THE TERROIR OF THE NAPA VALLEY AVA

The Napa Valley is bounded by the Mayacamas Mountains on the west the Vaca Mountains on the east, and Mt. St. Helena in the north. The elongate, curvilinear valley trends NW. The valley opens to San Pablo Bay in the south and is enclosed by mountains to the north. The Napa Valley lies in the buffer zone between cooler coastal areas to the west and the hotter Central Valley to the east. Temperatures decrease with elevation and show a general warming northward along the valley floor, with highest summer temperatures recorded in a zone along the base of the eastern hills. Degree Days in the valley can be as high as 4000 and in the immediately surrounding hills as low as 2500. Rainfall reflects topography decreasing north to south and increasing with elevation. The region is underlain by three rock bodies, upper Mesozoic Franciscan complex and Great Valley sequence – including its basement, the Coast Range Ophiolite, and the Pliocene Sonoma Volcanics. The former two are composed mostly of marine sandstone and shale and each may be associated with serpentinite and greenstone. The younger volcanics range in composition from basalt to rhyolite. Soil, in the agrarian sense, is not a good discriminator. What is important is the nature of the underpinning of vineyards, the material into which a soil profile is made. Three basic divisions are recognized: the various units of bedrock, alluvial fans composed of sediment derived from one or more of the bedrock bodies; and homogenized fluvial deposits. The Napa Valley AVA has been subdivided into 14 sub appellations, 10 of which correspond to the valley and adjoining hillside properties. Their boundaries are defined both geographically and politically. In terms of terroir, Napa Valley continues to evolve as the winemakers learn more about the physical constraints. One trend is to use a variety of viticultural interventions to create greater uniformity, thus, overshadowing some fundamental terroir attributes. Others are striving to honor the fundamental features as well as the nuances provided by solar aspect, afternoon breezes, and particular rock patterns to produce small lots of specific wines, true to the Earth from which they arise. How the tension between large-scale uniformity and small-scale distinctiveness resolves will fundamentally portend the fate of terroir.