After the Civil War, efforts began to survey and mark the actual borders. In 1868, the General Land Office (GLO) had Ehud N. Darling survey and set markers along the border between the Colorado and New Mexico Territories (the 37th parallel north); this border over time has become known as the "Darling Line". In 1875, another GLO surveyor, Chandler Robbins, surveyed and marked the border between the Arizona and New Mexico Territories (the 32nd meridian west of Washington). Robbins began near the Mexico–United States border, and worked his way north marking the border every so often. Near the 37th parallel north he intersected the Darling Line, and here he erected a sandstone shaft. This sandstone shaft marked today's location of the Four Corners Monument.