P.J. Ealey

PERIGLACIAL BEDROCK FEATURES OF THE LIZARD PENINSULA AND SURROUNDING AREA

P.J. E ALEY

Ealey, P.J. 2012. Periglacial bedrock features of the Lizard Peninsula and surrounding area. Geoscience in South-West England , 13 , 52-64.

Hitherto Quaternary studies in Cornwall have largely focussed on the coastal raised beaches, overlying ‘head’ and loess deposits. Abundant, widespread, and diverse periglacial features are described here from the Palaeozoic bedrock dominated coastal slopes and deeply incised valleys of the Lizard Peninsula and surrounding area. These include brecciation, rock-head deformation (overturning/terminal curvature), block fields/boulder lobes, rock fall/slides, and tors or tor-like features. This range of periglacial features is attributed to the diversity of the local geology and the variation in degree of weathering. The bedding, cleavage/foliation planes and joints, developed during and after the Variscan orogeny in the igneous rocks and metasediments, facilitated the observed bedrock fragmentation. Ground ice and segregated ice, as suggested by the common occurrence of frost susceptible loessic silt within the periglacially affected bedrock, are considered the main agents. The development of these periglacial features is attributed to Devensian/Weichselian periglaciation. Frost jacking beneath autochthonous block fields suggests evidence of permafrost conditions and the serpentinite and gabbro allochthonous block fields and lobes may be related to permafrost degradation after the Late Devensian glacial maximum. The ubiquity of brecciation and rock-head deformation in the area implies severe seasonal frost, if not permafrost. Cumulatively the periglacial features in the area have resulted in a clearly recognisable periglacial landscape that almost certainly extended beyond the current coastline and affected the development of the current offshore coastal zone.

8 Minster Fields, Manaccan, Helston, Cornwall, TR12 6JG, U.K. (E-mail: [email protected]).

Keywords: Lizard Peninsula, bedrock brecciation, rock-head deformation, block fields, tors, Crousa Gravels, shore platforms.

INTRODUCTION

Hitherto, Quaternary studies in Cornwall have largely peninsula (Figure 2). The Lizard Complex is largely formed of focussed on the coastal raised beaches, overlying head and basic and ultra-basic rocks. More than half (78 km 2) of the loess deposits (James, 1995; Campbell et al ., 1998). Little complex is dominated by peridotites, interpreted as upper attention has been paid to the areas, peripheral to these mantle rocks (Cook et al ., 1998) and now serpentinised. These localised coastal embayment depocentres, that are dominated serpentinites are massive but characterised by particularly well- by bedrock, formed during the Devonian/Carboniferous developed metre-scale cuboidal jointing (Flett, 1946). The Variscan orogeny, and now overlain by rare localised Tertiary virtual absence of a weathering zone, associated with the Lizard outliers (Walsh et al ., 1987) and laterally more extensive ‘head’ serpentinites, has been noted by many workers (Flett, 1946; and/or loess deposits. Periglacial features in these bedrock Coombe et al ., 1956; Staines, 1984; Ealey and James, 2008, areas are well developed and far more widespread than those 2011). The coarser grained gabbro, interpreted as a Devonian in the Quaternary coastal deposits (James, 2004) and yet to date oceanic crust overlying the serpentinised mantle peridotites, have received little systematic attention. Indeed French (2007) occupies a significant part (18 km 2) of the eastern complex and in a global context notes “ the occurrence of ground ice in is deeply weathered inland away from coastal promontories unconsolidated sediments is much better understood than in and headlands. Bounding the northern part of the Complex are bedrock ”. The purpose of this paper, therefore, is to describe amphibolites with a smaller component that also includes mica and discuss the periglacial bedrock features exposed on the schists in the western part of the Complex around Pen Olver Lizard Peninsula and surrounding area (Figure 1). (Figure 2). The amphibolites for the most part are characterised by well developed planar foliation and subsequent steep to vertical fractures producing typical castellated cliffs. Inland in BACKGROUND former marl pits there is evidence that the amphibolites were also deeply weathered. The term deep weathering is used Geology in the sense that exposures are dominated by weathering The geology of the Lizard Peninsula is dominated by the Grade V (Anon, 1995) where the rock has decomposed and/or Lizard Ophiolite Complex which was thrust northwards during disintegrated to soil, with the original rock structure still the Variscan orogeny onto a series of underlying slate discernible. dominated thrust sheets, now exposed on both sides of the

52 Periglacial bedrock features of the Lizard

Figure 1. Distribution of periglacial features on the Lizard Peninsula and surrounding area.

