A linear feature in the form of a shallow, embanked, rush-filled ditch can be followed from the cutting of the river Usk for more than 2 km. Its line is visible on air photos.
From the river (the given NGR) the leat follows the contour, roughly 335m, along a north-west to northerly line, across the Dunant Wysg before turning sharply to continue its northerly course. Its form varies, fading completely in places before reappearing. Where it is clearly visible it forms a curvilinear hollow 1m-1.5m wide, embanked on its downhill side to a height of about 0.5m but only about 0.2m above the peat-filled ditch. It fades at around SN81532686 close to an unnamed stream near the Trecastle-Talsarn road. Stream courses and bogland make it difficult to follow from here, on the ground or on APs. However, a well-defined line can be picked up on the north side of the road at SN81532716 from where it eventually links with a water course previously noted (1), before disappearing into forestry above the Usk Reservoir.
The purpose of the feature is problematic. The river now lies about 3m below the point at which the leat meets the river cutting, and it now intersects the river cutting at a right angle. Erosion may have led to the disappearance of further evidence of the leat-river junction, or else some means of raising water from the river was employed. If so, the leat is likely to be of some antiquity, possibly Roman. However no mineral deposits, aside from building stone, are known from this part of the upper Usk that might explain such an undertaking.

(1) Leighton, Mynydd Du & Fforest Fawr (RCAHMW 1997), p.30; additional information provided by Andrew Davies.

David Leighton, RCAHMW, 13 March 2012