While on the subject of boundaries across islands:

Islands can not only appear and dissappear, they can also move!

The island of Schiermonnikoog, like all other West Frisian islands in
the Netherlands, has a tendency to move to the east. On the western
side, they get smaller, but on the eastern side, they grow. It has to
do with tides and currents and stuff. Now east of Schiermonnikoog is
the Frisian-Groningen provincial border. Schiermonnikoog is in
Frisia. Or is it?

Apparently, not entirely. The island walked over, at least partly, to
the other side! It crossed the border sometime in the last ten years
or so, and now the provincial boundary runs over the island!

A nice example of "boundary is fixed, but the land changed". One has
only to look at various river boundaries (e.g. the Mississippi) to
see what meandering can do to the once logical boundary of your
territory.

Still, I expect the Dutch to be doing something about the
Schiermonnikoog case. They did so in the past with the provincial
boundary between Gelderland and North Brabant, where the boundary was
at places following an old riverbed of the river Meuse. The river
changed course since the fixing of the boundary.

No respect for boundaries, our lot.

Peter S.