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Named stone

Brushy Basin chert

Also called Morrison Formation chert (Brushy Basin Member), JMC (Crow Canyon code)

A kind of chert (and flint)

Brushy Basin chertChert (and flint)Texas & Southwest

Description

Brushy Basin chert comes from the Brushy Basin Member of the Jurassic Morrison Formation along the Utah-Colorado border, and Crow Canyon Archaeological Center ranks it among the finest stones of the Mesa Verde region. It formed from very fine volcanic ash that settled in shallow lakes and was completely replaced by silica. Look for white and soft pastel tans, pinks, greens and oranges mixed with darker browns, red-browns and greys, often three or more colours swirled or banded together. It is always opaque, smooth and waxy, and usually shiny; polished pieces take a very high gloss. Ancestral Pueblo people used it for cores, bifaces, projectile points, tchamahias and pendants. It is the finest of four Morrison materials that grade into coarser Morrison chert, mudstone and silicified sandstone.

How to identify it

Colour
White and pastel tan, pink, green and orange combined with darker brown, reddish brown and grey; three or more colours in swirled or banded patterns are common (Crow Canyon).
Texture
Non-granular: all grains replaced by silica. Very smooth and waxy. Always opaque (Crow Canyon).
Luster
Usually shiny; polished artifacts have a very high lustre (Crow Canyon).

Geology

Brushy Basin Member of the Jurassic Morrison Formation. It derives from very fine volcanic ash deposited in shallow lakes and wholly replaced by silica (Crow Canyon). Crow Canyon sorts all Morrison materials along a grain-size scale: Morrison silicified sandstone, Morrison mudstone, Morrison chert, then Brushy Basin chert as the finest.

Where it comes from

Texas & Southwest, Rockies & Great Basin

States

Quarries and outcrops

  • Brushy Basin Member outcrops along the Utah-Colorado border (Crow Canyon)

Used by

  • Ancestral Pueblo, Mesa Verde region: cores, debitage, bifaces, projectile points, tchamahias and pendants (Crow Canyon). Morrison Formation stones make up most chipped stone in Crow Canyon excavations.

Look-alikes

Stones that are easy to confuse with Brushy Basin chert.

  • Morrison chert: maroon, green, tan and brown, often mottled, very fine but with occasional grains and frequent flaws (Crow Canyon)
  • Burro Canyon chert: tan, brown, green and grey with small white opal-like chalcedony blebs, often translucent (Crow Canyon)
  • Morrison mudstone and Morrison silicified sandstone: same colours, coarser, duller (Crow Canyon)

Sources

  1. Ortman, Scott G., Erin L. Baxter, Carole L. Graham, G. Robin Lyle, Lew W. Matis, Jamie A. Merewether, R. David Satterwhite, and Jonathan D. Till. 2005. The Crow Canyon Archaeological Center Laboratory Manual, Version 1. Chapter 8, Stone Material Identification. Crow Canyon Archaeological Center, Cortez, Colorado.crowcanyon.org (opens in a new tab)

All materials