Section 1 - Passamaquoddy Bay
The 40 kilometres (25 miles) of international boundary passing between the islands of Passamaquoddy Bay is made up of 14 straight-line segments. The turning points between boundary segments are ranged by on-shore range marks. A number of the range marks are located on islands.
Section 2 - St. Croix River
From Passamaquoddy Bay, the boundary follows a system of rivers and lakes to the source of the St. Croix River near Houlton, Maine. This 210-kilometre (129-mile) section of the boundary is made up of 1,090 straight-line segments along the deepest channel of the waterway. The many turning points or heading changes between boundary courses are referenced by 256 boundary reference monuments. Much of the St. Croix River boundary passes through heavily forested countryside, thus making access to the range markers difficult.