A stone-revetted causeway crossing Cwm Clydach, perhaps the most impressive monument on this early horse-worked railway. It also carried the parallel cartroad built by Christie to gain landowners' agreement to the building of the tramroad. When last seen in 1990 the arch seemed on the point of total collapse.

Built in the early 1820s probably by the engineer Joseph Jones of Ystradgynlais and John Jeffreys of Sglydach, and funded by the London Indigo Merchant John Christie (originally from Stirling), as part of the Brecon Forest Tramroad from the Swansea Canal at Cae'r-lan over the uplands of the newly enclosed Fforest Fawr to the Tramroad Wharf near Sennybridge. Published as part of Stephen Hughes, 'The Archaeology of an Early Railway System: The Brecon Forest Tramroads' (RCAHMW, Aberystwyth, 1990), pages 250-51.

Stephen R. Hughes, 08.09.2007.