In the United States, there are three levels of appellations defined by the Bureau of Alcohol, Tobacco and Firearms (oddly enough, wine is considered a dangerous substance and is regulated by the same people who address gun control ... go figure). The most specific appellation is American Viticultural Area (AVA). Sonoma Valley, Dry Creek or Napa Valley are among the more familiar AVAs; since the climate and soil is different in each of these areas, BATF allowed them to make distinctions so that consumers could note quality differences. (Note: to completely immerse yourself in wine geekness, check out the 80-plus California AVAs at wineinstitute.org.)
Sonoma County has 1,050,000 acres of land, divided into 14 AVAs. The 14,000-acre Rockpile in northern Sonoma has rugged hillsides and dry, warm weather, great for hearty red grape varieties like zinfandel, syrah and petite sirah. You won't see much of the wine outside of California just yet -- there are no established wineries and only 200 acres currently planted -- but watch for Rockpile on the label.
Bennett Valley, in the southeastern part of the county near Santa Rosa, is considered "cool climate." Early morning fog cools down the grapes, creating higher acidity, and the well-drained soil allows the roots to grow unhindered, so full-frontal flavor develops. Growers cultivate lots of different grapes within the 850 planted acres, but the best wines to emerge lately are merlots, chardonnays and syrahs. One well-known winery in Bennett Valley is Matanzas Creek Winery.
Other recognized AVAs in Sonoma County are Alexander Valley, one of my favorites, where warm-weather grapes like cabernet, zinfandel and merlot thrive; cold-climate Russian River Valley, which produces some of the best (albeit expensive) pinot noirs and chardonnays California has to offer; and temperate Dry Creek Valley, which has fantastic sauvignon blancs and zinfandels.
These AVAs are taken pretty seriously. In 2002, BATF defeated a California Coast AVA saying, "The proposed California Coast viticultural area is not a unified geographical area with viticultural features that distinguish it from surrounding areas." The 22,000-square-mile California Coast AVA would have encompassed 68 existing AVAs and extended from Mendocino County to the Mexican border, so perhaps they're right. That's like saying Florida's sandy soil is the same as Georgia red clay. Not.
Recommended Wines
Rosenblum Cellars 2001 Zinfandel Rockpile Road Dry Creek Valley So fresh it wants to jump into your lap. Creamy with vanilla, ripe raspberries and blackberries, with enough alcohol content to knock you on your ass. In other words, a great wine. $28. 
Valley of the Moon 2002 Pinot Blanc Sonoma Valley Pinot blanc is unfortunately unknown, yet it needs to come out of the closet. This one drinks easy, with crisp lemon, fresh green grass and mineral flavors. If you're looking for a rounder, fuller sauvignon blanc, sip on this. $14.
Alexander Valley Vineyards 1999 Cyrus Alexander Valley A blend of cabernet, merlot and a couple of other grapes, this luscious offering is so profoundly elegant, you'll want to lick the bottom of your glass. Soft, smooth, supple acids and tannins unite with blackberry, lavender and black cherry to create a near perfect wine. $55. 1/2
Ramey 2001 Chardonnay Russian River Valley A sultry, sexy chard with toasty vanilla, tropical fruit and a tart green apple finish. The flavor changes so many times in your mouth, you'll think it's a Cher concert. $34.
