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ISSN 1750-855X (Print) ISSN 1750-8568 (Online)

Field Meeting Report: Charnwood Forest, led by Anthony Evans 18 th May 1985

Les Dolamore 1

DOLAMORE, L. (1986). Field Meeting Report: Charnwood Forest, led by Anthony Evans 18 th May 1985. Proceedings of the Shropshire Geological Society , 5, 11–17. The purpose of the field meeting was to visit key exposures within the Precambrian inlier of Charnwood Forest, at , and Beacon Hill.

1affiliation: Member of the Shropshire Geological Society

The group assembled at the car park adjacent to south-east, however, it strikes parallel to the fold Hallgate Farm and the reservoir, by axis. The cleavage may therefore represent a later Bradgate Park. The area to be visited lies within phase of deformation than that during which the Bradgate Park and is open to the public park. fold was formed. However, no hammers are allowed . We arrived On entering Bradgate Park, the first outcrops at 10.30 am after a two hour car drive from encountered – marked 'A' on the second map Shrewsbury. The weather was fine and warm and (Figure 2) – are of banded pyroclastic rocks: tuffs the bluebells were out in the woods. and lapilli-tuffs, with some intermixed The stratigraphical succession in the sedimentary material. Stratigraphically they are Charnwood Forest is shown in Tables 1 and 2. near the top of the Hallgate Member of the Broadly speaking the Precambrian rocks of Bradgate Formation. These are in The Maplewell Charnwood Forest form a plunging anticline, Group and are low grade metamorphosed tuffs or much faulted, and obscured by the over-lying lithic greywackes. They are very fine grained, mantle of Triassic rocks (Keuper Marl). As shown probable subaqueous sediments, well jointed on the maps (Figures 1 and 2), the oldest rocks some quartz veined but all intensively cleaved. (Blackbrook Group) crop out in the centre of the The exposure at 'B' is in two parts, the upper anticline, while the younger rocks appear on the showing well defined folding but the lower was a north-east, south-east and south-west, forming a mixture of a fine sediment and a conglomerate horseshoe distribution around the Blackbrook which showed the filling in by coarse material of Group. a small channel, the of which showed The Brand Group consists of sedimentary having been rotated by the pressures with their rocks. The rocks of the lower two groups are long axes in the direction of cleavage. wholly or partially composed of pyroclastic The next exposure 'C' showed that the cleavage material. The rocks of all three groups are usually running through the greywacke changed direction well bedded. Besides sedimentary and pyroclastic slightly in places, this change is associated with rocks, there are intrusions of dacite (a type of change in the grain size. Also in this exposure rhyolite, sometimes called "porphyroid"). These other changes in cleavage direction are thought to are often strongly sheared and cleaved. Later be due to folding after metamorphism which has comparatively non-sheared intrusions of dragged the cleavage over in varying directions granophyric diorite (markfieldite, formerly combined with the well marked differential referred to as syenite) are especially well cleavage. developed in the south-western part of the Forest. A short walk brought us to an old quarry at site In addition to the large anticlinal fold affecting 'D', where the cleavage was less intensive and the the Charnian rocks, there are a number of other jointing blocky. The feature illustrated here was secondary structures. These include small folds, a plumose (feathery) markings on the joint faces, cleavage (very well developed in the finer grained showing tension breaks. We then walked on past rocks) and jointing. The cleavage does not show a some buildings in the local stone, some converted close relationship to the major anticline, but to public conveniences, past the door enclosures usually crosses the fold axis obliquely. In the to the ruins of Bradgate House.

Proceedings of the Shropshire Geological Society, 5, 11─17 11  1986 Shropshire Geological Society

