Outline The Evidence For Permafrost Conditions Having Extended Beyond The
South Coast Of Mainland Britain
At various times during the Pleistocene epoch, it is argued that that the permafrost conditions of the northern hemisphere extended beyond their present limits and affected large areas of mid-latitude Europe and North America (Harry, in
Clark, 1988). In Europe, southeastern England (Williams, 1965; in Péwé, 1969), the
Channel Islands (James & Worsley, 1997), northern France, Belgium and The
Netherlands (Lautridou et al , 1986; Lowe & Walker, 1997) are believed to have been underlain by permafrost during the last glacial stage (See Figure 1). French (1996) and Pitty (1988) suggest that the extent of permanently frozen ground during the
Pleistocene and associated paleo-climatic conditions, can be mapped from the distribution of relict ice wedges, involution structures and other remnant landforms.
Consequently, this paper will consider the nature and processes associated with contemporary permafrost regions and then discuss the evidence, through case studies, for those conditions having extended beyond the south coast of mainland
Britain during the last glacial stage.
[Figure 1: Distribution of Permafrost in England (A) and northern France,
Belgium and The Netherlands (B) during the last glacial maximum (Lowe &
Walker, 1997: p108]
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Permafrost is defined by Muller (1947, in Washburn, 1973) as “a thickness of soil or other superficial deposit… at a variable depth beneath the surface… in which a temperature below freezing has existed continually for a long time” (p18).
Washburn (1973), however, notes that this definition is usually used to mean
‘perennially’ rather than ‘permanently’ frozen ground as climatic and surface changes can cause permafrost to thaw rapidly. During the summer, for example, when temperatures are above 0 o C, seasonal thawing creates a saturated active layer, which refreezes once the temperature drops (Washburn, 1973). Harry (in Clark,
1988) also suggests that although s ome researchers (e.g. Péwé, 1969) have equated the periglacial domain with the extent of permafrost, “the relationship is complex since not all periglacial processes are restricted to the permafrost zone”
(p115). In other words, permafrost should not necessarily be equated with the proximity and climatic influence of glaciers (Harry, ibid ). In Alaska, for example, there is an increase in permafrost intensity and activity away from the cordilleran glaciers
(French, 1996; Briggs et al , 1996).
[Figure 2: Modern Distribution of Permafrost in the Northern Hemisphere.
Briggs et al, 1997: p269]
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Figure 2 shows the current distribution of permafrost to include Alaska, northwest
Canada, northern Scandinavia and Siberia (Pitty, 1988). Globally, there is approximately 40 million km 2 of permafrost (Briggs et al , 1996), which forms continuous cover on non-glacial polar landscapes and cold, dry continental interiors, but becomes discontinuous, as the amount of unfrozen ground increases, equatorwards and near coastal regions (Briggs et al , 1996; Pitty, 1988; Washburn,
1973). As French (1996) suggests, the conditions necessary for the formation of permafrost differs from place to place. For example, in Norway, permafrost exists at
-6 o C, whilst in Siberia it exists at -8 o C (Lowe & Walker, 1997). Although it is accepted that mean annual air temperature must drop below 0 o C, the formation of permafrost ultimately depends on the porosity and thermal conductivity of the local earth materials (Harry, in Clark, 1988). Washburn (1979) also notes that current permafrost areas are not necessarily good analogues for paleo-climates as different solar radiation cycles in the mid-latitudes would have meant more diurnal freeze-thaw action than in the present arctic. Consequently, conclusions drawn about paleoclimatic conditions and the formation of permafrost must take into account these locational differences (Washburn, 1973; Pitty, 1988).
French (1996) argues that permafrost related features can be divided into those associated with either its aggradation or its degradation and that in both cases,
“the landforms are often associated with the build up or degradation of ground ice”
(p69). French (1996) also notes that although patterned ground and solifluction are commonly associated with permafrost, these features are not restricted to permafrost regions. Pitty (1988), however, argues that they are best developed in permafrost zones and it is for this reason that both patterned ground and solifluction will be considered.
The features associated with ground ice in permafrost areas include ice wedges and veins, ice-cored mounds and Palsas (Washburn, 1973; See Figure 3).
Within the continuous zone, ice wedges, Pingos and cryostructures are the dominant features (Harry, in Clark, 1988). Ice wedges form where water seeps into cracks in the permafrost table and then subsequently freezes (French, 1996). The ideal conditions for the development of ice wedges are poorly drained tundra lowlands within the continuous permafrost zone. Size varies from place to place and is dependent on the availability of water and the age of the ice wedge (French, 1996).
In Siberia, ice wedges approximately 3-4m wide at the surface and extending 5-10m downwards have been discovered (French, 1996). When the ice melts, it is replaced
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by material falling into the cracks from above and from the sides (French, in Harry,
1988) and consequently, the original form of the ice wedge is preserved to become a relict feature. Pitty suggests th at relict ice wedges are the “major and almost the sole reliable indicator of former permafrost conditions in lowland areas” (1988: p9). In
England, examples found in East Anglia, the South East and Midlands have been used to argue for permafrost conditions during the last glacial stage (Williams, 1965
& in Péwé, 1969; French, 1996; See Figure 1).