The Lizard Complex is bounded to the north by a series of Lizard Formation. This last formation is the most widespread in south-east dipping meta-sedimentary formations. These the area and in many places thicker than previously thought include from north to south the Mylor Slate, Porthscatho, Carne (Ealey and James, 2011). The thin loess soils on the Lizard and Roseland Breccia Formations (Leveridge and Shail, 2011). plateau have been dated as 15.9±3.2 ka BP (Wintle, 1981), but The slate content decreases in the same direction with this date may not be representative of the coastal thicker increasing intercalations of sandstone, conglomerate, and sequences. Indeed these may well be coeval with the St Loy olistoliths of differing rock types. A period of late Variscan (late Member at the type locality farther west on the Land’s End Carboniferous to early Permo-Triassic) extensional faulting Peninsula (Scourse, 1996; Campbell et al ., 1999) within which (Shail, 1999) followed the emplacement of the Lizard Complex loess intercalations and organic material, dated as and underlying slate dominated nappes. 29,120+1,690/-1,400 years BP, are observed. There is no record of the Mesozoic period in the area except possibly the Crousa Gravels, an anomalous quartz-rich deposit on the ultra-basic to basic rocks of the Lizard Complex. They Geomorphology occupy a small area (less than 1 km 2) and form a local The southern part of the Lizard Peninsula, underlain by the watershed on the gabbro outcrop (Figure 2). Containing rocks of the Lizard Complex, forms a topographic high. With accessory cassiterite and tourmaline minerals, they are present maximum heights of more than 100 m near its northern considered to be derived from the Carnmenellis Granite to the boundary this high is separated from the upland granite massifs north, but are currently presumed to be of Palaeogene age to the north by a topographically lower zone of predominantly (Bristow, 2004). Intense weathering, indicated by the Devonian slate-rich beds (Figure 2), commonly referred to as decomposed nature of the Crousa Gravels, possible local uplift the Meneage. The east-west trending Helford and adjacent Fal accompanied by faulting, and development of the current estuaries are entrenched into this slate dominated zone. A drainage followed their deposition. Due to the long erosional number of short streams, rarely more than 5 km long, drain the period, involving the whole of the Tertiary and Quaternary, it Lizard ‘plateau’ north into Helford Estuary and south, east and cannot therefore be assumed that all the weathering in the area west into the English Channel (Figure 2). The streams, often occurred only in the tropical climates associated with the with broad shallow valleys inland, become deeply incised Palaeogene. The weathering may well have continued under towards the coast due to uplift and/or coastal retreat more temperate conditions as Gerrard (1990) has noted (Westaway, 2010). These valleys have a long history predating elsewhere in SW Britain. Mid-Late Pleistocene high level sea stands (Ealey and James, Relatively laterally extensive Quaternary coastal deposits are 2011) but postdating the deposition of the Crousa Gravels, and preserved in the embayed coasts on the eastern side of the were an important geomorphological element in the periglacial Lizard peninsula and extend northwards into the outer parts of environment. They are frequently filled by ‘head’ deposits the Helford and Fal estuaries. Farther west on the peninsula which, derived from the steep coastal valley slopes, have been they become discontinuous and confined to the protected incised by the post-glacial streams. valley mouth coves of Kynance, Caerthillian, Poltesco and The cliffs bounding the peninsula rarely exceed 60 m in Kennack Sands. They reappear westwards as significant height. The steepest cliffs occur on the west facing slopes deposits at Praa Sands (Reid and Flett, 1907; Scourse, 1996) and where coastal erosion has been strongest. Elsewhere they are Marazion (Round, 1944). Typical coastal stratigraphic characterised by both convex and concave slope-over-wall sequences comprise raised beaches, overlain by ‘head’, profiles, the vertical walls of which vary in height from 30 m in assigned by Campbell et al . (1999) respectively to the Godrevy the more exposed western areas to 15 m or less in the more and St Loy members of the Penwith Formation, and loess of the sheltered sectors in the south and east (Malloch, 1971). The

53 P.J. Ealey

Figure 2. Map of Lizard Peninsula, showing simplified geology and localities mentioned in text.

