one of the largest and oldest coalfields in Great Britain and the basis for the development of the industry of south Wales. The coalfield is situated along the northern shore of Bristol Channel, extending partly onto its shelf. It occupies an area of 2,300 sq km.

The total estimated geological reserves of the coalfield are 9.5 billion tons. The coal, which is associated with Carboniferous beds, has been worked since the 13th century. There are three parallel strata, the first two of which (Carboniferous limestones of the Lower Carboniferous and buhrstone of the Namurian Stage) do not contain working coal seams. The Upper Carboniferous coal-bearing stratum consists of three series: the lower coal series (900 m thick), the Pennant sandstones (180–1,200 m thick), and the upper coal seam (more than 300 m thick). The last has been highly eroded by water in places. The rocks have been compressed into an asymmetric synclinal fold, which extends lati-tudinally and is greatly complicated by secondary folds and diagonal fractures. Overthrusts have developed in the western part, as a result of which certain parts of the Carboniferous beds are overlain by Precambrian and Silurian rocks.

Industrial coal reserves are concentrated in the lower coal series and in the Pennant sandstones, which have almost the same coal distribution. The upper series has a few thin coal seams, which have been virtually depleted. There are 12–20 seams in the South Wales coal basin, with an average thickness of 1–2 m, bulging out to 9 m in some places. The coals are hard, ranging from bituminous coals with a high content of volatile substances to anthracite. The ash content of the anthracite and highly metamorphosed coals is 2–5 percent; in the other coals it increases to 11 percent. The sulfur content ranges from 0.6 to 3.0 percent, and the phosphorus content, from 0.1 to 4.0 percent. The heat value is 35.5–36.5 megajoules/kg (8,450–8,700 kilocalories/kg). The coal is mined by underground methods.

The maximum extraction in the coalfield was 54.7 million tons, reached in 1913. Subsequently, production declined sharply owing to the mining of deeper horizons under increasingly difficult conditions. In 1976, 8.3 million tons were extracted. The principal center is Rhondda.