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Landslips near Lyme Regis
121
Landslips near Lyme Regis by MURIEL A. ARBER Presidential Address delivered 2 March 1973
CONTENTS J. 2. 3. 4.
INTRODUCTION THE CLIFF-SECTIONS SLIPPING BEFORE 1940 SLIPPING SINCE 1940 (a) Stonebarrow (b) Black Yen . (c) Lyme Regis . (d) Ware Cliffs to Axmouth 5. CONCLUSIONS REFERENCES
page 121 122 124 125 125 125 127 128 131 132
ARBER, M.A. 1973. Landslips near Lyme Regis. Proc. Geoi. Ass., 84 (2), 121-133. A brief summary is given of the history of the earlier landslips on the coast of south-east Devon and west Dorset, which have already been described by the author (1940, 1941). During the last thirty years there has been a great increase in the rate and scale of the slipping, and there has also been a landslide precipitated by excavations in the town of Lyme Regis itself. A description is given of all these slips, with special reference to the light that they have thrown on the mechanism involved. Some of the movements have been obviously rotational, but there is still no evidence of tilting in the great 'island' beyond the chasm at Dowlands. However, further knowledge of the undercliffs between Lyme Regis and Axmouth has led to the recognition that they consist of a series of ridges tilted inland and running parallel to the coast-line. Study of these ridges, and of miniature chasms that have developed in certain fields, suggests that the 'island' is a horst-like mass which remained unmoved between the tilted ridges which slid down both on its seaward side and in the chasm on its landward side. 18 Sherlock Close, Cambridge CB30HW.
I. INTRODUCTION I HAVE CHOSEN to
give this address on the same subject as that on which I read my first paper to the Association, because, in the third of a century that has since elapsed, there has been a great increase in the rate and scale of the coastal landslipping of west Dorset and south-east Devon. I have been observing these slips throughout this time and what I have seen has caused me to alter the views which I expressed in 1940. This tract of landslides is probably the largest in Britain, and the great 'island' left beyond the chasm at Dowlands (Plate 3) is, so far as I know, a unique feature, the existence of which has still not been satisfactorily explained. As a result of the slipping, access to the cliffs is in some ways more difficult than it was; the footpath between Lyme Regis and Charmouth no longer exists, and that between Lyme and Axmouth is very much broken. PROC. GEOL. ASS., VOL. 84, PART 2, 1973
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MURIEL A. ARBER
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On the other hand, the Nature Conservancy in 1955 declared the area between Lyme and Axmouth a National Nature Reserve, and I am indebted to the Conservancy for permission to make observations there year after year, and to Dr. W. A. Macfadyen for discussion of the problems of the slipping. For information and for help in the field, I wish to express my gratitude to the Warden of the Reserve, Mr. L. A. Pritchard, and to the late Dr. W. D. Lang, F.R.S., of Charmouth. Thanks to the kind co-operation of Professor H. B. Whittington, F.R.S., the maps and sections illustrating this address have been drawn from my sketches by Mr. John Lewis in the Sedgwick Museum, Cambridge. 2. THE CLIFF-SECTIONS
The cliffs considered here are those (Fig. 1) in south-east Devon, between Axmouth and Lyme Regis, and those in west Dorset consisting of Black Ven between Lyme and Charmouth and of Stonebarrow between Charmouth and the St. Gabriel's valley. They are formed (Fig. 2) of Lias beds, with some Rhaetic and Keuper Marls west of Pinhay, overlain unconformably by Cretaceous strata (Ager & Smith, 1965). The Cretaceous consists of Upper Greensand chert beds and sands; from Bindon eastwards there is a thin bed of sandy Gault at their base, and from Lyme
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LANDS LIPS NEAR LYME REG IS
westwards they have a capping of Chalk. Apart from minor folding, the general dip is of the order of 5° towards the south-east. CHALK
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MURIEL A. ARBER
3. SLIPPING BEFORE 1940 The Lias consists of alternating clays, shales, marls and limestones; certain limestone bands are more resistant than the rest and form waterbearing horizons. On Stonebarrow and Black Yen, these have produced terraces above the Birchi nodules near the base of the Black Yen Marls, and above the Stellare nodules near the top of the Black Yen Marls. There are thus two major terraces in the cliff-face, above each of which the clays are washed out. In general these slips are of the nature of mud-flows, but the relation of such mud-flows to landslides has been demonstrated in a slip precipitated by excavations for building in the town of Lyme itself in 1962. Above the Lias comes the major plane of slipping, in which the sands of the Upper Greensand on Stonebarrow and Black Yen have receded, leaving the brilliant yellow scarps on the summit of the cliffs. This topmost terrace forms Fairy Dell on the crest of Stonebarrow, while on Black Yen it was used for the old coast-road through the cutting known as the Devil's Bellows; this road was finally abandoned as unsafe in 1924, although it could still be followed on foot until 1937 (Arber, 1941, pl. 20, B). West of Lyme, there is comparatively little slipping within the Blue Lias and Rhaetic beds, and very much more slipping at the base of the Cretaceous. From Ware westwards, through Pinhay, Whitlands and Rousdon, there is a belt of undercliff stretching some 400 m. from the base of the Cretaceous inland cliff to the sea-cliffs of the Lias, Rhaetic and Keuper Marls. All this area of displaced and jumbled rocks is densely overgrown, and continues westwards for some 7 km. by Dowlands and Bindon to Axmouth. At Dowlands, however, there appears the remarkable feature which was formed at Christmas 1839 (Arber, 1940). About fifteen acres of land, now known (Wallace, 1966) as Goat Island (Plate 3), became separated from the main cliff by a chasm some 64 m. deep and extending about 800 m. from east to west, and some 60 m. across from the inland cliff to the island at the eastern end and 120 m. at the western end. In the chasm were ridges and pinnacles of Chalk, and certain blocks of two or three acres which were tilted backwards towards the main cliff, but the island itself was so little disturbed that hedges and trees survived and the crops which had been sown on it were harvested the following summer. No one saw the slip take place but it was immediately inspected and recorded by Buckland and Conybeare (Conybeare & others, 1840), and it was also described by the local historian George Roberts (1840), and illustrated in a series of lithographs and woodcuts (Plate 4). The explanation given by Conybeare, and generally accepted until comparatively recently, was that the lower levels of the Foxmould sands of the Upper Greensand, overlying the impervious Lias beds, had been reduced by exceptionally heavy rain to the condition of a quicksand, into which the
LANDSLIPS NEAR LYME REGIS
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chasm had foundered. At the foot of the seaward cliffs, a reef of Upper Greensand, nearly 1200 m. long, was forced up at sea some 12 m. above high-water level (Arber, 1940, pl. 14, A); this only survived for a few months. In February 1840, a large slip occurred at Whitlands (Arber, 1940, pl. 14, B). There was much disturbance of the undercliff, and again a shortlived reef of Upper Greensand, though on a smaller scale than that at Dowlands, was upheaved on the foreshore.
