This is the correct description https://fr.wikipedia.org/wiki/Dolmen_de_La_Croix_Blanche_(Eure)
Personally, I don't think it is a megalith at all though it may well mark the site of a destroyed monument. (There is another of these not far away - also shown on the drawing of several such monuments - at top left)
The Captain's photo of the notice board refers to another site altogether.
Something is not right. This message is just to keep things from messing up down the road