Shortly after the South Wales Railway opened in 1850, a considerable slip occurred in the deep cutting near Llansamlet between Neath and Swansea. To stop further movement and to avoid the enormous excavations that would have been needed to flatten the slopes, the line's engineer, I.K.Brunel, erected four flying arches across the cutting. Each arch is about 12m wide and 20 to 25m long, of rubble stone with cut stone voussoirs and is weighted on top with copper slag to resist the sideways pressure.
The arches are in a group of three with a fourth one on its own some 100m to the west; this is the most westerly arch.
Source: History of the Great Western Railway, E.T.MacDermot, Vol. 1, 1927, p.297.
B.A.Malaws, RCAHMW, 18 February 2010.