coastal slopes above the basal walls are well developed and Cornwall the brecciation occurs in Palaeozoic igneous and referred to in historic landscape characterisation (Herring, 1998) metamorphosed sediments. Unlike Chalk and other Mesozoic as “ Coastal Rough Ground ”. These slope-over-wall or bevelled lithologies, porosity and permeability in these metamorphosed cliffs are a characteristic feature of South-West England and older rocks can be assumed to be negligible in the matrix and have long been regarded as an important signature of confined to open bedding and/or cleavage planes and fractures subaerial/periglacial weathering (Hawkins, 1827; Arber, 1949; (joints/faults). Brecciation is widespread throughout the area Savigear, 1962; Griggs and Trenhaile, 1994; Bird, 1998). (Figure 1) and affects all lithologies (Figures 3a-h) and not just During the Pleistocene lowered sea level maxima, the region slates (Figure 3a), which are known to be particularly frost can be inferred to have stood up to 100 m higher above sea susceptible and prone to ground ice segregation (Scourse, level than today. 1987). Virtually all examples observed underlie a cover of ‘head’ and/or loess in the area with one exception, involving a sub-aerially exposed serpentinite buttress (Figure 3b). PERIGLACIAL BEDROCK FEATURES The depth of brecciation itself varies from less than 1 m to These features have been found extensively on the coastal more than 2 m. The degree of brecciation generally decreases slopes and valley slopes in the area. Bedrock structures, downwards as Murton (1996) noted in porous Chalk brecciation, wedges and cracks and terminal brecciation, and is paralleled by an increase of clast size curvature/overturning, are commonly observed beneath ‘head’ (Figures 3a, c and d). The structural planes of weakness and and/or loess and in the exposed bedrock above the deposition interstices between individual brecciated clasts are often filled sites of the latter. with silt (Figures 3a, c, e and f), well known for its frost susceptibility (Ballantyne and Harris, 1994). In view of the widespread distribution of loess on the Lizard Peninsula Brecciation (Staines, 1984; Roberts, 1985) it is considered more likely that The non-generic term ‘brecciation’ used in this study has this silt is loess infiltrating downwards from the overlying been variously referred in the past to as the periglacially loess deposits rather than a comminution product of the host weathered, frost shattered zone or even eisrinde/ice rind by bedrock during brecciation (Catt and Staines, 1982). It is not Büdel (1982). Reported occurrences of brecciation per se uncommon for the uppermost part of the brecciated zone elsewhere in lowland Britain are largely restricted to Mesozoic to be cryoturbated and/or show vertically heaved clasts rocks (Murton, 1996). In the Lizard area and elsewhere in (Figures 3a and g).

54 Periglacial bedrock features of the Lizard

a b

cd

e f

g h

Figure 3. Examples of brecciation in ( a) slates of the Carne Formation at The Herra, Gillan Creek ( b) serpentinite buttress, south side of Gew Graze ( c) sandstone and slate beds of Carne Formation at Poldhu Cove ( d) Carne conglomerate at Flushing Cove, Gillan Creek (e) Roseland Breccia, north bank of Gillan Creek with centimetre to decimetre loess infill between bedding planes ( f) metabasites at Maen- du Point, Perranuthnoe ( g) Landewednack amphibolites, east of Pen Olver with upper frost heaved horizon ( h) serpentinite between Gew Graze and Kynance Cove with periglacial serrated surface. The hammer is 33 cm long and the scale pole is decimetre spaced.

