an old, deep-seated landslip [K 73] which had cut across the cliff-path (TR 197684). It is called the Miramar slip, after the nearby hotel, and took place on 4 February 1953, shortly after the tidal surge of that winter. Here the cliffs are about 110 ft. (34 m.) high and consist of the lower parts of the London Clay, the junction with the underlying fine sands of the Oldhaven Beds lying only about 15 ft. (4.6 m.) below Ordnance Datum. The Director then described the landslip and called attention to some of its more notable features. The failing mass extended about 220 yds. (200 m.) along the cliffs and involved a strip of the cliff-top averaging about 20 yds. (18 m.) in width. This, together with the upper part of the cliff-face, sank about 50 ft. (15 m.) between very steep main and secondary slip surfaces to form a pronounced graben. The sunken mass was separated from the sea by a sharp ridge of London Clay, of which the degraded remains still form a pronounced feature of the landslip. Mudflows from the rear scarp now partially infill the graben and spill around each end of the degraded ridge into the sea. It seems that during the years before the 1953 surge, this length of Beltinge Cliff was occupied by a succession of mudflow corries such as have been shown to characterise a moderate intensity of erosion. The stormwaves and high sea-levels associated with the 1953 surge, however, apparently intensified erosion at the cliff-foot to bring about this major, deep-seated landslip. The development, during landslipping, of a pronounced graben is a well-known sign that failure has occurred on a markedly non-circular slip surface (Ritchie, 1958; Morgenstern & Price, 1965). Confirmation that this is the case for the Miramar slip has been obtained from borings (Hutchinson, 1965); the development of a deep-seated circular slip is prevented by the shallow depth to the Old haven Beds. The last part of the London Clay cliffs examined was the eastern part of Beltinge Cliff (TR 2(0685), about 200 yds. (I 83 m.) east of the Miramar slip. Here [K 74] the Oldhaven Beds have risen to just below beach level and the JOO-ft. (30.5 m.)-high cliffs again exhibit a succession of mudflow corries. The attention of the party was drawn to the mudflow just west of The Rand, a linear accumulation, predominantly of rounded black pebbles, which projects across the foreshore approximately at right-angles to the coast at about TR20 1688and coincides closely with the outcrop of the Oldhaven Pebble Bed. Observations have been made here since early 1962 (Hutchinson, 1965), which indicate that shallow landslides and falls from the corrie sides provide the main source of debris supply to the flow, with a smaller contribution made by a process termed basal incorporation.
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Measurements of the velocity profile across the surface of the flow and throughout its depth , using a specially designed inclinometer, demonstrate that its movement takes place predominantly by marginal slip, and thus approximates to plug flow. Measurements of the volume of debris discharged by the mudflow were quoted to illustrate its great seasonal variability. In an average late summer the flow is stationary; its maximum discharge so far observed in a twelvemonth period is about 44,000 cubic ft. (1250 cu. m.). Finally, the party examined an inclined, approximately planar surface of blue-grey London Clay in the lower cliff, which was traversed by a small branch of the main mudflow. The surface has a seaward inclination of 45° to 50° and a strike bearing of 43° to 48°. The Director described how measurements had been taken of the profile of the groo ve eroded in this surface by the passage of the mudflow, which provide good evidence that mudflows deepen their beds (Hutchinson, 1965). A discussion followed on the origin of the planar surfaces and there was general acceptance of Mr. S. C. A. Holmes's view that they are erosional features, characteristic of the rather shaly lithology of the lowest 30 ft. (9 m.) or so of the London Clay. From Beltinge Cliff the party rejoined the coach and returned to Canterbury for dinner. Sunday, 3 July This day was spent on the Cretaceous cliffs of east Kent. A visit was first made to an undefended cliff of Upper Chalk at Joss Bay, Isle of Thanet, where a small fall had taken place in the preceding February or March. At this point (TR398704) the cliffs, about 50 ft. (15 m.) high, consist of highly jointed Chalk of the Mieraster cor-anguinum and Uintacrinus zones (Peake , 1961), somewhat frost-shattered in the uppermost few feet. The northerly dip is locally about 1°. At the site of the fall, 'Whitaker's 3-in. band' is located in the base of the cliff. The Director outlined the probable mode of failure. A survey of the fresh scar, made previously with the help of Mr. D. G. Farquharson, indicated that the fall had been preceded by the opening up of a joint-eontrolled tension fissure parallel with the line of the cliff and located at a distance of about 0.15 H behind the cliff edge (H being the cliff height). This process was probably assisted by the formation of a wave-eut notch, I to 2 ft. (30.561 cm.) deep, at the cliff-foot. When the tension fissure reached a depth equal to 0.5 H, the lower half of the clifffailed in shear on a relatively plane slip surface inclined at about 60° to 65° to the horizontal. The fall had a length along the cliff of about 1.3 H. The high secondary permeability of the cliff implies that ground-water pressures are unlikely to have played a significant role in the failure . Members of the party then examined the remains of the fall. Of particular
