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Some comments on the raised beach platform of south-west Caernarvonshire and on an unrecorded raised beach at Porth Neigwl, North Wales
Some Comments on the Raised Beach Platform of South-West Caernarvonshire and on an Unrecorded Raised Beach at Porth Neigwl, North Wales by J. B. WHITIOW Received 13 May 1958 CONTENTS 1. THE PRE-DRIFT RAISED BEACH PLATFORM
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31 36 38 38 38
THE POST-GLACIAL RAISED BEACH ACKNOWLEDGMENTS EXPLANATION OF PLATE ... REFERENCES
ABSTRACT: The discovery of a raised beach deposit in the Lleyn peninsula has prompted the author to re-examine the raised platform of marine abrasion which has been recorded at a few localities in south-west Caernarvonshire. The latter platform was found to be entirely devoid of beach deposits and is undoubtedly of pre-glacial or interglacial age, whereas the raised beach deposit of Porth Neigwl appears to have been formed in post-glacial times.
1. THE PRE-DRIFT RAISED BEACH PLATFORM ON THE western coast of the Lleyn peninsula between Bardsey Island and Nevin a marine abrasion platform occurs at an average elevation of ten feet above H.W.M. and in every locality it is overlain by glacial drift. It was first mentioned by Jehu (1909),and later by Matley and others (1939), but it appears that none of the authors examined it at all the sites where it is uncovered from beneath the drift. The present writer has mapped all the localities where the platform is clearly exposed (Fig. 1), but only in one place are traces of the old cliff distinguishable. Near the lifeboat station of Porth Dinlleyn the drift has been removed (possibly by human agencies) sufficiently to uncover a wave-cut notch which can be traced for a few yards at a height of approximately twenty-five feet O.D. It is possible that at this site the rock exposure on which the notch is displayed may be an old sea-stack partially buried by the drift. Indeed it is interesting to speculate whether the curiously shaped peninsula of Porth Dinlleyn is in reality a series of sea-stacks rising above the raised beach platform, and now tied to the mainland by a thick cover of drift. The relationship between the Pre-Drift Raised Beach Platform and the modem wave-cut rock bench is not always a simple one. In the majority of cases the higher platform has been notched at its seaward end by wave attack of the present sea-level, but there are a few sites where this modem notching is not clearly marked and where the seaward end of the raised 31
32
J. B. WHITTOW
THE RAISED BEACH PLATFORM OF SOUTH-WEST CAERNARVONSHiRE AND A RAISED BEACH AT PORTH NEIGWL 1----1- - -- -3'0 sco re
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rock platform is rugged and broken . It is noteworthy that on the opposite shore of the Irish Sea the raised beach platform of the Wicklow-Wexford coast sometimes exhibits a similar characteristic (Martin, 1955). A feature of the utmost importance is that the raised platform stands at an elevation beyond the reach of all but the most exceptional storm waves, and as
PROC. GEOL. ASSOC., VOL. 71 (1960)
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RAISED BEACHES OF NORTH WALES
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a result it is often overgrown with lichens, mosses and grass. In some localities a few small sea-stacks are relict on the platform and they may be indicative of the fact that the still-stand at approximately twenty-five feet 0.0. probably existed for only a relatively short period, the sea having withdrawn while the coastline was still in the youthful stage of the marine erosion cycle. Nevertheless, it has been pointed out that whilst sea-stacks '... reach their most abundant development before maturity is attained, they may also be present on fully mature shores' (Johnson, 1919). A definite decision on this point, however, cannot be made, since the presence of the drift makes it impossible to distinguish the width of the platform or to determine the plan of the raised beach coastline. The most that can be said from the available evidence is, firstly, that the failure to discover more than one exposed example of a wave-cut notch behind the platform may mean that the width of the platform beneath the drift is considerable, therefore implying a lengthy period of marine abrasion; secondly, that the platform has been so perfectly planed across steeply dipping and contorted rocks at some sites that one is persuaded to think in terms of a prolonged marine cycle. Since the platform has been severelygrooved by ice-action at a number of localities it is also possible that ice may have played no small part in the smoothing of the platform surface. But the fact that the small sea-stacks, which rise from the platform at other places, have not been swept away suggests that glacial action was very localised and could not have been of such great significancein the general bevelling of the platform. The southernmost example of the raised rock platform in Lleyn occurs on Bardsey Island, where the low level bench west of Mynydd Enlli seems to be a composite feature, i.e. drift overlying the raised beach platform. The latter is well exposed near the landing-place, but circumstances prevented the author from making a full investigation of the remainder of the island. The platform is also exhibited on one of the Ynys Gwylan (Gull Islands) in Aberdaron Bay. It is covered by a remnant of drift on Ynys Gwylan Fawr but a remarkable stack rises to 100 feet in height above the rock platform, together forming a conspicuous feature when seen in profile from Aberdaron village. From Bardsey Sound to Porth Oer the sea cliffs are precipitous and the platform is not represented except possibly at two drift-capped islets, Dinas Fawr and Dinas Bach. Farther north, however, an excellent example occurs at Porth Oer, at either end of the so-called 'Whistling Sands'. The average height of the rock platform, which here cuts across highly contorted Pre-Cambrian rocks, is some twenty feet 0.0., but nowhere is the wave-cut notch exposed from beneath the drift. An interesting feature at Porth Oer is the way in which the platform is seen to truncate the top of the modern sea-stacks a short distance off-shore (Plate 3A). PROC. GEOL. ASSOC., VOL. 71, PART I, 1960
