The Role of
Geological Structure in Tor Formation Some Observations from Charnwood Forest
The Role of
Geological Structure in Tor Formation
Some
Observations from Charnwood Forest
Mike Lewis (1996 - extract from The Rocks of Charnwood Forest: A Study in Structural Geomorphology published in Trent Geographer)
Beacon Hill represents one of the highest points of the Charnwood district of Leicestershire at 248m. It is a favourite location for rock climbers and walkers and provides the individual with breath-taking views across the East Midlands plain. The summit of Beacon Hill represents a remnant of a bed of rocks now almost completely denuded from the Charnwood periclinal dome. The rocks belong to the Maplewell series and are mostly fine and coarse ashes and hornstones. Agglomerates, which make up a significant fraction of the Maplewell series, do not occur noticeably in outcrop on Beacon Hill but further west around Timberwood Hill and Charnwood Lodge drive. The banding of the ashes caused by the deposition of volcanic ash in precipitation is particularly marked on the highest outcrops. Hornstones also occur on these high outcrops, but are a more dominant feature of the western free-faces described below and the Broombriggs to the south of Beacon Hill.
Click here for geological notes on Charnwood Forest.
The cleavage of the Beacon Hill outcrops is marked and has been imparted by repeated periods of folding. This cleavage has been accentuated by dilatation brought about by successive periods of denudation and burial and has been aided by the natural laminations imparted by the ash banding. These joints have acted as a focus for different forms of weathering: chemical weathering during the tropical conditions of the early Cenozoic and physical weathering by congelifraction during the Pleistocene. The role of folding in governing the degree of cleavage is clear in the Charnwood district as evident by the high degree of cleavage present at High Sharpley and other rock outcrops in the north- western area where folding is highly complex. The low degree of cleavage at Beacon Hill, in contrast to High Sharpley, is one factor in the maintenance of significant free- faces, although consideration must be given to the overall dip of the bedding planes in relation to the ground surface slope as is described later.
Figure 1 illustrates how the rock outcrops on Beacon Hill can be divided into four parallel groups running roughly north-west to south-east. They are all low features rising to no more than 2 to 3 metres above the general level of the ground. The highest outcrop supports an Ordnance Survey triangulation pillar. All four groups consist of consolidated rock surrounded by clitter which is more widespread on the northern flank. Much of this clitter is seen to have been broken down by weathering into gravel. The angularity of this material is suggestive of congelifraction as a weathering agent. The highest outcrop, being more exposed, bears considerably more gravel than the other groups.
Figure 1: Spatial variations in weathering characteristics at Beacon Hill.
The westernmost outcrop has developed at the point of inflection of the slope and forms an overhanging free-face of some 10 to 15m in height. The base of the free-face extends beneath a thick accumulation of material derived from the weathering of the face. This material comprises large angular boulders and logans, some of which appear to have been freshly prised from the free-face indicating that weathering of the free-face is still active. In places a thin lithosol has developed over the weathered material aided by the extensive growth of mosses and algae which have colonised the area owing to runoff from the free-face. As on the summit the larger boulders are interspersed with smaller fragments and there are considerable quantities of gravel- sized material. Active disintegration of the larger boulders by congelifraction once removed from the free-face thus appears to be taking place. The free-face itself has been roughened by congelifraction concentrating along the joints in the rock opened up by dilatation following uplift of the Charnian beds.
Beneath the free-face the ground slopes away steeply to the valley bottom only occasionally revealing further outcrops which, although forming clearly defined free- faces, are evidently less weathered than the outcrops of the summit. The less exposed nature of these outcrops together with the deeper acid podzols and dense coniferous woodland have no doubt afforded some protection from congelifraction. These lower outcrops would appear to be in an area of net accumulation of material rather than the outcrops at the summit where net removal of material is occurring.
The average height of the rock outcrops on the summit of Beacon Hill tends to increase from east to west. This increase in height is also associated with greater decomposition and more extensive areas of clitter. There seems little doubt that the weathering of these rocks has been occurring continuously since uplift and exhumation and also selectively on those outcrops where jointing is more significant. Figure 1 shows this spatial variation in weathering characteristics.
A second marked break in relief exists on the westernmost edge of Beacon Hill at a lower elevation than the submittal free-face. The cliff rises to a height of 10m in places and the base lies buried beneath an accumulation of material derived from the weathering of the free-face, although the quantity of this material is not as great as observed on the higher free-face.
There is little doubt that active free-face retreat has been taking place on the north- western edge of Beacon Hill. However, it is unlikely that the current climate of Beacon Hill is conducive to any extensive landform modification by a combined process of congelifraction and solifluction as proposed for the southern Pennines by Palmer and Radley (1961). The congelifraction occurring now is limited to the slow decomposition of the surface of the free-face and the previously weathered boulders.
It is clear on inspection of the largest boulders that extensive weathering typical of a periglacial environment would have been needed to detach them from the free-face. Many of these large boulders have lost the angularity typical of the smaller logans and lie partially buried beneath a thin lithosol indicating that they have been in their current positions for some time, possibly since they were first weathered under the periglacial conditions of the Devensian over 10000 years BP. Furthermore, the congelifraction now occurring is such as to lead to the reduction of the free-face, not its preservation, as the active removal of weathered material by solifluction, possible under periglacial conditions, has long ceased.