4. SLIPPING SINCE 1940 For about a century after this major slipping at Dowlands and Whitlands, there was comparatively little movement in the cliffs west of Lyme. In the last thirty years, however, there has again been a great increase in instability, and recent movements have thrown much light on the mechanism of the slipping (Arber, 1971). Its nature as a rotational shear was first recognised by W. H. Ward (1945). (a) Stonebarrow
On 14 May 1942 (Lang, 1943)the crest of Stonebarrow broke away, and a slice about 500 m. from east to west and 18 m. from north to south slid down some 15 m. into the undercliff of Fairy Dell beneath. It left a slipscarp of yellow Upper Greensand, far more brilliant than the summit of the cliff had previously been in its slightly vegetated condition. The part which fell carried down with it two radio-location huts which had been built on the top of the cliff during the Second World War. The displaced slice came to rest tilted backwards towards the main cliff behind the undercliff, and thus demonstrated that the slipping here was due to rotational shear. Such mass movement of the Upper Greensand is unusual in west Dorset. (b) Black Yen
On Black Yen, as on Stonebarrow, it is rare for masses of Upper Greensand to collapse en bloc. However, in January and February 1944 (Lang, 1945), as a result of a slip on the seaward side (358933) of the DeviI's Bellows cutting, a single unbroken slice of Upper Greensand, approximately 90 m. long, subsided, leaving a scarp some 6 m. high at its western end and decreasing in height towards the east. Within the Lias, the precise nature of much of the movement is masked by mud-flows. The great amphitheatre-shaped hollow in the terrace lying above the Black Yen Marls (Plate 5) west of the DeviI's Bellows and extending from 355932 to 358932, is Black Yen sensu stricto, the
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MURIEL A. ARBER
black bog which gives its name to the whole cliff (Lang & Arber, 1942). It is a treacherous area of muddy clay, water and vegetation, from which unwary walkers have occasionally to be rescued. In the hollows and gullies leading into and from the grea t amphitheatre there is a constant downward movement of mud and small stones. There had already been a considerable fall of sand and mud here in 1957 when the largest recorded slip was observed by J. F. Jackson on 9 February 1958 (Lang, 1959, and personal communication). Mr. Jackson was on the beach when he saw material falling and heard a sound like thunder. A great mass of Upper Greensand, complete with gorse-bushes and trees, was moving down from the topmost terrace. There followed a sort of explosion in the cliff and a mass of Lias was hurled out. A river of liquid Lias clay began to descend over the lower cliff on to the shore between himself and Charmouth whence he had corne. He made good his retreat just before the mud-flow blocked the beach. The slip continued all night, bringing trees down with it, and the fan of debris is still there (plate 5), forming a small promontory under Black Yen, between Lyme and Charmouth, and now very much overgrown . Above it, about 120 m. a.D., is the place where the old road ran until its last relic was cut in 1937; after that it was still possible to cross the cliff by the old Roman road 10 m. farther inland at 353934. This road in turn was cut in 1958 (Lang, 1959), and by 1968 there was merely space where once the old Roman road had run (Plate 6). A path was made across the fields behind, thence following the golf-course (354934) down to Timber Hill above Lyme. Rapid erosion to the east of the golf-course then set in. In 1968 the edge of the fields had been eaten away, and the wire fence that had been put up beside the path was suspended over space. The path was moved still farther back more than once (Plate 7, A) until it was finally closed altogether in 1970; at the same time the path at the edge of the golf-links slipped away and now ends in a sheer cliff-face (349934). The Upper Greensand scarp is thus receding rapidly. Within the undercliff below, from Rhodehom Plantation (348933) to the great amphitheatre (355933), the movement has been less clear-cut among the flow of the Lias clays. Below the old road, one field (349932) is fissured in a series of small-scale scarps, some facing seawards , others 1andwards. There is, however, evidence of rotational shearing in the concrete blocks which once formed steps leading to the stile (350932) where the footpath from opposite the cemetery at Lyme used to join the old Charmouth road. These blocks, formerly horizontal, were tilted back towards the inland side. Mr. B. W. Conway, of the Institute of Geological Sciences, has made detailed observations on Black Yen, and exhibited some of his work at the Annual Reunion of the Geologists ' Association in 1971. In this exhibit he described how he was able on one occasion to watch the upheaval of Lias
EXPLANATION OF PLATES PLATE 3 Rotated blocks are seen in the chasm at Dowlands. The unmoved mass of 'Goat Island' lies to seaward of the chasm. Beer Head on the horizon PLATE 4 The chasm at Dowlands from Roberts (1840, 5th Edn.) PLATE 5 The Upper Greensand scarp of Black Yen is on the skyline; below comes the uppermost terrace of the Lias, above the Belemnite Marls; below the Belemnite Marls is the amphitheatre of Black Yen sensu stricto (from which black streaks issue down the cliff-face) above the Black Yen Marls; the third and lowest terrace is above the Shales-with-Beef; on the foreshore in the foreground is part of the 1958 tongue of slipped clay PLATE 6 The Upper Greensand scarp of Black Yen above the Belemnite Marls, backing the space where the Roman road and the coast-road once ran. (Compare Arber, 1941, pI. 20, B.) Stonebarrow and Golden Cap behind PLATE 7, A The previous year, the footpath over Black Yen had run along the edge of the cliff, now eaten away; the path had formerly crossed the hedge by the clump of trees PLATE 7, B The mass of black Lias clay in the foreground had been upheaved in 1961 among the fallen Greensand boulders on the foreshore at the eastern side of Charton Bay. By 1962 it had been much reduced in height PLATE 8 The Cliff House site two months after the slip occurred. The ruins of Cliff House stand on the terrace, with the slip-scarp which cut Stile Lane to the right behind. Marine Parade, below, had been cleared, but the black mud-flow still extended over the foreshore. Photograph taken from the North Wall of the Cobb (see Fig. 3) PLATE 9, A Looking over the undercliff (to right) from Goat Island (to left). The ridges in the foreground appear to be very narrow blocks which have suffered rotational shear. The narrow masonry-like ridges, to seaward of these blocks, are not shown in this photograph PLATE 9, B Slipping within the fallen Cretaceous material in the field below the entrance to the Reserve (329916) has left these upstanding horst-like blocks flanking miniature chasms [To face p, 126
PROC. GEOL. ASS., VOL. 84 (1973) PLATE 3
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PROC. GEOL. ASS., VOL. 84 (1 973) PLATE 5
PROC. GEOL ASS., VOL 84 (1973)
PLATE 6
PROC. GEOL. ASS., VOL. 84 (1973)
PLATE 7
A. Summit of Black Yen, II April 1969. Photo by M.A.A.
B. Upheaval of Lias on the foreshore at Humble Point, 23 April 1962. Photo by
M.A.A.
PROC. GEOL. ASS., VOL. 84 (1973) PLATE 8
PROC. GEOL. ASS., VOL. 84 (1973)
