The Moelwyn Slate Quarry, or mine, is connected by tramways with inclines to slate works at SH66834431 and thence to Ffestiniog Railway (NPRN 34660) at SH67884423. It is shown as disused on the Ordnance Survey County Series map of 1889 (Merioneth. III.16). The construction of Llyn Stwlan Reservoir (NPRN 306315) has inundated parts of the quarry complex.
J.Wiles, RCAHMW, 19 July 2004
The search for slate here began in the 1820s in a number of trials on the north-east flank of Moelwyn Bach. In the 1860s new workings were developed under the east face of Moelwyn Mawr, and a tramway incline system was built to carry slabs to the mill, and finished slates down a further series of four inclines to the Ffestiniog Railway. The quarry was worked in the 1870s and again in the 1890s, but closed c1900. The raising of Llyn Stwlan for the Festiniog pump-storage scheme has drowned parts of the tramway and inclines between the 1860s developments and the mill.
W J Crompton, RCAHMW, 30 July 2009.