Tors in glaciated lands

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Du continent au bassin versant Théories et pratiques en géographie physique

Hommage au Professeur Alain Godard

From Continent to Catchment Theories and Practices in Physical Geography

A tribute to Professor Alain Godard

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Du continent au bassin versant. Théories et pratiques en géographie physique (Hommage au Professeur Alain Godard) 2007, Presses Universitaires Blaise-Pascal, ISBN - 978-2-84516-335-5

The signi¿cance of tors in glaciated lands: a view from the British Isles

ADRIAN M. HALL 1, DAVID E. SUGDEN 1

A tor is a residual, wart-like mass of bare bedrock rising conspicuously above its surroundings from a basal rock platform generally buried by regolith. Tors are landforms of differential weathering and erosion which occur in all climatic zones on Earth (Godard, 1966). Only in formerly and presently glaciated regions are tors rare and here these often delicate rock structures have been largely swept away by glacial erosion (Godard, 1982). Where tors do survive, their significance has prompted debates which stretch back over 50 years relating to ice limits, glacier basal thermal regime, depths of glacial erosion and the origins of the tors themselves (Godard, 1961, 1965, 1966; Linton, 1949, 1950, 1952, 1955). This paper examines the significance of tors in glaciated lands, with particular reference to the British Isles (fig. 1). 1. Tor location in glaciated lands At the limits of Pleistocene glaciation, tors have been used as an indicator of former ice margins. On suitable rock types, tors may be widespread outside glacial limits but ruined or erased

within (Daveau, 1971; Scourse, 1987; Veyret, 1978). In landscapes of glacial erosion tors are absent from zones of areal scouring but may be present in landscapes of alpine glaciation (Small et al., 1997), selective linear erosion (Sugden, 1968; Sugden and Watts, 1977) and in areas of little or no erosion (André, 2004; Stroeven et al., 2002). In areas of restricted or alpine glaciation, tors are often located on ridges and summits free of glacier ice today or in the past. The tors may take the form of delicate pinnacles or gendarmes rising from narrow ridges. In the British Isles, the finest examples occur on the ridges of the Cuillin Hills of Skye and Glyder Fach and Fawr in North Wales (Ballantyne and Harris, 1994). In areas of selective linear glacial erosion, tors are typically found on plateaux and summit surfaces and may occur in association with extensive debris mantles. Plateaux may carry evidence of former ice cover from glacial erratics. Tors are generally absent from valley sides and floors in these landscapes, where landforms of glacial erosion instead become widespread (Hall and Phillips, 2006a). Where clear evidence exists of former ice cover,

1. School of GeoSciences, University of Edinburgh, U.K. ([email protected] ; [email protected]).

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the preservation of tors and regolith at high elevations has been attributed to former cold-based ice (Sugden, 1968). Where evidence is absent or equivocal, doubts remain over the existence and thickness of former ice covers. Such landscapes exist in Baffin Island in the Canadian Arctic (Briner et al., 2003) and also include isolated hill masses around which the British ice sheet streamed at the Last Glacial Maximum, for example the hills of northern Arran (Godard, 1969) and Ben Loyal in northern Scotland (Godard, 1965) and the Mourne Mountains in Ireland (Le Cœur, 1980 - fig. 1). In all these cases, however, the existence of tors has sometimes been attributed to these areas standing as nunataks above the last ice sheet (Boyer and Pheasant, 1974; Godard, 1965; Ives, 1958, 1966). In areas of little or no glacial erosion, tors again occur in association with regolith that has survived the passage of overriding ice. In Scandinavia, tors, together with boulder fields, patterned ground and old morainic deposits have been recognized as evidence of cold-based ice patches beneath the last ice sheet (Hättestrand and Stroeven, 2002; Kleman and Borgström, 1994). Tors may occur in association with saprolites which predate at least several phases of glaciation. In Scotland, such lowland tors occur only in the Cabrach, north-east Scotland, where they are being exhumed from deeply weathered gabbro (Basham, 1974). The scarcity of tors in the Buchan lowlands is noteworthy, as this area carries extensive and deep weathering mantles and remnants of Tertiary gravels (Hall and Sugden, 1987). In the lowlands of County Galway, western Ireland, a palaeosol is developed on saprolites adjacent to tors has yielded pollen of Pliocene affinities. This pre-Pleistocene landsurface is buried by soliflucted granite debris and tills (Coxon, 2001).