[Figure 3: Permafrost Ground Disturbances. Briggs et al, 1997: p289]
The remains of Pingos and other frost mounds (see Figure 3) also provide evidence for the previous existence of permafrost. In England, the remnants of frost mounds have been found at Walton Common in Norfolk (French, 1996). Palsas, which are mounds with a core of alternating ice and peat, are regarded by French
(1996) as one of the few reliable indicators of permafrost in the discontinuous zone.
Pingos are formed when unfrozen water that has collected in the substrate begins to
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freeze. As the water freezes, an ice lens forms and the resulting cryostatic pressures push up the land to create domes between 10-80m high (Washburn, 1973; French,
1996). Contemporary pingos are found in the Mackenzie Delta and on the Yukon coastal plain (French, 1996). Frost mound remnants posses a rampart where material has moved down the slope by either solifluction or creep and a central depression in which the ice core developed (French, 1996). The ramparts are particularly important because they are the main difference between the depression associated with thermokarst (i.e. a general thawing of the ice) and the remains of a pingo (French, 1996). However, as French (1996) points out, there are several problems associated with relying solely on remnant frost mounds as evidence for former permafrost conditions. There is a difficulty in “distinguishing between the remnants of seasonal and perennial types, since only the latter unambiguously requires permafrost for their formation” (French, 1996: p250) and there may be alternative, non-permafrost explanations for the features (French, ibid ).
Other processes associated with the permafrost zone can be divided into those associated with the active layer and those associated with movement and slopes
(French, 1996). In the active layer, cryofracture, patterned ground, and cryoturbation are the main features, whilst frost creep and gelifluction are the primary slope processes (Washburn, 1973; French, in Clark, 1988). Cryofracture, or frost wedging, is the prying apart of rock, into angular fragments of varying size, by the expansion of freezing water (Lautridou, in Clark, 1988). The creation of patterned ground and the up freezing of stones are associated with frost heaving and cryoturbation (See Figure
3). Cryoturbation is defined as the “lateral and vertical displacement of soil which accomp anies seasonal and/or diurnal freezing and thawing” of the active layer
(French, 1996: p143). Examples of cryoturbation can be found in the Axe Valley at
Broom in Devon (Bowen, 1999). The freezethaw cycle leads to “annual ground displacements of several centimetres” (French, 1996: p131) and creates uneven, patterned ground. At the same time, stones in the soil are gradually pushed upwards, in a vertical position, as the active layer seasonally thaws and refreezes.
Once the stones have reached the surface, they roll down to the edges of the hummocks and create sorted polygons, circles or nets (French, 1996; Washburn,
1973). On slopes, these shapes become elongated to form stripes (See Figure 4).
French (1996) argues that the paleo-environmental significance of patterned ground is questionable since “most patterned ground forms are not unique to periglacial regions” (p146). However, French (1996) does accept that they are usually indicative of cold climate conditions and Lowe & Walker (1997) suggest tha t there is “often a close field relationship between cryoturbations and other undoubted periglacial
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phenomena” (p105). The Breckland polygons and stripes in East Anglia, for example, coincide with evidence of other former cold climate features (French, 1996).
[Figure 4: Stone Circles and Stripes. French, 1996: p141]
Two slope processes are particularly linked with permafrost zones.
Gelifluction, a form of solifluction, has been described by Benedict as “the net downward displacement that occurs when the soil, during a freeze thaw cycle, expands normal to the surface and settles in a more nearly vertical position (1970, cited in French, 1996: p151). The associated landform is a uniform sheet of locally derived surficial materials in a tongue shaped lobe (French, 1996). Unstratified deposits, which usually consist of angular material within a sediment matrix, are known by a variety of names including ‘Head’ and ‘Coombe Rock’, whilst stratified deposits are scree or gr èzes litées (Dewolf, in Clark, 1988). Frost creep is the movement of particles down slope caused by the freeze-thaw process (French,
1996). As with patterned ground, the deposits are good indicators of cold climate conditions, but are not without ambiguity. As French argue s, “direct interpretation in terms of frost action is not always easy” (French, 1996: p240) as it is difficult to differentiate between a deposit caused by seasonal frost creep with retrograde movement and one caused by gelifluction where permafrost is present.
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The distribution of relict ice wedges, pingo remnants, cryoturbations and other periglacial landforms have been used to reconstruct the environment of western
Europe during the last cold stage (Lowe & Walker, 1997; French, 1996). Williams
(1965, & in Péwé, 1969), for example, used this data to establish the coverage of permafrost in England and Wales (See Figure 1A), whilst similar data has established the southern extent of permafrost in France, The Netherlands and
Belgium (See Figure 1B) (Lowe & Walker, 1997; Lautridou et al , 1986). The latter reconstruction, “suggests that the continuous permafrost limit may have been much further west than envisaged by Williams” (Van Vliet-Lanoe, 1988, in Lowe & Walker,
1997: p107) and it is within this context, and of the evidence outlined above, that the research undertaken in Brittany and the Channel Islands is considered.
