Huddart, D. & Glasser, N.F. 2007. Quaternary of Northern England. Geological Conservation Review Series No. 25, JNCC, Peterborough, ISBN 1 86107 490 5.
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W. Mitchell and D. Huddart
The northern Pennines upland landscape of high, open and exposed plateau is, arguably, England's last wilderness. At 893 m OD, Cross Fell
Cross Fell is part of the geological unit termed the 'Alston Block' (Johnson and Dunham, 1963; Johnson and Hickling, 1970; Taylor et al., 1971; Burgess and Wadge, 1974; Dunham, 1990). It is bounded to the north by the Stublick Fault and the Tyne Gap and to the south by the Lunedale Fault and the Stainmore Trough. To the west is the Pennine Fault, which has produced a major displacement and an impressive, although degraded, fault-line scarp with an estimated displacement of 600 m near Cross Fell (
Although there is little direct evidence of glaciation within the summit area of Cross Fell, a series of giant erratics, known as the Tullman Hills'
A number of large-scale periglacial features, such as blockfields, blocky scree slopes and frost-riven cliffs, large-scale stone stripes, polygons and nivation terraces can be found in the summit area. These features are thought to be relict and associated with the severe climatic conditions of the Late-glacial period (Lowe and Walker, 1997a, b). However, the smaller scale landforms, such as gelifluction terraces, polygons, stripes, erected stones, ploughing blocks and thufur, continue to be active under present climatic conditions, because the climate is cold enough to generate frost processes (Tufnell, 1985). In places, contemporary gelifluction terraces are superimposed over large-scale 'fossil' stone stripes.
The summit plateau of Cross Fell
Certain of the limestone units also have been fragmented into large clasts near their outcrop. These, however, tend not to form the plateau surface but tend to form cliffs, with the underlying mudstones obscured by talus slopes of limestone clasts. Talus slopes also have developed below outcrops along the northern and western flanks of Cross Fell, terminating at a break of slope at 750 m, particularly below a massive sandstone known as the 'Six Fathom Hazel' (Pounder, 1989). Lobate forms of boulders downslope of the talus slopes on the southwestern side of Cross Fell indicate the operation of debris-flow processes on these slopes (
To the south and south-east of Cross Fell summit there is discernible patterned ground, particularly sorted stone polygons
A particular type of patterned ground is the vegetation covered hummocks or thufur found on many west-facing slopes at altitudes over 800 m OD (Tufnell, 1966). On Great Dun Fell they occur on slopes with a gradient of 14–17°. In upper Knock Ore Gill, between 680 and 750 m OD, thufur are found in association with wet flushes on the eastern side on a 6–9° slope
A widespread feature of upland slopes in Britain is the downslope movement of material by seasonal freezing and thawing of the upper soil layers. This may also be termed 'gelifluction', where slow saturated flow of an ice-rich soil occurs during thaw consolidation (Ballantyne and Harris, 1994) and is termed 'congelifluction' by Tufnell (1969). The presence of mudstones in the area has provided large amounts of fine-grained material to be broken down by microgelivation. Mechanical fracturing of the interbedded sandstones and limestone has provided a number of angular clasts that mix with the mud to form a widespread diamicton that generally is regarded to be the result of periglacial processes. This clay-rich diamicton has been subjected to flow and deformation under gelifluction processes, which have resulted in a number of different surface forms. Tufnell (1969, 1985) identified five different types of 'gelifluction terrace', which he divided according to whether they are convex in plan down-slope (lobe) or parallel to slope (terrace), whether they are vegetated or expose sediment on the riser. Lobes are reported from Great and Little Dun Fell whereas terraces are exemplified above 730 m OD in Knock Ore Gill (Tufnell, 1985).
