There are many ways of understanding wine, but you’ve never really finished the job until you lace up a pair of boots and leave the car behind. Walking from one vineyard to the next enables you to see, smell and feel how a landscape works, measuring the light and warmth with your own body, gauging the play of soil and stone underfoot, testing the steepness of a slope with your calf muscles. I never understood the rugged mineral power of the Douro, the cloud-shaded, soft-soiled nuance of Burgundy or the blinding solar splash of Marlborough until I felt them for myself. Sonoma had always intrigued and puzzled me in equal measure, so when the chance came to spend three days walking some of its AVAs, I took it with gratitude.

Terroir in California is unusually complicated, and never more so than in here. Geologically speaking, Sonoma has been a crash zone for the last 135 million years. Pangaea, the Earth’s most recent supercontinent, was breaking up at that late Jurassic moment, and the ancient Farallon plate was subducting under the North American plate, creating the Rocky Mountains and the Coast Ranges as it did so. Nowadays, the colossal Pacific plate is doing the same thing – and the result is the celebrated San Andreas Fault, which actually bisects Sonoma County. Vines growing in the Sonoma Coast AVA, for example, root in rock formations which once lay 330 miles south of those found in the Knights Valley AVA. The San Andreas is the biggest but by no means the only fault in Northern California’s geological chaos, and the work of all the rivers, beaches and seas that have formed here subsequently, plus intense volcanic activity over the last 3-8 million years, multiply those complexities. (The Médoc or Chablis is, by comparison, geologically simple.) The thirteen AVAs which lie within Sonoma County mark a first step in trying to make wine sense of the plethora of different growing conditions here, but it won’t be the last.

My Sonoma Vineyard Walk was an abbreviated press preview of the full five-day tour offered by Zephyr Adventures as part of its wine programme. The tour takes you through five of the County’s AVAs (Dry Creek, Rockpile, Alexander Valley, Russian River and Sonoma Valley), and the walking is seasoned with picnics, open-air wine tastings and relaxed dinners; I stayed on afterwards to make a few more visits. The routes are easy rather than arduous, mostly undertaken in the company of a vineyard owner or manager, enabling you to ask the kind of questions (simple or complicated) you want – and get straight answers. As we walked the benchlands of the Alexander Valley, for example, I wondered why the perfectly exposed hillsides just above those deep loams weren’t also planted – as they would be in any of France’s classic wine regions. The hills have about three feet of topsoil over sandstone, whereas the loams can be 20 feet deep and make a much richer soil medium. Simple, explained Alexander Valley Vineyards’ estate manager Mark Houser. You might get double for your wine, but you’d triple your costs. It wasn’t worth it.

There are other philosophies, of course. After the trip was over, I visited Kenny Kahn’s Blue Rock Vineyards towards the north of Alexander Valley. The ‘blue rock’ in question is serpentine. Its chemistry means it is high in magnesium which in turn implies low levels of potassium, making it a difficult growing medium (serpentine soils in otherwise forested areas show up as dry grassland with scattered conifers known as ‘serpentine barrens’). Parts of Kenny’s vineyard are uncultivatable, and much of the rest requires extensive manuring and always gives low yields; Kenny took it on against local advice. The Blue Rock wines are, though, true vins de terroir; indeed, since serpentine is California’s official ‘state rock’, they probably ought to be served at state banquets. I tasted the debut vintage 1999 Cabernet Sauvignon and the 2004 Syrah and both were distinctively different from the relatively lush and generous Alexander Valley norm: aromatically delicate, with intense, almost austere flavours with a distinctive drily mineral, almost salty finish (in addition to magnesium, serpentine is also high in nickel, chromium and cobalt; indeed New Caledonian serpentine is the source of most of the world’s nickel ore).

As in most of California, of course, the most important terroir factor is not the exact composition of soil and bedrock but proximity to the ocean. The churning of the North Pacific Gyre, which sends cold waters plunging southwards from British Columbia towards Mexico’s Baja California, is what creates the state’s famous coastal fogs -- as well as being the reason why those attempting to swim from Alcatraz to freedom never made it. Think of Sonoma County, which fronts the ocean, as a crash-zone maze of hills and valleys interfingered by rivers and creeks; the exact extent to which fog can creep in overnight along those watercourses is what conditions the warmth of a site, the choice of grape variety and the constitution of the finished wine. In addition to the Sonoma Coast, the Russian River Valley (and its sub-zone Green Valley) as well as Sonoma-Carneros further south are the coolest and most fog-prone parts of Sonoma County: Chardonnay and Pinot Noir are both unproblematically successful here. Knights Valley to the east is the warmest AVA, closely followed by Alexander Valley; while the rest of the AVAs lie somewhere inbetween – including Chalk Hill (whose ‘chalk’ is in fact volcanic ash).

