Four Corners Monument

The Four Corners Monument marks the quadripoint in the Southwestern United States where the states of Arizona, Colorado, New Mexico, and Utah meet. It is the only point in the United States shared by four states, leading to the area being named the Four Corners region. The monument also marks the boundary between two semi-autonomous Native American governments, the Navajo Nation, which maintains the monument as a tourist attraction, and the Ute Mountain Ute Tribe Reservation.

The origins of the state boundaries marked by the monument occurred just prior to, and during, the American Civil War, when the United States Congress acted to form governments in the area to combat the spread of slavery to the region. When the early territories were formed, their boundaries were designated along meridian and parallel lines.

Source: Wikipedia