'Ploughing blocks' are the most widely distributed of the currently developing periglacial phenomena and have been observed down to an altitude of 450 m OD (Johnson and Dunham, 1963; Tufnell, 1969). 'Ploughing block' is a term used by Tufnell (1966) to describe large blocks on slopes that travel faster than the finer soil material. Such large clasts are able to move downslope under gravity, particularly when the soil is soft owing to high moisture content, and form distinctive mounds of ploughed material in advance of the block and a notable track or depression upslope behind the block. They indicate differential slope movement with mean annual rates of 1–5 cm a−1, with a maximum movement in the spring when frozen soil is melting (Tufnell, 1969, 1972, 1985). In plan, the depressions can be niche-shaped, or elongate, depending on the differences in speed of block movement relative to that of the surrounding ground. Blocks travelling faster than adjacent parts of the slope will create an elongate depression, whereas a niche-shaped depression will form where movement just exceeds that of the surrounding slope. These landforms usually occur on grassy slopes where there is sufficient moisture; they are rarely found in areas that lack vegetation. The possible causes of ploughing block movement are summarized in
The Cross Fell area is well known for its late-lying snowdrifts and snow patches associated with distinct bedrock benches on the upper slopes of the mountains (Tufnell, 1971) and in some cases the snow is associated with semicircular hollows. They compare well with cryoplanation terraces and nivation hollows in present periglacial environments (Ballantyne and Harris, 1994) and have the following features in common:
Some of the best terraces in the Dun Fell area are situated on the north-western slopes of the Upper Knock Ore Gill on ground that repeatedly experiences snow-patch formation (
During the last glaciation, Cross Fell acted as an area of local ice accumulation that was surrounded by ice from Scotland and the Lake District (Dwerryhouse, 1902; Raistrick, 1931b; Beaumont, 1968; Vincent, 1969; Lunn, 1996). This local ice was not thought to be very important in these original reconstructions. A similar situation in the western Pennines, however, has shown that the local ice was not just a summit ice cap but a linear ice divide, which extended along the mountains to the south of the Vale of Eden where it joined an ice divide over the Lake District (Mitchell, 1991c, 1994).
The presence of ice flow off the summit area is confirmed by drumlins to the east of the summit area in the headwaters of the River Tees, which indicate a flow direction generally southeastwards off the summit into the Tees valley (WA. Mitchell, unpublished data). A local origin for this ice is confirmed by the erratic content in till exposures near Moor House
Recent papers on other areas of the British Isles, particularly north-west Scotland (Ballantyne, 1997, 1998; Ballantyne et al., 1997, 1998) and the Lake District (Lamb and Ballantyne, 1998) have identified a series of periglacial trimlines. This is defined by an upper limit on slopes of features attributable to glacial action, with the summit areas ice free and characterized by frost-shattered regolith, such as blockfields. Periglacial trimlines allow the delimitation of areas that existed as nunataks above the surface of the last ice sheet and have been shown to occur between 800 and 870 m in the Lake District (Lamb and Ballantyne, 1998). This may well have been the case with the Cross Fell summits, although it also is possible that these plateau areas were buried under cold-based ice, which allowed the preservation of the periglacial forms (cf. Rea et al., 1998). The height at which glacial features become apparent is therefore a reflection of a change to temperate, basal ice.
Earlier workers were convinced that Cross Fell had remained unglaciated during the last glaciation and had been used to explain the presence of a number of species of alpine plants in Upper Teesdale. The idea was taken up by Raistrick (1931b), who produced a map showing the distribution of nunataks, such as Cross Fell, based on evidence such as the lack of evidence of till above c. 650 m and the extensive and abundant periglacial evidence described previously. However, this idea is now discounted and the arctic–alpine plants probably migrated to these areas during the cold phase of the Late-glacial. An early description of a supposed interglacial peat on the eastern slopes of Cross Fell by Lewis (1904) has since been re-examined and discredited (Godwin and Clapham, 1951). Turner (1984) in fact demonstrates from pollen analysis of peats that woodland was present to the summit of Cross Fell in the mid-Holocene. A.G. Lunn (pers. comm., 1999) favours a complete Late Devensian ice cover, based on evidence of an ice-scoured pavement in the midst of blockfields just north of Cross Fell and a meltwater channel, presumed to be subglacial, that cuts through the main divide north of Green Fell
During the Late-glacial period there was no ice in the Cross Fell area, although nearby small glaciers may have formed during the Loch Lomond Stadial, such as the glacier associated with the scarp at Cronkley Scar
The significance of periglacial processes in shaping the environment of upland Britain is clearly demonstrated by the range of associated landforms found on the highest ground of the Pennines. The Cross Fell area is important because of the range of periglacial geomorphological landforms found within such a small area. It has been one of the most actively studied periglacial landscapes in northern England.