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J. B. WHITTOW
Continuing northwards the platform can be traced for some distance near to Penrhyn Colmon, and at Porth Colmon can be found the finest example in the Lleyn peninsula. It is at this locality that small sea-stacks are exhibited. The platform appears to be present in a fragmented state at Porth Ysgaden and also at Penrhyn Cwmistir, although better and more accessible exposures are to be found at Aber Geirch. At this point the platform is somewhat higher than elsewhere on the coast of Lleyn, but it must be borne in mind that this is a headland site (despite its name), and that the Afon Geirch is at present probably following a channel excavated as a result of glacial diversion (Whittow, 1957). The slightly higher elevation of the platform at a headland locality agrees with the findings on the east coast of Ireland (Stephens, 1957). On the small peninsula of Porth Dinlleyn several houses have utilised the narrow site offered by the rock platform, being perched precariously on the brink of the modern highwater mark and overshadowed by precipitous cliffs of drift. The platform, which near the lifeboat station is finely glaciated, disappears beneath these drift cliffsas it is traced towards the centre of the bay, only to reappear on the headland of Penrhyn Nevin. This is the northern limit of the platform in the Lleyn peninsula so far as is known. Failure to distinguish the platform elsewhere in the Lleyn peninsula must be attributed to two reasons. In the first place the present cycle of marine erosion may have been responsible for its destruction on some stretches of the coastline. Secondly, in some areas it may be present beneath the extensive drift cover, particularly along the south coast of Caernarvonshire. Nevertheless, at no point has the author been able to identify the platform with certainty between the Ynys Gwylan and Portmadoc. It might be expected that such headlands as Careg-y-Defaid and Careg-yr-Imbill (near Pwllheli) would display notching at the appropriate elevation but unfortunately both have been extensively refashioned by quarrying. Farther east the headlands of Pen-y-Chain and Criccieth Castell are also without convincing evidence although it has been suggested that a rock bench two feet above high-water mark and overlain by 'head' at Criccieth may be the equivalent of the 'pre-glacial raised beach platform' (Fearnsides, 1910). The author has examined this site and also the headlands of Rhiw-for-Fawr and Craig-Ddu to the east of Criccieth but is not convinced that the rock platform under discussion is present. Although it has been suggested by Jehu that certain superficial deposits in Porth Oer may represent actual raised beach deposits, the author has found no evidence to support this possibility. At two localities (Porth Oer, Penrhyn Nevin), a thin layer of angular rock rubble or 'head' lies on the platform and in each case is overlain by a drift of uncertain age. Elsewhere the so-called 'Upper and Lower Boulder Clays' (Jehu, 1909) alternately rest immediately on the platform, and nowhere has the author discovered
RAISED BEACHES OF NORTH WALES
35
any calcreted or concreted beach material associated with the latter. This is hardly surprising, perhaps, for the glacial striations indicate that the platform must have been severely scoured by Irish Sea ice moving on-shore from a northerly direction. In elevation and form this platform of Lleyn is similar in every respect to the so-called Patella beach rock platform which has been widely recognised in the southern half of the British Isles. Since it has not been recorded elsewhere in North Wales the nearest localities for comparative purposes are to be found in eastern Ireland and in Gower, South Wales. On the south-east coast of Ireland a rock platform at a similar elevation to the one in southern Caernarvonshire is overlain by beach deposits which themselves pre-date two glacial drifts. The lower of these drifts (Eastern General) has been referred to the Riss glaciation by Mitchell (1951), so that the Irish rock platform is at least as old as the Mindel-Riss Interglacial. The Patella beach platform of Gower has been tentatively placed in the Mindel-Riss Interglacial by reason of its overlying deposits (Arkell, 1943) but doubts are now being expressed for this dating. It is noteworthy that raised beach investigation in Ireland is emphasising the danger of assuming that beach deposits are everywhere contemporaneous with the rock platform on which they rest (Stephens, 1957; Stephens & Synge, 1958). In conclusion it may be said that the Pre-Drift Raised Beach Platform in the Lleyn peninsula cannot be precisely dated. It lacks beach material which even if present would not necessarily be of the same age as the platform, whilst, in addition, the age of the overlying glacial deposits has not been accurately determined in all localities. In the author's opinion the 'Lower Boulder Clay' in the Lleyn peninsula appears to have been deposited by the initial advance into the peninsula of an Irish Sea ice-sheet, and it may be possible, therefore, to regard it as the oldest of the 'New Drifts' (Whittow, 1957). If tentative correlations must be made then the 'Lower Boulder Clay' of Lleyn may be equivalent in age to the Cornovian glaciation (Arkell, 1943) and the Midland General glaciation of Ireland which Charlesworth equates with the South Wales end-moraine (Charlesworth, 1929). Although deposits of head underly the 'Lower Boulder Clay' in a few localities in Lleyn it cannot necessarily be assumed that the head represents an 'Old Drift' deposit or is equivalent to part of Arkell's Catuvellaunian glaciation or the Eastern General glaciation of Ireland. The most that can be said from the available evidence is that the platform in the Lleyn peninsula is probably older than the British equivalent of the Wlirm glaciation. By comparison with similar rock platforms at approximately the same elevation on other coasts of the Irish Sea it appears that the platform may even be as old as the Mindel-Riss Interglacial, and there is no reason to suppose that it is not much older.
