Sliding (la glissade), the best-preserved of the pre-megalithic forms of worship, is characterized by the contact, at times brutish, of a part of the person of the believer with the stone itself. The most typical examples which have been preserved (and as the rites have no doubt generally been carried on in secret, much has escaped the observer) are in relation to love and fecundity.
In the north of Ille-et-Vilaine are a series of large blocks, at times, but not always, worn into cups, which have received the significant name of "Roches Ecriantes" because the young girls, that they may soon be married, climb to the top of them and let themselves slide (in patois ecrier) to the bottom; and some of them, indeed, are to a certain extent polished because of the oft- repeated ceremony, observed by numberless generations, which we are assured has been practised there.
At Mell( (Ille-et-Vilaine) the " Roche Ecriante " was worn full of basins; on the rock of the same name at Montault, a neighbor- ing parish, inclined at an angle of 45 degrees, there were visible evidences of numberless girls who had there ecriees. After the sliding it was necessary to place on the stone, which, however, no one must see done, a little piece of cloth or ribbon.
The Worship of Stones in France
Paul Sébillot and Joseph D. McGuire
American Anthropologist, New Series, Vol. 4, No. 1 (Jan. - Mar., 1902), pp. 76-107
Visited this on a cycling holiday in Brittany this summer. Just before the bend in the D7 going out of Saint-Suliac, walk up the farm track to the right, and you will see a home-made sign to "Menhir". That leads you into an orchard (there was nobody around when we were there) surrounding this massive stone. It is a very rugged and irregular shape. Whether really a menhir or just a prominent erratic I know not, though the maps show it as prehistoric. There was a huge pumpkin nearby with the name Gargantua carved into it. Menhir a true word spoken in jest, as Obelix might have said (in English translation).
Fortunately, some enterprising soul has slighly vandalised the chainlink fence next to the locked gate opposite the chamber entrance. There's now enough room to shove a camera lens through, or for those individuals with slightly less respect for authority, it might be possible to utilise the damaged areas of fence as footholds for climbing over the top. Obviously, I'd never recommend anything so foolhardy, but a little leprachaun told me that it's a very easy climb and well worth the effort.