Clearly, a special stone was required for the production of axes, and several so-called axe factories have now been identified in the UK where stone of just the right quality was worked on an almost industrial scale in the 3rd millennium BC. ‘Rough outs' of approximately the right shape and size would be prepared from the local rock, then worked on to produce the final axe. The final axes were so valuable that they were sent all over the country and provide the first real evidence of trade in UK prehistory. Large axe factories occur at Tievebulliagh in Northern Ireland, Graig Lwyd in Snowdonia and at Great Langdale in the Lake District, where the distinctively grey-green local Borrowdale volcanic tuff was exploited. Langdale, or Group VI axes, are the commonest ones found in Yorkshire.