 The other major factors in the decline of coal were related to the massive under @-@ investment in Rhondda mines over the past decades . Most of the mines in the valleys were sunk between the 1850s and 1880s , which , as a consequence , meant they were far smaller than most modern mines . The Rhondda mines were in comparison antiquated , with methods of ventilation , coal @-@ preparation and power supply all of a poor standard . In 1945 the British coal industry cut 72 per cent of their output mechanically , whereas in South Wales the figure was just 22 per cent . The only way to ensure the financial survival of the mines in the valleys was massive investment from the NCB , but the ' Plan for Coal ' paper drawn up in 1950 was overly optimistic in the future demand for coal , which was drastically reduced following an industrial recession in 1956 and an increased availability of oil . 
