I call that yellow lichen Xanthoria - it grows very slowly (and I'm not sure that is the correct name for it). The grey lichen is faster growing - I just call that 'grey stuff'. One of the glories of studying (and making) Rock Art is that it is very difficult to discriminate between that which is fifty years old and that which is five thousand. Thank heavens for this forum - may all county archaeologists balls drop off - sadly though I've only rearranged 100 tonnes of stone this summer rather than the 200 I had intended. I was straight-backed and angry in May - now bent and blathering !