55 P.J. Ealey The brecciation process, present in the area, is clearly attenuated beds merging downslope to form the basal layer of facilitated/guided by the structural fabric of the bedrock the overlying ‘head’. Following Gallop (1991) and Ballantyne whether it be bedding/cleavage/foliation planes or subsequent and Harris (1994), the downslope movement in these cross-cutting joints. In the slate dominated meta-sediment weathered granular rocks is attributed to frost creep, and frost formations the role of bedding/cleavage planes is a major factor creep and/or gelifluction at the distal end of the attenuated as is schistosity in the amphibolites. The angle of dip, relative beds. The slope parallel inflection point between the deformed to the ground surface and freezing front, may well be significant and intact bedrock does not seem to have been previously (Hall, 1986) and needs to be investigated further. The steep addressed in the literature. Possibly it represents the boundary cross-cutting joints are also of significance and facilitate between the active layer and permafrost table. fragmentation into rectilinear blocks (Figures 3c and f). In the Type 2 occurs in less weathered and more brittle lithologies. igneous rocks (serpentinite, gabbro and pre-orogenic volcanics This type of deformation can be further subdivided into two on of Floyd et al ., 1993), jointing is clearly the main structural the basis of the angle of bedrock bedding/cleavage dip relative discontinuity involved in the brecciation. This is particularly to the slope. well demonstrated in the serpentinites. Ealey and James (2008) Type 2A forms where the beds dip in the same direction as reported a serpentinite breccia from an abandoned quarry the slope but usually at a higher angle and this type involves a (Penruddock). Their Figure 3d clearly shows, beneath the complete dip reversal, i.e . ‘bedrock overturning’ (Figures 4c and breccia there, an uneven basal bedrock surface caused by the d). It is the most widespread form of terminal curvature in the sub-vertical jointing. Between Kynance Cove and Pengersick study area, and common in the Mylor Slate and Porthscatho Point, the serpentinite exposed at the surface close to the cliff Formations, but has even been observed by the writer in edge has a characteristic saw tooth appearance (Figure 3h), fractured granite on the valley sides of Watermill Cove, Isles of which is interpreted as the same type of basal bedrock Scilly. Significantly reported examples of Type 2 rock-head brecciated surface exhumed from beneath a thin loessic cover. deformation/terminal curvature elsewhere (FitzPatrick, 1987; The latter is seen locally exposed in the uppermost parts of the Ballantyne and Harris, 1994; Sauer et al ., 2002; Knight, 2009) are cliff in the area above sub-vertical jointed serpentinite on the all of Type 2A. The depth of deformation varies from 10 cm up northern side of Gew Graze. to 2 m. In its simplest form where the deformation is relatively Although the rocks of the Lizard are more deformed and thin, it comprises a unit of overturned slates, underlain by a metamorphosed, their observed brecciation overall is very sub-horizontal fracture plane (Figures 4c and d). Thicker zones similar in appearance to that described by Büdel (1982), Murton are more complex, exhibiting more than one sub-horizontal (1996) and French and Shur (2010), in less deformed, un- fracture plane, virtually identical to that illustrated by FitzPatrick metamorphosed Mesozoic rocks in current and past permafrost (1987, figure 13.2). In places (Figure 4e), physical displacement zones. The processes involved in the brecciation in the study appears to have occurred along these fractures. The area appear to be: opening of inherent sub-horizontal and sub- deformation reported by Cresswell (1983) from the Devonian vertical parting planes by prior weathering and/or ground ice, Trevose Slates at Constantine Bay, which some workers creation of voids, subsequent introduction of frost susceptible (Scourse, 1987; Ballantyne and Harris, 1994) cite to explain sediment and segregated ice, followed by consequent frost rock-head deformation mechanics, falls into the Type 2A heave. The occurrence of a more highly cryoturbated zone and category employed here. However, the structural regime in this vertically aligned clasts in the upper parts of the brecciated part of North Cornwall (Bedruthan Steps to Trevone Head) is zone at some localities may indicate the existence of an active characterised by a large flat-lying recumbent fold, ~8 km long, layer at the surface. dominated by sub-horizontal axial plane cleavage at marked angle to the bedding (Fig. 2 in Beese, 1982; Fig. 29 in Leveridge, 2011). It is these cleavage planes that have been involved in Rock-head deformation/terminal the rock-head deformation. This structural style is uncommon curvature/overturning or even unknown in West Cornwall and not applicable to the These structures, where the upper 1-2 m of bedrock is bent meta-sediments in the area surrounding the Lizard Complex over in the downslope direction, although often seen only in with their strongly dipping isoclinal bedding folds and axial temporary exposures, are surprisingly abundant in the study plane cleavage. In the study area the deformation seems to area and elsewhere in Cornwall. They are developed not only involve predominantly a progressive upwards increase in shear in Devonian meta–sediments but also in the meta-basic igneous strain. The upward decrease in strain style, which Cresswell rocks and later granites. They occur on the coastal slopes and (1983) also recognised in the Constantine Bay area, has not valley sides, and are particularly well developed around the been identified. This decrease in strain style represents plug- banks of the Helford Estuary. A review of the literature coupled like flow (Cresswell, 1983; Ballantyne and Harris, 1994) which with the observations in the study area has identified two Matsuoka (2001) considers evidence of two-sided frost action, principal types of rock-head deformation, based on differences i.e . downward freezing of the active layer and upward freezing in weathering grade of the host bedrock. from permafrost below. Suggested mechanisms in the Lizard Type 1 occurs in deeply weathered rocks with granular area involve initial frost/ice wedging (Worsley, 2007, figure 2) texture, usually of igneous origin (Ballantyne and Harris, 1994; of the dipping beds via bedding/cleavage planes, frost heaving Bristow, 2004; Knight, 2005), and involves downslope of widened beds, coupled with an increasing downslope bending/movement of the decomposed bedrock, associated gravitational component, leading to rotation and stresses with downslope thinning of individual layers. However, in the causing development of surface parallel fractures that cut across study area excellent examples occur also in the Menaver the bedding/cleavage planes. Conglomerate Member (Figure 4a) of the Carne Formation and Type 2B occurs where the beds dip into the slope in an the Mylor Slate Formation (Figure 4b). Close examination of opposite direction, which is seen also in many reported the 80 m long Mylor Slate outcrop indicates that the weathered examples of Type 1. Although there is no shortage of localities nature of the slates is the result of intense brecciation (Figure with the requisite structural setting, examples of Type 2B are 4b, insert). The conglomerate example is similar in texture to extremely rare in the area (Figure 4f). This sparsity appears to the weathered granular granite examples in the literature, being be reflected in the literature reviewed and may indicate that this associated with a deeply weathered fault zone that has been is more than a coincidence. Whether this rarity reflects a poor preferentially eroded out by the sea. The steps involved in the preservation potential or some other cause needs further development of Type 1 deformation appear to be: firstly, investigation. It is possible that the downslope orientation of bedrock weathering either prior to or during periglacial cold the beds at the ground surface facilitates downslope movement climate conditions; and secondly, individual weathered bed and/or incorporation into the overlying solifluction deposits. movement downslope accompanied by bed thickness attenuation. The Mylor Slate example clearly shows the