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J. B. WHITTOW
2. THE POST-GLACIAL RAISED BEACH The bay head of Porth Neigwl (Hell's Mouth) is composed almost entirely of cliffs cut in glacial drift, except where these are broken by the sand dunes blocking the former outlet of the Afon Soch. The high drift cliffs are flanked at either end of the bay by the great rocky headlands of Trwyn Talfarach in the west and Trwyn Cilan in the east. Both of these promontories are capped with glacial drift , and frequently the more gentle of their seaward slopes have thicknesses of similar material plastered over the rocks. Trwyn Talfarach is the seaward extension of a massive intrusion of hornblende-diorite which forms the eminences of Mynydd Penarfynydd and Creigiau Gwineu, and it was at the foot of the latter crag that a conglomerate was discovered which bore every resemblance to a raised beach deposit. The foreshore at this locality consists of massive blocks of hornblende-diorite similar to those which are scattered beneath the crags at a higher elevation. Consequently, these blocks are thought to have fallen from the cliffs of Creigiau Gwineu, possibly during the glacial period, but a diligent search failed to discover a wave-cut rock platform beneath this bulky debris. Behind and overlying this block material are unstable grass-covered slopes of glacial drift and hill-wash at the foot of which occurs the conglomerate, its landward extension obscured by slumped material (Plate 3B). It is about thirty feet in length, four feet in thickness, and its base is some three feet above high-water mark. The latter is somewhat difficult to distinguish on the rocky foreshore, but the upper limit of the Fucus Zone was taken as a line of demarcation (Plate 3B). The constituent rocks of the conglomerate are varied, and a petrological analysis distinguished the following: the smaller cobbles and pebbles consisted of dolerite, gabbro, quartz-felsite , rhyolite( ?), greywacke, grit, limestone, greenstone-crush basalt and low grade slate. Although many of these are derived from the Pre-Cambrian and Ordovician series in Lleyn, there are several pebbles which are undoubtedly glacial erratics from North Britain. The water-smoothed larger cobbles and small boulders consist mainly of hornblende-diorite and are almost certainly derived from the local intrusion described above. In addition there is a variety of marine and terrestrial mollusca (Table I). All these materials are cemented together by a matrix of calcareous tufa which also serves to fix the deposit to the underlying blocks of the foreshore. The matrix almost certainly derives from the boulder clay. The base of the conglomerate has been undercut and several blocks of it have become detached from the main mass. This undercutting may have resulted from the Romano-British transgression of which Godwin has found evidence in Southern Britain (Godwin, 1941), but since the deposit is so near to present high-water mark it may be the result of modem storm waves. There was at first some doubt about the origin of the conglomerate for
RAISED BEACHES OF NORTH WALES
37
it bore a close resemblance to certain cemented gravels and calcreted sands which occur in the cliff face at both Nevin and Llanbedrog. These examples, however, are undoubtedly of fluvio-glacial origin, are unfossiliferous, and probably owe their cementation to percolating water highly charged with calcareous minerals from the overlying shelly drift. It might be argued that the marine shells in the Porth Neigwl conglomerate were dredged from the floor of the Irish Sea by ice action, and therefore that the deposit is of glacial age, but an analysis of the Lleyn drifts shows that ice-dredged shells are usually so crushed and fragmented that they can only be distinguished under a microscope (Jehu, 1909). The mollusca of the Porth Neigwl conglomerate, on the other hand, are frequently preserved intact. At this point it is important to establish the relationship which exists between the Porth Neigwl conglomerate and the raised beach platform of south-west Caernarvonshire. TABLE
I. Mollusca from the Raised Beacb, Portb Neigwl, Caernarvonsbire MARINE SPECIES
Type Littorina littorea Littorina littoralis Littorina rudis Nucella lapillus Patella vulgata
TERRESTRIAL SPECIES
Approximate Percentage of Species 60%
10% 10% 10% 10%
Helix aspersa Oxychilus cellar/us Retinella pura Goniodiscus rotundatus Carychium minimum
