The coalfield area 
It comprises a fully exposed synclinorium with a varying thickness of "coal measures" (Upper Carboniferous / Pennsylvanian) deposits with thick, workable seams in the lower parts and generally thinner and sparser seams in the upper parts, together with a development of sandstones (Pennant Sandstone). See also the Geology of South Wales. These sandstones have been much used in building construction (including the characteristic terraces of former miners' houses) and give rise to bleak uplands rising 300-600 metres above sea level between the steep-sided valleys in which most deep mines were developed.
The coal generally increases in grade or "rank" from east to west, with bituminous coals in the east, and anthracite in the west. The Rhondda Valley was particularly known for steam coals which fuelled steamships of the 19th and early 20th centuries.
Exploitation of the coalfield 
Communications along the valley floors provided the main routeways for exporting coal south to ports and docks such as Newport Docks, Cardiff Docks and Barry Docks. Early activity was mainly by levels or adits driven into coal seams from outcrops in the valley sides. Development of the coalfield proceeded very actively from about 1850, when deep mining become significant in the previously entirely rural Rhondda Valley. Tramway-fed canals such as the Swansea Canal and Glamorganshire Canal were supplemented, and then superseded, by the development of numerous competing railway branches which fed docks principally at Swansea, Cardiff, Newport, Llanelli and Barry. These towns grew dramatically as a result. Later colliery shafts were sunk as deep as 800 Yards (732 Metres) in order to reach the thicker, better quality seams.
The coal industry engendered many serious mining accidents, which included Britain's worst at Senghenydd, claiming 439 lives, and others at Abercarn, Risca, in the Rhondda Valleys, in the Cynon Valley, Tondu and Aberbeeg, culminating in the Aberfan disaster. Some collieries, e.g., Morfa Colliery, near Port Talbot, Glamorgan, and Black Vein Colliery, Risca, Monmouthshire, suffered three disasters before their closure on safety grounds. The main problem was firedamp explosions, often followed by ignition of coal dust.
Iron ore was also extracted from the coal measures, principally from the north crop area (including Merthyr Tydfil and Blaenavon). The availability of coal and nearby limestone (as a flux) gave rise to a substantial local iron and steel industry which was perpetuated in the 20th century by the location of modern steelworks at Ebbw Vale, Newport and Cardiff and Port Talbot. These used imported iron ore.
Coal fuelling of Royal Navy ships was increasingly challenged from 1904 when strategists including Admiral "Jacky" Fisher and, later, Winston Churchill successfully argued for oil-firing of the steam engines in new ships. This trend, which was later extended to railway locomotives, was a factor in the economic hardship which struck the coalfield after the First World War. Coal workings were over-expanded in the late nineteenth century, but the Welsh coal owners had failed to invest in mechanisation. By the inter-war period the South Wales Coalfield had the lowest productivity, highest costs and smallest profits in Britain. Hardship continued through the 1926 general strike, the great depression of the 1930s, World War II and thereafter. The 1937 novel The Citadel and the 1939 novel How Green Was My Valley (later filmed, with an inaccurately represented "colliery village") describe such hardship, as do the poems of Idris Davies the miner, teacher and poet of Rhymney.
New collieries, particularly in the western part of the coalfield where anthracite is found, were developed into the 1960s by the National Coal Board (for instance, Cynheidre Colliery No 1 shaft, at 798 yards (730 m) deep was sunk in 1954/6). Following the general collapse of the UK coal industry, most pits closed during the 1980s and the last deep mine, at Tower Colliery on the north crop, ceased mining in January 2008. However, a few small licensed mines continue to work seams, mostly from outcrop, on the hillsides. Although some areas of the coalfield are effectively worked out, considerable reserves remain. However, geological difficulties make the cost of significant further extraction high. The coalfield experienced a late-stage development when opencast mining was commenced on a large scale, mostly on the gently-dipping north crop. Most of the old sites have been filled and landscaped, but new operations continue.
Following the Aberfan disaster of 1966, when a coal-tip slurry flow buried a school, mine-waste tips, which had been piled precariously on hilltops in many cases, were extensively regraded and reclaimed. This work continues. Landslipping of the steep valley slopes, and subsidence caused by the coal extraction, have also posed problems.
A subsidiary of Western Coal (which mainly operates in the British Columbia and West Virginia coalfields), Energybuild plc works a drift mine near the old Tower Colliery, the Aberpergwm Colliery. The mine produces high quality anthracite, and is sold primarily to Aberthaw power station and Port Talbot Steelworks and into the wholesale and retail sized coal markets. Energybuild also operates the Nant y Mynydd opencast coal site nearby. The company is now (as of April, 2011) a subsidiary of Walter Energy Inc, a large US coal producer based in Alabama which took over Western Coal.
Gleision Colliery tragedy 
On 15 September 2011, seven miners were working a narrow seam at the Gleision Colliery drift mine in the Tawe Valley, near Swansea, when a sudden ingress of water filled the passage in which they were working. Three miners were immediately able to escape to the surface. Despite extensive efforts to rescue the remaining miners, on 16 September South Wales Police confirmed that all four had died. They have been named as Charles Breslin, 62; David Powell, 50; Garry Jenkins, 39; and Phillip Hill, 45.
- "South Wales (geological map)". Geological Maps of Selected British Regions. Southampton University website. Retrieved 9 April 2013.
- John Fisher biography at First World War.com
- Churchill, Sir Winston Biography at The encyclopedia of Earth
- Davies p.156
- Jenkins, p.366
- News, Sky (16 September 2011). "Trapped Miners: Two Men Found Dead In Wales". Sky News. Retrieved 16 September 2011.
See also 
- Coal mining
- History of coal mining
- 1926 United Kingdom general strike
- UK miners' strike (1984-1985)
- Miners' institute
- Geology of South Wales
- List of British coalfields