Geology and man, introduction, Bristol and Gloucester region
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
Coal mining was, for about one hundred years, the most important extractive industry of the district. Now the only major activity is quarrying for roadstone, concrete aggregate and, to a much lesser extent, lime. Conforming with twentieth century trends, the former innumerable small workings, using a wide variety of local sources and scattered throughout the district, have now given way to a relatively small number of very large, highly mechanised quarries.
In Victorian times, and until the First World War, the quarrying and mining of Jurassic freestones was widespread and employed large numbers of workers. Nowadays, the use of the natural freestone is confined to the highest quality construction and repair work. In the Cotswolds and adjacent areas, however, the Jurassic limestones still provide material for pulverised and reconstituted facing blocks that are widely used where planning controls decree conformity with the original stone buildings.
The digging of clay for brickmaking and other uses, has also greatly diminished as the focus of the industry has moved to the vast Oxford Clay pits of Bedfordshire. Sand and gravel continues to be dug locally, mainly from the river terrace and fan gravel deposits, but also in Gwent from the Quartz Conglomerate (Upper Old Red Sandstone). Peat is dug from the Somerset Levels for horticultural purposes. The annual production of celestite from north of Bristol is around 10 000 tons. A long history of fuller’s earth mining has now come to an end. At Puriton, near Bridgwater, salt (sodium chloride) was obtained in the earlier part of this century by circulating water through boreholes that penetrated the underlying salt beds and then concentrating the brine solution so obtained. Subsequent drilling has indicated the presence of an extensive saltfield in the Central Somerset Basin. Gypsum, present as nodular masses and veins in the Blue Anchor Formation, was formerly worked on the foreshore at Watchet. Barite (BaSO4), which is found filling fissures in the Carboniferous Limestone of Cannington Park, near Bridgwater, was also worked on a small scale. Other mineral-based industries have been established within the district from time to time and the most interesting or important occurrences are discussed below.