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Named stone

Monterey chert

Also called Monterey Formation chert, Tuqan chert (San Miguel Island variety), Monterey chert and siliceous shale

A kind of chert (and flint)

Monterey chertChert (and flint)West Coast & Plateau

Description

Monterey chert comes from the Miocene Monterey Formation of coastal California. Chumash people and their ancestors mined chert and siliceous shale from its outcrops for millennia, and Monterey chert artifacts from Daisy Cave on San Miguel Island date to the end of the Ice Age. Archaeologists long assumed all of the island's Monterey chert was imported from the mainland, from the western Santa Barbara coast near Gaviota and Vandenberg. In 2008 Erlandson and colleagues reported rounded cobbles of what they called Tuqan chert, eroding from raised beach deposits on San Miguel, that look just like the mainland stone except for a thin white cortex. They are dark banded or mottled brown, black and gray, and some pieces are tan, buff or laminated with shale.

How to identify it

Colour
Dark banded or mottled chert in shades of brown, black and gray at the San Miguel (Tuqan) source; artifacts also tan or buff and laminated chert and siliceous shale (Erlandson and others 2008). Mainland varieties from Gaviota to Vandenberg resemble these.
Texture
Tuqan cobbles are very high-quality chert with well-developed conchoidal fracture, thanks to beach wave action breaking weak pieces.
Inclusions
Some pieces are laminated, with bands of chert and siliceous shale.
Cortex
Tuqan beach cobbles carry a continuous, opaque white or buff weathering rind about 1 to 2 mm thick, caused by long burial in calcium-carbonate-rich beach sediment; it often hides the chert inside.

Geology

Marine chert and siliceous shale of the Miocene Monterey Formation (bedded calcareous shales with chert and siliceous shale layers). On the mainland the outcrops mined by the Chumash lie in western Santa Barbara County (Gaviota to Vandenberg). On San Miguel Island 'Tuqan' cobbles of Monterey chert come from raised beach deposits at Cardwell Bluff.

Where it comes from

West Coast & Plateau

States

Quarries and outcrops

  • Gaviota area to Vandenberg Air Force Base, western Santa Barbara County (mainland sources)
  • Cardwell Bluff, eastern San Miguel Island (Tuqan chert)

Used by

  • Terminal Pleistocene and Early Holocene: Monterey chert artifacts at Daisy Cave, San Miguel Island
  • Chumash and their ancestors used it for millennia
  • San Miguel Island: Channel Island Barbed points and chipped-stone crescents are common in lithic scatters at the Tuqan source, suggesting mining for more than 10,000 years (Erlandson and others)

Look-alikes

Stones that are easy to confuse with Monterey chert.

  • Cico chert (a chalcedony source on San Miguel Island)
  • Fused shale (the other mineral commodity the Chumash worked)
  • Franciscan chert (a different, older chert)

Sources

  1. Erlandson, Jon M., Todd J. Braje, and Torben C. Rick. 2008. Tuqan Chert: A 'Mainland' Monterey Chert Source on San Miguel Island, California. Pacific Coast Archaeological Society Quarterly 40(1).pcas.org (opens in a new tab)
  2. Braje, Todd J., and others. Channel Islands National Park: Archaeological Overview and Assessment. National Park Service.npshistory.com (opens in a new tab)

All materials