56 Periglacial bedrock features of the Lizard

b

a

c d

ef

Figure 4. Rock-head deformation. ( a) Type 1 terminal curvature in weathered granular conglomerate of Carne Formation, Flushing, Gillan Creek. ( b) Type 1 terminal curvature in brecciated (see insert), Mylor Slate Formation, Falmouth road cutting. ( c) Type 2a terminal curvature in slates of Carne Formation, Poldhu Cove. ( d) Type 2a terminal curvature in Porthscatho Formation above Gunwalloe Fishing Cove. ( e) Slippage plane in periglacially deformed Porthscatho Formation, Helford Passage. Note sandstone beds above plane, juxtaposed against slates below. ( f) Type 2b terminal curvature in Porthscatho beds, Helford River Sailing Club. Hammer 33 cm long. Scale rod marked in decimetres.

57 P.J. Ealey Block fields/lobes On the Lizard Peninsula block fields are best developed in areas underlain by gabbro and serpentinite (Figure 1). The best examples of gabbroic block fields occur seaward of the palaeo- cliff between Coverack and Polcries where they overlie a well developed coastal head terrace, known as Lowland Point. These block fields are notable for their relatively large size, extending some 200 m seaward with comparable widths. Lying towards the top of the well developed head terrace, they comprise blocks, varying from equi-dimensional to tabular in shape and from 1-5 m in dimension. They are embedded in a matrix of loess (Scott et al ., 2011). Their dimensions and density are best appreciated on the foreshore where the head terrace they overlie has been eroded by the sea and they lie directly on the foreshore (Figure 5). Landward of the low head terrace cliffs they are not so obvious partly because they are currently obscured by vegetation, and partly because they were in the past used to form Bronze Age field boundaries, and Figure 5. Gabbroic (metre scale) boulders on the foreshore perhaps more importantly were dynamited and extensively seaward of eroded blockfield at Polcries. exported in the 19th Century, prior to the advent of the coastal quarries in the area. They are interpreted as being derived partly through frost wedging of the palaeo-cliff and partly through solifluction of core stones from the deeply weathered coastal slope behind. They are therefore clearly allochthonous (Ballantyne and Harris, 1994). The only other known example of a gabbroic block field in a current coastal setting occurs on the south-west side of the Lankidden/Carrick Luz promontory. Here a well known gabbroic dyke, characterised by ductile deformation of the rock fabric, Variscan low angle shear zones and post Variscan faulting (Gibbons and Thompson, 1991; Floyd et al ., 1993), dominates the promontory. This block field, comprising blocks up to a metre in size (Figure 6), is developed on the south-facing coastal slope and appears to have formed in situ . With marked structural discontinuities, a mechanism of frost jacking (Michaud and Dionne, 1987; Worsley, 2007) is envisaged. Inland extensive gabbroic block or clitter fields occur on the south- and east-facing slopes of Crousa Common in the area of the Crousa Gravels (Figure 2). Referred to as Figure 6. Gabbroic blockfield at Lankidden with fractured gabbro gabbroic debris or greystones by Flett and Hill (1912) and bedrock below. crusairs by Flett (1946), probably as a result of a misinterpretation of the local pronunciation of crousa (croosa), these block fields are largely obscured by heath vegetation. Those at Maindale, where the ground slopes south-eastwards away from Crousa Gravel outcrop, are better exposed. With similar boulder dimensions to those previously described on the coast, they are considered to have formed by solifluction of core stones and frost wedging of the underlying bedrock exposed by post Crousa Gravel erosion. Block fields have been reported on the extensive concave coastal slope between Coverack and Pedn Boar, underlain by serpentinite (Ealey and James, 2008, figure 3f; Ealey and James, 2011, figure 4). These block fields are probably better described as boulder lobes as they have well developed treads (up to 20 m wide and 35 m long) and risers. They are backed landwards and laterally by bedrock scarps and tor-like features from which they are interpreted as having originated by frost wedging. Their setting is identical to amphibolite block fields described from Start Point in Devon by Mottershead (1971). Previously unreported serpentinite boulder lobes occur on the Figure 7. Serpentinite boulder lobe with turf exfoliated riser (>1m sides of the deeply incised valleys at Kynance Cove, and on the high). The brecciated buttress behind is shown in Figure 3b. coastal slopes to the north. A remarkable series of stone lobes occurs on the steep south side of the Gew Graze coastal Rock falls/slides re-entrant. Comprising blocks generally <1 m, they are Evidence for lateral rock falls from the cliffs bounding local characterised by well developed treads and risers (Figure 7) and head filled coastal indentations is readily apparent. Essentially can be directly related to the brecciation of local buttresses this setting is identical to that of Facies B, described in a granitic upslope (Figure 3b) on this former valley side. On the slopes breccia facies model for the Isles of Scilly and Cornwall uninterrupted by buttresses, successions of lobes are arranged (Scourse, 1987, figure 21.6). The presence of basal raised longitudinally downslope (Figure 8), similar to those described beaches indicates that the bedrock slopes bounding these local by Quinn (1987) in south-west Ireland. head filled coastal indentations were initially sediment free and that rock fall or rockslide blocks were incorporated from the bedrock slopes above during the infill stage (Knight, 2005). Particularly good examples on the Lizard occur on serpentinite

