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You have seen them then Tiompan, extraordinary scary Celtic heads are what they are presumed to be, 2000 years old. Looking at the graffiti the stone must be very soft. Must look up how far North they are......
Just north of Glasgow , Moss , a couple of miles from Milngavie .Difficult to date the heads , interestingly the first mention of the "lifts " is Ure's "The History of Rutherglen and East-Kilbride; Published With a View to Promote the Study of Antiquity and Natural History" ( 1783 ) and he doesn't mention the heads at all , and they are quite obvious and Alcock did say "perhaps celtic iconography" . There are nearby quarries and the suggestion that quarry men were the engravers seems likely .
Yes the stone is very soft , a knife could easily carve it . Lots of natural "cup marks " in the area too .