The South Wales Coalfield () extends across Pembrokeshire, Carmarthenshire, Swansea, Neath Port Talbot, Bridgend, Rhondda Cynon Taf, Merthyr Tydfil, Caerphilly, Blaenau Gwent and Torfaen. It is rich in coal deposits, especially in the South Wales Valleys. Description It comprises a fully exposed synclinorium with a varying thickness of coal measures (Upper Carboniferous/Pennsylvanian) 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). 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, mostly to the north and west of Neath. The Rhondda Valley was particularly known for steam coals which fuelled steamships of the 19th and early 20th centuries. History Industrial and transport expansion Ways along the valley floors provided the main routes for exporting coal south to ports and docks such as Newport Docks, Cardiff Docks and Barry Docks. Early mining 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 became significant in the previously entirely rural Rhondda Valley. Coal was moved from mines on tramways to canals such as the Swansea Canal and Glamorganshire Canal. These were supplemented, and then superseded, by numerous competing railways which fed the docks at Swansea, Cardiff, Newport, Llanelli and Barry. Later colliery shafts were sunk as deep as in order to reach the thicker, better quality seams. 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. Decline 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. Oil, having higher energy density