The Soil & Terrain of Sonoma Coast
The Soil & Terrain of Sonoma Coast Wine Country
The Wilson Grove Formation is located in Western Sonoma County. It is made up of shallow, ancient ocean floor soils. Lime or gypsum can be added to increase pH in very acidic soils. Between 3 and 6 million years ago, the Wilson Grove Sea covered the town of Sebastapool and the Sonoma Volcanics were erupting. The petrified forest, located west of Calistoga, was buried by lava and ash from one of these explosions.
The San Andreas Fault runs through Sonoma County. West of the fault, soils are very old and are primarily made of sand, gravel and granite. East of the fault, soils are much younger and are eroding ocean floor and beaches. The terrain of Sonoma Coast is east of the fault.
The igneous rock in Napa and Sonoma contains a lot of iron. The Great Valley Sequence is gravelly and sandy. Some oxidized, red clay will develop with weathering over time in iron rich, volcanic rocks.