Thus the free-faces observed at Beacon Hill can be viewed as relic features of a periglacial climate formed by a combination of congelifraction working within the joints of the free-face and solifluction to remove the products of this weathering. It is likely that cold conditions suitable for this type of erosion would have persisted more or less continuously throughout the Devensian glacial period when Charnwood would have been close to the margins of the Devensian ice sheet further north. This type of erosion has also been used by Palmer and Neilson (1962) for the formation of tors out of periglacial free-faces on Dartmoor and Bodmin Moor. Precisely why the remains of the former free-faces tend to be better preserved in Cornwall is poorly understood, but the reasons may lie in both differences in geology and the time-scale for periglacial weathering. Although Charnwood and Dartmoor are both areas of igneous rock, the granite of Dartmoor differs from the volcanic tuffs of Charnwood by being more massively and regularly jointed. Large boulders have thus tended to be better preserved on Dartmoor and the regular horizontal and vertical jointing has aided stacking. The closer and more irregular jointing of the Charnian rock has inevitably resulted in its decomposition. Also, Dartmoor, being further from the ice margin during the Devensian would have experienced a shorter periglacial cycle and the presence of tors may be indicative of a cycle cut short of the full destruction of the tor as occurred at Charnwood. Figure 2 shows a scheme for the type of free-face retreat proposed for Beacon Hill.
Figure 2: A mechanism for free-face retreat and tor isolation proposed for Beacon Hill.
The mechanism of free-face retreat and tor isolation (in the case of Dartmoor) described above does not however represent a perfectly satisfactory explanation for the landforms observed at Beacon Hill. Those familiar with research on the formation of tors will know that the periglacial ideas of Palmer et al represent only one view. Earlier work, notably by Linton (1955), suggests that tors are the product of differential chemical weathering under a warm, tropical climate. The reason for the divergent views rests with the presence of tors in areas which are known to have been glaciated since the Pliocene (the period proposed by Linton for chemical weathering of the bedrock) and the absence of chemically weathered material in the immediate vicinity of many tors. However, whether or not tors were formed as a by-product, chemical weathering undoubtedly occurred during the early Cenozoic before significant cooling took place and would have led to considerable destruction of the bedrock where the jointing would have been extensive. The repeated rejuvenation known to have occurred in the Pliocene would have aided the removal of the regolith particularly from interfluves such as Beacon Hill. Furthermore, this continuous process of exhumation would have encouraged the enlargement of joints and the pre- disposition of the bedrock to weathering as a result of dilatation. Indeed dilatation joints form an obvious feature of the rock outcrops on Beacon Hill and occur roughly parallel to the general dip of the strata.
The dilatation joints are particularly noticeable within the free-face on the western edge of Beacon Hill. These joints would have provided a focus for not only extensive chemical weathering (mostly hydration and hydrolysis) during the early Cenozoic, but also extensive congelifraction during the cold periods of the Pleistocene. The repeated rejuvenations during the Pliocene would have facilitated the opening up of free-faces at different elevations on the slope, these free-faces then later being maintained through congelifraction and solifluction removal during the Pleistocene. Only now in the Holocene are these free-faces grading slowly into the general gradient of the slope. Indeed the absence of free-faces on the northern, eastern and southern flanks of Beacon Hill indicates that this grading is well advanced.
The picture described above thus brings together the two main divergent opinions on tor formation. Chemical weathering of bedrock, of the type proposed by Linton (1955), would have occurred undoubtedly during the early Cenozoic. As climate cooled repeated rejuvenation led to extensive dilatation joints which aided weathering and the opening up of a free-face probably encircling Beacon Hill. The onset of the Pleistocene saw a change in weathering process to the congelifraction of Palmer et al (1961, 1962) which would have been concentrated within the joints enlarged following exhumation. Solifluction would have removed the weathered material and maintained the free-faces. Following the Pleistocene congelifraction remained as a weathering agent, although occurring less powerfully and without the aid of solifluction it acted destructively rather than constructively resulting in the grading of the free-face. It is likely that tors would have been characteristic of Beacon Hill when free-face retreat was active, although grading of slope during the Holocene has led to their destruction. Indeed the dismembered components of relic tors may be found amongst the clitter below the western free-face. Figure 3 shows the scheme of exhumation and tor formation and destruction as is put forward by this discussion.
Figure 3: Stages in free-face retreat at Beacon Hill.
However, the above picture is complicated as there is evidence that exhumation may not have been continuous and that burial occurred on one if not more than one occasion. The effects of the climatic optimum of c.7000 years BP would have been experienced just as much here as elsewhere in the British Isles. Higher temperatures and rainfall would have halted congelifraction and allowed a secondary succession of vegetation to deciduous woodland. Indeed many boulders around the summit bear the imprint of tree roots etched by the release of fulvic acids. Such features associated with tor groups are not uncommon and have been described for both Plumpton Rocks and Brimham Rocks by Palmer. Brimham Rocks, one of the most noted tor groups in the central Pennines, was known to have supported deciduous woodland up until the 17th century. However, it still remains unclear whether the presence of vegetation resulted in the general protection of the tors or their destruction in whatever location be it Brimham, Plumpton or Charnwood.