58 Periglacial bedrock features of the Lizard

Figure 8. Downslope series of boulder lobes adjacent to previous lobe at Gew Graze. at Downas Cove and Pengersick Zawn (Figure 1) and in the hornblende schists north of Porthoustock. An example has been previously illustrated at Porthoustock Cove (Ealey and James, 2011, figure 8), which was attributed to freeze thaw activity within the bedrock of the lateral palaeoslopes bounding the narrow head filled embayments in the area. Subsequent field work has indicated more examples farther north around Pencra Head where even larger foliation parallel hornblende slabs (Figure 9) have slid from lateral bedrock buttresses, indicating that not only vertical but also sub-horizontal fractures, parallel to the foliation, were in the involved in creating the bedrock instability. Smaller wedge-like accumulations of head are seen in the cliffs bounding the hornblende schists well above the raised shore platforms and associated raised beaches. Here the steeply dipping joint or fault zones have been widened by erosion and filled with head. During the infill stage angular boulders, dislodged from the Figure 10. Head-filled wedge in eroded fault zone, located in upper bedrock wedge sides, were incorporated into the head amphibolite cliffs near Bass Point coast watch station, east of Pen Olver. (Figure 10). An ice wedge cast (Figure 11) is exposed within the loessic fill of a related joint controlled cleft on the side of the gabbroic Leggan Head [SW 806 210] between Nare Point and Pencra Head, possibly indicating the presence of permafrost. Periods of enhanced freeze-thaw cycles and frost heave are generally accepted as a mechanism for widening fractures and prising blocks from intact bedrock. Their final detachment and movement downslope, however, may be triggered by thunderstorms/earthquakes and/or increased precipitation (Knight, 2005).

Figure 9. Large amphibolite slabs prised from lateral bedrock cliff within head deposit. Schistosity parallel to upper and lower surfaces Figure 11. Small wedge, filled with loessic head on south side of Leggan of slabs. Hammer is 33 cm in length. Point. Note frost wedge, developed in infill. Hammer is 33 cm in length.

59 P.J. Ealey Tors and tor-like features serpentinites (Figure 13a) with their lack of significant chemical weathering, pronounced cuboidal jointing, and local adjacent The term ‘tor’ is used here for features that have resulted clitter are almost certainly of periglacial cold climate origin, an from non-uniform weathering of bedrock in pre- or interglacial interpretation supported by the frost shattered buttress (Figure periods followed by removal of the softer more deeply 3b) with an associated stone lobe below on the south side of weathered material by gelifluction under Pleistocene periglacial Gew Graze (Figure 7). The mylonitised amphibolites are conditions (Linton,1955). The term ‘tor-like’ (Ball and Goodier, deeply weathered inland as seen in the legacy of former ‘marl’ 1970) is used for other craggy features apparently resulting from pits, but on the well developed coastal slopes, cliffs, and shore a combination of frost-riving and mass movement under platforms these more severe weathered zones have obviously Pleistocene periglacial conditions only (Palmer and Nielson, been completely eroded. Tor-like features are not uncommon 1962). These terms, though admittedly not widely employed, between Church and Caerthillian coves (Figure 13b) and were are particularly useful in the study area, as the tors and tor-like noted by Bird (1998). The presence of brecciation (Figure 3g) features have different morphological expression (rounded and local clitter (Figure 13b) suggest an active periglacial versus angular) and involve different lithologies with different environment. weathering characteristics. Both features are common on the coastal slopes and on the upper slopes of the deeply incised valleys near the coast. Inland summit features on the Lizard PERIGLACIAL STRUCTURES WITHIN TERTIARY AND plateau have yet to be found, although they may well have QUATERNARY SEDIMENTARY SEQUENCES existed in earlier Tertiary and/or Quaternary times. Tors appear to be restricted to areas underlain by gabbro, Brecciation and rock head deformation are closely related where their two stage origin can be demonstrated between with the overlying or downslope unconsolidated ‘head’ Porthoustock and Coverack as a result of the local coastal deposits as these are the repository for their frost-shattered end quarrying (Figures 12a and b). They also occur on the upper products. The principal indicators of past cold phase climatic slopes of the valley in the Gwenter area (Figure 2). They have conditions (Table 2 in Renssen and Vandenberghe, 2003) are manifestly resulted from non-uniform weathering of the commonly based elsewhere on better understood periglacial gabbroic bedrock in pre- glacial or interglacial periods followed structures in unconsolidated sediments (French, 2007). by removal of the softer more deeply weathered material by Therefore the spatial and stratigraphic distribution of these gelifluction under Pleistocene periglacial conditions (Linton, structures within the unconsolidated Quaternary sediments of 1955). Tor-like features are common on the coastal slopes and the Lizard Peninsula and surrounding areas are briefly reviewed deeply incised coastal valleys in areas underlain by the to provide a framework to assess the palaeoclimatic significance serpentinite, hornblende and mica schists. These features in the of the periglacial bedrock features discussed in this paper.