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2. Tor form All tors are products of long term differential weathering and erosion and develop in rocks which are more resistant than those in the immediate surroundings. In the British Isles, tors are most common in orthogonally jointed rocks, notably granite, where local rock resistance is largely determined by joint density. Tors are also developed in many other types of rock, including conglomerate, sandstone, basalt and schist (Ballantyne and Harris, 1994). In former glaciated regions, tors range in height from 1 m to more than 25 m, with most tors less than 10 m high. Tor form in plan and section, together with individual block sizes are usually controlled by the spacing of vertical and horizontal joints and fractures. Large tors often have a core of blocks 1-100 m3 in size which develop from zones of unusually massive rock. These monolithic structures rise from summits and spurs to dominate the skyline. The rounded shapes of granite boulders have been linked to an origin as core stones within former deep saprolites (Linton, 1955) but the rounding probably relates more to the propensity of many coarse-grained igneous rock types for granular disintegration (Ballantyne and Harris, 1994). Other rock types, such as quartzite and schist, give smaller, more angular outlines to tors and boulders. 3. Processes of tor formation Linton (1955) emphasised the primacy of deep chemical weathering in tor formation. His now classic model was developed primarily on Dartmoor but was earlier applied in formerly glaciated regions (Linton, 1949, 1950, 1952). Tors were prepared for exhumation by prolonged weathering under humid subtropical conditions in the Neogene. Deep saprolites formed, leaving residual core stones in zones of widelyspaced joints. Stripping of the saprolites by periglacial slope processes took place during the Pleisto-

THE SIGNIFICANCE OF TORS IN GLACIATED LANDS: A VIEX FROM THE BRITISH ISLES

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Fig. 1. Location of major tor groups in the British Isles. England. 1 – Isles of Scilly (Granite): 2 – Dartmoor (Granite); 3 – Exmoor (Sandstone); 4 – Weald (Sandstone); 5 –Charnwood Forest (Granite); 6 – Tabular Hills (Sandstone); 7 – Derbyshire (Dolomite); 8 – Southern Pennines (Gritstone); 9 – Stiperstones (Quartzite); 10 – Cheviot Hills (Granite). Wales. 11 – Central Wales (Various); 12 – Pembrokeshire (Rhyolite); 13 – Preseli Hills (Dolerite); 13a – Glyders (Schist). Scotland. 14 – Northern Arran (Granite); 15 – Ochil Hills (Andesite); 16 – West Lomond (Sandstone); 17 – Clachnaben (Granite); 18 – Bennachie (Granite); 19 – Ben Rinnes (Granite); 20 – Cairngorms (Granite); 21 – Southern Caithness (Conglomerate); 22 – Ben Loyal (Syenite); 23 – Trotternish (Basalt). Ireland. 24 – Mourne Mountains (Granite); 25 – Slieve League (Quartzite); 26 – Leinster (Granite); 27 – Wicklow (Granite); 28 – Cnoc Mordain, Co. Galway (Granite) References listed in BALLANTYNE and HARRIS (1994, p. 178).

