West Sonoma Coast

Geological Influences

The landforms and soils of the West Sonoma Coast AVA are predominantly related to the Franciscan Complex –  the subduction of the Pacific tectonic plate beneath the North American plate which took place from 190 million to 16 million years ago. This long, slow process created mountains and soils, along with the San Andreas and other earthquake faults.

As the Pacific Plate moved east, much of the seafloor was scraped off and either wound up on top of the North American plate or accreted to its edge. This formed coastal mountain ranges from marine sedimentary and metamorphic soils. The Pacific Plate’s deeper bedrock went below the North American Plate.

There are many different Franciscan Complex soils within the West Sonoma Coast AVA. However, when it comes to the actual vineyards there is less diversity. Since the vineyards tend to be at altitude, due to the climate, they have thin, well-draining top soils on subsoils of sedimentary rock.

Mountains/River/other key influences

The two main geographical influences on the West Sonoma Coast AVA are the Pacific Ocean and the Coastal Mountain Ranges. The ocean has a profound cooling and moderating effect. The AVA boundaries were drawn with this in mind, including only territory with direct maritime influence. So, the area is notably cooler and somewhat more humid than portions of the county further east. 

The effects of the mountains on vineyards in the AVA are based on slope and altitude. Low altitude areas in the AVA have a cool marine climate and spend much of the day enveloped in fog. Low temperatures and sunlight, along with damp air, make viticulture in many spots challenging, though it can work in some. In contrast, the high-altitude vineyards have a cool Mediterranean climate with plenty of sun and a long growing season. Slopes influence water drainage, soil depth, and aspect to the sun.

Climate

High-altitude areas of the West Sonoma Coast AVA are marked by a warm-summer Mediterranean climate and long, sunny, dry growing seasons. Winters are moderate, but that’s when almost all of the precipitation comes. Though the AVA shares this climate type with other  California growing areas with coastal influence, the West Sonoma Coast is cooler than most.

From a Winkler degree-day standpoint, vineyards in the West Sonoma Coast AVA are Region I, cool climate. Accumulation varies with vintage and location, but generally runs from 927-1093° C (1,700 to just over 2,000° F). On a daily basis, the AVA experiences cooler days but warmer nights and thus a much smaller diurnal shift than zones further inland. Warm nights also mean that there is little risk of frost.

The West Sonoma Coast is also less windy. To the south, wind rushes through the Petaluma Gap to fill the low-pressure zone caused by rapidly rising, hot air well inland. Since West Sonoma Coast’s back is to the mountains and its air is cool, there’s little wind generation.