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Earth Notes: Prescott's Argillite Source Mine

Turquoise, obsidian, chert and other stones and minerals have been valued by people across the Southwest for thousands of years. A lesser known but equally valuable rock is argillite.

It’s made of compacted mud or clay, and in Arizona most argillite comes from deposits in a rock formation called Mazatzal Quartzite. In other parts of the country it’s called pipestone, and like its namesake suggests, is used for carving smoking pipes.

A distinctive deposit of argillite was mined at least a thousand years ago near Prescott. The orange rock with tan inclusions is soft and easily worked…perfect for making pendants, bracelets and beads. At the Prescott site, thousands of tiny argillite beads were discovered, each with a hole drilled through it by an obsidian tool.

Excavators in the 1930’s found another cache of beads at the Tuzigoot pueblo on the Verde River. The late anthropologist Katharine Bartlett with the Museum of Northern Arizona documented an argillite mine near Prescott which turned out to be the source of many artifacts in the area, as well as other sites in the Verde Valley and Flagstaff.

More recently, archaeological artifacts found throughout the state were analyzed by x-ray diffraction, which identified the same argillite mine as the source. At least one trade route was traced from the mine to a series of Sinaguan pueblos downstream along the Verde River. Some archaeologists think this area may have been a production center for the entire Southwest.