After the US won the Mexican American war, we annexed most of CA and the southwest. Only afterwards did we realize the best route for the southern railroad was still across the border in Mexico. Sooooo we bought the land we wanted from Mexico in what is called the Gadsden Purchase. This marker is one of the original ones, we think, which means it dates from the mid 19th century.
The road right on the border is not great, but is pretty good from here to Lochiel, and the walk would not be prohibitive. Nice views.