a b

Figure 12. (a) Incipient tor revealed during stripping of weathered zone at Dean Quarry. ( b) Tor on coastal slope above palaeo-cliffs, revealed by solifluction of weathered zone and redeposition as head on Lowland Point below.

a b

Figure 13. (a) Serpentinite tor-like feature on Chynnals Point, south of Coverack. ( b) Tor-like feature in Landewednack amphibolites (hornblende schists) on east side of Pen Olver.

60 Periglacial bedrock features of the Lizard Quaternary sediments on the Lizard Peninsula have reported possible fragipan horizons in the type section of the Goonhilly Member of the Lizard (Loess) Formation (Roberts, Probable cryoturbation structures and the presence of 1985; Campbell et al ., 1999) and in the impermeable lower, polygon structures have long been reported from the area denser loess unit at Polcries underlying the bog iron deposit, underlain by the Tertiary Crousa Gravels (Flett and Hill, 1912; described by Scott et al . (2011). However subsequent Coombe et al ., 1956; Ealey et al ., 1999), unfortunately with microscopic examination of pedological soil blocks was unable virtually no detailed documentation. Recently British to confirm the expected characteristics of ground ice Geological Survey archive photos, taken in 1907 and showing segregation laminations (H.C.L. James pers. comm.). the pit described by Flett and Hill (1912), have been obtained. At this time the pit was at least 3 m deep and the contortions/folding, described by Flett and Hill (1912) as 8 feet Quaternary sediments elsewhere in West Cornwall (i.e . >2 m) deep and underlying horizontal strata, can be clearly Within the better known Quaternary sections elsewhere seen (Figure 14) beneath an upper layer of loess. They (Scourse, 1996), periglacial structures have been recognised at resemble the involution/plumes reported by James (2004) and different stratigraphic levels. At Godrevy on the north coast, are almost certainly related to the sorted patterned ground, ice/sand wedge casts, although apparently not formally reported by Coombe et al . (1956). The large size of such reported, have long been known to occur at the base of the involutions has been taken as evidence of former permafrost by head in the littoral sands. Here in the uppermost 2 m of head some workers (Vandenberghe, 1988; Van Vliet-Lanoë, 1988; vertical aligned stones and plumes are also particularly well Renssen and Vandenberghe, 2003). Since the Lizard loess as developed (James, 2004). Both involutions and probably well as the Tertiary Crousa Gravels is involved in these Lizard related plumes occur in horizons, 1 m or more thick, at Par (St based structures, they must be of Quaternary and probably Late Austell), Pendower, Towan, and Marazion in the upper half of Devensian age. Involutions have also been reported (Ealey and the head sections. Ice wedge casts within the ‘head’ have also James, 2011) from Nare Point immediately above the raised been reported from Par (James, 2004), Towan (James, 1981) beach and towards the top of the laterally extensive section and Tregunna (Clarke, 1973; Scourse, 1987) in the Camel ‘head’ sequence there. The upper examples, involving slate Estuary. clast dominated head beds, are plume like and of metre scale rather than the decimetre scale seen in the basal head here. Vertically inclined clasts, up to 1 m in size, occur at a number DISCUSSION AND CONCLUSIONS of localities in metre thick horizons, usually in the upper parts of sections, where the matrix is dominated by loessic silt. The review of periglacial severe cold phase indicators (ice- Similarly inclined smaller clasts (centimetre scale) have been wedge casts, large scale involutions, and metre thick heaved observed at the base of the loess at Countybridge (Ealey and vertical stone horizons) in the unconsolidated James, 2008) and Jangye-ryn. Ealey and James (2008, 2011) Tertiary/Pleistocene sediments indicates that these indicators are particularly evident in the upper part of the ‘head’ and loess on the Lizard and farther afield suggesting the presence of at least sporadic and probably discontinuous permafrost (James 2004) during this period, here tentatively assigned to the Late Devensian glacial maximum. This period coincides with the southward advance of the Irish Sea ice sheet onto the northern Isles of Scilly (Scourse, 1991) and the recently proposed possible glaciation of Dartmoor (Evans et al ., 2012). There is some evidence, limited to date, to indicate that there was also at least one other earlier cold phase following the deposition of the raised beaches, on both the north and south coasts of Cornwall. Due to the paucity of age control for the Pleistocene deposits in West Cornwall its chronostratigraphic position within the Devensian is unclear. In previous discussions concerning the presence or absence of permafrost in South-West England (see James, 2004) the role of loess may have been underestimated. Loess is widespread in the study area (Catt and Staines, 1982; Roberts, 1985; Ealey and James, 2011) and is clearly present within the brecciated bedrock. Renssen and Vandenberghe (2003) indicate the mean annual air temperature (MAAT) requisite for permafrost is less (-4°C or lower) in sediments dominated by loess/silt than it is for sand/gravel deposits (MAAT –8°C or lower). This would imply, for instance, that the ice wedge in loessic ‘head’, shown in Figure 11, could have developed when the MAAT was only -4°C. The evidence for periglacial activity in the bedrock of the area is far more abundant than that in the unconsolidated Quaternary sediments (Figure 1). The coastal allochthonous block fields, overlying the ‘head’ terraces and bounded by landward cliffs or cliff top scarps are clearly some of the youngest Pleistocene deposits in the area. It is postulated that they resulted from the climate amelioration following the severe cold period of the Late Devensian glacial maximum. Recent work on permafrost (Gruber and Haeberli, 2007, Harris et al ., 2009) in steep mountainous areas suggests that permafrost degradation is an important factor in bedrock slope destabilisation. Indeed the presence of large involutions, Figure 14. British Geological Survey archive photograph involving gravels, and ice wedge casts in the study area at this (P200621) of Crousa Gravel pit, taken on 1st January 1907, and stratigraphic level suggests the MAAT may have been -8°C or simplified line drawing. even colder during the Last Glacial Maximum. Other

