A Boundary Monument between the United States and Mexico at San Diego, California, USA. The monuments erected by the boundary survey played a pivotal role in securing the line after the Mexican-American War. These obelisks and stone mounds literally marked on the ground the southernmost edges of the nation; they became fundamental points of reference in subsequent boundary disputes (of which there were many) and in the resurvey of the border that took place at the end of the 19th century. Those which survive are often securely protected behind barbed wire and stout fencing!