Economic minerals other than coal, Bristol and Gloucester region
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
Roadstone, aggregate, lime
The Carboniferous Limestone accounts for the greatest tonnage of stone sold. There are many large quarries in it on both sides of the River Severn, but the largest concentration is in the eastern Mendips. The limestone is mainly sold as hardcore, roadstone, concrete aggregate, tarmacadam-coated stone and agricultural dust; there is one large lime producer. All the limestone formations are used. The total annual production, which peaked in the early 1970s at about 18 million tons, is of national significance and serves a wide area of southern England.
The Silurian volcanics of the eastern Mendips, the Cromhall Sandstone of north Bristol and the Upper Old Red Sandstone at one locality, though worked on a much smaller scale, are of importance as roadstone because their wearing properties are different from the limestone.
The Blue Lias continues to be worked for hardcore, building stone and lime, principally along the Polden Hills, south of the Mendips. Quarries in the Middle Jurassic limestones of the Cotswolds mainly produce roadstone of various types for local use.
The iron ores of the Forest of Dean have been worked since Roman times, the ancient outcrop workings being known as ‘scowles’. Most of the mining ceased at the beginning of the present century with the approaching exhaustion of the orefield.
The main ores, called ‘brown haematites’, consist of hydrated ferric oxides with which are included the crystalline form known as goethite. The metallic iron content varies from 15 to 65 per cent. The ore occurs as irregular pockets, lodes and veins, partly replacing limestone in the Carboniferous Limestone (P948990). The Crease Limestone and the basal beds of the Whitehead Limestone were the chief repositories: ore was also found in the Drybrook Sandstone, Lower Dolomite and the basal limestone of the Lower Limestone Shale. The orebodies, which decrease with depth, were deposited from descending iron-bearing solutions, the open-jointed limestones being highly susceptible to permeation and metasomatic replacement. The iron carbonates and pyrite of the Coal Measures shales appear to have been the primary source of the iron: weathering under the desert conditions of Permo-Carboniferous times appears to have given rise to an iron-rich surface from which acidic solutions descended to the underlying rocks.
In the Bristol and Somerset area, hematite lodes in the fissured Pennant Sandstone were formerly worked at Iron Acton, Frampton Cotterell, and Temple Cloud, and on a smaller scale elsewhere. The red and yellow earthy iron oxides known as ‘oxide’ and ‘ochre’ have been worked at several localities in the region for the manufacture of pigments. Most recently, Winford and Wick were the chief centres of ochre workings, the material being dug from pockets in the Carboniferous Limestone or from bedded replacement deposits in Triassic rocks adjacent to the limestone.
Lead and zinc ores
The working of lead ores in the Mendips dates from Roman times; the heyday of the industry was probably in the 17th century, but activity continued until the early years of this century. A conservative estimate of the tonnage of lead concentrates produced is in the order of a quarter of a million tons. Serious exploitation of the zinc ores occurred between the beginning of the 17th century and the middle of the 19th century, and the amount produced must have approached that of lead, but no figures are known. Mining apparently ceased with the exhaustion of the readily accessible ore. The depths of the workings do not appear to have exceeded about 100 m.
The main lead ore was galena (PbS) and that of zinc was smithsonite (‘calamine’, ZnCO3). They occur as veins or fissure-fillings in the Carboniferous Limestone and Dolomitic Conglomerate (Green, 1958), with a gangue of calcite and to a lesser extent, of barite and, rarely, barytocelestite.
The main orefield lay in the Central Mendips between Charterhouse and Pen Hill. An important offshoot in the Rowberrow-Shipham area was the main centre of the zinc workings. The unrestored old mine workings are known as ‘gruffy grounds’ and, though many have been levelled, some can still be seen in the Charterhouse–Lamb Leer cavern area. The ore was taken to be washed and smelted at the ‘mineries’ near Charterhouse, Priddy and East Harptree, where extensive deposits of the old slags and fines (‘slimes’) accumulated. The last phase of the industry consisted of reworking these deposits, and all the remaining ruined mine buildings and ‘washeries’ date from this period.
In addition to the main central Mendips orefield, similar, though more scattered occurrences of ore, mainly galena, have been worked in the Carboniferous Limestone over a very wide area, including much of the remainder of the Mendips, the Weston–Worle ridge and Broadfield Down; also at Pen Park Hole, Brentry and near Westbury-on-Trym, in the Bristol area.
Although there is general agreement that the source of the mineralising fluids is far travelled, it remains uncertain whether juvenile (magmatic) or connate (formation) water, or a combination of both is involved. The Triassic age of the mineralisation, obtained by isotopic dating of the galena, which corresponds to the age of the youngest strongly mineralised rocks (Dolomitic Conglomerate), is no longer accepted as conclusive. It has long been known that small-scale galena-sphalerite-barite-calcite mineralisation affects Penarth Group, Lower Lias and Upper Inferior Oolite rocks in the Mendip–Bristol area, including their secondarily silicified facies (Harptree Beds) within, or close to the main ore-bearing areas of the Mendips and Broadfield Down. The question as to whether the minor and major mineral occurrences are of the same age would be solved if the latter could be linked to the extensive metasomatisation represented by the Harptree Beds. Recent evidence (Stanton, 1981) favours this, and a Middle Jurassic (or later) age for the mineralisation appears increasingly possible.
Small quantities of the earthy oxide of manganese, pyrolusite (MnO2), occur as diffuse pods within iron-hydroxide ore masses in the Dolomitic Conglomerate of the Mendips at East Harptree, Higher Pitts near Ebbor, Croscombe and Wad- bury. Much of the ore was used by the potteries for imparting a black colour to the ware, but at Wadbury it was used in the local ironworks to harden steel. It is no longer mined. Of purely scientific interest is the occurrence of a suite of rare minerals, notably oxychlorides of copper and/or lead including mendipite, in cavities within the pyrolusite at Higher Pitts. Comparable occurrences have now been recorded from manganiferous iron ore veins up to 2 m across in Carboniferous Limestone in the eastern Mendips.
Brick, tile and pottery clays
A large brickmaking industry using the wide variety of brick-clays and marls was formerly active within the region. Now fewer than six brickyards remain operational. Household bricks, tiles and pots were made from clays in the Old Red Sandstone, Mercia Mudstone Group, Lias and Alluvium, whilst Coal Measures clays were used for the manufacture of engineering bricks and large pipes. Around Bridgwater, the fine silty alluvium of the River Parret was exploited to make ‘Bath Bricks’, used for scouring purposes.
Many of the Lower Lias shales are bituminous to some degree. In the early 1920s, sampling of shales in the bucklandi Zone on the west Somerset coast showed an oil content of 40 gallons (182 litres) to the volumetric ton (1.15 m3). Commercial retorts were built at Kilve and produced some hundreds of barrels of oil, but then ceased through lack of financial backing.
- Green, G W. 1958. The central Mendip lead-zinc orefield. Bulletin of the Geological Survey of Great Britain, No. 14, 70–90.
- Stanton, W I. 1981. Further field evidence of the age and origin of the lead-zinc-silica mineralisation of the Mendip region. Proceedings of the Bristol Naturalists’ Society, Vol. 41, 25–34.