Local tradition has it that the menhir was one of three that blocked the entry to Hell. A curious custom involved young girls who wished to marry within the year having to climb to the top and slide down 'in their christening knickers' ....
In Brittany are monoliths about which women dance in a state of nudity, and rub themselves against them in hopes of thereby becoming mothers.
Near Dinan is the stone of St Samson. Girls slide down it, as it is on an incline, and if they can reach the bottom without a hitch, they believe that they will be happy mothers when married.
Some of these stones are pitted with artificially cut hollows. The stones are washed, to produce rain, are anointed, and the cup-marks filled with butter and honey. Most in France are now surmounted with crucifixes, or have a niche cut in their faces into which an image of the Virgin is inserted.
From p37 of Sabine Baring-Gould's 'Book of Folklore' (1911).