The Geology of Pecos National Historical Park, New Mexico

Pecos National Historical Park preserves 12,000 years of human history at a crossroads in north-central New Mexico. It was established in 1965 as Pecos National Monument to protect the remains of an ancient pueblo and a 17th-century Spanish mission. In 1990 it was expanded to tell the wider story of the Pecos area, a gateway between the Great Plains and the Rio Grande valley where many peoples met, and to preserve the site of the Battle of Glorieta Pass, a key Civil War battle in the Far West.

Two units

Pecos National Historical Park's two units: the Pecos Unit, crossed by the Pecos River, and the Glorieta Unit, whose Pigeon's Ranch and Cañoncito subunits are crossed by Glorieta and Galisteo creeks. National Park Service.

• The Pecos Unit, about 5,000 acres, holds the original 341-acre monument with the pueblo and mission, and the Forked Lightning Ranch, a 1900s ranch that tells the area's cattle-ranching history. Most of it is underlain by the Paleozoic Alamitos and Sangre de Cristo formations, except where Tertiary gravels and Quaternary deposits cover them. The Pecos River runs north to south through the unit and is joined there by Glorieta Creek.
• The Glorieta Unit, 682 acres, has two subunits. Pigeon's Ranch holds the main battlefield; the Sangre de Cristo Formation is its only mapped bedrock. Cañoncito, site of the Confederate camp, has the park's most complex geology, with rocks from the Proterozoic to the Triassic, faults of the Picuris-Pecos fault zone, and Quaternary deposits along Galisteo Creek.

Where three provinces meet

New Mexico's physiographic provinces. Pecos lies where the Southern Rocky Mountains, the Basin and Range (including the Rio Grande Rift) and the Great Plains meet. NPS image by Rebecca Port.

The park sits in a broad, rolling valley between the Sangre de Cristo Mountains and Glorieta Mesa.

• To the north, the Sangre de Cristo Mountains, the southern end of the Southern Rocky Mountains, run from south-central Colorado into northern New Mexico. They rose during the Laramide Orogeny about 40 to 70 million years ago, and their southern foothills reach into the Pecos Unit and Pigeon's Ranch. At their southern end they are marked by large asymmetrical folds, several of which extend into the Pecos Unit.
• To the south and east, the Great Plains. Glorieta Mesa, part of the plains, rises 1,200 feet to a top of 7,559 feet (2,304 m) and fills the Pecos Unit's southern view. Its gently tilted Paleozoic and Mesozoic rocks face north in an escarpment and slope away to the east and south. The Park Service calls this edge of the Rockies perhaps the most dramatic boundary between physiographic provinces in North America.
• To the west, the Rio Grande Rift, part of the Basin and Range, a series of roughly north–south mountain ranges separated by broad basins. Basin and Range extension near Albuquerque began about 22 million years ago, and the Española Basin west of the park has undergone about 10 to 17% extension. Santa Fe, about 30 miles northwest, lies in the rift.

The Picuris-Pecos fault zone, traced for more than 52 miles (84 km) from the Picuris Mountains south of Taos to south of Cañoncito, is arguably New Mexico's most significant strike-slip fault. It has moved many times, from the Proterozoic through the Ancestral Rocky Mountains and Laramide mountain-building to the opening of the Rio Grande Rift.

The rocks

• Precambrian: granite and granitic gneiss, roughly 1.6 to more than 1.7 billion years old, form the core of the southern Sangre de Cristos and appear in a narrow band at Cañoncito. They formed as chains of small landmasses collided with the growing North American continent.
• Alamitos Formation, Pennsylvanian, about 306 million years old: the park's oldest sedimentary rock, in the eastern Pecos Unit. Gray, fossil-rich marine limestone with coarse sandstone and shale, up to 1,500 feet thick, laid down in shallow seas and rivers.
• Sangre de Cristo Formation, about 280 to 305 million years old: multicolored mudstone with sandstone, limestone and conglomerate, up to 3,000 feet thick near Cañoncito, shed from the Ancestral Rocky Mountains to the northwest.
• Permian rocks at Cañoncito: the Yeso Formation (reddish-brown sandstone and siltstone from tidal flats and coastal plains), the Glorieta Sandstone (clean sand from coastal dunes, a cliff-former whose type section is on Glorieta Mesa), the thin San Andres limestone from shallow seas, and the orange to brick-red siltstone, sandstone and gypsum of the Artesia Formation, from tidal flats and lagoons.
• Triassic rocks at Cañoncito: the red Moenkopi Formation from rivers and lakes, the Santa Rosa Formation that caps Glorieta Mesa, and about 615 feet of the Chinle Group.
• Younger deposits: gravels washed from the Sangre de Cristos filled the valley before the Pecos River and Glorieta Creek cut their modern channels. They cover much of the Pecos Unit, including the pueblo and mission site, and sit as high as 1,500 feet above the river. Quaternary alluvium, terraces and valley fill line today's streams.

