Tuscan obsidian

Also called Source X (older archaeological name), Tuscan Formation obsidian, Backbone Ridge obsidian, Cow Creek obsidian

A kind of obsidian

Tuscan obsidianObsidianWest Coast & Plateau

Description

Tuscan obsidian comes from the Tuscan Formation, a Pliocene pile of volcanic mudflow (lahar) breccias on the east side of the northern Sacramento Valley, east of Red Bluff and Redding. Archaeologists used to call it Source X. Hamusek-McGann's study of 1995 found eight new glass localities in the formation and showed that the groups can be separated chemically but not reliably by eye. The glass is variable at every locality: dense black, mahogany, red-and-black banded and transparent pieces occur together with dull gray, opaque stuff. Most of it is small water-worn nodules, but the Backbone Ridge and Cow Creek areas have some of the largest nodules and excellent flaking glass. Inks Creek and Paynes Creek glass is crystal-bearing and crumbles under percussion.

How to identify it

Colour: Dense black opaque, mahogany, red and black flow-banded, and transparent glass, with dull gray opaque material in the same locale; Cow Creek has gray banded and mahogany pieces (Hamusek-McGann; Moore).
Texture: Variable; mostly small water-worn nodules, some boulder-sized. Highly vitreous glass occurs with dull gray opaque glass in the same locality. Inks Creek and Paynes Creek glass is vitrophyric, non-vitreous and unpredictable to knap; softness helps pressure flaking.
Inclusions: Inks Creek and Paynes Creek glass contains crystals (vitrophyric).
Cortex: Mostly small water-worn nodules.

Geology

The Pliocene Tuscan Formation: tuff breccias formed by lahars from volcanoes near Lassen Peak (Mount Yana and Mount Maidu and other source areas), in beds 40 to 100 feet thick and up to about 1,000 feet in total, exposed over about 2,000 square miles on the east side of the northern Sacramento Valley. Artifact-quality glass can occur anywhere in the formation, so source areas are hard to map. Chemical groups include Backbone Ridge and Cow Creek.

Where it comes from

West Coast & Plateau

States

California

Quarries and outcrops

Tuscan Formation foothills east of Red Bluff and Redding, northern Sacramento Valley
Backbone Ridge and Cow Creek localities (largest nodules; Cow Creek is in Shasta County)
Inks Creek and Paynes Creek localities (poorer quality)

Used by

Lithic procurement of the prehistoric people of north-central California (the study discusses the implications; the opened pages give no specific periods)

Look-alikes

Stones that are easy to confuse with Tuscan obsidian.

Medicine Lake Highland obsidian (long thought better; Hamusek-McGann says Tuscan glass was undervalued)

Sources

Hamusek-McGann, Blossom. 1995. The Tuscan Obsidian Source of Northern California: Archaeological Implications and Geochemical Variability. Proceedings of the Society for California Archaeology 8.californiaprehistory.com (opens in a new tab)
Moore, Joe. 2009. Great Basin Tool-Stone Sources: The NDOT Obsidian and Tool-Stone Sourcing Project, 2002 Progress Report. Nevada Department of Transportation, Cultural Resource Section.deschutesmeridian.com (opens in a new tab)

All materials