Or slate mine, or standing stone? Well wonder no more. Found this interesting website that sheds a bit of light on some of the ancient, and not so ancient, landmarks we often come across on our Welsh wanders. The interface is a bit clunky and takes some getting used to but there's some interesting information in there.
http://www.coflein.gov.uk/For example, that slate quarry wot I found at the head of Cwm Maen Gwynedd, on my last Berwyns walk, is described as follows:
"Complex record for mining site. (Maengwynedd slate mine. First operated in 1866-7. (Williams 1985,119; Silvester, R J 1992d). Includes CPAT PRN 17031 and PRN15201:Operated in 3 phases, 1867-71, 1876-86, 1899-1910. Originally the quarry worked the exposed outcrop, with some attempt to go underground, then slightly lower down was further open working. Subsequent working underground, with trimming and handsawing.)"