PLATE 9
A. Seaward faee of Goat Island, 21 April 1958. Photo by M.A.A.
B. Miniature 'islands' below Ware Cliffs, 14 April 1971. Photo by M.A.A. To face p, 127J
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LANDS LIPS NEAR LYME REGIS
from below low-tide mark in a syncline on the foreshore. This, so far as I know, is the only record of upheaval at the toe of a slip in the cliffs east of Lyme Regis. The Black Ven Marls have long been foundering in the re-entrant known as Raffey (361932) on the Charmouth flank of Black Ven (Arber, 1941, pl. 19, A). In 1943 they began to move in a manner comparable to that of a lava-flow (Lang, 1944, and personal communication), with knocking noises inside the mass. The fallen mass left a clean section of the upper part of the Black Ven Marls, below the footpath on the cliff-edge above. In 1969 the movement was again accelerated here, and the footpath itself was being eaten away. The slipping is rapidly approaching an estate of newly built houses, so that efforts are being made to check its progress. (c) Lyme Regis
In the town of Lyme itself, sapping by springs at the junction of the Upper Greensand and Lias clays has made the ground unstable, as was seen in the goods shed and coal pens of the now disused railway station (333926). The area in which slipping is most liable to occur, however, has always been within the slope of the Lower Lias itself which forms a hillside (Fig. 3) which is virtually a cliff-face between the Cobb (338917) and Broad Street (341921). This slope is formed of Shales-with-Beef and is capped at the top of the Langmoor Gardens (339920) by a small terrace which is presumably formed by the Birchi nodules, though since Lang's detailed
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Fig. 3. Sketch-map of part of Lyme Regis. (Grid reference of the site of Cliff House: 338919)
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MURIEL A. ARBER
maps (Lang, 1914, 1924) do not extend into the town, this must be speculative in the absence of exposures of material in situ. Above this terrace again is a steep slope leading to a certain flattening about at the level of the junction of Cobb Road and Pound Street (337921); one might hazard the guess that this is conceivably the horizon of the Stellare beds. This hillside, between the Langmoor Gardens and the Cobb hamlet, has been so unstable that very little building has ever been done on it. The house called Wings, in which Jane Austen stayed, was in Stile Lane towards the foot of the slope which became so unsafe in the nineteen-forties that the house had to be demolished. In 1962, however, a speculative builder excavated, with a view to erecting houses there, the land belonging to Cliff House (338919) just below the terrace level that is presumably formed by the Birchi nodules. About two months after the excavation began, a landslide occurred on 12 February 1962; Cliff House itself, which was standing empty, was ruined, and other houses were extensively damaged. A great mud-flow poured down over Marine Parade below and on to the beach (Plate 8). The lower part of this slip appeared to be mainly a mud-flow, of the type so familiar in the Lyme area as to be known locally as a rougement, but some of the buildings were tilted back into the cliff, showing that here again the movement was a rotational shear and not merely a surface flow. Above the terrace, a scarp appeared (Plate 8), cutting Stile Lane (338920). After this slip the ground between Cliff House and Marine Parade was laid out as public gardens named after Lord Lister. Further small-scale movement has since occurred, breaking the paths in the Gardens, but the largest collapse has been on the other side of the Langmoor Gardens, 200 m. to the east, where a mud-flow descended on Library Cottage (341920) early in 1971, engulfing the kitchen at the back of the house. Where the Cliff House site adjoins Cobb Road there is a comer which has been unstable for over fifty years. A house which stood there, to the west of the road (337918), gradually became so twisted that it had eventually to be demolished. Thirty metres above its site is Pine Walk (Fig. 3). Houses have now been built on its seaward side, and about 1970 a slip developed beyond the southernmost of these. (d) Ware Cliffs to Axmouth The undercliff at the junction of the Jurassic and Cretaceous beds extends all the way from the Ware Cliffs to Axmouth, and movement at this level has been active in recent years. In 1968 and 1969, Ware Lane (330917)was broken just east of the entrance to the Nature Reserve (Fig. 3), and the bungalow known as the Orchard, immediately outside the gateway of the Reserve, has had to be demolished. The last two of the fields through which the footpath from Lyme passes (331916) before it reaches Ware
LANDS LIPS NEAR LYME REGIS
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Lane, and the field (329916) immediately east of Underhill Farm, have become fissured with a series of small scarps. Miniature chasms have developed among the ridges (Plate 9, B) in the field east of the farm and these will be considered later in relation to the great chasm at Dowlands. About one kilometre west of the entrance to the Reserve, the public footpath was broken about 1968 by a small but sheer drop, and west of this point the old path has become impassable, so that an alternative parallel route has been opened farther inland. At Pinhay Bay a slip which occurred in 1966 destroyed the path to the beach and ruined the ram which used to pump water to the house above. A great mud-flow of clay, remarkably like a glacier in surface structure, has poured over the cliffs, carrying down trees in its course. All along the undercliff from Pinhay to Bindon, parallel to the inland cliff, there developed about 1961 a slip-scarp varying from about 1 to 6 m. in height. Below this line everything appeared to be lowered en masse. East of Whitlands (about 313907) the trees in the undercliff sank bodily some 5 m. so that the sea became visible from the cliff-top at a point from which it had formerly been hidden. In 1961 a slip at Humble Point (305899) produced an upheaval of Lias clay (Plate 7, B) to a height of some 3 to 4 m. on the foreshore, extending from Humble Point for a distance of about 400 m. eastwards. This upheaval of the toe of the slip occurred not far from the site of the reef of Upper Greensand forced up on the foreshore at Whitlands in 1840. Until 1969 there was a cart-road down to the shore at Charton Bay. On 22 June 1969 a landslide occurred here, so that the cart-road was broken by a sheer drop of about 14 m., and the only way down to the beach was by ropes. On the seaward side of this new scarp there rise two great ridges of rock resembling masonry, rather similar to those which will be described seaward of Goat Island at Dowlands. At Charton Bay there is no 'island' apart from these 'masonry' blocks and there is no apparent complementary rise of the beach or sea-floor. All the fallen material, between the precipice and the ridges, seems to have sunk right down. Immediately west of Charton Bay come the cliffs of Rousdon, and beyond these is Dowlands, with its great chasm and Goat Island. No spectacular slipping has taken place along here in recent years. Finally, before Axmouth is reached, there comes the land of Bindon, where deep crevasses and miniature chasms developed on the cliff-top (at approximately 268897) about 1961. These crevasses, running roughly parallel to the cliff-edge, were in one case at least nearly 4 m. deep. In some examples the outer scarp formed a small-scale knife-edge rather similar to the large-scale rock-edges in the undercliff. These did not herald any immediate break-away of the cliff-edge as did the cracks on the cliff-top that
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MURIEL A. ARBER
were reported before the 1839 slip at Dowlands (Arber, 1940), but, like the still smaller subsidences in the field below Ware Cliffs, it is possible that these chasms at Bindon may throw light on the formation of the great chasm at Dowlands. Dowlands remains the outstanding problem, because Goat Island (Plate 3) survives intact, to provide the seaward wall of the chasm which would otherwise have been merely part of the fallen material in the undercliff. If Goat Island were tilted landwards there would be no mystery, for then it would be obvious that it was merely an exceptionally large block detached by a rotational shear. But the surface of Goat Island is more or less level, with no inland tilt. It was this that led to my earlier acceptance of Conybeare's theory that the sliding was simply a direct movement of massive Upper Greensand down the slip-plane above the argillaceous Lias and Gault. The chasm has now become so completely overgrown that it is difficult to see the structure of its floor, but Conybeare's diagram (Arber, 1940, fig. 34) shows the blocks of the floor as being tilted landwards, and they are depicted thus in some of the contemporary views (Plate 4). This indicates therefore that the slip was of a rotational shear type as far as the chasm was concerned. The fact that a reef of Upper Greensand rocks was upraised at sea, at the toe of the slope, confirms the method of slipping. A hypothesis as to how Goat Island has remained in its present form may be made on the evidence of other structures in the undercliffs. Looking seawards from the top of Goat Island (Plate 9, A), one sees two big ridges of Cretaceous material, running parallel to the Island and with their crests a little lower than its surface. Beyond them are one or two more ridges that are extraordinarily narrow and look like walls of masonry. This series of ridges could therefore be interpreted as very narrow blocks which have suffered rotational shear on the seaward side of the Island, complementing the more massive rotated blocks in the chasm on the landward side. When one recognises the character of these ridges at Dowlands, one recognises the structure of the whole undercliff from Bindon to Ware. H is so densely overgrown that one can get no general view of the surface, and in traversing it one's first impression is of an irregular jumbled mass of fallen rocks. However, on closer acquaintance one finds that the ridges and hollows of the undercliff in fact lie in lines parallel to the coast. There is a series of roughly parallel ridges with narrow crests, which indicates that they are rotated masses. Two examples of these are the ridge followed by the main footpath through a bluebell wood under Bindon cliffs, and the ridge flanking the hollow known as Long Bottom at Pinhay. Of course there is also a great deal of tumbled and jumbled material, but the undercliff has a basic structural plan, demonstrating the nature of the slipping.