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interest was the slip surface, accessible at the top of the fallen debris, on which striations and traces of chalk paste were still visible. The irregular, blocky nature of the rear face of the tension scar left by the fall was also noted. The extensive landslips at Folkstone Warren [KD 81] were next visited. They extend for nearly 2 miles (3.2 km.) along the coast and have a maximum breadth of about a quarter of a mile (402 m.). From a viewpoint on the crest of Abbot's Cliff, above the east end of the Warren (TR 268386), the Director gave a brief history of the landslips and outlined their engineering geology. Members were reminded that the cliffs in the vicinity of the Warren truncate the scarp of the North Downs and consist of about 450 ft. (137 m.) of the Middle and Lower Chalk, underlain by the Gault and Lower Greensand (Osman, 1917; Smart, Bisson & Worssam, 1966). The landslips are due to the presence of the argillaceous Gault, here between 120 and 160 ft. (37 m. and 49 m.) thick, in the base of the cliffs. The regional dip is about 1 to the north-east and the base of the Gault rises from approximately -190 ft. (- 58 m.) O.D. at the eastern extremity of the landslips to about sea-level at their western end. Since the opening of the railway through the Warren in 1844, over thirty landslides are known to have occurred there. In the light of the extensive subsurface investigations of Toms (1946, 1953) and Wood (1955), it is possible to divide the failures into four categories. The largest are deep-seated renewals of movement in the extensive multiple rotational landslips which form the Warren. The last failure of this type took place in December 1915 and closed the railway for nearly four years. Smaller failures of rotational type involve a renewal of movement in only the seaward part of the Warren, the last significant movements occurring during the period 1940 to 1952. The work of Toms (1953) and Wood (1955) has shown that in both the above types of landslip, the slip surface penetrates approximately to the base of the Gault. The remaining two types of failure are associated with the precipitous rear scarp of the Warren landslips, known as the High Cliff. Chalk-falls from this cliff comprise the more catastrophic of the two, but they seem commonly to be preceded by settlements of up to a few feet in those parts of the cliff-top immediately landward of the rear scarp. Movements of this fourth category are known as 'sets', and doubtless also involve the underlying Gault (Toms, 1953). A mechanism for their formation has been proposed by Hutchinson (1965). Knowledge of the ground-water conditions in the landslips also rests largely on the work of Toms (1946, 1953) and Wood (1955). Within the slipped masses a phreatic ground-water level reflects, to some extent, the fluctuations of the water-table in the Chalk inland. Good correlation has been obtained between the occurrence of peak levels in this phreatic 0
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system and renewals of rotationallandslip movement (Hutchinson, 1965). One of the stabilisation measures carried out at the Warren consists of draining away some of the phreatic water by driving in headings from about the level of the top of the sea-wall. A second, confined ground-water level is found beneath the Gault in the Folkestone Beds. Ground-water pressures in the confined system are generally fairly low, because the sandy, rather permeable Folkstone Beds outcrop in the sea-bed just off the coast. From Abbot's Cliff the party enjoyed a good view of the 300 to 400 ft. (91 m. to 122 m.) high, rather weathered rear scarp, fronted by the irregular shelf or undercliff of slipped masses on which the railway runs. Attention was drawn to the bare scar running across the otherwise grassy talus slopes at the foot of the High Cliff, which represents the landward limit of large-scale movements in the 1915slip. Along the coastline the continuous, groined sea-wall, supplemented in its central parts by extensive toeweighting structures, was pointed out. The sea-wall was completed in 1939 and the toe-weighting added between 1948and 1955. The party then drove to Capel-Ie-Ferne where, after lunch, members descended into the Warren by a path down the High Cliff (TR 254385), and reassembled on the eastern part of the toe-weighting structure (TR 258382). From that point the junction between the Middle and Lower Chalk was indicated at about mid-height of the High Cliff, being clearly marked by the plenus Marls and the overlying Melbourn Rock. An examination was then made of the nearby chalky bluff, known as the Horse's Head (TR 256382). It was seen to consist at the base of intact Chalk, back-tilted at an angle of over 60° by the rotationallandslip movements. Miss A. E. Ward found the lamellibranch Inoceramus labiatus in this material, which indicates that it belongs to the lower zone of the Middle Chalk. This shows that the Horse's Head, which is one of the most seaward component blocks of the multiple rotational slip complex, has suffered, in addition to its backward rotation, a vertical component of downward displacement of the order of 300 ft. (91 m.). It was suggested that the time of initiation of the Folkestone Warren landslips is unlikely to have been earlier than the restoration of sea-level to its present position, after the severe eustatic depression associated with the last glaciation (Hutchinson, 1965). According to West (1963) the restoration occurred between about 3500 and 3000 B.C. The Director then drew attention to some interesting features in the middle and upper parts of the Horse's Head, from which it is hoped eventually to arrive at a more precise date for the initiation of the Warren landslips. The back-tilted intact Chalk there is overlain by a thickness of 25 to 30 ft. (7.6 to 9.1 m.) of chalk debris, in which three immature soil profiles are distinguishable. These are also back-tilted, though not quite so steeply as the intact Chalk, and they seem likely to have been formed at
