The California state rock, serpentine is also typical. Magnificent examples of these rocks are exposed at Shell Beach on the Sonoma County coast south of Jenner. The San Andreas Fault moved the Sur series and underlying Diorite at least 320 miles north relative to the mainland during the last 29 million years. These ancient beaches are being uplifted and eroded today along the coast. Look for Native American white shell middens in dark organic soil above the sands at Bodega Head.
Alluvium is sand and gravel forming today in river valleys and on beaches.
Its contents formed on the ocean floor, some many thousands of miles southwest from here. They mixed along faults and stuck to the continental edge as the ocean floor slid down under western North America, visible at Shell Beach on the coast. With yet more uplift and erosion, the Russian River cut its path across rising mountains to the sea. Alluvium is the eroded sand and gravel that washes into the river valleys and up onto the beaches of Sonoma County , part of the ongoing dynamics of geologic change.