LANDSLIPS NEAR LYME REGIS
131
Without borings, it is impossible to tell how deep-seated these slipped ridges are, or to know their relationship to the underlying Rhaetic and Lias. At Pinhay Bay, before the recent slipping which has produced large screes, the sea-cliffs formed a clean and clear section through the White Lias (uppermost Rhaetic) and Blue Lias beds. The cliffs were about 50 m. high, and were capped by the undercliff known as Pinhay Warren, made largely of Chalk. It was slipping that had brought Chalk down so comparatively near to sea-level. Farther west, near the Waterworks (312903), an irregular and unstable mass of Chalk actually forms the seaward cliffs; this too has evidently fallen from above and masks the Jurassic behind it. In Charton Bay, the Lias, Rhaetic and Keuper Marls reappear to form the cliffs, but to the west of Charton Bay the fallen Chalk is seen very near sea-level. Its superficial appearance is solid, giving an almost downlike character to the part of the undercliff called the Plateau (Wallace, 1966), with the spring known as the Gusher emerging at the junction of the Cretaceous and Lias below. These Whitlands cliffs have been the scene of large-scale slipping in the past, and presumably the Chalk here is a very massive displaced block. Still farther west, at Dowlands, fallen Chalk and Upper Greensand overlie great slabs of Blue Lias on the foreshore (285894) below the ruins of Rock Cottage (otherwise known as Cliff or Gapper's Cottage). The range of inland cliffs behind the undercliff shows Upper Greensand capped with Chalk. In front of them are ridges of Chalk which have slipped forward but have not moved far away; an example of this is the Great Cleft (Arber, 1940). The Chapel Rock at Whitlands is the summit of another ridge which has slipped seawards parallel to the main cliff.
5. CONCLUSIONS
Thus the structure of ridges and hollows is integral to the whole of the landslips west of Lyme. It does not occur in the same way on Black Yen and Stonebarrow east of Lyme, because there, in the absence of the Chalk, the Cretaceous beds are less massive; although the fallen material does form some rotated blocks, it disintegrates more rapidly. In returning to the problem of the existence of Goat Island, one has again to consider the question of its level. There is no evidence that it has subsided. Conybeare (1840) stated that the strata on the inland flank of Goat Island are some 60 ft. (about 18 m.) lower than their counterparts in the main inland cliff, and he suggested that this might be explained by the dip. Dr. Macfadyen has pointed out to me that, according to his section (Macfadyen, 1970, fig. 5), a dip of only 5° would be sufficient to lower the strata by 60 ft. between the inland cliff and the seaward side of the Island.
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MURIEL A. ARBER
Conybeare also suggested the alternative possibility of a fault along the line of the chasm. If there were such a fault it seems conceivable that this might be responsible for one, at least, of the shear planes which gave way, thus causing the slipped mass to subside into the chasm. Conybeare makes one other observation which possibly holds the key to the character of the Island: he says that it was rifted but not broken. Since it is flanked by the sunken and rotated masses in the chasm on its landward side and by the narrow rotated ridges on its seaward side, it seems possible that Goat Island itself was rifted but had not yet split into its component ridges by the time that the movement had ceased. Evidence that a chasm can develop without displacement of its seaward flank is to be found in the miniature examples at Ware and Bindon. In the field under the Ware Cliffs (329916), as has already been described, there have developed little chasms flanked by upstanding horst-like 'islands' (Plate 9, B) about 1'5 m. high. In the field on the cliff-top at Bindon, the chasms were about 4 m. deep, and their outer flanks again were in some cases not tilted. This seems to show that it is possible for a chasm to develop without necessarily tilting the land beyond it, so that the 'island' has something of the character of a horst. It is, however, very unsafe to draw such large-scale conclusions from such small-scale evidence. The only decisive indication of the mechanism of the slipping must come from deep borings; and one can but hope that some day it will be considered worth while for the expense to be borne, perhaps by the Engineering Unit of the Institute of Geological Sciences. The work of the Unit on the cliffs of west Dorset is already providing detailed measurements of the slipping, and a technical study of its causes. When the results of the work are published, many of the observations and speculations made in this address should be replaced by precise facts and well-based theories. Nevertheless there may always be room for the amateur to make his own records over the years, and to have the joy of forming his own working hypotheses to explain the problems, in the full knowledge that they will be displaced when further evidence provides better solutions. REFERENCES AOER, D. V. & W. E. SMITH. 1965. The Coast of South Devon and Dorset between Branscombe and Burton Bradstock. Geologists' Association Guide No. 23. ARBER, M. A. 1940. The Coastal Landslips of South-East Devon. Proc. Geol. Ass., 51,257-71. - - - . 1941. The Coastal Landslips of West Dorset. Proc, Geol. Ass., 52, 273-83. - - - . 1971. The Plane of Landslipping on the Coast of South-East Devon. 153-4 in Applied Coastal Geomorphology, ed. by J. A. Steers. London. CONYBEARE, W. D. & others. 1840. Ten Plates . • • Representing the Changes Produced on the Coast of East Devon, between Axmouth and Lyme Regis by the Subsidence of the Land and Elevation of the Bottom of the Sea, on the 26th December, 1839, and 3rd of February, 1840. • • • London.
LANDSLIPS NEAR LYME REGIS
133
LANG, W. D. 1914. The Geology of the Charmouth Cliffs, Beach and Fore-Shore. Proc, Geol. Ass., 25, 293-360. - - - - . 1924. The Blue Lias of the Devon and Dorset Coasts. Proc. Geol. Ass., 35, 169-85. - - - - . 1943. Geological Notes. Proc, Dorset nat. Hist, archaeol, Soc., 64, 129-30. - - - - . 1944. Geological Notes. Proc. Dorset nat. Hist, archaeol. Soc., 65, 147-9. - - - . 1945. Report on Dorset Natural History for 1944 (except Birds). Proc. Dorset nat. Hist. archaeol. Soc., 66, 127-9. - - - - . 1959. Report on Dorset Natural History for 1958. Geology. Proc. Dorset nat. Hist. archaeol. Soc., 80, 22. - - - - & M. A. ARBER. 1942. Names of the West Dorset Cliffs. Notes & Queries for Somerset & Dorset, 23, 278-81. MACFADYEN, W. A. 1970. Geological Highlights of the West Country. A Nature Conservancy Handbook. London. ROBERTS, G. 1840. An Account of and Guide to the Mighty Land-slip of Dowlands and Bindon, In the parish of Axmouth, near Lyme Regis, December 25, 1839.... 5th ed. Lyme Regis. WALLACE, T. J. 1966. The Axmouth-Lyme Regis Undercliffs National Nature Reserve. Allhallows School, Rousdon (Pamphlet). WARD, W. H. 1945. The Stability of Natural Slopes. Geogr, J., 105, 170-97.
