The Afon Twrch is a river which rises in the Black Mountain in south Wales.

[2] Settlements on the lower section of the river include Ystradowen, Cwm-twrch-uchaf, Cwm-twrch-isaf and Gurnos.

Parts of Cwm Twrch on its Carmarthenshire side are designated as an SSSI because they expose a good example of a boundary between strata.

[3][4] A colliery was established at Henllys Vale (OS grid ref SN762137) from which tramroads or waggonway ran down the valley to Cwm-twrch-isaf.

A bank of limekilns is also located at this spot, which can be reached by a walk along the line of the tramway running upstream from Brynhenllys Bridge (OS grid ref SN 756125).