One of six documented seigneurial mills known to have been established around the margins of the Great Forest of Brecknock by the fourteenth century, and named `Pwllcoch? in early sources (1). The site lay in Cwm Taf in what became the hamlet of Ynys-y-felin, north of Merthyr Tydfil, at an altitude of 800 feet.
The mill's proximity to the moorland edge of the Forest (a few hundred metres to the west) suggests not only that the modern interface between farmland and open land was already taking shape here by this early date but also that upland agriculture hereabouts in the medieval period may have been much more extensive than was previously realised.
The mill was documented as a customary corn mill in the sixteenth century (2). From the 1660s it was apparently suffering from competitor mills and becoming less viable. It was still operating as a corn mill in 1701 but by 1714 it had been converted for use as a fulling mill, a change perhaps hastened by a run of bad harvests. By the nineteenth century it was being called a woollen mill.
On the first edition OS map the mill building is revealed not only by the place-name but also by its leat, still visible. However, the map makes no mention of the mill itself, whether in use or disused, and nor does the Tithe Award map, suggesting it had already gone out of use by the mid-nineteenth century. The site now lies submerged beneath the Llwyn-on Reservoir.

(1) The National Archives, Ministers? Accounts, SC6/1156/17: farmers of mills in Brecon Forest, 1371-2.
Davies, R.R. Lordship and Society in the March of Wales 1282-1400, p.403
(2) A.M.Selwood. The Social and Economic History of the Parish of Penderyn, Brecs, 1500-1851. University of Wales Masters thesis
(National Library of Wales).

David Leighton, RCAHMW, 22 July 2011