Landslips near Lyme Regis by MURIEL A. ARBER Presidential Address delivered 2 March 1973
CONTENTS J. 2. 3. 4.
INTRODUCTION THE CLIFF-SECTIONS SLIPPING BEFORE 1940 SLIPPING SINCE 1940 (a) Stonebarrow (b) Black Yen . (c) Lyme Regis . (d) Ware Cliffs to Axmouth 5. CONCLUSIONS REFERENCES
page 121 122 124 125 125 125 127 128 131 132
ARBER, M.A. 1973. Landslips near Lyme Regis. Proc. Geoi. Ass., 84 (2), 121-133. A brief summary is given of the history of the earlier landslips on the coast of south-east Devon and west Dorset, which have already been described by the author (1940, 1941). During the last thirty years there has been a great increase in the rate and scale of the slipping, and there has also been a landslide precipitated by excavations in the town of Lyme Regis itself. A description is given of all these slips, with special reference to the light that they have thrown on the mechanism involved. Some of the movements have been obviously rotational, but there is still no evidence of tilting in the great 'island' beyond the chasm at Dowlands. However, further knowledge of the undercliffs between Lyme Regis and Axmouth has led to the recognition that they consist of a series of ridges tilted inland and running parallel to the coast-line. Study of these ridges, and of miniature chasms that have developed in certain fields, suggests that the 'island' is a horst-like mass which remained unmoved between the tilted ridges which slid down both on its seaward side and in the chasm on its landward side. 18 Sherlock Close, Cambridge CB30HW.
I. INTRODUCTION I HAVE CHOSEN to
give this address on the same subject as that on which I read my first paper to the Association, because, in the third of a century that has since elapsed, there has been a great increase in the rate and scale of the coastal landslipping of west Dorset and south-east Devon. I have been observing these slips throughout this time and what I have seen has caused me to alter the views which I expressed in 1940. This tract of landslides is probably the largest in Britain, and the great 'island' left beyond the chasm at Dowlands (Plate 3) is, so far as I know, a unique feature, the existence of which has still not been satisfactorily explained. As a result of the slipping, access to the cliffs is in some ways more difficult than it was; the footpath between Lyme Regis and Charmouth no longer exists, and that between Lyme and Axmouth is very much broken. PROC. GEOL. ASS., VOL. 84, PART 2, 1973
9
MURIEL A. ARBER
122
On the other hand, the Nature Conservancy in 1955 declared the area between Lyme and Axmouth a National Nature Reserve, and I am indebted to the Conservancy for permission to make observations there year after year, and to Dr. W. A. Macfadyen for discussion of the problems of the slipping. For information and for help in the field, I wish to express my gratitude to the Warden of the Reserve, Mr. L. A. Pritchard, and to the late Dr. W. D. Lang, F.R.S., of Charmouth. Thanks to the kind co-operation of Professor H. B. Whittington, F.R.S., the maps and sections illustrating this address have been drawn from my sketches by Mr. John Lewis in the Sedgwick Museum, Cambridge. 2. THE CLIFF-SECTIONS
The cliffs considered here are those (Fig. 1) in south-east Devon, between Axmouth and Lyme Regis, and those in west Dorset consisting of Black Ven between Lyme and Charmouth and of Stonebarrow between Charmouth and the St. Gabriel's valley. They are formed (Fig. 2) of Lias beds, with some Rhaetic and Keuper Marls west of Pinhay, overlain unconformably by Cretaceous strata (Ager & Smith, 1965). The Cretaceous consists of Upper Greensand chert beds and sands; from Bindon eastwards there is a thin bed of sandy Gault at their base, and from Lyme
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123
LANDS LIPS NEAR LYME REG IS
westwards they have a capping of Chalk. Apart from minor folding, the general dip is of the order of 5° towards the south-east. CHALK
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124
MURIEL A. ARBER
3. SLIPPING BEFORE 1940 The Lias consists of alternating clays, shales, marls and limestones; certain limestone bands are more resistant than the rest and form waterbearing horizons. On Stonebarrow and Black Yen, these have produced terraces above the Birchi nodules near the base of the Black Yen Marls, and above the Stellare nodules near the top of the Black Yen Marls. There are thus two major terraces in the cliff-face, above each of which the clays are washed out. In general these slips are of the nature of mud-flows, but the relation of such mud-flows to landslides has been demonstrated in a slip precipitated by excavations for building in the town of Lyme itself in 1962. Above the Lias comes the major plane of slipping, in which the sands of the Upper Greensand on Stonebarrow and Black Yen have receded, leaving the brilliant yellow scarps on the summit of the cliffs. This topmost terrace forms Fairy Dell on the crest of Stonebarrow, while on Black Yen it was used for the old coast-road through the cutting known as the Devil's Bellows; this road was finally abandoned as unsafe in 1924, although it could still be followed on foot until 1937 (Arber, 1941, pl. 20, B). West of Lyme, there is comparatively little slipping within the Blue Lias and Rhaetic beds, and very much more slipping at the base of the Cretaceous. From Ware westwards, through Pinhay, Whitlands and Rousdon, there is a belt of undercliff stretching some 400 m. from the base of the Cretaceous inland cliff to the sea-cliffs of the Lias, Rhaetic and Keuper Marls. All this area of displaced and jumbled rocks is densely overgrown, and continues westwards for some 7 km. by Dowlands and Bindon to Axmouth. At Dowlands, however, there appears the remarkable feature which was formed at Christmas 1839 (Arber, 1940). About fifteen acres of land, now known (Wallace, 1966) as Goat Island (Plate 3), became separated from the main cliff by a chasm some 64 m. deep and extending about 800 m. from east to west, and some 60 m. across from the inland cliff to the island at the eastern end and 120 m. at the western end. In the chasm were ridges and pinnacles of Chalk, and certain blocks of two or three acres which were tilted backwards towards the main cliff, but the island itself was so little disturbed that hedges and trees survived and the crops which had been sown on it were harvested the following summer. No one saw the slip take place but it was immediately inspected and recorded by Buckland and Conybeare (Conybeare & others, 1840), and it was also described by the local historian George Roberts (1840), and illustrated in a series of lithographs and woodcuts (Plate 4). The explanation given by Conybeare, and generally accepted until comparatively recently, was that the lower levels of the Foxmould sands of the Upper Greensand, overlying the impervious Lias beds, had been reduced by exceptionally heavy rain to the condition of a quicksand, into which the
LANDSLIPS NEAR LYME REGIS
125
chasm had foundered. At the foot of the seaward cliffs, a reef of Upper Greensand, nearly 1200 m. long, was forced up at sea some 12 m. above high-water level (Arber, 1940, pl. 14, A); this only survived for a few months. In February 1840, a large slip occurred at Whitlands (Arber, 1940, pl. 14, B). There was much disturbance of the undercliff, and again a shortlived reef of Upper Greensand, though on a smaller scale than that at Dowlands, was upheaved on the foreshore.
