Bounded on both sides by mountains, the Napa Valley stretches approximately 30 miles in a northwesterly direction, its width ranging from five miles at the widest point near the city of Napa to just a mile where the valley narrows near the town of Calistoga.
Bisecting the valley is the Napa River, which follows the valley's tapered contour, and dwindles from a fully navigable river in its southern stretches to little more than a creek at its northern beginnings. The valley's topography changes with its length, from the windswept estuarine flats and gentle hills in the south to the valley's narrow tip at the town of Calistoga, cradled between the sheer walls of the Palisades at the foot of Mount St. Helena to the east and the forested Mayacamas Mountains to the west.
The Napa Valley's amiable climate makes it a veritable garden. A wide variety of fruits and vegetables thrive here: oranges, prunes, apples, olives and more. But what makes the Napa Valley truly unique is its remarkable suitability for the production of wine grapes. A maritime climate that produces cool nights and warm days combines with soils that are deep but not excessively fertile to yield grapes that are singular in their intensity, complexity and balance.
Several different microclimates and a wide array of soil profiles mean that different vineyards produce grapes that are unique in style and character - but uniformly high in quality.
The vineyard environments of the Napa Valley have evolved through geologic time. Like the rest of California, Napa Valley has had a very active and eventful geologic history. Many tectonic plates (large pieces of the earth's crust) have collided with North America to form California. As a result, there are many geological faults in the area, which have molded the topography of the Napa Valley and the mountains that surround it.
A great deal of volcanic activity occurred in the area about two million years ago. These volcanic eruptions deposited a series of ash and lava called the Sonoma Volcanics over much of Napa and Sonoma Counties, especially along the axis of the Mayacamas Range. The small hills which emerge from the valley floor north of Yountville were created by this volcanic activity.
Changes in sea level caused San Pablo Bay to alternately advance and retreat over the southern part of the valley several times. This resulted in the deposition of bay sediment (clays and sand) as soil parent material in the southern valley. The bedrock varies from coarse sandstones to marine conglomerates to volcanic basalts and tuff. These different parent materials give rise to soils with very different ability to retain water, texture and fertility.