Rubble stone causeway contemporary with the deep cutting of the Tennant canal of 1821 over which it passes, the early horse railway which crosses its deck was opened in 1809 from works on North bank of River Clydach to Dyffryn Clydach (2 1/2m).

Interpretion / significance. Horse-worked railways of the early nineteenth-century were largely confined to Britain. Without steam-powered machinery it was difficult to build huge earthworks and lines were quite often supported on tall masonry walls or causeways of which this is one of the largest surviving intact examples. Other examples survive intact at Hirwaun Ironworks; on the Ffestiniog Railway; at Cwm Nant Llwyd near Alltwen, Pontardawe and at Nunninglow on the Cromford and High Peak Railway in Derbyshire in the English Midlands. For that reason this structure is of some international significance. (See Stephen Hughes, 'The Archaeology of an Early Railway System: the Brecon Forest Tramroads' (RCAHMW, Aberystwyth) 1990 pp. 316-20 for more contextual details of significance).

It would have been designed by the engineer William Kirkhouse, in 1821, as he had designed the rest of the Tennant Canal.

The Early railway causeway at Skewen is a high stone structure built causeway of coursed green Pennant sandstone rubble built over a deep, still watered canal cutting on the Tennant Canal. In design terms it bears a family resemblence to the same engineer's ruinous Cefn Morfydd inclined causeway on his 'Parson's Folly Tramroad' at Aberdulais.

The cutting at Skewen is bottomed by an inverted arch built to support the canal over quicksand. The south elevation of the causeway seems to have been buttressed by the addition of a heavy wall to the east of the canal: butt-jointed onto the original structure. The face-plate of one wrought-iron tie-rod, designed to reinforce the high narrow structure of the causeway, survives, five more holes in the fabric on two levels may indicate where more were. The south-eastern end of the bridge is terminated by rectangular copper-slag blocks and these may have continued as a parapet. However this is now irregular and very patched with concrete. Three cast copper-slag blocks are built into the south elevation. The arch-stones, or 'voussoirs', are of tooled green Pennant sandstone. The abutment walls of the bridge are of coursed rubble of the same material.

The inverted-arch is hidden from view. The long curving canal cutting is deepest at the point of the bridge crossing and this probably is largely responsible for the form of the bridge. The location was determined by the line of the early railway crossing the new canal formation at this point. There are smaller flat-decked bridges at the south and north ends of the cutting. The form of another flat-decked bridge at the entrance to the Crown Copper Works to the south is explained by the fact that another early railway crossed into that works there. At the causeway the Mines Royal Works stands on rising ground to the east of the canal: there must have been an early railway embankment to the west (Stephen R. Hughes, 18.01.2007 using notes made on 29.07.1977).