61 P.J. Ealey indications of permafrost in the area are frost jacking in the into the former higher emergent shore platforms and lowered autochthonous block field at Lankidden (Figure 6), and possibly them by up to 2 m within the last 5000 years. At Nare Point beneath thin ‘head’/loess sections overlying jointed gabbros and Flushing the >1 m thick brecciated zone above HWM has and metabasic rocks elsewhere (Figure 3f). Frost jacking has been eroded, resulting in a similar amount of lowering and an previously only been reported in the literature from bare former apparent shore platform (Figure 15). It is further suggested that glaciated surfaces, probably because it is more readily the current narrow shore platforms around the inner estuaries identifiable there, and appears to be restricted to locations with in the area have resulted from Holocene marine erosion of the at least near continuous permafrost present (Dionne, 1983; brecciated and rock-head deformation zones (Figure 1), visible Worsley, 2007). in the low cliffs immediately landward of these platforms. Rock-head deformation and brecciation and are particularly widespread in the study area and are almost certainly related in origin in that together they form a widespread frost shattered zone or eisrinde/ice rind of Büdel (1982). Indeed in one exposure both occur within 20 m of each other (Figures 3d and 4a). Their occurrence on the coastal and valley slopes above the thicker ‘head’/loess deposits below suggests that their development may have been contemporaneous with the deposition of the upper levels of the Quaternary unconsolidated sediments with their probable indications of permafrost. The bedrock brecciation is tentatively attributed to repeated ice segregation near the top of a permafrost zone (Murton, 1996) and/or to repeated freeze/thaw in the active layer above. The field evidence suggests that the rock-head deformation in deeply weathered bedrock (Type 1) developed in the active layer. Where the bedrock is less weathered and deformation takes the form of complete brittle downslope overturning (Type 2A), active layer gelifluction (Sauer et al ., 2002), two sided freezing with an active layer above a permafrost table (Cresswell, 1983) and downslope movement solely in the permafrost zone (FitzPatrick, 1987) have been Figure 15. Platform developed as a result of coastal erosion of cited as mechanisms. periglacially brecciated zone, Nare Point coast watch station. References cited in this study ( e.g . Murton, 1996; French and Shur, 2010; Harris et al ., 2009) indicate that there is increasing global interest in the occurrence of ground ice in bedrock and REFERENCES its effects. This study has shown that periglacial rock-head deformation, brecciation and bedrock slope stability are ANON. 1995. The description and classification of weathered rocks for influenced by fracture/joint set geometry, angle of dip of the engineering purposes. Geological Society Engineering Group Working Party cleavage to relative the bedding, and angle of bedding/cleavage Report. Quarterly Journal of Engineering Geology, 28 , 207-242. dip relative to the ground surface. The angle of bedrock dip, ARBER, M. 1949. Cliff profiles of Devon and Cornwall. 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