A Day of Total Terroir: Geology in the Wine Country

Geologists ponder the ineffable factors behind wine

David Howell of the U.S. Geological Survey was preparing to guide us through California's Napa Valley on a cool June morning. "On today's trip, we won't be visiting any rock outcrops, just wineries. So before we get on our buses, I'm going to compress 200 million years of history into a few minutes, and say a little about how the geology of Napa Valley makes its wine industry possible. But basically, we're going to be tasting the geology through the wine itself."

Terroir and Napa Valley Geology

This field trip was part of a scientific convention, a meeting of the Geological Society of America's Cordilleran Section, being held in Berkeley near my home. There were many tempting trips in the days before and after the meeting itself—a weekend in the gold country, a tour of old mercury mines, a rare visit to the privately owned Sutter Buttes, and a close look at the sea cliffs south of San Francisco—but "Geology and Wines of Napa Valley" got my vote.

I was hoping for some insight into the winemaker's concept of terroir, a vaguely defined term even in its original French, let alone in this English version I found on the web: "soil, climate, microclimate, aspect, rootstock—a myriad number of checks and balances of almost impossible intricacy that go towards producing an ideal quantity of absolute quality."

Howell told our group that Napa Valley geology is one piece of the bigger picture of California.

A combination of tectonic forces leaves the valley sinking between two strands of the San Andreas fault complex, a long zone more than 100 kilometers wide where the edge of the North American plate is shredded like old stockings. The forces involve not just horizontal motion along the direction of the faults, but pushing and pulling across the faults as well—combinations called transpression and transtension.

"Napa Valley is a transtensional pull-apart basin that happens to have mostly marine sedimentary and metamorphic rocks on the west side, and lava flows and other eruptive products on the east," Howell said. "In the middle, along the Napa River, is alluvium—mixed sediments derived from both sides of the valley. This creates a wide variety of soils in an area one-sixth the size of Burgundy. And the valley is oriented north-south, with its south end open to cool Bay air and its north end hot and dry, which makes for an equally wide variety of microclimates."

Pine Ridge: The Volcanic Side

Our first stop—not counting the time Howell dashed to the shoulder to fetch chunks of obsidian—was at Pine Ridge Winery, deep in the lava zone of the Stag's Leap district. The winery's guide explained that the whole valley is being replanted in the wake of a phylloxera infestation. This is an opportunity to do things right, he said, where previously the vineyard practices were haphazard. Now growers can choose from hundreds of available rootstocks and grape clones to match the local soils and landscape. They can use different patterns of trellising and distances between vines. They can respond to subtle soil changes even within a single vineyard, he said, and the value of today's grapes makes the expense worthwhile.

While he spoke, we tasted crisp and fruity white wines from the cool southern Carneros district, rich ripe cabernet from the Rutherford bench on the west side of the middle valley, and a rugged, tannic cab from the northern volcanic Howell Mountain area. Could we taste the geology, the terroir? We sipped and pondered. Certainly there are differences, but the grapes and climates are different, the vintages are different—everything is different.

As we walked into the winery's cavern, the bare exposures of ancient lava flows got our attention. We all hurried to finger the outcrop. Bedrock—we knew something about that. And we looked forward to the next stop, where a rigorous experiment with terroir was taking place.

Diamond Creek: The Winegrower's Side

Our next stop at Diamond Creek Vineyards seemed like a promising experiment.

Here, in a little ravine at the north end of the valley, winegrower Al Brounstein has planted cabernet sauvignon vines in three completely different soils—white volcanic ash, red lava soil, and barren stony alluvium—around a little pond. From that pond, a frog could land on any of the three soils in one leap.

We gathered at the pond and Howell poured us the 1992 Volcanic Hill, Red Rock Terrace, and Gravelly Meadow vintages. (He also thanked the corporate sponsor who underwrote this costly tasting—to make good on a golf bet that Howell won long ago in Jamaica.) The same grape, the same vintage, the same location, the same winemaker, and yet we could tell the wines apart.

All were tight, bold cabernets of good structure, of course, still young seven years after vinting. But one was more rounded, another relatively bright and forward, the third more closed and nuanced. Were these vague-sounding impressions the best that our scientist's palates could do? Or were our logical temperaments being swayed by the vivid surroundings, the romance of the vineyards in spring, and possibly the many other wines we'd been tasting with lunch beforehand?

Brounstein himself was little more help, an elderly wistful winegrower who did not share our language or outlook. He could only muse that after all, the gravel bed got more shade and the terrace faced the northwest. The wines were 5 to 10 years early for American tastes although the French loved their wines young, and in a few years the wines might be very different. . . . It seemed that even this experiment had too many variables.

Livingston: A Geologist on the Rutherford Bench

Our remaining hope for answers was the third and final stop, Livingston Wines, near Rutherford at the foot of the western hills on the Rutherford bench, an alluvial fan with coarse soil and sharp drainage. John Livingston is a retired geologist, after all, and surely had pondered the problem of terroir in ways we would find congenial.

In Livingston's kitchen we tasted his 1995 Moffett Vineyard Cabernet, from land right next to a vineyard owned by Pine Ridge Winery.

And indeed this rich, ripe, assertive wine was very much like Pine Ridge's Rutherford cabernet we'd had that morning, including the distinctive flavor of "Rutherford dust." Yet we definitely knew they were different. Of course, the vintages weren't the same, and neither was the time of day, or the winemaking practices, and yadda-yadda-yadda . . .

So, does the geologist have special insight into the winemaker's struggle with terroir? Livingston hardly paused at the question. "Not really," he said. Perhaps sensing a slight disappointment, he added, "although I am able to size up a vineyard quickly, for instance. I can see right away if it has all the elements that I want." We nodded, had some more wine, and presently we were studying the large "black granite" tabletop in Livingston's kitchen and asking him about the Ubatuba stone it was made from.

From Art to Science

At the end of the day, it seemed that terroir is just what the French say it is—all the million factors that influence the grape. The pessimist might feel that therefore winemaking is basically a hopeless muddle of "checks and balances of almost impossible intricacy," and that personal taste and marketing flimflam and accidents of nature make the whole thing a crap-shoot. But I think optimistically of other fields of practice that started out as art before they became science—alchemy, medicine, geology itself. Progress is inevitable, and I hope to live long enough to taste the greatest wines ever made.

PS: I've continued to explore the question, of course; a wine drinker with geological awareness can hardly do otherwise. Some further evidence is presented in my gallery of geological wine labels and in the Geology and Wine category. And David Howell has gone on to publish "The Winemaker’s Dance: Exploring Terroir in the Napa Valley."