Pedernal chert

Also called Cerro Pedernal chert, Pedernal chalcedony, Pedernal Chert Member of the Abiquiu Formation

Pedernal chertChert (and flint)Texas & Southwest

Description

Pedernal chert is the signature chert of northern New Mexico, named for Cerro Pedernal, the flat-topped peak in the Chama Valley. 'Pedernal' is Spanish for flint. The chert is white to pearly gray, often with bands or patches of black, red, maroon or yellow, and it is remarkably free of flaws. It breaks easily in any direction with conchoidal fracture. Geologists call it unusual because it is a land-formed layer of silica, probably formed when volcanic glass broke down and silica replaced calcareous soil. People quarried it from the mountain's slopes and from San Pedro Mountain for ten to thirteen thousand years, and rounded cobbles of it wash far downstream in the Rio Grande and onto the Colorado Plateau. Do not confuse it with the Texas point type Pedernales.

How to identify it

Colour: Commonly white to pearly gray; black bands near the base and sometimes the top; weathering can turn it pink, red or yellow; red flecks occur; yellow tones go with clear, translucent phases (Bryan, quoted by Warren 1974). Smith and Huckell (2005) describe translucent gray and yellowish gray with patches and swirls of opaque red, maroon and black.
Texture: Vitreous chert (microcrystalline quartz) and chalcedony; vugs filled with chalcedony, drusy quartz or calcite. Remarkably free of flaws and joints; breaks by conchoidal fracture with almost equal ease in any direction (Bryan).
Inclusions: Red flecks or spots in the pale mass; the chert is mostly massive with scattered pebbles, and in places forms a nodular cement within gravel.

Geology

The Pedernal member of the Oligocene-Miocene Abiquiu Formation: a 4 to 8 m interval of gravel containing one to four discontinuous bodies of chalcedonic and cherty silica, each 20 cm to over 2 m thick. It crops out around the base of Cerro Pedernal, on San Pedro Mountain near Gallina, and along the Sierra Nacimiento at San Pedro Parks. Silica probably replaced calcareous soil (calcrete) as volcanic glass broke down. Resistant chert cobbles are widespread in alluvium of the Rio Grande rift and the Colorado Plateau.

Where it comes from

Texas & Southwest

States

New Mexico

Quarries and outcrops

Cerro Pedernal, Rio Arriba County (quarries on the western and southern slopes)
San Pedro Mountain and San Pedro Parks, Sierra Nacimiento
Redeposited cobbles in the Albuquerque, San Juan and Espanola basins

Used by

Clovis (a cache of Clovis-complex bifaces tentatively identified; Warren 1974)
Paleoindian through Archaic, Puebloan and historic periods: Warren says use spans ten thousand years or more; Smith and Huckell say over 13 millennia
Archaic quarry-area camps with Bajada, San Jose, Armijo and En Medio phase points (the chert itself produced only one Archaic point fragment there)

Look-alikes

Stones that are easy to confuse with Pedernal chert.

Jemez Mountains obsidians (often found together at Archaic sites)
Other New Mexico cherts and silicified rhyolite such as Socorro jasper (not described in these sources)

Sources

Warren, A. Helene. 1974. The ancient mineral industries of Cerro Pedernal, Rio Arriba County, New Mexico. In New Mexico Geological Society Guidebook 25, Ghost Ranch, pp. 87-93.nmgs.nmt.edu (opens in a new tab)
Smith, Gary A., and Bruce B. Huckell. 2005. The geological and geoarchaeological significance of Cerro Pedernal, Rio Arriba County, New Mexico. In New Mexico Geological Society Guidebook 56, pp. 425-431.nmgs.nmt.edu (opens in a new tab)

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