 The southern coalfield of Wales is the largest continuous coalfield in Britain , extending some 113 kilometres ( 70 mi ) from Pontypool in the east to St Brides Bay in the West , covering almost 2 @,@ 600 square kilometres ( 1 @,@ 000 sq mi ) . This coalfield took in the majority of Glamorgan , and the entirety of the Rhondda was situated within it . Although neighbouring areas such as Merthyr and Aberdare had already sunk coal mines , it was not until Walter Coffin initiated the Dinas Lower Colliery in 1812 that coal was first exported from the Rhondda Valleys on any sort of commercial scale . This coal was originally taken by packhorse , before the extension of Dr. Griffiths ' private tramline , to Pontypridd and then by the Glamorganshire Canal to the port at Cardiff . The lack of any transportation links was one of the main problems that curtailed exploitation of the Rhondda Valley coal fields , along with the belief that the coalfields beneath the valley were thought to be too deep for economic working . It was therefore seen as an expensive risk and deterred anyone looking for a quick profit . The exploration of the Rhondda was undertaken by the Bute Trustees , agents of the third Marquess of Bute , who not only owned large tracts of valley farmland but also possessed a large financial interest in the Cardiff Docks which would export the coal . The trustees sank the Bute Merthyr Colliery in October 1851 , at the top of the Rhondda Fawr in what would become Treherbert . The Bute Merthyr began producing coal in 1855 , the first working steam coal colliery in the Rhondda . 