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an early stage in the rotational slipping. An examination of a sample of the lowest soil layer by Dr. M. P. Kerney revealed the temperate mollusc Pomatias elegans in some quantity. On the basis of this and other aspects of the fauna Dr. Kerney considers the soil layer to be probably of Atlantic or Sub-boreal age. The party rejoined the coach at the bottom of the road leading into the Warren, just south-east of the Martello Tunnel, and followed the coast towards the west. While passing through Sandgate, brief mention was made of the two very large landslips [KD 88] of 1827 (Topley, 1893) and 1893 (Blake, 1893), which together affected almost the whole area of the town. Both failures seem to have involved renewals of movement in old landslips, and were probably seated in the Atherfield Clay. Houses tilted by the 1893 movements were pointed out on the north side of the coast road. To the west of Sandgate this line of cliffs is now protected from the sea by the alluvial flat of Romney Marsh and its protective fringe of shingle. The last halt of the excursion was made at Lympne to see a typical section of this abandoned cliff [KA 93]. The party alighted near Lympne Castle (TR 119347) and walked down the footpath to Stutfall Castle. The steep feature produced by the Hythe Beds was noted at the top of the slope. They are underlain by the Atherfield Clay, here probably about 30 ft. (9.1 m.) thick, and the Weald Clay, which forms the major part of the abandoned cliff. At Stutfall Castle the Director outlined the history of the cliff since its abandonment by the sea, which, according to Gallois (1965), took place some 3000 to 4000 years ago. Subsequently free degradation moulded the cliff, broadly as described earlier for the London Clay (Hutchinson, 1967), which had achieved some degree of stability by the time the Romans chose to build Stutfall Castle on its slopes in the latter part of the third century A.D. (Bushe-Fox, 1932). The present ruinous condition of the fort and the considerable dislocations suffered by its walls show that at least shallow landslide activity continued after the third century. Smith (1852) considers that the present dislocation of the fort was completed before the Norman Conquest. In the last millenium evidence is scanty. It would appear that the upper slopes have been subject to occasional, fairly small, rotational landslips. One slip of this type is reported by Collinson (1728) to have involved a farm near Lympne (TR 11253470) in about 1725. Such failures in the crest of a slope are typical of the later stages of the free degradation process. The clay slopes above Stutfall Castle now exhibit the smooth undulations of degraded old slips and appear to have a high ground-water table. The average inclination of the slopes is about 8io, and they are undoubtedly close to their angle of ultimate stability against landsliding. A drop in ground-level of up to 5 or 6 ft. (1.5 to 1.8 m.) occurs between
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the uphill and downhill sides of the remaining Roman walls which lie across the slope. The Director suggested that the level-difference may have resulted from an accumulation produced by hill-wash and soil creep against the barrier formed by the walls, during the past 1700 years. In conclusion, the Director recalled that the last period of intense solifluction in south-east England was in zone III of the Late-glacial, between about 8800 and 8300 B.C. (Kerney, Brown & Chandler, 1964). Whatever solifluction deposits then mantled the Lympne slopes, however, are likely to have been partly or wholly removed by the marine erosion which continued at the slope foot for another six or seven thousand years. Support for this view is provided by a comparison of the morphology of the Lyrnpne slopes with that of the inland continuation of the Hythe escarpment at Sevenoaks. There the slopes are draped by major solifluction lobes, of which no sign is seen on the Lympne slopes. The party returned to the coach, where Mr. S. C. A. Holmes proposed a vote of thanks to the Director and to Mr. N. S. Farrar, who acted as Secretary of the Meeting. REFERENCES BLAKE, J. F. 1893. The Landslip at Sandgate. Nature, 47, 467-9. BUSHE-Fox, J. P. 1932. Some Notes on Roman Coast Defences. J. Roman Studies, 22, 60-72. COLLINSON, P. 1728. An Uncommon Sinking of the Ground in Kent. Phil. Trans. R. Soc., 35,551-2. DUVIVIER, J. 1940. Cliff-Stabilisation Works in London Clay. J. Instn civ, Engrs, 14, 412-26. GALLOIS, R. W. 1965. The Wealden District (4th Edn.). Br, reg. Geol. London. HULL, F. 1967. (County Archivist of Kent.) Personal Communication. HUTCHINSON, J. N. 1965. The Stability of Cliffs Composed of Soft Rocks, with Particular Reference to the Coasts of South-East England. Ph.D. Dissertation, University of Cambridge. - - - . 1967. The Free Degradation of London Clay Cliffs. Proc, Geotechnical Con! Oslo, I, 113-18. - - - . Mass Movement. In: Encyclopedia of Earth Sciences (Edited by R. W. Fairbridge), Reinhold Publishing Corp., U.S.A. (in press). - - - . Surveys of Coastal Landslides: Kent. Building Research Current Papers (Building Research Station, Watford) (in preparation). KERNEY, M. P., E. H. BROWN & T. J. CHANDLER. 1964. The Late-Glacial and PostGlacial History of the Chalk Escarpment near Brook, Kent. Phil. Trans. R. Soc. B., 248, 135-204. MORGENSTERN, N. R. & V. E. PRICE. 1965. The Analysis of Stability of General Slip Surfaces. Geotechnique, 15, 79-93. OSMAN, C. W. 1917. The Landslips of Folkestone Warren and the Thickness of the Lower Chalk and Gault near Dover. Proc: Geol. Ass., 28, 59-84. PEAKE, N. B. 1961. The Coastal Chalk of North-East Thanet. Geologists' Ass. Guides. No. 30: The London Region, 18-22. REDMAN, J. B. 1859a. Landslip at Warden Point, Sheppey, The Times, 22 September 1859. - - - . 1859b. The Landslip at Warden Point. Reprinted in: Bingham, R., 1861-2. On the Geology of the Isle of Sheppey, Proc, Geol. Ass., I, 92-106. and 167. RITCHIE, A. M. 1958. Recognition and Identification of Landslides. In: Landslides and Engineering Practice (edited by E. B. Eckel). Highway Research Board Special Report 29, NAS-NRC Publication 544, Washington, D.C.
