There are a series of natural outcrops of spotted dolerite ('bluestone') which naturally fragments into pillars, blocks and screes. Noted as the geological source of the Stonehenge 'bluestones' but debate is still active as to whether the stones reached Salisbury Plain in Wiltshire through glacial or human agency.

Survey work by Wainwright and Darvill in 2003 and 2004 for the Strumble-Preseli Ancient Communities and Environments project (SPACES) has identified possible sites of quarrying, or hollows from which loose blocks have been extracted, undated evidence for mining in the form of deep cuttings resembling adits (in the eastern part of the Carn) and a possible axe flaking floor at the south-east foot of the Carn where there are intrusive outcrops of a rhyolite beneath the dolerite outcrops. Most interesting has been the identification of three or four large orthostats which appear to have been abandoned on the southern slopes below the Carns during transportation. All show signs of quarrying along one side, with the other sides retaining the natural, weathered stone surface. Some are propped up on other stones; one is split in two. It would appear from this evidence that blocks of spotted dolerite were being worked free from the outcrops at some point in the past, and dragged south from the outcrops towards the lowlands. Initial field visit by members of RCAHMW Survey Branch on 21st April 2004.

Main reference: Darvill, T and Wainwright, G, 2002, SPACES - exploring Neolithic landscapes in the Strumble-Preseli area of southwest Wales, Antiquity 76, 623-4.

T Driver, RCAHMW, 24 August 2004.