most boundary scholars agree that the modern system of negotiated &
monumented international boundaries dates only from about the treaty of
westphalia in 1648 following the thirty years war
however
http://www.esj-lille.fr/atelier/explo/EPOQUE/NEP_06.HTM shows a
photo & tells the story in french of a 1375 boundary monument on the french
spanish border in the pyrenees that is said to have been the focal point of
an annual ceremony of friendship held without fail ever since this monument
was first installed according the terms of the treaty of roncal
it is known today as monument 262 or saint martins stone
in honor of the patron of boundary stones
as also of drunks & innkeepers
but there may also be an even older boundary marker on the other side of france
http://republicaine-lorrain.fr/tourisme/bitche/breintenstein.htm shows a
photo & tells also in french of a prehistoric druidic monument still
standing on or near the present alsace lorraine subdivisional boundary
it was renovated with christian art in 1787
after which it became known as the rock of the 12 apostles or of the 3 peters
apparently it has marked intermittently in its different guises a shifting
international frontier during many centuries
please let me know of any other candidates you may be aware of too
m