L. DOLAMORE

The house was built ca. 1490-1505 by Thomas long contorted irregular pieces, typical of Grey, Marquis of Dorset and father of Lady Jane slumping. Grey, who was born here. Bradgate House was The last exposure 'J' we looked at in Bradgate one of the first great country houses to be built of Park was a small group of large rocks in the brick. It did not suffer in the Civil War, but was Tuffaceous Pelites Member showing a structure left to decay after 1739. The only part which has described as "pull apart" and "sedimentary been completely preserved is the chapel in which boudinage", thought to occur as the pyroclastic occasional services are held. sediment was subjected to down-slope tension To the south of the house, we crossed the whilst still unlithified. stream to examine exposure 'E' of the Stable Pit Woodhouse Eaves is a village shown on the Quartz Arenite. This is a medium-grained last map (Figure 3) and here we looked at two indurated quartzite which weathers to a dark sites. The first was a small quarry beneath the purple colour. It is cut by many quartz veins and a church, probably slates were extracted from here. dyke of very much altered markfieldite. Cross These were interbedded with sub-greywackes – bedding can be seen in these exposures. there were no special features seen within this Slickensides in two directions showed that the Formation of the Brand Group. quartzite formed a small syncline. The outcrop is We then walked out of the village, up possibly part of an exhumed Triassic landscape. Windmill Hill, where the outcrop is an ignimbrite Close by the side of the ruined house we looked at in the Beacon Hill Formation. This appears exposure 'F' which is part of a laccolithic intrusion to be a devitrified pumice-tuff in which flattened of markfieldite or a granophyric diorite of chloritized pumice fragments can be seen. It was andesite and hornblende with usually 10% quartz presumably deposited by a glowing avalanche and alkaline feldspar, all subjected to (nuée ardente) such as engulfed St. Pierre, in hydrothermal alteration. This "markfieldite" is Martinique, during the 1902 eruption of Mount identical to the intrusion at Nuneaton, thus Pelée. No stratification is visible in the exposure. indicative of a Precambrian age. Recent The last site visited was the Type Locality for anomalies in the Rb-Sr dating have cast doubts on the Beacon Hill Formation. This is the second the accuracy of the figures given. highest hill (818 feet) in Charnwood Forest. From We then had a long trek uphill to the War it we can see other parts of the Forest including Memorial area where the view is splendid in clear the highest point – (912 feet) – Ives weather, but the day was rather misty so bad luck Head (660 feet), the town of , the for us! We ate our lunch here as it gradually Castle Donnington Power Station, etc. On clear clouded over. days Boston "Stump" is visible from here. After lunch we looked at exposure 'G' in the At Beacon Hill we saw the main formation of Bradgate Formation, dipping steeply southwards. the Maplewell Group. The rocks are laminated The large exposure of the single bedding plane green or buff siliceous tuffs which weather white contains examples of the Charnia discus . or cream. They showed the well-cleaved nature of The frond-like structure found in 1952 is not the rocks, the "refraction" of the cleavage as it present here but the associated discs were present, traverses bands of differing grain size, the lines on not too difficult to see and of varying sizes. bedding surfaces due to the intersection of The next exposure 'H' by the side of the folly cleavage and bedding. These lines are "Old John", still in the Maplewell Group, approximately parallel to the fold-axis and are illustrates slumping in the sediment – quite small therefore called b-lineations. in extent but the slightly coarser sediment, semi- Most of the folding in Charnwood Forest is lithified, had broken into a breccia in a matrix of concentric in type, (synonyms: flexural or buckle finer sediment lying in the beds above and below folding). This is the type of fold produced by which had maintained coherence in the buckling a pack of cards. Towards the end of the disturbance. phase of concentric folding the cleavage was Further exposures at 'I' showed up as very developed and minor cleavage folding then coarse lumps of breccia in the dacite tuffs and the occurred. This folding took place by shearing whole was structureless. The lumps were often along the cleavage planes. It has produced the

Proceedings of the Shropshire Geological Society, 5, 11−17 12  1986 Shropshi re Geological Society

CHARNWOOD FOREST numerous puckers to be seen in many of the Disclaimer - The information contained in this account exposures. has been prepared from notes taken during the field meeting. Its sole aim is to provide a record of what was seen and provide an insight into the diversity of Precambrian exposed within Charnwood Forest. It should not be used for any other purpose or construed as permission or an invitation to visit the sites or localities mentioned.

Table 1. Stratigraphical succession within Charnwood Forest.

Post-Glacial Alluvium Glacial and

Trias Keuper Marls, and Breccia Basic dykes in Granodiorite at Grace Dieu

Silurian Mountsorrel and Granodiorite and associated hypabyssal and plutonic rocks Microgranite dyke at Lubscloud

Rb-Sr date = 433 ±17 m.y.

Ordovician Tremadocian sediments known just east of the Fault in the Merry Lees Colliery and in boreholes in . Possibly hornfelsed Cambrian in aureole of Mountsorrel Grandiorite Late Precambrian Granophyric diorites of Bradgate, , Rb-Sr date = 552 ±58 m.y.

Late Precambrian The Charnian Supergroup (formerly the to (?) Cambrian Charnian System)

Proceedings of the Shropshire Geological Society, 5, 11−17 13  1986 Shropshire Geological Society

L. DOLAMORE

Table 2.

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CHARNWOOD FOREST

Figure 1: Geological map of Charnwood Forest.

Proceedings of the Shropshire Geological Society, 5, 11−17 15  1986 Shropshire Geological Society

L. DOLAMORE

Figure 2: Geological map of Bradgate Park within Charnwood Forest.

Proceedings of the Shropshire Geological Society, 5, 11−17 16  1986 Shropshi re Geological Society

CHARNWOOD FOREST

Figure 3: Location map for Charnwood Forest.

Copyright Shropshire Geological Society © 1986.

ISSN 1750-855x

Proceedings of the Shropshire Geological Society, 5, 11−17 17  1986 Shropshire Geological Society