Sonoma Valley Soil and Geology

Sonoma Valley Tours of the AVA’s geology is a complicated but enlightening process. The soils are part of a geological phenomena called the Franciscan Complex.

Over the millenia, crust from the Oceanic Farallon Plate has been subducted under the the Continental North American plate. This process occured between 150 and 15 million years ago and has resulted in ancient iron-rich igneous and metamorphic rocks exposed on the valley walls.

The subduction of the Farallon Plate has caused an unusual amount of volcanism in the valley. The Sonoma Volcanics are a layer of lava flow and volcanic ash that resulted from a particulary active time several million years ago.

Long-term exposure to weather patterns also has a major geological impact. Soils can be influenced by a number of factors including being eroded, leeched of minerals and nutrients, or being impacted by alluvial deposits and organic matter.

 Sonoma Valley Soil and Geology

So what does this all mean for current wine production is the Sonoma Valley? The best wines start in the vineyard, and depending on the varietal, the best vineyards have certain geological characteristics. Critical decisions such as grape type, rootstocks and clone selection are all heavily influenced by the composition of a vineyard’s soil.

For example, the thin soils of the Eastern Sonoma Mountains are heavily planted with Cabernet Sauvignon, Merlot and Zinfandel. The Western Mountains are almost exclusively planted with Cab. Louis M Martini’s Monte Rosso Vineyard is located in the Western Mayacamas Mountains.

Over the years, this has probably been the most consistently excellent Cabernet Sauvignon produced in the Sonoma Valley. Arrowood Vineyards also makes an excellent Cab from the western part of the AVA.

Soils in the southern part of the Sonoma Valley have more clay influence. They are well-suited for lighter-bodied varietals, including Pinot Noir and Chardonnay. Riesling and Gewurztraminer are also planted near the San Pablo Bay.