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SMART, J. G. 0., G. BISSON & B. C. WORSSAM. 1966. Geology of the Country around Canterbury and Folkestone. Mem. geol. Surv., U.K. SMITH, C. R. 1852. Report on Excavations made on the Site of the Roman Castrum at Lympne in Kent in 1850. London. STEERS, J. A. 1964. The Coastline of England and Wales (2nd Edn.), Cambridge. TOMS, A. H. 1946. Folkestone Warren Landslips: Research carried out in 1939 by the Southern Railway. Proc. Instn civ, Engrs, Railway Paper No. 19,3-25. - - - - . 1953. Recent Research into the Coastal Landslides at Folkestone Warren, Kent, England. Proc. 3rd Int. Conf. Soil Mech, and Foundn. Engrg., Zurich, 2, 288-93. TaPLEY, W. 1893. The Landslip at Sandgate. Proc. Geol, Ass., 13,40-7. WEST, R. G. 1963. Problems of the British Quaternary. Proc. Geol. Ass., 74,147-86. WHITAKER, W. 1908. The Water Supply of Kent. Mem, geol. Surv, U.K. WISE, E. V. A. 1957. The Stability of Natural Slopes in Over-Consolidated, Fissured Clay. OJ.C. Oisseration, Imperial College, University of London. WOOD, A. M. M. 1955. Folkestone Warren Landslips: Investigations, 1948-50. Proc, Instn civ, Engrs, Railway Paper No. 56,410-28.
J. N. Hutchinson Department of Civil Engineering Imperial College London S.W.7
Saturday, 2 July This day was spent entirely on the London Clay cliffs of north Kent. Attention was drawn particularly to the strong influence of the intensity of marine erosion on the character of the coastal landslides, which is well revealed in these relatively homogeneous cliffs. The cliffs visited were accordingly categorised as those suffering strong marine erosion , those exposed only to moderate erosion , and the abandoned cliffs, from the foot of which no debris is removed. The party travelled by coach first to Warden Point, Isle of Sheppey (TR 018725). This is the eastern, most exposed, part of the London Clay cliffs of north Sheppey and affords an excellent example of the behaviour of this formation under conditions of strong marine erosion . According to Steers (1964), the average rate of recession of the cliffs at Warden Point is as high as 9.7 ft./year (3 m.). The behaviour has been described by Hutchinson (1965). It is characterised by the occurrence of deep-seated rotational landslips, bench-shaped in plan. They usually extend along the coast for distances of between four and eight times the cliff height, here about 130 ft. (40 m.). The London Clay extends well below the foot of the cliffs, and at Warden Point its base is estimated from Whitaker (1908) and regional evidence to lie over 300 ft. (91 m.) below sea-level. Most of the landslips involve base failure. No subsurface investigations have been carried out , but the back-tilted and little-broken nature ofthe slipped masses suggests strongly that the failures take place on surfaces which are approximately circular. 227
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With time, the steep rear scarp of such a landslip is broken down by shallow slides and mudflows. The debris from these slowly fills the hollow left at the rear of the slipped mass and eventually spills past each of its extremities into the sea in the form of secondary mudflows. The continuing marine erosion removes the tongues of these mudflows and gradually forms a fresh wave-cut platform in the slipped mass. This effects a progressive overall steepening of the cliff, which eventually completes the cycle of degradation by precipitating a further, deep-seated landslip. Various lines of inquiry indicate that the length of the cycle at Warden Point lies between thirty and forty years (Hutchinson, 1965). The party gathered at a vantage point on the cliff-top, above a wellmarked arete of London Clay (TR 020725). The Director drew attention to the deep-seated rotationallandslip immediately to the south-east [K 40].1 It took place in 1950,and illustrates many of the features mentioned above. Its slipped mass has a back-tilt of about 40° and is now partly buried by mudflow deposits deriving from the rear scarp. The regional dip here is less than 1° northward. The adjacent landslip to the north-west [K 39] is of similar type, but is larger and in a more advanced stage of the degradation cycle. As the tide was fairly low, it was possible to see the interesting structures in the wave-cut platform off the present coast. They consist of bands of cementstone in London Clay, running approximately parallel with the coast and back-tilted at angles of up to 40° or 50°. The exposures represent the planed-off remnants of earlier landslips, and reveal their deep-seated, rotational character. The coastwise extent of the cementstone bands corresponds closely, in general, with that of the present-day landslips, a relationship which is particularly clearly illustrated at the junction between landslips [K 38] and [K 39]. Thus, once formed, the pattern of landslips and dividing aretes tends to maintain the same lateral position through successive cycles of degradation. The habit probably derives largely from the lateral drainage pattern in the cliff away from the aretes. Before leaving the cliff-top, the Director indicated the site of the former St. James's Church at about the position of the present cliff-foot immediately to the north of where the party was standing. The church was dismantled in 1875shortly before the site slipped down the cliff (Hull, 1967). Landslides just seaward of the church in 1856 and 1859 have been described by Redman (1859a and b). The party then descended the cliffs to the beach by way of the dried-up mudflow on the north-west side of the 1950 slip. Both this route and that used for ascent become impracticable, and even dangerous, in the winter months. The small cliff cut by the sea in the slipped mass of that failure 'The symbols in square brackets are the site references used in the survey of coastal landslides in Kent (Hutchinson, in preparation).
