O A K V I L L E S O I L
The geology and soils of Oakville and the greater Napa Valley are notoriously complex. They are the product of slow collisions between the North American plate and the Farallon plate, and later the Pacific plate, over the past 80 million years. The friction between these land masses formed the inland Sierra Nevada mountain range. But they also gave rise to the volcanoes that created the Vaca Mountains on the eastern border of Napa Valley and the upthrust ancient sea bed rock formations—and the wildly heterogenous Franciscan rock series—that comprise the Mayacamas Mountains on the western side of the valley.
The soils of the Oakville district are primarily comprised of broken down or decomposed rock that has been carried downhill from the eastern Vaca range and the western Mayacamas range into the base of the valley, where they ultimately intermingle with the silt, clay and gravel deposits of the Napa River flood plain.
On the one hand, much of Oakville is distinguished by its relative uniformity. Most of the district lies on a gravel base overlaid with alluvial soils washed down from both sides of the valley. This gravel base provides excellent water drainage and allows rooting depths of more than one hundred feet. On the other hand, there are significant differences between the rock formations and resulting soils on the eastern and western sides of Oakville.
The Mayacamas range to the west is composed primarily of a combination of ancient Pacific sea bed rock formations and the heterogenous assortment of greenstones, deep sea cherts, sandstone, limestone, serpentines, shales and other metamorphic rocks known as the Franciscan formation. At the base of the Mayacamas, the eastern side of the Oakville AVA is dominated by two large alluvial fans made up of clay loams and bale loams—sediment and rock carried down the mountains by gravity and streams on Mt. St. John, flowing down toward the Napa River.
Comprised of fine clay, sand and other types of disintegrated rock, these sedimentary fans spill out across the Oakville district and create a subtle slope of less than two degrees down to the Napa River plain. This gentle slope allows excess rainfall to flow into the Napa River from the western side of the valley.
On the eastern side of the Oakville AVA, the soils are influenced by the primarily volcanic composition of the Vaca Mountains, which are largely made up of exposed volcanic tuffs, lava flows and pyroclastic flows, with some recurrences of the Franciscan formations seen on the western side of the valley. On Oakville’s eastern side there are also alluvial fans, but they’re shorter and less broad. The flats of the Napa River flood plain transition more quickly to the exposed rock of the Vaca range, and do so in more sudden step-like formations, the result of the Vaca range having broken down from its previous heights long ago.