ALDERNEY (UK)
(UK)
NORMANDY
BRITTANY
[Figure 5: Location of the Channel Islands, Brittany and Normandy. Microsoft
Encarta Atlas, 2001]
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The Channel Islands are located in the Golfe de St-Malo between the west coast of Normandy and the north coast of Brittany. Alderney is the furthest north of the islands and is situated off the Normandy coast near the Cap de la Hague (See
Figure 5). Figures 1 suggests that the Channel Islands were in or very close to the continuous permafrost zone during the last cold stage and Gurney et al (1998) believe there is “good reason to expect evidence of periglacial structures, indicative of at least discontinuous permafrost, if not continuous permafrost, in the Channel
Islands” (p1).
[Figure 6: Exposed Ice Wedge Cast at Fort Richmond, Guernsey. Gurney et al,
1998: p4]
Gurney et al (1998) note the existence of either seasonal or perennial frozen ground structures in Alderney and Guernsey, including sorted patterned ground, sediment displacement and ice wedge casts. In Alderney, the evidence for permafrost is based upon the distribution of head and loess deposits (Keen, 1978;
James & Worsley, 1997), a frost thrust boulder and “macrofabric evidence relating to
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former active layers above a permafrost table” (James & Worsley, 1997: p177). As discussed above, head deposits alone are not enough to indicate the existence of permafrost (French, 1996) and it is the disturbed raised beach at Fort Clonque, where most of the pebbles lie near to the vertical, and frost thrust boulder at Fort
Tourgis that provides more concrete evidence (James & Worsley, ibid ). James and
Worsley ( ibid ) suggest that the boulder at the Fort Tourgis site is incomplete ejection from the active layer through frost heaving. The boulder was also surrounded by a coarse head deposit, where the long axes were vertically aligned with the boulder and a series of involutions caused by cryoturbation (James & Worsley, ibid ). Further work in The Channel Islands has provided more evidence, including head deposits at
Moulin Huet in Guernsey, Bonne Nuit Bay in Jersey and Hannaine Point in Alderney and disturbed ground on Guernsey (Gurney et al , 1998). In addition, a wedge feature was discovered at Fort Richmond in Guernsey (See Figure 6). Gurney et al , (1998) suggest that the feature is a relict ice-wedge and therefore indicative of permafrost, although they note that there is a danger in drawing conclusions from single structures (Worsley, 1996 in Gurney et al , ibid ). The sedimentological evidence from
Alderney and “the range of possible/probable frozen ground phenomena overlying former discontinuous or continuous permafrost in Guernsey” (Gurney et al, 1998: p4) suggest that “severe periglacial climatic conditions occurred on Alderney” (James &
Worsley, 1997: p180) and Guernsey.
[Figure 6: Sketch Map of C
ôtes D’Armor Sites. Renouf & James, 2001: p161]
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Brittany (See Figure 5) lies to the west of the continuous permafrost zone as shown in Figure 1. However, work by Renouf and James (2001) in C ôtes D’Armor and Lautridou et al (1986) suggests that the northern coastline of Brittany should, like
Guernsey and Alderney, now be within its limits. Evidence of cryoturbation was found on a raised beach at Port l’Épine (See Figure 6) that suggested past permafrost conditions. The subrounded clasts within the raised beach “display a striking vertically inclined macrofabric similar to that previously recorded at Alderney and Guernsey” (Renouf & James, 2001: p161). Although Renouf and James (2001) suggest that vertically inclined clasts could be the result of frost heaving within seasonally frozen ground or permafrost, they accept that the clasts may have been reoriented by alternative processes. At Toul ar Roussin and the adjoining North East
Cove fractured and displaced pebbles and large angular boulders, which may have been frost shattered, have also been recorded. T he pebbles have also been “sorted into plumes of vertically inclined material with regular spacing”, which may have been indicative of patterned ground (Renouf and James, 2001: p163).
Renouf and James (2001) argue that the evidence from the Channel islands, together with “the discovery of deep seated periglacial disturbances of raised beach sediments” at Toul ar Roussin , suggest that the “southern permafrost limit needs to be redrawn to include… the Channel Islands and the northern coastline of Brittany”
(p165). As discussed above, other mechanisms could have been responsible for the evidence found (Renouf and James, 2001; Washburn, 1973) and periglacial environments are not necessarily synonymous with permafrost conditions (Harry, ibid ). However, the existence of a relict ice wedge in Guernsey, which Pitty (1988) has described as the “major and almost the sole reliable indicator of former permafrost conditions” (p9) and compelling evidence from Alderney and Brittany seems to support Renouf and James’ (2001) conclusion that these areas were underlain with continuous permafrost.
[2,566 Words]
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