A slate boundary stone on Maesglase. A boundary marker, border marker, boundary stone, or border stone is a robust physical marker that identifies the start of a land boundary or the change in a boundary, especially a change in direction of a boundary.There are several other types of named border markers, known as pillars, obelisks, and corners. slate boundary stone on Maesglase. A boundary marker, border marker, boundary stone, or border stone is a robust physical marker that identifies the start of a land boundary or the change in a boundary, especially a change in direction of a boundary.