Geology of Gila Cliff Dwellings National Monument

Inside the cliff dwellings, looking out across Cliff Dweller Canyon. NPS.

The NPS Geodiversity Atlas, a product of the Geologic Resources Inventory, summarizes each park's rocks, minerals, sediments, fossils, landforms and soils to support education, geoconservation and management. This is its page for Gila Cliff Dwellings National Monument, New Mexico.

A land built by volcanoes

• The volcanic field: the monument lies in the Mogollon–Datil volcanic field, one of the world's great volcanic provinces, reaching from northern Mexico to Colorado. Around Gila it was active for roughly 35 million years, from 40 million to less than 5 million years ago.
• A caldera: the monument sits inside the 24-kilometer (15-mile) Gila Cliff Dwellings caldera, centered about 6 km (4 mi) west of the mouth of Cliff Dweller Canyon. Its buried eastern edge is tentatively placed on both sides of the Gila River at Melanie hot spring, about 0.8 km (0.5 mi) upstream from where the Alum Canyon trail reaches the river.
• Two collapses: the Gila Cliff Dwellings and Bursum calderas both date to 28 million years ago, but Gila collapsed first — it is filled with 300 meters (1,000 feet) or more of Bloodgood Canyon Tuff, erupted from the slightly younger Bursum caldera. The tuff was dated using sanidine crystals, which flash blue in sunlight and are popularly called "moonstones."
• An open question: the ignimbrite whose eruption collapsed the Gila caldera may be the Shelley Peak Tuff or the Davis Canyon Tuff.
• Today's landscape comes mostly from erosion and faulting of those volcanoes and from streams filling basins such as the Gila Hot Springs graben — a long, down-dropped block of crust and the main structure of the area.

The Geologic Resources Inventory's map of the monument. NPS.

The caves and the cliff dwellings

The monument's most striking feature, geologically and culturally, is the Gila Conglomerate and its caves. The rock readily forms cavities, overhangs and caves, which drew people in:

• Two occupations: an early Archaic group (before 500 CE) and the Mogollon (about the 1270s to 1290s CE).
• Seven caves, carved by streams and groundwater sapping, sit close together on the northwest side of Cliff Dweller Canyon, 400–600 m (1,300–2,000 ft) from its mouth and about 60 m (200 ft) above the narrow canyon floor.
• The dwellings: 50 coursed-rock and masonry rooms plus 50 open areas — courtyards, corridors, walkways and patios — with room shapes following the cave walls.

Park caves are protected under the Federal Cave Resources Protection Act of 1988.

Visitors exploring the cliff dwellings. NPS.

Fossils

Fossils are rare in volcanic country, but packrat middens in the caves may preserve a record of the past 20,000 years, and alluvium and terrace gravels may hold fossils. The Gila Conglomerate yields fossils elsewhere in the region, though none yet at the monument. NPS fossils are protected under the Paleontological Resources Preservation Act of 2009.

Hazards

Geologic processes still shape the park. Watch for rockfall, spalling, flooding, post-wildfire sediment and debris flows, and earthquakes.

Regional setting and resources

The monument is part of the Basin and Range province. The Geologic Resources Inventory provides scoping summaries, digital geologic maps, reports and posters (listed in the NPS DataStore), and a Soil Resources Inventory has been completed. See also the park's pages on geologic activity, the geology of Cliff Dweller Canyon and hot springs, and the USGS geologic map.

Sources

Based on "NPS Geodiversity Atlas — Gila Cliff Dwellings National Monument, New Mexico," National Park Service, citing Ratté (1997, 2008), Chapin and others (2004), Ratté and Stotelmeyer (1984), Ratté and others (2014), Bradford (1992), Nordby (2011) and Anderson and others (1986); a work of the United States government in the public domain.