Woofgang I'd say it is graffiti.
If I went to stone henge and carved my name and date into it that would be considered graffiti.
Same if I went to the local town and carved my name into a paving slab that would also be considered graffiti.
Obviously when the pilgrims landed this was monumental and to mark that and where and when, they carved the stone? That would be Graffiti. If someone carved it later with permission or even by request, then it would not be graffiti.