Named stone
Government Mountain obsidian
Also called Government Mountain, Kaibab National Forest
A kind of obsidian
Government Mountain obsidianObsidianTexas & SouthwestDescription
Government Mountain, in the Kaibab National Forest of northern Arizona, is described as perhaps the best known obsidian source in the state. It is a single rhyolite dome with obsidian, tuff and rhyolite in the alluvium around its base, and obsidian dated at about 2.7 million years. Nodules up to 15 cm are still common despite years of prehistoric and modern collecting. The glass is aphyric, but its fabric is distinctly granular, and thin flakes look cloudy gray in transmitted light. Some lighter, more vitreous glass came from the south slope. Cultural debris is heavy, with up to 200 pieces per 5 m2 on the west and south slopes, though some is modern knapping. Shackley notes that Government Mountain glass travelled over great distances in archaeological contexts.
How to identify it
- Colour
- Gray, thin flakes cloudy gray in transmitted light; some lighter gray, more vitreous glass on the south slope (Shackley).
- Texture
- Aphyric but megascopically granular and distinctive.
- Inclusions
- Aphyric, with microphenocrysts of alkali feldspar and iron oxide giving the granular look.
- Cortex
- Most nodules have little cortex; some are coated with ash or have a thin gray or brown weathered surface.
Geology
A single rhyolite dome of the San Francisco Volcanic Field, south-central Coconino County. Obsidian, rhyolite and tuff occur in the alluvium around the dome base. Potassium-argon dating gives 2.70 million years for obsidian on the west side. Secondary deposition reaches only about 2 km, mostly south into Government Prairie.
Where it comes from
Texas & Southwest
States
Quarries and outcrops
- Government Mountain dome, Kaibab National Forest, Coconino County
- Government Prairie (secondary deposits)
Used by
- Found in archaeological contexts over great distances (Shackley, Jemez page); RS Hill glass may be as common as Government Mountain glass in Preclassic Hohokam sites of central Arizona (Shackley 2005, via the lab page)
Look-alikes
Stones that are easy to confuse with Government Mountain obsidian.
- Cow Canyon obsidian (similar chemistry, but separable by Ti, Zr, Nb and Ba)
- Sitgreaves Mountain obsidian (grayer, with sparse crystals; reduces less efficiently)
- RS Hill obsidian (a separate San Francisco Volcanic Field source with similar glass)
Sources
- Shackley, M. Steven. Geoarchaeological XRF Laboratory, University of California, Berkeley. Southwestern obsidian source description: San Francisco Volcanic Field, northern Arizona (Government Mountain section).swxrflab.net (opens in a new tab)