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cene, leaving the unweathered blocks as tors. This two-stage model has been applied successfully in many extra-glacial regions (Godard, 1982; Jahn, 1974), and indeed derived in part from earlier observations in those areas (Branner, 1896), but soon came under criticism in its type area. Two elements, in particular, have significance for the origins of tors in former glaciated regions. Firstly, most tors on Dartmoor are not associated spatially with deep saprolites (Palmer and Neilson, 1962). Instead, tors are surrounded by boulder-strewn slopes where regolith thicknesses are seldom deeper than 2 m and deep saprolites are confined to valley floors (Eden and Green, 1971). Secondly, the geochemical and textural characteristics of the regolith and saprolites, with low fines contents, preservation of feldspars and micas and development of clay minerals such as kaolinite and gibbsite formed at the early stages of granite weathering (Green and Eden, 1971), are not consistent with chemical weathering under warm, humid environments (Eden and Green, 1971). This gruss weathering type is widespread in former glaciated regions (Hall, 1985) and developed under environments similar to those of the present (Migoń, 1997). In sharp contrast, Palmer and Neilson (1962) focussed on processes which operated on Dartmoor under the periglacial phases of the Pleistocene. Intense frost weathering produced large volumes of coarse clastic debris which was carried away from tor margins by solifluction to give « clitter », the boulder spreads found around the fringes of many Dartmoor tors (Gerrard, 1988; Waters, 1965). No substantial role was recognised for chemical weathering in the formation of tors, a view which ignores the widespread presence of weathering pits on the tors (Ormerod, 1859) and the evidence from clay minerals and groundwater solutes for former and contemporary chemical alteration of the Dartmoor regolith (Williams et al., 1986).

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Far less attention has been given to the emergence of the Dartmoor tors through the repeated formation and stripping of thin regolith similar in properties to that which exists today. The granite regolith comprises boulders set within a matrix of granular gruss (Waters, 1965) and is typical of the characteristics of debris developed on coarse-grained granites in other periglacial areas (Ballantyne and Harris, 1994). The debris mantle is of complex origin, with elements that derive from multiple phases of chemical weathering, from stress release and from frost weathering. The occurrence of chemical weathering under present and former interglacial conditions is shown by Holocene soil formation, the visible signs of alteration on biotite and feldspar, and solute loads in springs (Williams et al., 1986). In addition, the block streams around many tors point to the significance of past phases of macrogelivation and solifluction (Gerrard, 1988). Textures indicative of mechanical breakage on quartz grains (Doornkamp, 1974) may indicate that the gruss matrix of the debris mantle has a significant component derived from microgelivation, frost creep and inter-granular stress release. Although the residence time of regolith on the Dartmoor slopes is currently largely unknown (Croot and Griffiths, 2001), studies in other humid, unglaciated granite terrains indicate rates of regolith formation of > 5 m/Ma (Bierman, 1994; Pavich, 1989; Summerfield, 1991). Such rates, when combined with evidence of significant Holocene chemical weathering and Late Devensian frost weathering and mass movement (Murton and Lautridou, 2003), suggest that the 0-2 m of regolith found on Dartmoor has formed mainly in the Late Pleistocene. By implication, the thin regolith is largely renewed and removed during each interglacial-glacial cycle, as in other terrains south of the Pleistocene glacial limits (Braun, 1989). The exhumation of tors from deep saprolites, as envisaged by Linton (1955) can be identified mainly in areas lying towards the southern and lower limits of glaciation in the Northern Hemis-


phere in the Serra da Estrella, Portugal (Daveau, 1971), the Massif Central, France (Godard, 1982; Veyret, 1978) and the Colorado Front Range. Tors are found associated with deep sandy saprolites outside glacial limits. Inside the limits tors are demolished or reduced to roots and saprolites are thinned, or in patches or removed altogether (Godard, 1982). Further north, the association of tors with deep saprolites has been demonstrated in only a few lowland areas and here the saprolites may be inherited from the Mesozoic or even Cambrian weathering covers and have been exhumed from beneath cover rocks by Pleistocene glacial erosion (Lidmar-Bergström, 1982; Söderman et al., 1983). There are many indications that tors in formerly-glaciated mid latitudes and the polar regions are forming under contemporary process of weathering and mass wasting (Watts, 1983). In the Cairngorms, the presence of notches around the bases of tors, together with the existence of clay minerals and partially altered primary minerals in the mountain top soils (Mellor and Wilson, 1989) indicates significant chemical weathering. This is consistent with the high solute loads carried by adjacent springs (Soulsby et al., 1998). In polar environments, tors may rise from platforms with very little debris cover or from thick-debris mantled slopes (Yuengling, 1998). Evidence from cosmogenic isotope analysis shows that tors on uplands protruding through the West Antarctic Ice Sheet in Marie Byrd Land, Antarctica, evolve under interglacial conditions similar to that of the present day (Sugden et al., 2005). The implication is that weathering and mass wasting proceeds in an Antarctic periglacial climate. We conclude that tors in glaciated lands form under the changing environments of the Pleistocene. Tors are products of non-glacial processes and tor formation is shut down during periods of ice cover. Tor development resumes under icefree conditions and the nature of these conditions varies widely across formerly glaciated regions.