4. SLIPPING SINCE 1940 For about a century after this major slipping at Dowlands and Whitlands, there was comparatively little movement in the cliffs west of Lyme. In the last thirty years, however, there has again been a great increase in instability, and recent movements have thrown much light on the mechanism of the slipping (Arber, 1971). Its nature as a rotational shear was first recognised by W. H. Ward (1945). (a) Stonebarrow
On 14 May 1942 (Lang, 1943)the crest of Stonebarrow broke away, and a slice about 500 m. from east to west and 18 m. from north to south slid down some 15 m. into the undercliff of Fairy Dell beneath. It left a slipscarp of yellow Upper Greensand, far more brilliant than the summit of the cliff had previously been in its slightly vegetated condition. The part which fell carried down with it two radio-location huts which had been built on the top of the cliff during the Second World War. The displaced slice came to rest tilted backwards towards the main cliff behind the undercliff, and thus demonstrated that the slipping here was due to rotational shear. Such mass movement of the Upper Greensand is unusual in west Dorset. (b) Black Yen
On Black Yen, as on Stonebarrow, it is rare for masses of Upper Greensand to collapse en bloc. However, in January and February 1944 (Lang, 1945), as a result of a slip on the seaward side (358933) of the DeviI's Bellows cutting, a single unbroken slice of Upper Greensand, approximately 90 m. long, subsided, leaving a scarp some 6 m. high at its western end and decreasing in height towards the east. Within the Lias, the precise nature of much of the movement is masked by mud-flows. The great amphitheatre-shaped hollow in the terrace lying above the Black Yen Marls (Plate 5) west of the DeviI's Bellows and extending from 355932 to 358932, is Black Yen sensu stricto, the
126
MURIEL A. ARBER
black bog which gives its name to the whole cliff (Lang & Arber, 1942). It is a treacherous area of muddy clay, water and vegetation, from which unwary walkers have occasionally to be rescued. In the hollows and gullies leading into and from the grea t amphitheatre there is a constant downward movement of mud and small stones. There had already been a considerable fall of sand and mud here in 1957 when the largest recorded slip was observed by J. F. Jackson on 9 February 1958 (Lang, 1959, and personal communication). Mr. Jackson was on the beach when he saw material falling and heard a sound like thunder. A great mass of Upper Greensand, complete with gorse-bushes and trees, was moving down from the topmost terrace. There followed a sort of explosion in the cliff and a mass of Lias was hurled out. A river of liquid Lias clay began to descend over the lower cliff on to the shore between himself and Charmouth whence he had corne. He made good his retreat just before the mud-flow blocked the beach. The slip continued all night, bringing trees down with it, and the fan of debris is still there (plate 5), forming a small promontory under Black Yen, between Lyme and Charmouth, and now very much overgrown . Above it, about 120 m. a.D., is the place where the old road ran until its last relic was cut in 1937; after that it was still possible to cross the cliff by the old Roman road 10 m. farther inland at 353934. This road in turn was cut in 1958 (Lang, 1959), and by 1968 there was merely space where once the old Roman road had run (Plate 6). A path was made across the fields behind, thence following the golf-course (354934) down to Timber Hill above Lyme. Rapid erosion to the east of the golf-course then set in. In 1968 the edge of the fields had been eaten away, and the wire fence that had been put up beside the path was suspended over space. The path was moved still farther back more than once (Plate 7, A) until it was finally closed altogether in 1970; at the same time the path at the edge of the golf-links slipped away and now ends in a sheer cliff-face (349934). The Upper Greensand scarp is thus receding rapidly. Within the undercliff below, from Rhodehom Plantation (348933) to the great amphitheatre (355933), the movement has been less clear-cut among the flow of the Lias clays. Below the old road, one field (349932) is fissured in a series of small-scale scarps, some facing seawards , others 1andwards. There is, however, evidence of rotational shearing in the concrete blocks which once formed steps leading to the stile (350932) where the footpath from opposite the cemetery at Lyme used to join the old Charmouth road. These blocks, formerly horizontal, were tilted back towards the inland side. Mr. B. W. Conway, of the Institute of Geological Sciences, has made detailed observations on Black Yen, and exhibited some of his work at the Annual Reunion of the Geologists ' Association in 1971. In this exhibit he described how he was able on one occasion to watch the upheaval of Lias
EXPLANATION OF PLATES PLATE 3 Rotated blocks are seen in the chasm at Dowlands. The unmoved mass of 'Goat Island' lies to seaward of the chasm. Beer Head on the horizon PLATE 4 The chasm at Dowlands from Roberts (1840, 5th Edn.) PLATE 5 The Upper Greensand scarp of Black Yen is on the skyline; below comes the uppermost terrace of the Lias, above the Belemnite Marls; below the Belemnite Marls is the amphitheatre of Black Yen sensu stricto (from which black streaks issue down the cliff-face) above the Black Yen Marls; the third and lowest terrace is above the Shales-with-Beef; on the foreshore in the foreground is part of the 1958 tongue of slipped clay PLATE 6 The Upper Greensand scarp of Black Yen above the Belemnite Marls, backing the space where the Roman road and the coast-road once ran. (Compare Arber, 1941, pI. 20, B.) Stonebarrow and Golden Cap behind PLATE 7, A The previous year, the footpath over Black Yen had run along the edge of the cliff, now eaten away; the path had formerly crossed the hedge by the clump of trees PLATE 7, B The mass of black Lias clay in the foreground had been upheaved in 1961 among the fallen Greensand boulders on the foreshore at the eastern side of Charton Bay. By 1962 it had been much reduced in height PLATE 8 The Cliff House site two months after the slip occurred. The ruins of Cliff House stand on the terrace, with the slip-scarp which cut Stile Lane to the right behind. Marine Parade, below, had been cleared, but the black mud-flow still extended over the foreshore. Photograph taken from the North Wall of the Cobb (see Fig. 3) PLATE 9, A Looking over the undercliff (to right) from Goat Island (to left). The ridges in the foreground appear to be very narrow blocks which have suffered rotational shear. The narrow masonry-like ridges, to seaward of these blocks, are not shown in this photograph PLATE 9, B Slipping within the fallen Cretaceous material in the field below the entrance to the Reserve (329916) has left these upstanding horst-like blocks flanking miniature chasms [To face p, 126
PROC. GEOL. ASS., VOL. 84 (1973) PLATE 3
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PROC. GEOL. ASS., VOL. 84 (1 973) PLATE 5
PROC. GEOL ASS., VOL 84 (1973)
PLATE 6
PROC. GEOL. ASS., VOL. 84 (1973)
PLATE 7
A. Summit of Black Yen, II April 1969. Photo by M.A.A.
B. Upheaval of Lias on the foreshore at Humble Point, 23 April 1962. Photo by
M.A.A.
