MOUNTAIN ASH  , an urban district of Glamorganshire, south Wales, in the Aberdare valley on the Cynon, a west bank tributary of the Taff, with stations on the Taff Vale and Great Western railways, 18 m . N.E. of Cardiff . Pop . (1901), 31,093 . A branch of the Glamorganshire canal passes through the place . At the beginning of the 19th century Mountain Ash was a small village known only by its Welsh name of Aberpenar, but from 1850, with the development of its collieries, the population rapidly increased . The district has an area of 10,504 acres and comprises; besides Mountain Ash proper, a string of villages, the chief being Cwmpenar, Penrhiwceiber, Abercynon or Aberdare Junction (at the confluence of the Cynon with the Taff) and Ynysybwl, 3 M. to the west on the Clydach . The public buildings include St Margaret's (1862) and St Winifred's (1883), the parish churches of Mountain Ash and Penrhiwceiber respectively; old and new town halls (1864 and 1904), cottage hospital (1896), and a library institute and public hall erected in 1899, at a cost of 8000, by the workmen of Nixon's Navigation collieries . There is a park of 7 acres given in 1897, by Lord Aberdare, whose residence, Duffryn, is in the district . There are also a workmen's institute and a public hall at Penrhiwceiber . The older part of the urban district is included in the parliamentary borough of Merthyr Tydfil, and also shares with Merthyr and Aberdare the services of a stipendiary magistrate .