Altitude, of course, plays a role in Sonoma too, and sometimes a paradoxical one. In most of the world’s vineyards, a higher site is a cooler site. Here, though, height can lift a vineyard above the fogline, giving it extra sunshine hours compared to the vineyards below. You wouldn’t expect Zinfandel to do well in foggy Russian River, for example – yet, grown above the fogline, it can flourish. As the cool of night drew on, we Zephyr walkers drank an unusually shapely, poised 2004 Trenton Station Lion Ridge Zinfandel from the McLelland-Kunde ranch made by Rod Berglund at Joseph Swan. We were sitting on picnic tables on top of Lion Ridge to enjoy the view – and the vines were just a few feet away, enjoying the last of the light with us.

That Zinfandel, as it happened, was a perfect contrast to one we had tried the day before in Rockpile, the only AVA in Sonoma where the fog never penetrates. This young AVA promises to be one of the county’s most interesting, though there are no wineries there as yet: we met Lori de Mello, sister of Rockpile Road Vineyard owner Jack Florence, who sells to Rosenblum (recently acquired by Diageo) among others. Rockpile is, in effect, the back end of Dry Creek, where the tarmac runs out high above the reservoir of Lake Sonoma. The lowest point of the AVA is at 800 feet (244 m), and almost all of the 160 acres (65 ha) so far planted lie at over 1000 feet (305 m). Despite its foglessness, there is still a pronounced coastal influence (the Pacific is less than 16 miles away) in the cool winds which harry the vines; soils are relatively thin, too: just a foot or two of oxidized volcanic clay-loam over shale and sandstone, and I noticed quartzite and ironstone, too, as we walked. The end result is low yields of late ripening, small-berried fruit: the 2005 Rosenblum Rockpile Road Zinfandel was full of moist, fruitcake richness yet it is firmly structured, too, with ample tannins and sustained acidity giving it an inner architecture which the more flamboyant examples of this variety can sometimes lack. We tasted it up on a hilltop stand among the vines underneath two shading live oaks, surrounded by paper-barked madrones and tough manzanita bushes, the grass speckled with chamomile, buttercup and wild iris -- and the view beyond as endless as it was rumpled. Even in populous California, you can quickly leave the world of shopping malls, gas stations and Taco Bells to find natural solitude characterised by an airy spaciousness rare in Europe.

Dry Creek itself is now a managed sluice between the reservoir of Lake Sonoma and the Russian River, northern Sonoma’s main watercourse, so it’s never dry any more; indeed it had been overloaded to the point of flooding shortly before we arrived, and we saw the stick-strewn aftermath when visiting the newly established Truett-Hurst. This will be a biodynamic vineyard right next to Dry Creek, under the advisory tutelage of Heath Dolan. Owner Phil Hurst, whose Winery Exchange negociant business has been a major California supplier to Tesco, intends to plant Zinfandel and Petite Syrah in this relatively low-lying site. Our other Dry Creek walk was though the higher-sited vineyards of Michel-Schlumberger, with winemaker Mike Brunson and owner Jacques Schlumberger (who, despite his name, is from Houston). Their graceful 2006 Pinot Blanc marks a welcome break with varietal conformity, and his 2006 Chardonnay combines the breadth and depth that seems to be California’s birthright with a pure, green note and a sense of restraint and understatement which should play well in Europe. The effects of height are conventional in these two sites.

In size terms, Alexander Valley and Russian River Valley are the two largest AVAs in Sonoma, both with 15,000 acres (6,070 ha) planted – and my post-walk visit to Medlock Ames was the perfect place to compare the two, since this outstanding new winery on the southwestern cusp of the Alexander Valley has land in both (it also abuts Chalk Hill). Fog here is a more regular night visitor than in the northern part of Alexander Valley. The 2006 Chardonnay was the finest textured white I tasted during my short visit: supple, spilling and full on the tongue, delicately fruited and spice-nuanced. The 2004 red wines I tried included an enticingly voluptuous Merlot red and an outstandingly well-defined Cabernet with great aromatic finesse, typically soft but ample Alexander Valley tannins and a ripe, black-fruit finish, complex with spice and tobacco. All three wines perfectly exemplified why California’s smaller wineries merit better UK distribution than they get at present: outstanding quality, made with great site sensitivity. In every sense the obverse of California’s big brands.

Compared to Napa, with its wealth, self-importance and marketing overdrive, Sonoma is more homely and understated, particularly once you get beyond the big name wineries in the southern end of the County. It’s beautiful: a chessboard of hills and screening valleys where redwoods march up moist crevices and where, on rounded hills of golden grass, the lace lichen hangs in swags from oaks and madrones to feed on silky morning fogs. We never saw a cougar (the ‘mountain lion’ still endemic here) but met snakes, poison oaks, blue herons and turkey vultures; we even watched a bald eagle struggle for lift as a steelhead trout thrashed in its claws. The manifold complexities of Sonoma’s terroirs, correlated to all that natural beauty, will still be revealing themselves and surprising our grandchildren in fifty years’ time. Walk them while you can.