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was first examined and a probable back-tilt of 33° measured (TR 02107245) . The back-tilted bands of cementstone exposed in the foreshore were also inspected. In the cliffs to the north-west the Director pointed out that three distinct categories of material were being removed from the cliffs by mar ine erosion; London Clay in situ beneath the aretes, disturbed London Clay from the landslip masses, and mudflow material from each side of the slipped masses. A short halt was made by the tongue of the secondary mudflow issuing round the south-east end of the large landslip [K 39] immediately northwest of the 1950 failure (TR 020726). The characteristic texture of the mudflow deposit, consisting of discrete fragments of stiff to hard material in a matrix of structureless, softened clay, was well displayed , as was the tendency of the tongue to drain and stiffen and be overridden by the softer material from behind. Just to the north-west of this point , Dr. K. M. Clayton noticed a sharp, fairly level discontinuity, 15 to 20 ft. (4.5 to 6 m.) above the cliff-base. This proved to be the slip surface of a shallow mudflow which was overtopping the degraded, slipped mass, there of brown London Clay. The flow, which was about 2 ft. (0.6 m.) thick and fairly stiff, had clearly been moving as a block on one discrete and highly polished slip surface, well exposed beneath small, overhanging portions of the mudflow. The party returned to the coach by ascending the cliffs at the east end (TR 017727) of the next landslip to the west [K 38), another deep-seated rotational failure, which occurred in about 1945. The slopes were thinly spread with a pale-coloured sand which appeared to have been washed out of a sandy gravel capping the cliff. It was mentioned that similar examples of seepage erosion may be seen in the Bagshot and Claygate Beds capping the cliffs a few miles to the west. A short coach journey carried the party to the Ferry House Inn on the Isle of Harty, where lunch was taken. Harty was visited in order to see the very different types of landslide which take place on freely degrading slopes of London Clay; that is, slopes from which the removal of debris at the foot has ceased. Before lunch a short examination was made of the abandoned cliff of London Clay just west of the inn [KA 49]. The cliff is fronted by saltings and no longer suffers any appreciable erosion at its foot. The party walked to the highest point of the cliff (90 ft. (27.5 m.) O.D.) whence (TR 011661) it could be seen that most of the slope was affected by shallow landsliding, predominantly of successive rotational type. The average inclination of the slope varies between about t ItOand 12to. Recent studies on the abandoned London Clay cliffs of Kent and Essex were outlined: they indicate that shallow landsliding will continue until the cliff has degraded to an inclination of about 8° (Hutchinson, 1967, and in preparation).
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After lunch, the party travelled to the north-west part of Harty (TR 016676), to an abandoned cliff of London Clay [KA 50] in a more advanced state of degradation. There, in a grassy slope of 8io average inclination, a subdued, cross-slope undulation could be discerned with an amplitude of about 25 yds. (23 m.) and a pitch of roughly 1 ft. (0.3 m.). This represents the degraded remains of an old landslip, whose present smooth contours may well have been produced in part by ploughing. The last length of London Clay cliffs to be visited was that lying to the east of Herne Bay. With a present average recession rate of generally less than 5 ft.jyear (1.5 rn.), the cliffs provide an example of the behaviour of the London Clay under more moderate marine erosion. Their average height is about 100 ft. (30.5 m.), The party alighted from the coach on the top of the East Cliff (TR 188683) [KD 70], which is defended at its foot by a concrete sea-wall. The Director remarked that its main interest lay in the stabilisation works carried out on the cliff slopes, namely the trimming back of the slopes to inclinations of between about 12° and 19°, and the installation of deep counterfort drains with secondary feeders. The main stabilisation works are described by Duvivier (1940). The location of an old, deep-seated rotational landslip was pointed out (TR 18756840 to 18906840). This was probably initiated before about 1907, when, it is believed, the first seadefences were built. Subsequent renewals of movement in this landslip, involving upheaval of blue London Clay on the foreshore and damage to the sea-wall, occurred in the winter of 1935-6 (Duvivier, 1940) and during the period 1956-7 (Wise, 1957). Moving to the east along the cliff-top, the party next examined the length of coast known as The Lees [K 71) (TR 192684 to 194684). This is virtually unprotected: an intermittent line of old timber piles offshore probably represents the remains of the 1907 breastwork. It is the best remaining example (shortly due to be destroyed by an extension of the sea-defences to the east) of the behaviour of London Clay cliffs under moderate marine erosion. The Director pointed out that under these conditions, the effects of weathering on the intensely fissured, stiff clay is to produce a pattern of cliff degradation dominated by the shallower forms of mass movement. This is characterised by the formation of a succession of carries in the cliff slopes, each occupied by a mudflow and bounded by aretes of dried London Clay. At The Lees, five such mudflow carries were seen, with widths varying between about 0.8 and 1.8 times the cliff height. It was emphasised that the essential difference between this site and Warden Point is that here virtually the only material eroded by the sea is from the tongues of the mudflows. The mudflow activity is thus stimulated and perpetuated and the sea prevented from attacking London Clay in situ. In general, therefore, this length
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of cliff is never sufficiently steepened for a deep-seated rotational landslip to be initiated. A further 300 to 400 yds. (274 to 365 m.) to the east, above Beltinge Cliff, the party halted on the edge of an old, deep-seated landslip [K 73] which had cut across the cliff-path (TR 197684). It is called the Miramar slip, after the nearby hotel, and took place on 4 February 1953, shortly after the tidal surge of that winter. Here the cliffs are about 110 ft. (34 m.) high and consist of the lower parts of the London Clay, the junction with the underlying fine sands of the Oldhaven Beds lying only about 15 ft. (4.6 m.) below Ordnance Datum. The Director then described the landslip and called attention to some of its more notable features. The failing mass extended about 220 yds. (200 m.) along the cliffs and involved a strip of the cliff-top averaging about 20 yds. (18 m.) in width. This, together with the upper part of the cliff-face, sank about 50 ft. (15 m.) between very steep main and secondary slip surfaces to form a pronounced graben. The sunken mass was separated from the sea by a sharp ridge of London Clay, of which the degraded remains still form a pronounced feature of the landslip. Mudflows from the rear scarp now partially infill the graben and spill around each end of the degraded ridge into the sea. It seems that during the years before the 1953 surge, this length of Beltinge Cliff was occupied by a succession of mudflow corries such as have been shown to characterise a moderate intensity of erosion. The stormwaves and high sea-levels associated with the 1953 surge, however, apparently intensified erosion at the cliff-foot to bring about this major, deep-seated landslip. The development, during landslipping, of a pronounced graben is a well-known sign that failure has occurred on a markedly non-circular slip surface (Ritchie, 1958; Morgenstern & Price, 1965). Confirmation that this is the case for the Miramar slip has been obtained from borings (Hutchinson, 1965); the development of a deep-seated circular slip is prevented by the shallow depth to the Old haven Beds. The last part of the London Clay cliffs examined was the eastern part of Beltinge Cliff (TR 2(0685), about 200 yds. (I 83 m.) east of the Miramar slip. Here [K 74] the Oldhaven Beds have risen to just below beach level and the JOO-ft. (30.5 m.)-high cliffs again exhibit a succession of mudflow corries. The attention of the party was drawn to the mudflow just west of The Rand, a linear accumulation, predominantly of rounded black pebbles, which projects across the foreshore approximately at right-angles to the coast at about TR20 1688and coincides closely with the outcrop of the Oldhaven Pebble Bed. Observations have been made here since early 1962 (Hutchinson, 1965), which indicate that shallow landslides and falls from the corrie sides provide the main source of debris supply to the flow, with a smaller contribution made by a process termed basal incorporation.
