Geology of Chaco Culture National Historical Park

A Chacoan great house in Chaco Canyon. NPS.

The NPS Geodiversity Atlas, a product of the Geologic Resources Inventory, summarizes each park's rocks, landforms and soils to support education, geoconservation and management. This is its page for Chaco Culture National Historical Park, New Mexico.

The setting

• The Chacoan world: built in stages between A.D. 850 and 1150, Chaco's pueblos were the center of a world spanning much of the Southwest — the San Juan Basin of New Mexico and parts of the Four Corners in Colorado, Utah and Arizona.
• The San Juan Basin: more than 26,000 square miles (67,340 km²), a nearly circular, bowl-shaped depression filled with sedimentary rocks 570 million to 2 million years old. Its folded layers hold groundwater, oil and gas, coal and uranium.
• The Colorado Plateau: Chaco lies near the basin's center, at the southeastern edge of this high, little-deformed crustal block of about 140,000 square miles, ringed by the deformed Rocky Mountains and Basin and Range.

A 100-million-year record

Chaco's rocks and deposits record:

1. flooding by the Western Interior Seaway;
2. downwarping of the San Juan Basin in the Laramide Orogeny, and its filling with land-derived sediment;
3. widespread erosion and carving of today's drainages, including Chaco Canyon;
4. active sedimentation in the canyon;
5. the cutting of Chaco Arroyo and its inner channel;
6. human activity, itself now part of the geologic record.

The rocks: most exposed strata belong to the Mesaverde Group — the Point Lookout Sandstone, Menefee Formation and Cliff House Sandstone — with the Lewis Shale and Pictured Cliffs Sandstone also laid down in the seaway. The Menefee and Cliff House dominate what you see; the Lewis Shale, beneath windblown sand north of the upper Cliff House cliffs, is too thin to map at 1:24,000.

Features to see: Upper Cretaceous rocks and fossils, Quaternary fossils, Chaco Canyon and Chaco Arroyo, arroyo formation and alluvial fill, sheetwash and slope deposits, dunes, tinajas and charcos (natural rock basins that hold water), badlands, and cave shelters. See the park's Geologic Resources Inventory report.

The Geologic Resources Inventory map of the park. NPS.

Fossils

An inventory of the Upper Cretaceous rocks begun in 2005 has found abundant fossils, especially in the Menefee Formation and Cliff House Sandstone. By the end of the 2013 field season, more than 300 fossil localities had been mapped: 148 plant, 95 vertebrate, 49 invertebrate, and 10 with a mix. Park fossils are protected by the Paleontological Resources Preservation Act of 2009.

The birthplace of geoarchaeology

Geoarchaeology studies ancient people and their environment together, especially how sites formed — and Chaco Canyon is where it began:

For more than a century and a half Chaco has drawn researchers, with early uses of tree-ring dating, biogeochemistry, pollen analysis, anthropology, remote sensing, archaeological stratigraphy, and packrat middens to reconstruct environments over millennia.

Alcoves and soil pipes

Uneven weathering between the Cliff House Sandstone and Menefee Formation has carved alcoves, some of which may prove to be caves. They hold packrat middens and some of the park's earliest datable perishable cultural material — corn, basketry and more. Soil piping is also common; larger pipes may count as caves, can damage structures and artifacts, and sometimes shelter bats. Caves are protected by the Federal Cave Resources Protection Act of 1988.

Wind and sand

Wind has long shaped Chaco. The 2006 Pueblo Bonito trail guide notes that in the late 1800s, before the first archaeological protection laws, looters broke through Pueblo Bonito's back wall to empty rooms rather than dig down through many feet of windblown sand. Spring dust storms are frequent, dunes once buried the old north entrance road, windblown soils cap the mesas, and big dunes fill side canyons such as Weritos Rincon and Pueblo Pintado. The Crownpoint dune overlies a 6,900-year-old turtle fossil dated by radiocarbon.

Fajada Butte. NPS.

Regional geology and resources

Chaco belongs to the Colorado Plateaus province. The Geologic Resources Inventory offers scoping summaries, digital maps, reports and posters (NPS DataStore), and a Soil Resources Inventory is complete. See also the park's geology page.

Sources

Based on "NPS Geodiversity Atlas — Chaco Culture National Historical Park, New Mexico," National Park Service, citing Brister and Price (2002), Varela (2013), Love (2007) and the National Park Service (2006); a work of the United States government in the public domain.