It is difficult to assess the spatial relationship which exists between the two, except on a basis of height above high-water mark, a criterion always regarded as a tentative means of comparison on its own. From such a comparison, however, it would appear that the base of the conglomerate lies some five feet below the average height of the Pre-Drift Raised Beach Platform. Three other important points emerge from the comparison: first, no wave-cut rock platform similar to that described above has been discovered anywhere in Porth Neigwl; secondly, an analysis of its fossil content indicates that the conglomerate is more likely to be of post-glacial age than of interglacial or pre-glacial age; thirdly, the drift which overlies the landward end of the conglomerate is undoubtedly a slumped deposit, alternately burying and re-exposing sections of the conglomerate (Plate 3B). Thus there is every possibility that the latter was formed as a beach at the foot of a boulder clay cliff in post-glacial times, and it cannot therefore be correlated with the Pre-Drift Raised Beach Platform. It is to be expected that if a remnant of an early post-glacial beach were to be preserved in
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J. B. WHITTOW
Porth Neigwl it would be found under the western slopes, most sheltered from the waves of maximum fetch. The Porth Neigwl raised beach cannot be precisely dated in post-glacial time, and although raised beaches with a similar marine shell content have been described from the northern parts of the British Isles under various titles ('Littorina', 'Neolithic' and 'Early Post-Glacial') they are not necessarily contemporaneous. The O-isobase of the British post-glacial raised beach has been established in North Wales by Wright (1914,1937) and his map has been widely adopted. Wright leaves no published record of the exact location in North Wales on which his map is based, and since there is a curious discrepancy in the position of the O-isobase between the first and the second editions of his book it is difficult to understand on what evidence this line is drawn. It is certainly of interest to the present study that the first edition indicates the Lleyn peninsula as being within the compass of the post-glacial raised beach, whilst in the second edition the whole of Lleyn is excluded. Finally, it is noteworthy that recent research points to a post-glacial strand-line of sixteen to seventeen feet O.D. in south-west Lancashire and Flintshire (Gresswell, 1953; Rowlands, 1955), which may be related to the Porth Neigwl raised beach. ACKNOWLEDGMENTS The author wishes to express his gratitude to Mrs. E. M. Jope (The Queen's University, Belfast) for help in the recognition of the raised beach shell content, to Mr. N. Angus (The Queen's University, Belfast), for aid in the petrological analysis, and to Mr. J. Mbazira (Makerere College, Kampala , Uganda) for his cartographic assistance. EXPLANATION OF PLATE 3 A. The Pre- Drift Raised Beach Platform at Porth Oer. Note how the platform bevels the top of the modem sea-stack. B. The Post-Glacial Raised Beach in Porth Neigwl (Front elevation). Note the recent slumping of the drift which has ob scured part of the conglomerate. The Fucus Zone can be distinguished as a dark colouring on the boulders in the foreground.
REFERENCES ARKELL, W. J. 1943. The Pleistocene Rocks at Trebetherick Point, North Cornwall. Their Interpretation and Correlation. Proc. Geol. Ass., Lond., 54, 141-70. CHARLESWORTH, J. K. 1929. The South Wales End-Moraine. Quart . J. geol. So c. Lond. , 85,335-55. FEARNSIDES, W. G. 1910. The Tremadoc Slates and Associated Rocks of South East Caernarvonshire. Quart . J . geol. Soc. L ond., 66, 142-88. GODWIN, H . 1941. Studies of the Pos t-Glacial History of British Vegetation. VI. Correlations in the Somerset Levels. N ew Phytol., 40, 2, 108-32.
RAISED BEACHES OF NORTH WALES
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GRESSWELL, R. K. 1953. Sandy Shores in South Lancashire. Liverpool. JEHU, T. J. 1909. The Glacial Deposits of West Caemarvonshire. Trans. roy. Soc. Edinb., 47, 17-56. JOHNSON, D. W. 1919. Shore Processes and Shoreline Development. New York. MARTIN, S. 1955. Raised Beaches and their Relations to Glacial Drifts on the East Coast of Ireland. Irish Geogr., 3, 87-93. MATLEY, C. A. & others. 1939. Summer Field Meeting in Western Lleyn. Proc. Geol. Ass., Lond., 50, 83-100. MITCHELL, G. F. 1951. The Pleistocene Period in Ireland. Dansk Geologisk Forenig, 12, 111-14. ROWLANDS, B. M. 1955. 'The Glacial and Post-Glacial Geomorphological evolution of the landforms of the Vale of Clwyd.' Unpublished M.A. thesis, University of Liverpool. STEPHENS, N. 1957. Some Observations on the 'Interglacial' Platform and the Early Post-Glacial Raised Beach on the East Coast of Ireland. Proc. Ri Irish Acad., 58, B, 129-49. - - - & F. M. Synge. 1958. A Quaternary Succession at Sutton, Co. Dublin. Proc. R. Irish Acad., 59, B, 19-27. WHITTOW, J. B. 1957. 'The Lleyn Peninsula, North Wales. A geomorphological study,' Unpublished Ph.D. thesis, University of Reading. WRIGHT, W. B. 1914. The Quaternary lee Age. l st ed. London. - - - , . 1937. The Quaternary lee Age. 2nd ed. London.
