The country in which I live in is the Republic of Macedonia and its borders are based on physical features.
The founders of my state/country initially took the ethnic boundaries as to determine the borders of the country, but since big portion was already occupied by stronger countries, for the part that wasn't occupied as a markers were taken the geographical features.
Most of the borderline of my country is set on along the highest points of the mountains, so the one part of the mountains belongs to my country, while the other part of the mountain to the other country, kind of like the separation of the watersheds.