Geology of the Bath area: Applied geology: building stone and bulk minerals
|This topic provides a summary of the geology of the Bath area – covered by the British Geological Survey
1:50k geological map sheet 265.
Authors: A J M Barron, T H Sheppard, R W Gallois, P R M Hobbs and N J P Smith (BGS).
The various limestone and sandstone formations of the district have all, at some time or other, been exploited for building stone on a local scale. However, the Bath district is best known for the building stone worked from the rocks of the Chalfield Oolite Formation, the so-called ‘Bath Stone’ or ‘Great Oolite Freestone’; ‘freestones’ being limestones which can be sawn or trimmed in more than one direction, and which are thus suitable for complex carving or moulding. The best freestones in the Chalfield Oolite are found in the upper part of the Combe Down Oolite Member and within the Bath Oolite Member, where the rocks are composed of fine- to coarse-grained ooid-limestone with a sparry cement and little matrix.
The freestones of Bath have been worked since Roman times: villas at Box and Bathford used local stone, as did parts of the Great Baths. During medieval times, Bath Stone was used for the construction of several great buildings in the south-west, including Lacock Abbey and Longleat. During this period, the stone was won mostly from surface workings, but by the 18th century the rise in demand led to the development of underground stone mines, where the rocks are unweathered and the stone generally of better quality. Mining was constrained by various practical issues, including the need for a sound roof bed above the mined freestone horizon; consideration of the nature and thickness of the overburden with regard to the potential for subsidence, and the need to avoid areas of disturbance such as fault zones or areas of landslide and camber. The Chalfield Oolite is also a major aquifer (see above), and groundwater ingress presented a further problem. The principal quarries and mines were at Box, Corsham (P539467), Monkton Farleigh (Brown’s Folly) and Combe Down. The mined freestones were used in the construction of many buildings, including Buckingham Palace in London and the Royal Pavilion in Brighton, and were widely exported, being employed in the Town Hall of Cape Town, South Africa, and Union Station in Washington DC. Demand began to decline during the economic depression in the 1930s. During the Second World War, many of the mines were used for ordnance factories and storage. Production in the later part of the 20th Century has been relatively small, and today only three sites remain in work: the Upper Lawn Quarry [766 624] at Combe Down, and Elm Park Mine [885 682] and Hartham Park Quarry [855 701] at Corsham, the last two being underground mines.
The large quarries in Carboniferous Limestone at Wick continue to be worked for crushed rock aggregate and coated roadstone. Construction sand has formerly been extracted from sand beds in the Hazelbury Bryan Formation and in the Lower Greensand to the north-east of Melksham, but these workings are now disused.
The name ‘fuller’s earth’ derives from the ability of certain clays to absorb oil and grease, and therefore to be used for cleansing or ‘fulling’ woollen cloth. These clays are composed mainly of the mineral montmorillonite, and today they are used in oil refining, as a suspension agent in drilling muds and agricultural sprays, and as a component of grouts in civil engineering applications. Until 1980 a bed of fuller’s earth of relatively low grade was mined at Midford [754 616] and the Combe Hay Works [729 612], just south of the Bath district. Resources are still to be found south and east of the city, around Wellow, extending northwards as far as Bathampton Down [77 65]. A full account is given by Forster et al. (1985).
- Forster, A, Hobbs, P R N, Monkhouse, R A, and Wyatt, R J. 1985. An environmental geology study of parts of West Wiltshire and south-east Avon. British Geological Survey Technical Report, WA/85/25.