On this Greek site about Mt. Athos I found pictures of these strange “bas-relief” carvings and a round shape cut in mountain stone, in the neighbourhood of Lavra.
Surely all the shapes have a religious meaning and it is obvious on picture 2. with a Christian cross, but I am quite puzzeled by the other – primitive – images of men and animals and the unknown shapes. What could they mean and who made them? Who knows more?