Limestone of the Alamitos Formation along the Pecos River. NPS photo by Mario Armijo.

Rivers

About 3.5 miles of the year-round Pecos River run through the park. It rises at 12,000 feet in the Sangre de Cristos about 30 miles north and flows more than 900 miles through eastern New Mexico and West Texas to the Rio Grande, a little upstream of Amistad National Recreation Area. The ephemeral Glorieta Creek starts near Glorieta Pass and joins the Pecos in the park; Galisteo Creek crosses Cañoncito and flows west to the Rio Grande at Santo Domingo Pueblo.

Stone and people

The walls around Pecos Pueblo, built of cobbles from nearby terraces and Glorieta Creek and of Sangre de Cristo conglomerate and sandstone, which split naturally into good building blocks. NPS photo by Paul Varela.

The people of Pecos Pueblo used local rock for pottery, stone tools and building. The mission churches are adobe, with flagstone quarried from the Sangre de Cristo Formation, selenite or mica for windows, and local gypsum for plaster.

Glorieta Pass has been a travel route since prehistoric times. Its position let Pecos Pueblo trade with both Plains peoples and the Rio Grande pueblos. Francisco Vásquez de Coronado crossed it in 1541 on his way to explore the plains; the Santa Fe Trail used it from the 1820s to the 1880s, following the Pecos drainage up from the plains and then Glorieta Creek to the divide; the railroad and Interstate 25 followed later.

The Battle of Glorieta Pass was fought on March 26–28, 1862. Confederate troops advancing up the Rio Grande Valley toward Fort Union and the Colorado gold fields met Union forces that had marched down from Colorado. The Union army had to fall back, but the Confederates were beaten after a Union detachment climbed down the steep, rugged edge of Glorieta Mesa and destroyed their lightly guarded supply train at Cañoncito.

Fossils, caves and old mines

Marine invertebrate fossils in the Alamitos Formation. NPS photo by Mario Armijo.

Fossils are documented in the park only from the Alamitos and Sangre de Cristo formations, though most of the other Paleozoic and Mesozoic rocks could hold them. The Alamitos has rugose corals, bivalves, ammonites and other cephalopods; the Sangre de Cristo has yielded vertebrate fossils, and elsewhere it has large fossil trackways, though none have been found in the park. Archeological sites have produced brachiopods, crinoids, corals, bivalves and a mammal tooth. All park fossils are protected by the Paleontological Resources Preservation Act of 2009.

About 98.5% of the park lies on sedimentary rock with limestone and other soluble carbonates and counts as known or potential karst, but it has no sinkholes or other karst features. Its two small caves, both closed, formed along joints in the rock. One, Baca Cave, was enlarged by mining; the Baca Mine, probably a copper or uranium prospect with an adit and a shaft, is fenced off as the park's one abandoned mineral land site.

Hazards

• Floods can strike along the Pecos River and Glorieta and Galisteo creeks, though the cultural sites are likely far enough from the river to be safe. Seasonal arroyos can flood in heavy storms and cut away their banks.
• Erosion: the Pecos River and Glorieta Creek are cutting down and eating into their banks faster than normal in places, leaving narrow, deep channels that threaten streamside habitat and cultural resources. Slumps, slides and rockfalls occur along the river corridors.
• Earthquakes: north-central New Mexico has moderate seismic hazard. The USGS 2014 hazard map gives the area a 2% chance in 50 years of ground shaking of 14 to 20% of gravity, roughly VI to VII on the Modified Mercalli Intensity Scale, and the region can expect 10 to 20 damaging earthquakes in 10,000 years. A magnitude 3.0 quake struck about 5 miles west of Cañoncito in 2013, and a magnitude 2.9 about 12 miles north of the Pecos Unit in 2019.

Sources

• National Park Service, "NPS Geodiversity Atlas—Pecos National Historical Park, New Mexico." https://www.nps.gov/articles/nps-geodiversity-atlas-pecos-national-historical-park.htm
• Geologic Resources Inventory products for the park: https://irma.nps.gov/DataStore/SavedSearch/Profile/3141; a Soil Resources Inventory has also been completed (https://irma.nps.gov/DataStore/SavedSearch/Profile/3092).
• The Park Service's page spells the ranch "Forked Lighting"; the ranch is the Forked Lightning Ranch. The page lists gastropods among the Alamitos fossils as only possible in its text but identifies one in its photo caption, so this page names neither.