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At the southern limits of Pleistocene glaciation, such as in the Scilly Isles, the contemporary climate is temperate and humid and Pleistocene temperate minima were associated with cold and humid periglacial environments with an absence of permafrost (Scourse, 1987). In mountain areas of Scotland and Scandinavia, present interglacial conditions are cold and humid and tors develop entirely under periglacial environments, probably including former periods of permafrost (Ballantyne and Harris, 1994). In the Arctic and Antarctic and on high latitude nunataks, tors may evolve in dry and cold polar climates. The diversity of environments means that the processes of tor formation vary both spatially and temporally, requiring that specific evidence of morphogenic processes must be sought at each tor locality. 4. Tors as indicators of former ice covers A long-established view is that tors cannot survive over-riding by glacier ice: Localities where tors are now found cannot have been overrun by actively streaming ice. (Linton, 1952)

Alternatively, tors may survive if the basal ice within the glacier remains cold-based (Sugden, 1968). Where no sliding of ice takes place across the glacier bed then delicate features including tors and regolith may be preserved (Rapp, 1996). In this case, the tors provide no information on the limits of former glaciers. It is important for questions of former ice limits to be able to distinguish between these different hypotheses and this requires identification of criteria that indicate former ice covers at tor sites. A range of characteristics may be used to establish that tors and their immediate surroundings have or have not been covered by glacier ice (Hall and Phillips, 2006a). Only a few characteristics can be accepted as unequivocal evidence of former glaciation. The presence of glacial erratics on tors, for example, requires glacial

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transport and hence ice cover. Other criteria are less certain and alternative interpretations of the observed characteristics are possible which do not require ice cover. Often it is the combination of characteristics, perhaps from a number of adjacent localities that allows former covers of glacier ice to be inferred. Evidence of glacial transport to the tor site is provided by glacial erratics and by perched blocks. Erratics may relate to transport after or at the LGM (Briner et al., 2003; Sugden, 1968) or to earlier glacial phases (André, 2004). Glacial erratics will be absent from plateaux of monolithic geology formerly covered by independent ice domes. The highest summits or ridges in an area may also lack erratics because (i) they controlled the location of regional or local ice sheds or (ii) a massif may cause ice to diverge and flow around it or (iii) no higher ground existed up-glacier to act as a source of debris. Perched blocks may be identified as glacial in origin in areas of uniform lithology by slight differences in lithology (Hall and Phillips, 2006a), position on the tor (André, 2004) or by the lack of weathering compared to tor surfaces (Briner et al., 2003).

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this transformation (Rapp, 1996; André, 2004; Hall and Phillips, 2006a; Phillips et al., 2006). The first signs of glacial modification relate to loss of blocks from the summit and margins of tors. Further erosion involves the loss of superstructure until only the most massive parts of the tor remain. These may take the form of stumps, plinths or slabs according to joint geometry. At the later stages of glacial erosion, the tor may begin to be reshaped by abrasion of stoss surfaces and plucking of lee side cliffs. The form of small tors in other glaciated regions is often subdued and may indicate that glacial modification has gone unnoticed. Preliminary observations indicate that minor glacial erosion of tors is commonplace on British hill tops, including Lochnagar, northern Arran and the Mourne Mountains. 5. The age of tors in former glaciated regions

Evidence of glacial transport from the tor site is also provided by displaced tor blocks and boulder trains (Hall and Phillips, 2006a). Tor blocks may reach sizes of 100 m3 and show the weathered upper surfaces typical of tors. The presence of weathering pits shows the original attitude of the block on the tor surface (Hall and Phillips, 2006b). In the Cairngorms, such criteria can be used to trace the displacement of former tor blocks, allowing distances and directions of transport to be identified. Tors formed of smaller blocks may be partly or wholly demolished to form boulder trains (Hall and Phillips, 2006a).