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Measurements of the velocity profile across the surface of the flow and throughout its depth , using a specially designed inclinometer, demonstrate that its movement takes place predominantly by marginal slip, and thus approximates to plug flow. Measurements of the volume of debris discharged by the mudflow were quoted to illustrate its great seasonal variability. In an average late summer the flow is stationary; its maximum discharge so far observed in a twelvemonth period is about 44,000 cubic ft. (1250 cu. m.). Finally, the party examined an inclined, approximately planar surface of blue-grey London Clay in the lower cliff, which was traversed by a small branch of the main mudflow. The surface has a seaward inclination of 45° to 50° and a strike bearing of 43° to 48°. The Director described how measurements had been taken of the profile of the groo ve eroded in this surface by the passage of the mudflow, which provide good evidence that mudflows deepen their beds (Hutchinson, 1965). A discussion followed on the origin of the planar surfaces and there was general acceptance of Mr. S. C. A. Holmes's view that they are erosional features, characteristic of the rather shaly lithology of the lowest 30 ft. (9 m.) or so of the London Clay. From Beltinge Cliff the party rejoined the coach and returned to Canterbury for dinner. Sunday, 3 July This day was spent on the Cretaceous cliffs of east Kent. A visit was first made to an undefended cliff of Upper Chalk at Joss Bay, Isle of Thanet, where a small fall had taken place in the preceding February or March. At this point (TR398704) the cliffs, about 50 ft. (15 m.) high, consist of highly jointed Chalk of the Mieraster cor-anguinum and Uintacrinus zones (Peake , 1961), somewhat frost-shattered in the uppermost few feet. The northerly dip is locally about 1°. At the site of the fall, 'Whitaker's 3-in. band' is located in the base of the cliff. The Director outlined the probable mode of failure. A survey of the fresh scar, made previously with the help of Mr. D. G. Farquharson, indicated that the fall had been preceded by the opening up of a joint-eontrolled tension fissure parallel with the line of the cliff and located at a distance of about 0.15 H behind the cliff edge (H being the cliff height). This process was probably assisted by the formation of a wave-eut notch, I to 2 ft. (30.561 cm.) deep, at the cliff-foot. When the tension fissure reached a depth equal to 0.5 H, the lower half of the clifffailed in shear on a relatively plane slip surface inclined at about 60° to 65° to the horizontal. The fall had a length along the cliff of about 1.3 H. The high secondary permeability of the cliff implies that ground-water pressures are unlikely to have played a significant role in the failure . Members of the party then examined the remains of the fall. Of particular
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interest was the slip surface, accessible at the top of the fallen debris, on which striations and traces of chalk paste were still visible. The irregular, blocky nature of the rear face of the tension scar left by the fall was also noted. The extensive landslips at Folkstone Warren [KD 81] were next visited. They extend for nearly 2 miles (3.2 km.) along the coast and have a maximum breadth of about a quarter of a mile (402 m.). From a viewpoint on the crest of Abbot's Cliff, above the east end of the Warren (TR 268386), the Director gave a brief history of the landslips and outlined their engineering geology. Members were reminded that the cliffs in the vicinity of the Warren truncate the scarp of the North Downs and consist of about 450 ft. (137 m.) of the Middle and Lower Chalk, underlain by the Gault and Lower Greensand (Osman, 1917; Smart, Bisson & Worssam, 1966). The landslips are due to the presence of the argillaceous Gault, here between 120 and 160 ft. (37 m. and 49 m.) thick, in the base of the cliffs. The regional dip is about 1 to the north-east and the base of the Gault rises from approximately -190 ft. (- 58 m.) O.D. at the eastern extremity of the landslips to about sea-level at their western end. Since the opening of the railway through the Warren in 1844, over thirty landslides are known to have occurred there. In the light of the extensive subsurface investigations of Toms (1946, 1953) and Wood (1955), it is possible to divide the failures into four categories. The largest are deep-seated renewals of movement in the extensive multiple rotational landslips which form the Warren. The last failure of this type took place in December 1915 and closed the railway for nearly four years. Smaller failures of rotational type involve a renewal of movement in only the seaward part of the Warren, the last significant movements occurring during the period 1940 to 1952. The work of Toms (1953) and Wood (1955) has shown that in both the above types of landslip, the slip surface penetrates approximately to the base of the Gault. The remaining two types of failure are associated with the precipitous rear scarp of the Warren landslips, known as the High Cliff. Chalk-falls from this cliff comprise the more catastrophic of the two, but they seem commonly to be preceded by settlements of up to a few feet in those parts of the cliff-top immediately landward of the rear scarp. Movements of this fourth category are known as 'sets', and doubtless also involve the underlying Gault (Toms, 1953). A mechanism for their formation has been proposed by Hutchinson (1965). Knowledge of the ground-water conditions in the landslips also rests largely on the work of Toms (1946, 1953) and Wood (1955). Within the slipped masses a phreatic ground-water level reflects, to some extent, the fluctuations of the water-table in the Chalk inland. Good correlation has been obtained between the occurrence of peak levels in this phreatic 0