PROC. GEOL. ASS., VOL. 84 (1973) PLATE 8
PROC. GEOL. ASS., VOL. 84 (1973)
PLATE 9
A. Seaward faee of Goat Island, 21 April 1958. Photo by M.A.A.
B. Miniature 'islands' below Ware Cliffs, 14 April 1971. Photo by M.A.A. To face p, 127J
127
LANDS LIPS NEAR LYME REGIS
from below low-tide mark in a syncline on the foreshore. This, so far as I know, is the only record of upheaval at the toe of a slip in the cliffs east of Lyme Regis. The Black Ven Marls have long been foundering in the re-entrant known as Raffey (361932) on the Charmouth flank of Black Ven (Arber, 1941, pl. 19, A). In 1943 they began to move in a manner comparable to that of a lava-flow (Lang, 1944, and personal communication), with knocking noises inside the mass. The fallen mass left a clean section of the upper part of the Black Ven Marls, below the footpath on the cliff-edge above. In 1969 the movement was again accelerated here, and the footpath itself was being eaten away. The slipping is rapidly approaching an estate of newly built houses, so that efforts are being made to check its progress. (c) Lyme Regis
In the town of Lyme itself, sapping by springs at the junction of the Upper Greensand and Lias clays has made the ground unstable, as was seen in the goods shed and coal pens of the now disused railway station (333926). The area in which slipping is most liable to occur, however, has always been within the slope of the Lower Lias itself which forms a hillside (Fig. 3) which is virtually a cliff-face between the Cobb (338917) and Broad Street (341921). This slope is formed of Shales-with-Beef and is capped at the top of the Langmoor Gardens (339920) by a small terrace which is presumably formed by the Birchi nodules, though since Lang's detailed
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128
MURIEL A. ARBER
maps (Lang, 1914, 1924) do not extend into the town, this must be speculative in the absence of exposures of material in situ. Above this terrace again is a steep slope leading to a certain flattening about at the level of the junction of Cobb Road and Pound Street (337921); one might hazard the guess that this is conceivably the horizon of the Stellare beds. This hillside, between the Langmoor Gardens and the Cobb hamlet, has been so unstable that very little building has ever been done on it. The house called Wings, in which Jane Austen stayed, was in Stile Lane towards the foot of the slope which became so unsafe in the nineteen-forties that the house had to be demolished. In 1962, however, a speculative builder excavated, with a view to erecting houses there, the land belonging to Cliff House (338919) just below the terrace level that is presumably formed by the Birchi nodules. About two months after the excavation began, a landslide occurred on 12 February 1962; Cliff House itself, which was standing empty, was ruined, and other houses were extensively damaged. A great mud-flow poured down over Marine Parade below and on to the beach (Plate 8). The lower part of this slip appeared to be mainly a mud-flow, of the type so familiar in the Lyme area as to be known locally as a rougement, but some of the buildings were tilted back into the cliff, showing that here again the movement was a rotational shear and not merely a surface flow. Above the terrace, a scarp appeared (Plate 8), cutting Stile Lane (338920). After this slip the ground between Cliff House and Marine Parade was laid out as public gardens named after Lord Lister. Further small-scale movement has since occurred, breaking the paths in the Gardens, but the largest collapse has been on the other side of the Langmoor Gardens, 200 m. to the east, where a mud-flow descended on Library Cottage (341920) early in 1971, engulfing the kitchen at the back of the house. Where the Cliff House site adjoins Cobb Road there is a comer which has been unstable for over fifty years. A house which stood there, to the west of the road (337918), gradually became so twisted that it had eventually to be demolished. Thirty metres above its site is Pine Walk (Fig. 3). Houses have now been built on its seaward side, and about 1970 a slip developed beyond the southernmost of these. (d) Ware Cliffs to Axmouth The undercliff at the junction of the Jurassic and Cretaceous beds extends all the way from the Ware Cliffs to Axmouth, and movement at this level has been active in recent years. In 1968 and 1969, Ware Lane (330917)was broken just east of the entrance to the Nature Reserve (Fig. 3), and the bungalow known as the Orchard, immediately outside the gateway of the Reserve, has had to be demolished. The last two of the fields through which the footpath from Lyme passes (331916) before it reaches Ware
LANDS LIPS NEAR LYME REGIS
129
Lane, and the field (329916) immediately east of Underhill Farm, have become fissured with a series of small scarps. Miniature chasms have developed among the ridges (Plate 9, B) in the field east of the farm and these will be considered later in relation to the great chasm at Dowlands. About one kilometre west of the entrance to the Reserve, the public footpath was broken about 1968 by a small but sheer drop, and west of this point the old path has become impassable, so that an alternative parallel route has been opened farther inland. At Pinhay Bay a slip which occurred in 1966 destroyed the path to the beach and ruined the ram which used to pump water to the house above. A great mud-flow of clay, remarkably like a glacier in surface structure, has poured over the cliffs, carrying down trees in its course. All along the undercliff from Pinhay to Bindon, parallel to the inland cliff, there developed about 1961 a slip-scarp varying from about 1 to 6 m. in height. Below this line everything appeared to be lowered en masse. East of Whitlands (about 313907) the trees in the undercliff sank bodily some 5 m. so that the sea became visible from the cliff-top at a point from which it had formerly been hidden. In 1961 a slip at Humble Point (305899) produced an upheaval of Lias clay (Plate 7, B) to a height of some 3 to 4 m. on the foreshore, extending from Humble Point for a distance of about 400 m. eastwards. This upheaval of the toe of the slip occurred not far from the site of the reef of Upper Greensand forced up on the foreshore at Whitlands in 1840. Until 1969 there was a cart-road down to the shore at Charton Bay. On 22 June 1969 a landslide occurred here, so that the cart-road was broken by a sheer drop of about 14 m., and the only way down to the beach was by ropes. On the seaward side of this new scarp there rise two great ridges of rock resembling masonry, rather similar to those which will be described seaward of Goat Island at Dowlands. At Charton Bay there is no 'island' apart from these 'masonry' blocks and there is no apparent complementary rise of the beach or sea-floor. All the fallen material, between the precipice and the ridges, seems to have sunk right down. Immediately west of Charton Bay come the cliffs of Rousdon, and beyond these is Dowlands, with its great chasm and Goat Island. No spectacular slipping has taken place along here in recent years. Finally, before Axmouth is reached, there comes the land of Bindon, where deep crevasses and miniature chasms developed on the cliff-top (at approximately 268897) about 1961. These crevasses, running roughly parallel to the cliff-edge, were in one case at least nearly 4 m. deep. In some examples the outer scarp formed a small-scale knife-edge rather similar to the large-scale rock-edges in the undercliff. These did not herald any immediate break-away of the cliff-edge as did the cracks on the cliff-top that
130
MURIEL A. ARBER
were reported before the 1839 slip at Dowlands (Arber, 1940), but, like the still smaller subsidences in the field below Ware Cliffs, it is possible that these chasms at Bindon may throw light on the formation of the great chasm at Dowlands. Dowlands remains the outstanding problem, because Goat Island (Plate 3) survives intact, to provide the seaward wall of the chasm which would otherwise have been merely part of the fallen material in the undercliff. If Goat Island were tilted landwards there would be no mystery, for then it would be obvious that it was merely an exceptionally large block detached by a rotational shear. But the surface of Goat Island is more or less level, with no inland tilt. It was this that led to my earlier acceptance of Conybeare's theory that the sliding was simply a direct movement of massive Upper Greensand down the slip-plane above the argillaceous Lias and Gault. The chasm has now become so completely overgrown that it is difficult to see the structure of its floor, but Conybeare's diagram (Arber, 1940, fig. 34) shows the blocks of the floor as being tilted landwards, and they are depicted thus in some of the contemporary views (Plate 4). This indicates therefore that the slip was of a rotational shear type as far as the chasm was concerned. The fact that a reef of Upper Greensand rocks was upraised at sea, at the toe of the slope, confirms the method of slipping. A hypothesis as to how Goat Island has remained in its present form may be made on the evidence of other structures in the undercliffs. Looking seawards from the top of Goat Island (Plate 9, A), one sees two big ridges of Cretaceous material, running parallel to the Island and with their crests a little lower than its surface. Beyond them are one or two more ridges that are extraordinarily narrow and look like walls of masonry. This series of ridges could therefore be interpreted as very narrow blocks which have suffered rotational shear on the seaward side of the Island, complementing the more massive rotated blocks in the chasm on the landward side. When one recognises the character of these ridges at Dowlands, one recognises the structure of the whole undercliff from Bindon to Ware. H is so densely overgrown that one can get no general view of the surface, and in traversing it one's first impression is of an irregular jumbled mass of fallen rocks. However, on closer acquaintance one finds that the ridges and hollows of the undercliff in fact lie in lines parallel to the coast. There is a series of roughly parallel ridges with narrow crests, which indicates that they are rotated masses. Two examples of these are the ridge followed by the main footpath through a bluebell wood under Bindon cliffs, and the ridge flanking the hollow known as Long Bottom at Pinhay. Of course there is also a great deal of tumbled and jumbled material, but the undercliff has a basic structural plan, demonstrating the nature of the slipping.
