Glogue Quarry is located to the east of Crymych, in the upper reaches of the Afon Taf. It was a substantial quarry formed by the merging of two early, perhaps seventeenth-century, workings. Material was lowered by two inclines, the upper one abandoned when work deepened and a tunnel was cut to bring material out to the head of the lower incline. The workings were handicapped by a lack of transport. Originally slate was was carted to Blackpool on the Eastern Cleddau, and after 1853 to Narberth Road on the South Wales Railway. Expansion only became possible when in 1873 a siding on the Whitland & Cardigan Railway was laid.
The mill was powered by steam, later electricity. Roofing slates of good colour were produced but as they were heavy the main output was slab. During the 1920s attempts were made to make bricks from slate dust. But these became uncompetitive when, in 1927, the GWR demanded a transport premium due to their weight; forcing closure.
Bulk working has left little to be seen aside from vestiges of buildings in the mill area and the stone-built lower incline. Notably, a terrace of family dwellings was built by the company and is still occupied (in 1991).

Sources:
D.Gwyn, Welsh Slate: the Archaeology and History of an Industry (RCAHMW 2015), pp.184-5, Fig.222 (map) & 230.
A.J.Richards, A Gazeteer of the Welsh Slate Industry (1991), p.220.

David Leighton, RCAHMW, 4 February 2015