Area of Carboniferous Limestone, which has been subject to extensive quarrying, particularly in the northern part of the hillfort. A single shaft is recorded north of Foel Farm on the Ordnance Survey 2nd edition map, and it remains visible at SJ 0654 7807.

Large open cuts and levels extend from the uppermost southern slopes of Y Foel, where several intersecting veins were worked for haematite, nickel, cobalt, and pyrites. Excavations conducted by CPAT (Thomas, 1993) near the transmitter station at the southern end of the hillfort uncovered a hammer stone. The stone showed signs of use, including pitting and spalling at one end, with indications that it may have been hafted. Fragments of altered lead ore were also found nearby. The hill is known for its veins of iron and cobalt. Iron oxides have also been found in Roman bronzesmith workshops at Melyd Avenue, Prestatyn, 3.5 km away.

An examination of the hillfort area identified numerous shallow surface workings and larger undated excavations towards the southern end, likely following natural cavities in the limestone that had been infilled with iron-rich deposits. A disusedquarry at the northern end of Moel Hiraddug contains a large area of iron-rich rocks, probably haematite, occurring as the infill of cavities in the limestone.