Napa Valley located in Napa County, California is widely considered one of the top wine regions in California, and all of the United States with a history dating back to the nineteenth century. The combination of Mediterranean climate, geography and geology of the valley are conducive to growing quality wine grapes. Several microclimates exist within the area due to various weather and geographical influences. The open southern end of the valley floor is cooler during the growing season due to the proximity of San Pablo Bay while the sheltered, closed northern end is often much warmer. The valley floor is flanked by the Mayacamas Mountain Range on the western and northern sides while the Vaca Mountain Range is on the eastern side. Several smaller valleys also exist within these two ranges. The floor of the main valley gradually rises from sea level at the southern end to 362 feet above sea level at the northern end in Calistoga at the foot of Mount Saint Helena. The Oakville and Rutherford American Viticultural Areas (AVAs) lie within a geographical area known as the Rutherford Bench in the center of the valley floor. The eastern side of the valley tends to be more arid due to the fact that winter storms tend to drop much more precipitation on the western mountains and hills. The soil in the southern end of the valley consists mainly of sediments deposited by earlier advances and retreats of San Pablo Bay while the soil at the northern end of the valley contains a large volume of volcanic lava and ash. Several of the small hills that emerge from the middle of the valley floor near Yountville are indicators of the region's volcanic past.