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system and renewals of rotationallandslip movement (Hutchinson, 1965). One of the stabilisation measures carried out at the Warren consists of draining away some of the phreatic water by driving in headings from about the level of the top of the sea-wall. A second, confined ground-water level is found beneath the Gault in the Folkestone Beds. Ground-water pressures in the confined system are generally fairly low, because the sandy, rather permeable Folkstone Beds outcrop in the sea-bed just off the coast. From Abbot's Cliff the party enjoyed a good view of the 300 to 400 ft. (91 m. to 122 m.) high, rather weathered rear scarp, fronted by the irregular shelf or undercliff of slipped masses on which the railway runs. Attention was drawn to the bare scar running across the otherwise grassy talus slopes at the foot of the High Cliff, which represents the landward limit of large-scale movements in the 1915slip. Along the coastline the continuous, groined sea-wall, supplemented in its central parts by extensive toeweighting structures, was pointed out. The sea-wall was completed in 1939 and the toe-weighting added between 1948and 1955. The party then drove to Capel-Ie-Ferne where, after lunch, members descended into the Warren by a path down the High Cliff (TR 254385), and reassembled on the eastern part of the toe-weighting structure (TR 258382). From that point the junction between the Middle and Lower Chalk was indicated at about mid-height of the High Cliff, being clearly marked by the plenus Marls and the overlying Melbourn Rock. An examination was then made of the nearby chalky bluff, known as the Horse's Head (TR 256382). It was seen to consist at the base of intact Chalk, back-tilted at an angle of over 60° by the rotationallandslip movements. Miss A. E. Ward found the lamellibranch Inoceramus labiatus in this material, which indicates that it belongs to the lower zone of the Middle Chalk. This shows that the Horse's Head, which is one of the most seaward component blocks of the multiple rotational slip complex, has suffered, in addition to its backward rotation, a vertical component of downward displacement of the order of 300 ft. (91 m.). It was suggested that the time of initiation of the Folkestone Warren landslips is unlikely to have been earlier than the restoration of sea-level to its present position, after the severe eustatic depression associated with the last glaciation (Hutchinson, 1965). According to West (1963) the restoration occurred between about 3500 and 3000 B.C. The Director then drew attention to some interesting features in the middle and upper parts of the Horse's Head, from which it is hoped eventually to arrive at a more precise date for the initiation of the Warren landslips. The back-tilted intact Chalk there is overlain by a thickness of 25 to 30 ft. (7.6 to 9.1 m.) of chalk debris, in which three immature soil profiles are distinguishable. These are also back-tilted, though not quite so steeply as the intact Chalk, and they seem likely to have been formed at
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an early stage in the rotational slipping. An examination of a sample of the lowest soil layer by Dr. M. P. Kerney revealed the temperate mollusc Pomatias elegans in some quantity. On the basis of this and other aspects of the fauna Dr. Kerney considers the soil layer to be probably of Atlantic or Sub-boreal age. The party rejoined the coach at the bottom of the road leading into the Warren, just south-east of the Martello Tunnel, and followed the coast towards the west. While passing through Sandgate, brief mention was made of the two very large landslips [KD 88] of 1827 (Topley, 1893) and 1893 (Blake, 1893), which together affected almost the whole area of the town. Both failures seem to have involved renewals of movement in old landslips, and were probably seated in the Atherfield Clay. Houses tilted by the 1893 movements were pointed out on the north side of the coast road. To the west of Sandgate this line of cliffs is now protected from the sea by the alluvial flat of Romney Marsh and its protective fringe of shingle. The last halt of the excursion was made at Lympne to see a typical section of this abandoned cliff [KA 93]. The party alighted near Lympne Castle (TR 119347) and walked down the footpath to Stutfall Castle. The steep feature produced by the Hythe Beds was noted at the top of the slope. They are underlain by the Atherfield Clay, here probably about 30 ft. (9.1 m.) thick, and the Weald Clay, which forms the major part of the abandoned cliff. At Stutfall Castle the Director outlined the history of the cliff since its abandonment by the sea, which, according to Gallois (1965), took place some 3000 to 4000 years ago. Subsequently free degradation moulded the cliff, broadly as described earlier for the London Clay (Hutchinson, 1967), which had achieved some degree of stability by the time the Romans chose to build Stutfall Castle on its slopes in the latter part of the third century A.D. (Bushe-Fox, 1932). The