The Geological History of Charnwood Forest

Mike Lewis (extracted from The Rocks of Charnwood Forest by the same author)

500 millions years BP(early Palaeozoic) the English Midlands were on the southern fringe of a large continent of which Scandinavia and parts of northern Scotland also formed a part. Today, only vestiges of this fringe remain and Charnwood Forest together with the Longmynd in Shropshire are two such examples.

The Pre-Cambrian rocks of Charnwood Forest are of volcanic origin and can be attributed to a prolonged period of volcanic eruptions from one major crater (now thought to be located near High Sharpley ) and a number of lesser vents. The ejected materials were deposited for several kilometres around the crater, much of the material being carried south by the then northerly prevailing wind (figure 1(a)). The deposition of the finer ejected material would have been aided by precipitation to give rise to clear banding or laminations within the rock. These laminations are notable in the outcrops at Cademan Wood and High Sharpley, but can also be distinguished at Beacon Hill.

The volcanic material varied in size and texture and coarse material included in finer ash was later laid down in addition to the lava which flowed from the crater. This great variability of material produced the agglomerates, tuffs and ashes which are exposed notably at Beacon Hill, but elsewhere around Maplewell. This distinctive series of rocks has been classified as the Maplewell series from the type site.

As the volcanic activity subsided grading and sedimentation occurred on the flanks of the volcano giving rise to extensive areas of conglomerates which in places lie buried beneath volcanic muds of varying texture and composition. This series of rocks, termed The Brand series from the type site, are found typically in the western parts of the region outcropping at the Hanging Stones beneath Beacon Hill, Billa Barra and Groby.

After vulcanism had ceased the area was subjected to considerable folding and uplift during the middle Palaeozoic. The Charnwood rocks were folded into a large anticline inducing considerable cleavage to form the slates of the Brand series. Folding was also accompanied by significant igneous intrusions guided by the many faults which had now opened up. These intrusions later solidified to form the syenites found in many outcrops particularly Groby, the Altar Stones at Markfield and Cliffe Hill. Further thrusting occurred in the later Palaeozoic to induce considerable complexity in the overall structure especially in the northern-western area around High Sharpley (figure 1(b)). However, in the southern area the structure is more easily discernible as a denuded periclinal dome comprising concentric beds of similar age, yet even this has been severely masked by folding, faulting and thrusting movements. The nature of dipping at Beacon Hill, which forms a later discussion of this paper, is indicative, although after some considerable extrapolation, of periclinal folding.

It is probable that once folding and uplift had ceased the Charnwood periclinal dome may have been several thousand feet higher than its current elevation with its base over a thousand feet below the existing surface. Since then denudation occurred continuously, stripping the summit exposing rugged peaks and crags and filling the intervening valleys with deep thicknesses of colluvial material (figure 1(c)). The concentric nature of the volcanic beds would have been revealed as a ring with the oldest rocks at the centre. Drainage would have been radial as rivers cut their way through the resistant rock barriers away from the summit. This long period of denudation took Charnwood through the coal forming period of the Carboniferous when the surrounding valleys would have been occupied by marine lagoons and swamps. It was not until the Triassic period that the relief of Charnwood had eventually been denuded sufficiently for it to be buried beneath the dry desert clay- sands which later formed marls (figure 1(d)). There is evidence that these impermeable marls were deposited around Bardon Hill to a height of 250 metres and possibly as high as 270 metres. The impermeable nature of the marls allowed shallow lakes to form whose presence can be discerned from the laminations in the sandstones seen in cross-section at the many roadstone quarries such as that at Mountsorrel.

During the Cenozoic, however, denudation resumed exposing the rugged peaks (figure 1(e)). Many of the highest peaks had reappeared during the Pliocene. The tropical conditions of the early Cenozoic may have aided the further disintegration of the old rocks by chemical weathering by acidulated groundwater; hydration and hydrolysis being the two main processes. The opening of joints through dilatation during the Cenozoic exhumation would have pre-disposed to the rocks to decomposition.

The Pleistocene which followed with its successive glacial periods would have both aided weathering of the exposed crags through congelifraction and removed the products of this weathering as ice sheets advanced and receded in the surrounding plain (figure 1(f)). Much of the debris from the lower crags was mixed with the Upper Triassic marls by the ice and then strewn erratically over the entire area except for the highest peaks. Examination of the boulder clay has also revealed rounded boulders transported from outside the area, for example chalk and flint from north-eastern England, indicating the severity of glacial erosion. Indeed rocks from Charnwood have also been found in places far removed from the area.

The sequencing of the individual glacial periods of the Pleistocene is, however, fraught with problems. It is known that ice associated with the last glacial period, the Devensian, did not reach as far south as Charnwood, although the periglacial conditions of the period may have resulted in significant congelifraction particularly on the most exposed highest peaks. Furthermore, there is still considerable uncertainty over the previous glacial period, the Wolstonian. Although many have argued that the Wolstonian ice covered a far greater area and for longer, recent evidence (Rose, 1987) has suggested that a stratigraphic anomaly exists at the type site. The Baginton/Lillington gravels, which form the lower part of the Wolstonian at its type site have been traced to East Anglia where they lie beneath the glacial deposits of the earlier Anglian age. Thus the only significant glacial period in terms of landscape modification at Charnwood would appear to have been the Anglian which is thought to have occurred 450000 years BP (Eyles et al 1989).

Figure 1: Diagrammatic summary of the geological history of Charnwood Forest.