 Initially the shallower pits at Aberdare proved a bigger attraction for prospective mine owners , but once Aberdare became fully worked by the 1860s the Rhondda saw a rapid growth in development . During the 1860s @-@ 1870s 20 collieries opened in the Rhondda Valleys with the leading <unk> in the Rhondda Fach being David Davis of Aberdare , and David Davies in the Rhondda Fawr . In 1865 the output of coal from the Rhondda Valley was roughly one quarter of that of Aberdare ; ten years later the Rhondda was producing over two million tons , more than the Aberdare Valleys . These figures would later be dwarfed by the massive excavation rates seen in the last quarter of the 19th century up to the beginning of the First World War . In 1913 it was recorded that the Rhondda Valley 's output was 9 @.@ 6 million tons . 
