Outcrops of serpentinized peridotite uplifted from earth’s mantle rise from the western foothills of the Maacama fracture zone, eight miles east of unincorporated Yorkville in Mendocino County, Calif., on May 21, 2026. (Rowena Forest via Bay City News)

The Maacama fault system and fracture zone lies at the heart of Mendocino County, defining the landscape of the central valleys and framing the geography, ecosystems, industry and culture of inland Mendocino.

A sweeping and formidable tectonic corridor, this sister of the San Andreas Fault system delineates the valleys and towns of Laytonville, Willits, Ukiah, Hopland, and others along its path. Historic toponyms such as Long Valley and Little Lake Valley allude to the geography of the fractious Maacama.

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Rowena Forest is a physical geographer and science writer at Cal Geographic. To follow her in-depth explorations and investigations of California’s living landscapes, and for upcoming field trips please...