Tors in glaciated regions have sometimes been referred to as « preglacial » landforms often coexisting with « relict » surfaces (e.g. Sugden, 1968). The terms are used in two senses –pre-Pleistocene or pre-glaciation, the latter where the tor predates one or more periods of glaciation. The advent of cosmogenic isotope analysis has allowed constraints to be placed for the first time on the exposure ages of rock surfaces on tors and on erosion rates for tor summits and for surrounding regolith. It appears they are neither as old nor as relict as formerly believed. A number of studies of erosion rates and surface ages of tors in glaciated regions have been published in recent years and change our view of the relevant timescales and regional glacial histories (Bierman et al., 1999; Bierman et al., 2001; Briner et al., 2003; Gosse et al., 1995; Phillips et al., 2006).

On summits and ridges which lack glacial erratics, tor morphology may provide the only available indicator of the passage of glacier ice. Recently models been developed that identify the stages in

The results of cosmogenic exposure ages are often difficult to interpret due to a number of complicating factors. The exposure age is the accumulated period since a surface was exposed


to cosmogenic radiation. A rock surface will be shielded by ~ 3 m of overlying rock or ~ 7 m of ice and the term event surface has been proposed to describe situations where initial exposure occurs relatively quickly (Phillips et al., 2006). Tors are prone to auto-shielding, where upper tor blocks absorb nuclides prior to disintegration or toppling. Inheritance of nuclides is a particular problem on glacially modified tors, where glacial entrainment has removed blocks < 3 m thick. Nuclides are lost as rock surfaces erode and the burial history of the tor is also often largely unknown, in terms of snow and ice cover. Where tors occur in upland areas there may be no detailed record of the glacial history of the area, especially none that relates to plateaux and summits. Thus recourse is generally made to global climatic proxies such as the Greenland ice cores and deep ocean cores (Phillips et al., 2006; Stroeven et al., 2002). A clear distinction must be made between unmodified and glacially-modified tor surfaces in the interpretation of cosmogenic results. The summits of unmodified tors should represent the longest exposed surfaces in a locality (Phillips et al., 2006). The interpretation of exposure ages is complicated by the nature of tors and the processes of block removal. If tors are viewed simply as stacks of joint-bounded blocks of different sizes then the exposure of event surfaces reflects the toppling or sliding of large blocks. More generally, the blocks will be progressively reduced in size by removal of granules and thin sheets and so event surfaces will not be created. As the blocks thin so larger concentrations of nuclides penetrate the underlying block. The emergence of a surface will thus usually involve a component of nuclides inherited from before exposure (Yuengling, 1998). Cosmogenic isotope results from tor surfaces are more often viewed in terms of erosion rates for this reason (Small et al., 1997). All existing data confirm that tors in landscapes of selective linear erosion can survive multiple phases of glaciation. In the Cairngorms, at least