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page
31 36 38 38 38
THE POST-GLACIAL RAISED BEACH ACKNOWLEDGMENTS EXPLANATION OF PLATE ... REFERENCES
ABSTRACT: The discovery of a raised beach deposit in the Lleyn peninsula has prompted the author to re-examine the raised platform of marine abrasion which has been recorded at a few localities in south-west Caernarvonshire. The latter platform was found to be entirely devoid of beach deposits and is undoubtedly of pre-glacial or interglacial age, whereas the raised beach deposit of Porth Neigwl appears to have been formed in post-glacial times.
1. THE PRE-DRIFT RAISED BEACH PLATFORM ON THE western coast of the Lleyn peninsula between Bardsey Island and Nevin a marine abrasion platform occurs at an average elevation of ten feet above H.W.M. and in every locality it is overlain by glacial drift. It was first mentioned by Jehu (1909),and later by Matley and others (1939), but it appears that none of the authors examined it at all the sites where it is uncovered from beneath the drift. The present writer has mapped all the localities where the platform is clearly exposed (Fig. 1), but only in one place are traces of the old cliff distinguishable. Near the lifeboat station of Porth Dinlleyn the drift has been removed (possibly by human agencies) sufficiently to uncover a wave-cut notch which can be traced for a few yards at a height of approximately twenty-five feet O.D. It is possible that at this site the rock exposure on which the notch is displayed may be an old sea-stack partially buried by the drift. Indeed it is interesting to speculate whether the curiously shaped peninsula of Porth Dinlleyn is in reality a series of sea-stacks rising above the raised beach platform, and now tied to the mainland by a thick cover of drift. The relationship between the Pre-Drift Raised Beach Platform and the modem wave-cut rock bench is not always a simple one. In the majority of cases the higher platform has been notched at its seaward end by wave attack of the present sea-level, but there are a few sites where this modem notching is not clearly marked and where the seaward end of the raised 31
32
J. B. WHITTOW
THE RAISED BEACH PLATFORM OF SOUTH-WEST CAERNARVONSHiRE AND A RAISED BEACH AT PORTH NEIGWL 1----1- - -- -3'0 sco re
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rock platform is rugged and broken . It is noteworthy that on the opposite shore of the Irish Sea the raised beach platform of the Wicklow-Wexford coast sometimes exhibits a similar characteristic (Martin, 1955). A feature of the utmost importance is that the raised platform stands at an elevation beyond the reach of all but the most exceptional storm waves, and as
PROC. GEOL. ASSOC., VOL. 71 (1960)
A
B
To face p, 33]
PLATE 3
RAISED BEACHES OF NORTH WALES
33
a result it is often overgrown with lichens, mosses and grass. In some localities a few small sea-stacks are relict on the platform and they may be indicative of the fact that the still-stand at approximately twenty-five feet 0.0. probably existed for only a relatively short period, the sea having withdrawn while the coastline was still in the youthful stage of the marine erosion cycle. Nevertheless, it has been pointed out that whilst sea-stacks '... reach their most abundant development before maturity is attained, they may also be present on fully mature shores' (Johnson, 1919). A definite decision on this point, however, cannot be made, since the presence of the drift makes it impossible to distinguish the width of the platform or to determine the plan of the raised beach coastline. The most that can be said from the available evidence is, firstly, that the failure to discover more than one exposed example of a wave-cut notch behind the platform may mean that the width of the platform beneath the drift is considerable, therefore implying a lengthy period of marine abrasion; secondly, that the platform has been so perfectly planed across steeply dipping and contorted rocks at some sites that one is persuaded to think in terms of a prolonged marine cycle. Since the platform has been severelygrooved by ice-action at a number of localities it is also possible that ice may have played no small part in the smoothing of the platform surface. But the fact that the small sea-stacks, which rise from the platform at other places, have not been swept away suggests that glacial action was very localised and could not have been of such great significancein the general bevelling of the platform. The southernmost example of the raised rock platform in Lleyn occurs on Bardsey Island, where the low level bench west of Mynydd Enlli seems to be a composite feature, i.e. drift overlying the raised beach platform. The latter is well exposed near the landing-place, but circumstances prevented the author from making a full investigation of the remainder of the island. The platform is also exhibited on one of the Ynys Gwylan (Gull Islands) in Aberdaron Bay. It is covered by a remnant of drift on Ynys Gwylan Fawr but a remarkable stack rises to 100 feet in height above the rock platform, together forming a conspicuous feature when seen in profile from Aberdaron village. From Bardsey Sound to Porth Oer the sea cliffs are precipitous and the platform is not represented except possibly at two drift-capped islets, Dinas Fawr and Dinas Bach. Farther north, however, an excellent example occurs at Porth Oer, at either end of the so-called 'Whistling Sands'. The average height of the rock platform, which here cuts across highly contorted Pre-Cambrian rocks, is some twenty feet 0.0., but nowhere is the wave-cut notch exposed from beneath the drift. An interesting feature at Porth Oer is the way in which the platform is seen to truncate the top of the modern sea-stacks a short distance off-shore (Plate 3A). PROC. GEOL. ASSOC., VOL. 71, PART I, 1960
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J. B. WHITTOW
