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."
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."
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.
