Some AVAs group production from several regions. One such area is Northern Sonoma is an umbrella AVA, including Chalk Hill, and the Alexander, Knight's, Dry Creek and Russian River Valleys, with parts of Green Valley and Sonoma Coast. This AVA was created mainly as a labeling construct and describes little about the region.
Alexander ValleySonoma is subdivided into a number of sub appellations. One of the largest is Alexander Valley, located in the north of Sonoma County. This region starts south of Healdsburg and follows the Russian River to Mendocino County. 13,000 acres of vines are planted from a total of 75,000 available acres. Some sites have rich, fertile loam, while others are planted on good benchland sites with good drainage and lower yields.
Alexander Valley is a long, skinny region; the northern end hotter than the southern end. Geyserville is at the heart of region, with a big reputation for Zinfandel production and white wine is also successful. The wines are soft, generous, and drink well in their youth.
Knights Valley
Knights Valley is located to the east of the Alexander Valley and links Sonoma and Napa. It is a fairly warm sub-region and has less maritime influence, but the elevation helps moderate the climate. 1,000 acres are planted in mainly volcanic soils.
Dry Creek and RockpileOn the other side of the Alexander Valley lays Dry Creek Valley. The first vineyards in this region were planted in 1870. It is a fairly narrow valley with 6,000 acres planted out of approximately 80,000 total. Soil types include the particular Dry Creek Conglomerate, a type of gravel compound, over the benchland and sandier soils in the center of the valley. For climate, Dry Creek is fairly warm and dry, since it is protected from maritime influence by coastal mountains, and is warmer in the north than in the south.
Cabernet is the most widely planted red grape, but the region is also known for Zinfandel and Rhone blends. A smaller offshoot of Dry Creek is the Rockpile AVA, with 200 acres planted out of a possible 1,300 at high elevations. With extremely rocky soils and situated above the fog line, this area receives more sunshine paired with cooler temperatures, giving wines with ripeness and structure. This new AVA was created in 2002 and many winemakers feel it is on the short list as one of the best places to grow Zinfandel in California.
Russian RiverTo the south of the Dry Creek and Alexander valleys lays the Russian River valley. Here, 50 wineries and 200 growers have planted 10,000 acres near the river from Santa Rosa to Sebastopol. In terms of climate, this is mostly a cool Region I with morning fog, heavy maritime influence and high rainfall.
As with many California regions, the vineyards at the foot of the east-facing slopes are planted on well drained benchland, while those planted near the river have finer textured soils that hold more water. This is not as problematic here as it would be in Napa and the Russian River is known as a place of fine Pinot Noir and Chardonnay.
Chalk Hill
Contained within the boundaries of the Russian River AVA, Chalk Hill is actually located on the opposite side of the Russian River on west-facing hills. Distinct from the rest of the AVA, it is warmer (Region II), has no fog and receives only a slight influence from the ocean. In spite of this warmer climate, it is still cooler than Alexander and Knight's Valleys, according to Stephen Brook.
1,000 acres are planted, mainly to Chardonnay with some Merlot. The name Chalk Hill is somewhat of a misnomer since the soil contains no chalk, but rather volcanic ash, which gives the soil its white color.
Green Valley
Another subregion of the Russian River Valley is Green Valley, with 1,000 acres planted close to the Pacific. This area has a fairly cool Region I climate with large diurnal swings in temperature. Chardonnay predominates and the wines are marked by high acidity and a fresh, Chablis-like character.
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Carneros, Spanish for ram, is the name of the region that lies just north of the San Pablo Bay at the southern tip of both the Napa and Sonoma Valleys. The determining feature of Carneros is the cool maritime influence from the San Pablo Bay, which makes this a Region I or cool Region II. Once the fog burns off, however, this cool region receives a lot of sunshine, helping to ripen the Merlot, Chardonnay and Pinot Noir to perfection. 
Sonoma Mountain is a fairly high-altitude region located in the hills above Glen Ellen. Although it is above the fog line, the weather is fairly cool, and the climate is classed as a Region II. Primarily black grapes are produced. 