Continuing northwards the platform can be traced for some distance near to Penrhyn Colmon, and at Porth Colmon can be found the finest example in the Lleyn peninsula. It is at this locality that small sea-stacks are exhibited. The platform appears to be present in a fragmented state at Porth Ysgaden and also at Penrhyn Cwmistir, although better and more accessible exposures are to be found at Aber Geirch. At this point the platform is somewhat higher than elsewhere on the coast of Lleyn, but it must be borne in mind that this is a headland site (despite its name), and that the Afon Geirch is at present probably following a channel excavated as a result of glacial diversion (Whittow, 1957). The slightly higher elevation of the platform at a headland locality agrees with the findings on the east coast of Ireland (Stephens, 1957). On the small peninsula of Porth Dinlleyn several houses have utilised the narrow site offered by the rock platform, being perched precariously on the brink of the modern highwater mark and overshadowed by precipitous cliffs of drift. The platform, which near the lifeboat station is finely glaciated, disappears beneath these drift cliffsas it is traced towards the centre of the bay, only to reappear on the headland of Penrhyn Nevin. This is the northern limit of the platform in the Lleyn peninsula so far as is known. Failure to distinguish the platform elsewhere in the Lleyn peninsula must be attributed to two reasons. In the first place the present cycle of marine erosion may have been responsible for its destruction on some stretches of the coastline. Secondly, in some areas it may be present beneath the extensive drift cover, particularly along the south coast of Caernarvonshire. Nevertheless, at no point has the author been able to identify the platform with certainty between the Ynys Gwylan and Portmadoc. It might be expected that such headlands as Careg-y-Defaid and Careg-yr-Imbill (near Pwllheli) would display notching at the appropriate elevation but unfortunately both have been extensively refashioned by quarrying. Farther east the headlands of Pen-y-Chain and Criccieth Castell are also without convincing evidence although it has been suggested that a rock bench two feet above high-water mark and overlain by 'head' at Criccieth may be the equivalent of the 'pre-glacial raised beach platform' (Fearnsides, 1910). The author has examined this site and also the headlands of Rhiw-for-Fawr and Craig-Ddu to the east of Criccieth but is not convinced that the rock platform under discussion is present. Although it has been suggested by Jehu that certain superficial deposits in Porth Oer may represent actual raised beach deposits, the author has found no evidence to support this possibility. At two localities (Porth Oer, Penrhyn Nevin), a thin layer of angular rock rubble or 'head' lies on the platform and in each case is overlain by a drift of uncertain age. Elsewhere the so-called 'Upper and Lower Boulder Clays' (Jehu, 1909) alternately rest immediately on the platform, and nowhere has the author discovered
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any calcreted or concreted beach material associated with the latter. This is hardly surprising, perhaps, for the glacial striations indicate that the platform must have been severely scoured by Irish Sea ice moving on-shore from a northerly direction. In elevation and form this platform of Lleyn is similar in every respect to the so-called Patella beach rock platform which has been widely recognised in the southern half of the British Isles. Since it has not been recorded elsewhere in North Wales the nearest localities for comparative purposes are to be found in eastern Ireland and in Gower, South Wales. On the south-east coast of Ireland a rock platform at a similar elevation to the one in southern Caernarvonshire is overlain by beach deposits which themselves pre-date two glacial drifts. The lower of these drifts (Eastern General) has been referred to the Riss glaciation by Mitchell (1951), so that the Irish rock platform is at least as old as the Mindel-Riss Interglacial. The Patella beach platform of Gower has been tentatively placed in the Mindel-Riss Interglacial by reason of its overlying deposits (Arkell, 1943) but doubts are now being expressed for this dating. It is noteworthy that raised beach investigation in Ireland is emphasising the danger of assuming that beach deposits are everywhere contemporaneous with the rock platform on which they rest (Stephens, 1957; Stephens & Synge, 1958). In conclusion it may be said that the Pre-Drift Raised Beach Platform in the Lleyn peninsula cannot be precisely dated. It lacks beach material which even if present would not necessarily be of the same age as the platform, whilst, in addition, the age of the overlying glacial deposits has not been accurately determined in all localities. In the author's opinion the 'Lower Boulder Clay' in the Lleyn peninsula appears to have been deposited by the initial advance into the peninsula of an Irish Sea ice-sheet, and it may be possible, therefore, to regard it as the oldest of the 'New Drifts' (Whittow, 1957). If tentative correlations must be made then the 'Lower Boulder Clay' of Lleyn may be equivalent in age to the Cornovian glaciation (Arkell, 1943) and the Midland General glaciation of Ireland which Charlesworth equates with the South Wales end-moraine (Charlesworth, 1929). Although deposits of head underly the 'Lower Boulder Clay' in a few localities in Lleyn it cannot necessarily be assumed that the head represents an 'Old Drift' deposit or is equivalent to part of Arkell's Catuvellaunian glaciation or the Eastern General glaciation of Ireland. The most that can be said from the available evidence is that the platform in the Lleyn peninsula is probably older than the British equivalent of the Wlirm glaciation. By comparison with similar rock platforms at approximately the same elevation on other coasts of the Irish Sea it appears that the platform may even be as old as the Mindel-Riss Interglacial, and there is no reason to suppose that it is not much older.
