This drainage ditch runs for some 1.5 kilometres around Twyn yr Arian hill, at the northwestern corner of Mynydd Eglwysilan. It seems to have been cut in the late 20th century to control water flow around a large colliery spoil tip, now levelled and graded, which lies immediately to the west of the common. The ditch still holds water and measures up to 3 metres wide at the top and 1 metre wide at the base. At its northwestern end, it passes under a post-medieval, drystone field boundary wall via a sluice channel that has been cut without disturbing the wall, with a concrete lintel inserted beneath the wall base for support. J.J. Hall, Trysor, 28 October 2012