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In 2004 the EMRG excavated the site of Thomas Bonsall’s late 18th century water-powered (overshot 12 feet in diameter water wheel) stamp mill adjacent to the Nant-yr-onnen steam on Copa Hill, Cwmystwyth.

Here, harder quartz-veined lead ores were crushed. Water was used to lift a series of iron-shod stamps to crush the ore onto a grate as part of a wet stamping process. The lead concentrate was collected in a wooden tub beneath.

Cwmystwyth under snow - Stamp Mill excavation site, March 2004 (S. Timberlake)

Excavation of Thomas Bonsall's late 18th-century water-powered lead-stamping mill, Nant yr onnen, Cwmystwyth. (S. Timberlake)