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. Much is written about the influences that California soils have on vine growth and fruit quality.  The collective wisdom among California growers that climate is equally important as soil. To understand this, the concept of terroir, a French term that refers to the soil, comes into play. The French notion of terroir looks at all the natural conditions which influence the biology of the vine stock and thus the composition of the grape itself. The terroir is the coming together of the climate, soil and landscape. It is the combination of numerous factors: temperatures by night and by day, rainfall distribution, hours of sunlight, soil acidity, presence of minerals, depth, water-retention, exposure to sunlight and slope and drainage. All of these factors react with each other to form, in each part of a vineyard, what French wine growers refer to as terroir.