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three phases of glacial modification of tors are recognised on the basis of exposure ages (Phillips et al., 2006). On these and other granite hills in Scotland, glacially-modified surfaces carry weathering pits > 10 cm deep, indicating that glacial erosion took place before the last interglacial (Hall and Phillips, 2006b). The duration of ice cover is critical for questions of the ages of tors. In the north Swedish uplands, ice cover is estimated to span much of the last 1 Ma, so that 10Be exposure ages of 37-79 ka may imply ages of > 605 ka for first exposure of tor surfaces (Stroeven et al., 2002). In the Cairngorms, ice cover is estimated at > 50 % over the past 625 ka (Phillips et al., 2006). In Marie Byrd Land, Antarctica, summit tors on contemporary nunataks have been exposed for over 100 ka but buried beneath ice sheets for 150-250 ka (Sugden et al., 2005). Mean erosion rates for tor surfaces range from 1-10 m/Ma (Bierman et al., 1999; Phillips et al., 2006; Small et al., 1997; Staiger et al., 2004; Stroeven et al., 2002; Yuengling, 1998). The slowest rates of erosion come from the arid regions of the Canadian Arctic (Bierman et al., 1999) and Antarctica (Hall and Denton, 2005). These rates of erosion are slow in comparison to glacial erosion and explain the persistence of tors through multiple interglacial phases. Studies of tors in extra-glacial areas often indicate differential rates of erosion where the regolith is eroding faster than exposed tor surfaces. On alpine summits in North America, tors summits are eroding at mean rates of 7.6 m/Ma whilst regolith is eroding at mean rates of 13-14 m/Ma (Small et al., 1999; Small et al., 1997). In SE Australia, tor summits erode at 9 m/Ma, whilst regolith erodes at 26 m/Ma (Heimsath et al., 2000). This differential weathering and erosion promotes tor emergence. In former glaciated regions, the few available studies indicate comparable rates of tor emergence. One tor in the Cairngorms has yielded exposure ages at different levels above the current ground surface that indicate emergence at 31 m/Ma but other small tors clearly emerged more slowly at

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rates of 11 m/Ma (Phillips et al., 2006). On the Seward Peninsula, Alaska, Yuengling (1998) suggests that tors ≤ 6.5 m high have emerged since the Nome River Glaciation at ~ 400 ka. Existing data is limited but indicates a wide range of ages for tor surfaces in former glaciated regions. The Galway tors appear to be pre-Pleistocene in origin and to have lain buried through the Pleistocene (Coxon, 2001). Elsewhere in the British Isles, groups of large tors in the mountains of Mourne, Arran and Caithness show loss of blocks and superstructure to glacial erosion and must predate the Last Glacial Maximum. Cosmogenic isotope data show that all Cairngorm tors are older than the previous interglacial and the oldest tor surface has an apparent exposure age of 675 ka. It carries 1 m deep weathering pits and is unmodified by glacial erosion, yet its accumulated nuclides indicate that it cannot be regarded as a pre-Pleistocene feature (Phillips et al., 2006). Tors in the Cairngorms appear to be dynamic forms which have emerged entirely during the Middle Pleistocene. The substantial volumes of Middle and Late Devensian frost-generated debris derived from tors and their immediate surroundings in the Scilly Isles indicates that these small tors have emerged in the last tens of thousands of years (Scourse, 1987). No evidence exists currently in the British Isles that allows tors to be regarded as inherited Neogene landforms. Tors in areas with longer periods of ice cover or low precipitation may have longer exposure histories. Tors on Baffin Island (Bierman et al., 1999), the Torngat Mountains, Labrador (Staiger et al., 2004) and northern Sweden (Stroeven et al., 2002) may have been first exposed 1 million years ago. On mountain tops and in enclaves where the period of Pleistocene maximum glaciation predates the Late Pleistocene, tors may have re-emerged subsequent to glacial destruction.

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6. Tors and former glacier thermal regime The absence of tors from landscapes of areal scouring created by successive ice sheets which covered the shield regions of the Northern Hemisphere during the Pleistocene is a demonstration that sliding, warm-based glacier ice is capable of the removal of fragile, upstanding tors. On the Scilly Isles, where the maximum limit of Pleistocene glaciation was reached at the LGM (Scourse et al., 2004), the demolition of tors indicates that only a single phase of warm-based ice cover is sufficient to remove small tors. In Estes Park, Colorado, tors covered by only a few tens of metres of ice during the Bull Lake glaciation (130-160 ka) have lost superstructure or been reduced to plinths. Combined with evidence of tor destruction even at the maximum limits of glaciations, it is clear that even thin, sliding ice has sufficient tractive force to entrain and remove tor blocks. The survival of fragile tors from summits, ridges and lowlands in other formerly glaciated regions probably requires former covers of cold-based ice throughout the cold stages of the Middle and Late Pleistocene. The covers may be localised and equate to « cold-bed patches » (Kleman et al., 1994), where a carapace of cold ice protects the glacier bed from erosion. Where tors show no modification then the ice is not only cold-based but also barely deforming across the tor site. Such locations would be those covered by thin or diverging ice flow (Sugden, 1974, 1978). The first stages of modification involve the entrainment of blocks from the tor summit and margin. Modest distances of block transport and the absence of evidence of basal meltwater in form of plucking on the lee side of the form indicates erosion under cold-based ice (Hall and Phillips, 2006a), but the degree of modification of the tor remains modest. Only in the later stages of tor destruction, when the tor is reduced to a plinth or slab, do the processes of lee-side pluc-


