Fort Davis Geology: A Frontier Fort Beneath Volcanic Palisades

Fort Davis and the valley below the Davis Mountains. Image from the NPS's page.

A fort in a sky island

Fort Davis National Historic Site preserves one of the largest and most intact post-Civil War frontier forts in the American Southwest. It stands on the eastern side of the Davis Mountains, a sky island in the Trans-Pecos region of West Texas — higher, cooler and wetter than the Chihuahuan Desert around it. They are the largest mountain range entirely within Texas, rising to nearly 8,400 feet, and were built by volcanic activity over three million years, ending about 35 million years ago.

• The site: at the mouth of Hospital Canyon, a dry box canyon near Limpia Creek, where wood, water and grass were plentiful. Limpia Creek runs just outside the northern boundary, and Davis Mountains State Park borders the site to the northwest.
• Size: 523 acres (212 hectares), from about 4,830 feet (1,470 m) to 5,355 feet (1,630 m).
• Underfoot: the fort sits on alluvial fan deposits beneath the cliffs of Sleeping Lion Mountain and North Ridge, both of rhyolite.

The volcanic setting

The Davis Mountains Volcanic Field is part of the larger Trans-Pecos Magmatic Province, where volcanoes were active from 47 to about 17 million years ago — the earlier eruptions mostly to the south, in Big Bend. The Davis Mountains erupted mainly 37 to 35 million years ago, in the region's main phase.

• Flood rhyolites: huge rhyolite lava flows covering wide areas — rare in the geologic record.
• Ash-flow tuffs: produced in caldera-forming eruptions, when so much magma erupts that the ground above the chamber collapses. Four caldera complexes have been found in the Davis Mountains.
• Unusual chemistry: many of the rocks are peralkaline — rich in potassium and sodium. That makes the lava far less viscous than most rhyolite, which usually piles into domes rather than flowing far — surely a factor in the flood rhyolites. Such rhyolites also erupt hot: their ash-flow tuffs can land hot enough to flow again (rheomorphism), and then look so much like lava flows that geologists can't always tell them apart.

The site belongs to the Basin and Range Physiographic Province.

Three rock units

Bedrock geologic map of Fort Davis, Texas. NPS image.

• Frazier Canyon Formation: tuff-rich sediments and mafic lava flows, older than 36.5 million years, about 120 feet (37 m) thick at the base of North Ridge.
• Sleeping Lion Formation: the main rock of the site, making up Sleeping Lion Mountain and most of North Ridge. Low-silica peralkaline rhyolite with crystals in a glassy groundmass, erupted about 36.5 million years ago west of Fort Davis, it flowed east down ancient valleys. Its origin is a puzzle: it shows both the glass shards of an ash-flow tuff and the basal breccias of a lava flow. About 200 feet (60 m) thick here, with prominent columnar jointing.
• Barrel Springs Formation: the top of North Ridge and the neighboring state park, where it is about 260 feet (80 m) thick. At the site, a rheomorphic ash-flow tuff at its base lies under rhyolite lava flows. Some researchers call it a group and name the lower tuff the Fort Davis Tuff, erupted 36.2 million years ago from the Paradise Mountain Caldera.

The palisades

Columnar jointing in the Sleeping Lion Formation makes the cliffs of North Ridge and Sleeping Lion Mountain look like a palisade. As some lava flows and ash-flow tuffs cool, they contract and crack into regularly spaced joints that meet in a prism pattern. Fort Davis's columns are more irregular than the near-perfect ones at Devils Postpile and Devils Tower, but formed the same way.

Geology and the fort

The landscape is part of the fort's historic setting. It guarded the Trans-Pecos stretch of the San Antonio–El Paso Road and the Chihuahua Trail, and the area is still largely undeveloped, with views much as they were when the fort was active. Geology helped choose the site: water from Limpia Creek and a spring, and wood and building stone nearby. The fort's stone came from quarries in the local volcanic rocks within 1.5 miles (2 km).

Fossils and hazards

• Fossils: none have been found at the site, and none are likely.
• Rockfalls and floods: rockfalls and other mass wasting can occur on the rhyolite cliffs, and flash floods in Hospital Canyon.
• Earthquakes: a relatively low hazard — a 2% chance in 50 years of shaking of 12–14% of gravity (about VI on the Modified Mercalli scale), and 4 to 10 damaging shakings expected in 10,000 years. Texas's largest recorded earthquake, in 1931 near Valentine, about 40 miles east, reached an estimated VIII.

An agave on the slopes above Fort Davis. Image from the NPS's page.

Inventories

The Park Service's Geologic Resources Inventory provides maps, reports and posters for more than 270 natural resource parks, and a Soil Resources Inventory has been completed for Fort Davis.

Sources

Based on "NPS Geodiversity Atlas—Fort Davis National Historic Site, Texas," National Park Service; a work of the United States government in the public domain. A thumbnail of the inventory report's cover is not reproduced.