present ruinous condition of the fort and the considerable dislocations suffered by its walls show that at least shallow landslide activity continued after the third century. Smith (1852) considers that the present dislocation of the fort was completed before the Norman Conquest. In the last millenium evidence is scanty. It would appear that the upper slopes have been subject to occasional, fairly small, rotational landslips. One slip of this type is reported by Collinson (1728) to have involved a farm near Lympne (TR 11253470) in about 1725. Such failures in the crest of a slope are typical of the later stages of the free degradation process. The clay slopes above Stutfall Castle now exhibit the smooth undulations of degraded old slips and appear to have a high ground-water table. The average inclination of the slopes is about 8io, and they are undoubtedly close to their angle of ultimate stability against landsliding. A drop in ground-level of up to 5 or 6 ft. (1.5 to 1.8 m.) occurs between
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the uphill and downhill sides of the remaining Roman walls which lie across the slope. The Director suggested that the level-difference may have resulted from an accumulation produced by hill-wash and soil creep against the barrier formed by the walls, during the past 1700 years. In conclusion, the Director recalled that the last period of intense solifluction in south-east England was in zone III of the Late-glacial, between about 8800 and 8300 B.C. (Kerney, Brown & Chandler, 1964). Whatever solifluction deposits then mantled the Lympne slopes, however, are likely to have been partly or wholly removed by the marine erosion which continued at the slope foot for another six or seven thousand years. Support for this view is provided by a comparison of the morphology of the Lyrnpne slopes with that of the inland continuation of the Hythe escarpment at Sevenoaks. There the slopes are draped by major solifluction lobes, of which no sign is seen on the Lympne slopes. The party returned to the coach, where Mr. S. C. A. Holmes proposed a vote of thanks to the Director and to Mr. N. S. Farrar, who acted as Secretary of the Meeting. REFERENCES BLAKE, J. F. 1893. The Landslip at Sandgate. Nature, 47, 467-9. BUSHE-Fox, J. P. 1932. Some Notes on Roman Coast Defences. J. Roman Studies, 22, 60-72. COLLINSON, P. 1728. An Uncommon Sinking of the Ground in Kent. Phil. Trans. R. Soc., 35,551-2. DUVIVIER, J. 1940. Cliff-Stabilisation Works in London Clay. J. Instn civ, Engrs, 14, 412-26. GALLOIS, R. W. 1965. The Wealden District (4th Edn.). Br, reg. Geol. London. HULL, F. 1967. (County Archivist of Kent.) Personal Communication. HUTCHINSON, J. N. 1965. The Stability of Cliffs Composed of Soft Rocks, with Particular Reference to the Coasts of South-East England. Ph.D. Dissertation, University of Cambridge. - - - . 1967. The Free Degradation of London Clay Cliffs. Proc, Geotechnical Con! Oslo, I, 113-18. - - - . Mass Movement. In: Encyclopedia of Earth Sciences (Edited by R. W. Fairbridge), Reinhold Publishing Corp., U.S.A. (in press). - - - . Surveys of Coastal Landslides: Kent. Building Research Current Papers (Building Research Station, Watford) (in preparation). KERNEY, M. P., E. H. BROWN & T. J. CHANDLER. 1964. The Late-Glacial and PostGlacial History of the Chalk Escarpment near Brook, Kent. Phil. Trans. R. Soc. B., 248, 135-204. MORGENSTERN, N. R. & V. E. PRICE. 1965. The Analysis of Stability of General Slip Surfaces. Geotechnique, 15, 79-93. OSMAN, C. W. 1917. The Landslips of Folkestone Warren and the Thickness of the Lower Chalk and Gault near Dover. Proc: Geol. Ass., 28, 59-84. PEAKE, N. B. 1961. The Coastal Chalk of North-East Thanet. Geologists' Ass. Guides. No. 30: The London Region, 18-22. REDMAN, J. B. 1859a. Landslip at Warden Point, Sheppey, The Times, 22 September 1859. - - - . 1859b. The Landslip at Warden Point. Reprinted in: Bingham, R., 1861-2. On the Geology of the Isle of Sheppey, Proc, Geol. Ass., I, 92-106. and 167. RITCHIE, A. M. 1958. Recognition and Identification of Landslides. In: Landslides and Engineering Practice (edited by E. B. Eckel). Highway Research Board Special Report 29, NAS-NRC Publication 544, Washington, D.C.
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SMART, J. G. 0., G. BISSON & B. C. WORSSAM. 1966. Geology of the Country around Canterbury and Folkestone. Mem. geol. Surv., U.K. SMITH, C. R. 1852. Report on Excavations made on the Site of the Roman Castrum at Lympne in Kent in 1850. London. STEERS, J. A. 1964. The Coastline of England and Wales (2nd Edn.), Cambridge. TOMS, A. H. 1946. Folkestone Warren Landslips: Research carried out in 1939 by the Southern Railway. Proc. Instn civ, Engrs, Railway Paper No. 19,3-25. - - - - . 1953. Recent Research into the Coastal Landslides at Folkestone Warren, Kent, England. Proc. 3rd Int. Conf. Soil Mech, and Foundn. Engrg., Zurich, 2, 288-93. TaPLEY, W. 1893. The Landslip at Sandgate. Proc. Geol, Ass., 13,40-7. WEST, R. G. 1963. Problems of the British Quaternary. Proc. Geol. Ass., 74,147-86. WHITAKER, W. 1908. The Water Supply of Kent. Mem, geol. Surv, U.K. WISE, E. V. A. 1957. The Stability of Natural Slopes in Over-Consolidated, Fissured Clay. OJ.C. Oisseration, Imperial College, University of London. WOOD, A. M. M. 1955. Folkestone Warren Landslips: Investigations, 1948-50. Proc, Instn civ, Engrs, Railway Paper No. 56,410-28.
J. N. Hutchinson Department of Civil Engineering Imperial College London S.W.7