king and abrasion become important and this marks the onset of basal sliding (André, 2004; Hall and Phillips, 2006a). In an ice-sheet situation basal sliding is favoured by deeper converging ice flow, leading to a general absence of tors from glaciated lowlands and valleys and an inverse relationship between erosion rates and altitude (Briner et al., 2003; Kleman and Stroeven, 1997; Staiger et al., 2004; Sugden et al., 2005). Where tors or even tor roots survive in formerly-glaciated areas then the total erosion has been confined to removal of the protuberance, without significant lowering of surrounding surfaces. In glaciated tor fields such as the Cairngorms this means that total glacial erosion since tor emergence has been only a few metres (Hall and Phillips, 2006a). On spurs that merge down slope into larger roche moutonnée forms, the survival of tor roots identify a zone of very limited glacial erosion on the upper part of the spur (Sugden et al., 1992). Tors are therefore useful indicators of terrain that has experienced minimal glacial erosion. In such terrain, erosion during the Quaternary from non-glacial processes may exceed that from glacial processes, especially where ice cover has been restricted in duration and where levels of precipitation are high. Conclusions In this contribution to the Festschrift we have tried to place work on tors in the British Isles in a wider global context. There are several points that emerge. First, the alternative hypotheses of tor formation championed in the British Isles by Linton and Palmer are two extremes of a continuum. Rather it appears that tors are forming in a wide range of climatic environments by a combination of physical and chemical agents of weathering and mass wasting. The implication is that lithology and rock structure are the dominant controls on tor morphology and formation and that climatic environment plays a secon-

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dary role. Second, many tors in the British Isles, as elsewhere, have survived burial by ice sheets. Their coexistence with erratics is clear evidence of glacial overriding. The best working hypothesis is that they survive in areas overlain by coldbased ice. Minimal ice flow, for example on ice divides and beneath thin ice, can lead to perfect preservation beneath an ice sheet. Modest ice flow can lead to the erosion of tor blocks, transporting them often only a few metres. Third, summit tors bearing evidence of glacial modification constrain the vertical extent of ice sheets. In the British Isles, evidence exists for erosion of tors by the last and earlier ice sheets. Fourth, cosmogenic isotope analysis of tor surfaces reveals apparent exposure ages of several tens to hundreds of thousands of years. Using two isotopes and climatic proxies it is now possible to recreate the history of exposure. The main implications of such studies are that tors continue to evolve under a variety of climates. Moreover, British tors are landforms of the Pleistocene and not pre-glacial relicts. Finally, the preservation of tors beneath cold-based ice has an unusual benefit. It means that the upper surfaces of tors are the longest lasting and slowest evolving points on the landscape. This means that they can be used as natural erosion pins with potential to record the longest histories in rock landscapes. References André M.-F., 2004. The geomorphic impact of glaciers as indicated by tors in North Sweden (Aurivaara, 68º N), Geomorphology, 57: 403-421. Ballantyne C.K., Harris C., 1994. The Periglaciation of Great Britain, Cambridge University Press: 330 p. Basham I.R., 1974. Mineralogical changes associated with deep weathering of gabbro in Aberdeenshire, Clay Minerals, 10: 189-202. Bierman P.R., 1994. Using in situ produced cosmogenic isotopes to estimate rates of landscape evolution; a review from the geomorphic perspective. Journal of Geophysical Research, B, Solid Earth and Planets, 99: 13 885-13 896.

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