The Organ Mountains are a rugged mountain range in southern New Mexico in the Southwestern United States. Organ Mountains-Desert Peaks National Monument was declared a national monument on May 21, 2014. They lie 10 miles (16 km) east of the city of Las Cruces, in Doña Ana County.

The Organ Mountains are near the southern end of a long line of mountains on the east side of the Rio Grande's rift valley. The range is nearly contiguous with the San Andres Mountains to the north and the Franklin Mountains to the south, but is very different geologically. Whereas the San Andres and Franklin Mountains are both formed from west-dipping fault blocks of mostly sedimentary strata (with limestone most prominent), the Organ Mountains are made primarily of igneous rock (intrusive granite and extrusive rhyolite). Their name reflects their similarity in appearance (particularly the granite "needles" in the highest part of the range) with pipes that would be part of a pipe organ. The San Andres Mountains-(southern subrange of San Augustin Mountains), are separated from the Organ Mountains by San Augustin Pass, through which U.S. Highway 70 passes on its way to White Sands Missile Range, White Sands National Monument and Alamogordo. The Franklin Mountains are separated from the Organ Mountains by a 10-mile wide low area known as Anthony Gap. Much of this intervening land is part of Fort Bliss.

The Organ Mountains may be the most botanically diverse mountain range in New Mexico, with approximately 870 vascular plant species. Several of these, including the Organ Mountains evening-primrose (Oenothera organensis) and smooth figwort (Scrophularia laevis), are endemic to the mountain range and occur only in small, scattered populations. The Organ Mountains also have surprisingly high diversity in ferns, with 30 of the 56 species reported for New Mexico occurring within it. The high diversity and endemism of the range makes the Organ Mountains a very rewarding destination for the botanically-inclined, as well as a focus of botanical study. The flora differs greatly between the three sections of the mountain range, with the two igneous sections (The Needles and the central extrusive portion) sharing relatively few species with the southern limestone portions. The limestone section includes some of the northernmost populations of lechuguilla (Agave lecheguilla), often considered an indicator species of the Chihuahuan Desert, whereas the igneous sections of the range include all of the endemic taxa and have botanical affinity with Madrean flora typical of the southeastern Arizona sky islands.

en.wikipedia.org/wiki/Organ_Mountains_(New_Mexico)