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 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.