Soils and Geology
The Napa Valley appellation is unique for the diversity of soils found in such a small geographic area. Soils of volcanic, maritime and alluvial origin exist, each created by geological events that have occurred over a 60-million-year history. The valley was formed by tectonic plate movement and volcanic activity, by alluvial waters and the flooding of San Pablo Bay, which once reached as far as the site of the present-day town of Yountville. Rocky knolls and alluvial fans interrupt the level expanses of the valley floor, each bearing testimony to geologic occurrences in the past.
The area's topography supports a wide variety of soils-more than 30 different types have been identified. Ranging from well-drained gravelly loams to moisture-retaining silty clays, these soils vary in depth and fertility. The structure and composition of the soil greatly affects the vegetation, including grapes, which it sustains.
In general, the soils found on the level flood plain of the Napa River and valley floor are deeper than the rocky soils covering the slopes and hills. Experience has shown, however, that a highly fertile and deep soil is neither necessary nor always desirable for the growing of premium wine grapes. While yields may suffer as a grapevine's roots struggle to penetrate the thin and rocky soils of hillside vineyards, the resulting stress often results in smaller grapes of highly concentrated color and flavors.
Napa Valley Rocks: Soils and Geology
Although Napa Valley is one of the world's smallest wine regions, it is also one of the most diverse. Culled from the results of two scientific studies about the soils and geology of the Napa Valley appellation, this short video presents an easy-to-understand chronicle of 150 million years of the science behind the Napa Valley.