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2. THE POST-GLACIAL RAISED BEACH The bay head of Porth Neigwl (Hell's Mouth) is composed almost entirely of cliffs cut in glacial drift, except where these are broken by the sand dunes blocking the former outlet of the Afon Soch. The high drift cliffs are flanked at either end of the bay by the great rocky headlands of Trwyn Talfarach in the west and Trwyn Cilan in the east. Both of these promontories are capped with glacial drift , and frequently the more gentle of their seaward slopes have thicknesses of similar material plastered over the rocks. Trwyn Talfarach is the seaward extension of a massive intrusion of hornblende-diorite which forms the eminences of Mynydd Penarfynydd and Creigiau Gwineu, and it was at the foot of the latter crag that a conglomerate was discovered which bore every resemblance to a raised beach deposit. The foreshore at this locality consists of massive blocks of hornblende-diorite similar to those which are scattered beneath the crags at a higher elevation. Consequently, these blocks are thought to have fallen from the cliffs of Creigiau Gwineu, possibly during the glacial period, but a diligent search failed to discover a wave-cut rock platform beneath this bulky debris. Behind and overlying this block material are unstable grass-covered slopes of glacial drift and hill-wash at the foot of which occurs the conglomerate, its landward extension obscured by slumped material (Plate 3B). It is about thirty feet in length, four feet in thickness, and its base is some three feet above high-water mark. The latter is somewhat difficult to distinguish on the rocky foreshore, but the upper limit of the Fucus Zone was taken as a line of demarcation (Plate 3B). The constituent rocks of the conglomerate are varied, and a petrological analysis distinguished the following: the smaller cobbles and pebbles consisted of dolerite, gabbro, quartz-felsite , rhyolite( ?), greywacke, grit, limestone, greenstone-crush basalt and low grade slate. Although many of these are derived from the Pre-Cambrian and Ordovician series in Lleyn, there are several pebbles which are undoubtedly glacial erratics from North Britain. The water-smoothed larger cobbles and small boulders consist mainly of hornblende-diorite and are almost certainly derived from the local intrusion described above. In addition there is a variety of marine and terrestrial mollusca (Table I). All these materials are cemented together by a matrix of calcareous tufa which also serves to fix the deposit to the underlying blocks of the foreshore. The matrix almost certainly derives from the boulder clay. The base of the conglomerate has been undercut and several blocks of it have become detached from the main mass. This undercutting may have resulted from the Romano-British transgression of which Godwin has found evidence in Southern Britain (Godwin, 1941), but since the deposit is so near to present high-water mark it may be the result of modem storm waves. There was at first some doubt about the origin of the conglomerate for
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it bore a close resemblance to certain cemented gravels and calcreted sands which occur in the cliff face at both Nevin and Llanbedrog. These examples, however, are undoubtedly of fluvio-glacial origin, are unfossiliferous, and probably owe their cementation to percolating water highly charged with calcareous minerals from the overlying shelly drift. It might be argued that the marine shells in the Porth Neigwl conglomerate were dredged from the floor of the Irish Sea by ice action, and therefore that the deposit is of glacial age, but an analysis of the Lleyn drifts shows that ice-dredged shells are usually so crushed and fragmented that they can only be distinguished under a microscope (Jehu, 1909). The mollusca of the Porth Neigwl conglomerate, on the other hand, are frequently preserved intact. At this point it is important to establish the relationship which exists between the Porth Neigwl conglomerate and the raised beach platform of south-west Caernarvonshire. TABLE
I. Mollusca from the Raised Beacb, Portb Neigwl, Caernarvonsbire MARINE SPECIES
Type Littorina littorea Littorina littoralis Littorina rudis Nucella lapillus Patella vulgata
TERRESTRIAL SPECIES
Approximate Percentage of Species 60%
10% 10% 10% 10%
Helix aspersa Oxychilus cellar/us Retinella pura Goniodiscus rotundatus Carychium minimum
