 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 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 . 