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Without borings, it is impossible to tell how deep-seated these slipped ridges are, or to know their relationship to the underlying Rhaetic and Lias. At Pinhay Bay, before the recent slipping which has produced large screes, the sea-cliffs formed a clean and clear section through the White Lias (uppermost Rhaetic) and Blue Lias beds. The cliffs were about 50 m. high, and were capped by the undercliff known as Pinhay Warren, made largely of Chalk. It was slipping that had brought Chalk down so comparatively near to sea-level. Farther west, near the Waterworks (312903), an irregular and unstable mass of Chalk actually forms the seaward cliffs; this too has evidently fallen from above and masks the Jurassic behind it. In Charton Bay, the Lias, Rhaetic and Keuper Marls reappear to form the cliffs, but to the west of Charton Bay the fallen Chalk is seen very near sea-level. Its superficial appearance is solid, giving an almost downlike character to the part of the undercliff called the Plateau (Wallace, 1966), with the spring known as the Gusher emerging at the junction of the Cretaceous and Lias below. These Whitlands cliffs have been the scene of large-scale slipping in the past, and presumably the Chalk here is a very massive displaced block. Still farther west, at Dowlands, fallen Chalk and Upper Greensand overlie great slabs of Blue Lias on the foreshore (285894) below the ruins of Rock Cottage (otherwise known as Cliff or Gapper's Cottage). The range of inland cliffs behind the undercliff shows Upper Greensand capped with Chalk. In front of them are ridges of Chalk which have slipped forward but have not moved far away; an example of this is the Great Cleft (Arber, 1940). The Chapel Rock at Whitlands is the summit of another ridge which has slipped seawards parallel to the main cliff.
5. CONCLUSIONS
Thus the structure of ridges and hollows is integral to the whole of the landslips west of Lyme. It does not occur in the same way on Black Yen and Stonebarrow east of Lyme, because there, in the absence of the Chalk, the Cretaceous beds are less massive; although the fallen material does form some rotated blocks, it disintegrates more rapidly. In returning to the problem of the existence of Goat Island, one has again to consider the question of its level. There is no evidence that it has subsided. Conybeare (1840) stated that the strata on the inland flank of Goat Island are some 60 ft. (about 18 m.) lower than their counterparts in the main inland cliff, and he suggested that this might be explained by the dip. Dr. Macfadyen has pointed out to me that, according to his section (Macfadyen, 1970, fig. 5), a dip of only 5° would be sufficient to lower the strata by 60 ft. between the inland cliff and the seaward side of the Island.
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Conybeare also suggested the alternative possibility of a fault along the line of the chasm. If there were such a fault it seems conceivable that this might be responsible for one, at least, of the shear planes which gave way, thus causing the slipped mass to subside into the chasm. Conybeare makes one other observation which possibly holds the key to the character of the Island: he says that it was rifted but not broken. Since it is flanked by the sunken and rotated masses in the chasm on its landward side and by the narrow rotated ridges on its seaward side, it seems possible that Goat Island itself was rifted but had not yet split into its component ridges by the time that the movement had ceased. Evidence that a chasm can develop without displacement of its seaward flank is to be found in the miniature examples at Ware and Bindon. In the field under the Ware Cliffs (329916), as has already been described, there have developed little chasms flanked by upstanding horst-like 'islands' (Plate 9, B) about 1'5 m. high. In the field on the cliff-top at Bindon, the chasms were about 4 m. deep, and their outer flanks again were in some cases not tilted. This seems to show that it is possible for a chasm to develop without necessarily tilting the land beyond it, so that the 'island' has something of the character of a horst. It is, however, very unsafe to draw such large-scale conclusions from such small-scale evidence. The only decisive indication of the mechanism of the slipping must come from deep borings; and one can but hope that some day it will be considered worth while for the expense to be borne, perhaps by the Engineering Unit of the Institute of Geological Sciences. The work of the Unit on the cliffs of west Dorset is already providing detailed measurements of the slipping, and a technical study of its causes. When the results of the work are published, many of the observations and speculations made in this address should be replaced by precise facts and well-based theories. Nevertheless there may always be room for the amateur to make his own records over the years, and to have the joy of forming his own working hypotheses to explain the problems, in the full knowledge that they will be displaced when further evidence provides better solutions. REFERENCES AOER, D. V. & W. E. SMITH. 1965. The Coast of South Devon and Dorset between Branscombe and Burton Bradstock. Geologists' Association Guide No. 23. ARBER, M. A. 1940. The Coastal Landslips of South-East Devon. Proc. Geol. Ass., 51,257-71. - - - . 1941. The Coastal Landslips of West Dorset. Proc, Geol. Ass., 52, 273-83. - - - . 1971. The Plane of Landslipping on the Coast of South-East Devon. 153-4 in Applied Coastal Geomorphology, ed. by J. A. Steers. London. CONYBEARE, W. D. & others. 1840. Ten Plates . • • Representing the Changes Produced on the Coast of East Devon, between Axmouth and Lyme Regis by the Subsidence of the Land and Elevation of the Bottom of the Sea, on the 26th December, 1839, and 3rd of February, 1840. • • • London.
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LANG, W. D. 1914. The Geology of the Charmouth Cliffs, Beach and Fore-Shore. Proc, Geol. Ass., 25, 293-360. - - - - . 1924. The Blue Lias of the Devon and Dorset Coasts. Proc. Geol. Ass., 35, 169-85. - - - - . 1943. Geological Notes. Proc, Dorset nat. Hist, archaeol, Soc., 64, 129-30. - - - - . 1944. Geological Notes. Proc. Dorset nat. Hist, archaeol. Soc., 65, 147-9. - - - . 1945. Report on Dorset Natural History for 1944 (except Birds). Proc. Dorset nat. Hist. archaeol. Soc., 66, 127-9. - - - - . 1959. Report on Dorset Natural History for 1958. Geology. Proc. Dorset nat. Hist. archaeol. Soc., 80, 22. - - - - & M. A. ARBER. 1942. Names of the West Dorset Cliffs. Notes & Queries for Somerset & Dorset, 23, 278-81. MACFADYEN, W. A. 1970. Geological Highlights of the West Country. A Nature Conservancy Handbook. London. ROBERTS, G. 1840. An Account of and Guide to the Mighty Land-slip of Dowlands and Bindon, In the parish of Axmouth, near Lyme Regis, December 25, 1839.... 5th ed. Lyme Regis. WALLACE, T. J. 1966. The Axmouth-Lyme Regis Undercliffs National Nature Reserve. Allhallows School, Rousdon (Pamphlet). WARD, W. H. 1945. The Stability of Natural Slopes. Geogr, J., 105, 170-97.