The San Andres Mountains form part of the eastern edge of the rift valley of the Rio Grande, and are made up of west-dipping fault blocks made primarily of San Andres Formation limestone, but also with extensive exposures of reddish Abo Formation sandstone on the western side, and quartz monzonite on the eastern side.[1] Gypsum deposits washed from these mountains are the main source of the dunes in White Sands National Park.