Undisturbed deposits of lead ore were found when Grebe Swallet, an old mine at Charterhouse, was reopened in 1982. Abraded lumps of galena are present in a wide variety of sedimentary matrices filling voids in the Carboniferous Limestone. The deposits are residual orebodies derived from primary galena veins that once existed at levels above the present limestone plateau of the Mendip Hills. Formed in association with neptunian dykes, the primary lead veins were fragmented and concentrated as the plateau surface was lowered 90 metres or more by dissolution over a very long period of time. It is argued that all the lead orefields of the Mendip high plateau are likely to have consisted of secondary residual deposits similar to those at Charterhouse.