It is difficult to assess the spatial relationship which exists between the two, except on a basis of height above high-water mark, a criterion always regarded as a tentative means of comparison on its own. From such a comparison, however, it would appear that the base of the conglomerate lies some five feet below the average height of the Pre-Drift Raised Beach Platform. Three other important points emerge from the comparison: first, no wave-cut rock platform similar to that described above has been discovered anywhere in Porth Neigwl; secondly, an analysis of its fossil content indicates that the conglomerate is more likely to be of post-glacial age than of interglacial or pre-glacial age; thirdly, the drift which overlies the landward end of the conglomerate is undoubtedly a slumped deposit, alternately burying and re-exposing sections of the conglomerate (Plate 3B). Thus there is every possibility that the latter was formed as a beach at the foot of a boulder clay cliff in post-glacial times, and it cannot therefore be correlated with the Pre-Drift Raised Beach Platform. It is to be expected that if a remnant of an early post-glacial beach were to be preserved in
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J. B. WHITTOW
Porth Neigwl it would be found under the western slopes, most sheltered from the waves of maximum fetch. The Porth Neigwl raised beach cannot be precisely dated in post-glacial time, and although raised beaches with a similar marine shell content have been described from the northern parts of the British Isles under various titles ('Littorina', 'Neolithic' and 'Early Post-Glacial') they are not necessarily contemporaneous. The O-isobase of the British post-glacial raised beach has been established in North Wales by Wright (1914,1937) and his map has been widely adopted. Wright leaves no published record of the exact location in North Wales on which his map is based, and since there is a curious discrepancy in the position of the O-isobase between the first and the second editions of his book it is difficult to understand on what evidence this line is drawn. It is certainly of interest to the present study that the first edition indicates the Lleyn peninsula as being within the compass of the post-glacial raised beach, whilst in the second edition the whole of Lleyn is excluded. Finally, it is noteworthy that recent research points to a post-glacial strand-line of sixteen to seventeen feet O.D. in south-west Lancashire and Flintshire (Gresswell, 1953; Rowlands, 1955), which may be related to the Porth Neigwl raised beach. ACKNOWLEDGMENTS The author wishes to express his gratitude to Mrs. E. M. Jope (The Queen's University, Belfast) for help in the recognition of the raised beach shell content, to Mr. N. Angus (The Queen's University, Belfast), for aid in the petrological analysis, and to Mr. J. Mbazira (Makerere College, Kampala , Uganda) for his cartographic assistance. EXPLANATION OF PLATE 3 A. The Pre- Drift Raised Beach Platform at Porth Oer. Note how the platform bevels the top of the modem sea-stack. B. The Post-Glacial Raised Beach in Porth Neigwl (Front elevation). Note the recent slumping of the drift which has ob scured part of the conglomerate. The Fucus Zone can be distinguished as a dark colouring on the boulders in the foreground.
REFERENCES ARKELL, W. J. 1943. The Pleistocene Rocks at Trebetherick Point, North Cornwall. Their Interpretation and Correlation. Proc. Geol. Ass., Lond., 54, 141-70. CHARLESWORTH, J. K. 1929. The South Wales End-Moraine. Quart . J. geol. So c. Lond. , 85,335-55. FEARNSIDES, W. G. 1910. The Tremadoc Slates and Associated Rocks of South East Caernarvonshire. Quart . J . geol. Soc. L ond., 66, 142-88. GODWIN, H . 1941. Studies of the Pos t-Glacial History of British Vegetation. VI. Correlations in the Somerset Levels. N ew Phytol., 40, 2, 108-32.
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GRESSWELL, R. K. 1953. Sandy Shores in South Lancashire. Liverpool. JEHU, T. J. 1909. The Glacial Deposits of West Caemarvonshire. Trans. roy. Soc. Edinb., 47, 17-56. JOHNSON, D. W. 1919. Shore Processes and Shoreline Development. New York. MARTIN, S. 1955. Raised Beaches and their Relations to Glacial Drifts on the East Coast of Ireland. Irish Geogr., 3, 87-93. MATLEY, C. A. & others. 1939. Summer Field Meeting in Western Lleyn. Proc. Geol. Ass., Lond., 50, 83-100. MITCHELL, G. F. 1951. The Pleistocene Period in Ireland. Dansk Geologisk Forenig, 12, 111-14. ROWLANDS, B. M. 1955. 'The Glacial and Post-Glacial Geomorphological evolution of the landforms of the Vale of Clwyd.' Unpublished M.A. thesis, University of Liverpool. STEPHENS, N. 1957. Some Observations on the 'Interglacial' Platform and the Early Post-Glacial Raised Beach on the East Coast of Ireland. Proc. Ri Irish Acad., 58, B, 129-49. - - - & F. M. Synge. 1958. A Quaternary Succession at Sutton, Co. Dublin. Proc. R. Irish Acad., 59, B, 19-27. WHITTOW, J. B. 1957. 'The Lleyn Peninsula, North Wales. A geomorphological study,' Unpublished Ph.D. thesis, University of Reading. WRIGHT, W. B. 1914. The Quaternary lee Age. l st ed. London. - - - , . 1937. The Quaternary lee Age. 2nd ed. London.