John Livingston, a geologist and owner of Livingston Moffett, says the alluvial debris on his property drains rapidly, a condition that yields rich, concentrated Cabs.
Character Profiles -Unearthing Napa Valley's Dirty Little Secrets
By Jeff CoxThe beautiful and serene Napa Valley is one of the most violent places on earth. At least when viewed from the perspective of geological time. So if that favorite bottle of Napa Valley Cabernet Sauvignon is full of blood and thunder, it probably inherited those dramatic qualities from the soil in which it was grown.
The outer layer of the earth's crust in this narrow, north-south-running region was created by exploding volcanoes and fiery flows of molten rock, shattering earthquakes, upthrust chunks of primeval sea bottom and massive floods. Millions of years' worth of persistent driving rainstorms and baking heat further molded this dynamic landscape.
"These tumultuous events have created forces that are contained in the soils, just waiting to be released," says Aaron Pott, winemaker at St. Clement Vineyards in St. Helena, whose wines possess characteristics such as lively fruitiness and fine tannins. He takes a cosmic view. "This notion that a vineyard is imbued with the nature and experience of its past intrigues me. When I consider that this force reveals itself in the wines, it makes perfect sense to me. It's what gives a wine its essence, its vitality and, ultimately, its mystery."
But there are more down-to-earth reasons why the geology of the Napa Valley affects grapevines in such fortuitous ways. Primary among them is that the forces of nature have created a wonderland of diversity here. In a valley barely 25 miles long and only a few miles wide, more than 60 recognized soil types range from fertile, gravelly loams to soils so nutrient-poor that even weeds won't grow. In between these extremes are soils ideally suited to nurturing wine grapes - poor enough in character that ripened grapes are small and potent, but with just enough nutrition to sustain the vines.
Soil makeup isn't the only contributor to the great diversity of growing conditions in the Napa Valley. Varying environmental aspects and climates obviously make their contributions, too. Mountains fringing the east and west sides of the valley present very different angles to the daily crossing of the sun. Some vineyards are laid out on the valley's floor while others scale mountainous heights. Temperatures are significantly higher at the more protected north end of the valley than at the open south end that borders the cold waters of San Pablo Bay. These differences add up to a diversity of microclimates that spawn distinct wines, be they single vineyard or estate bottlings, such as Robert Mondavi's To Kalon Vineyard Cabernet from the gravelly alluvium of the Oakville appellation, or cuvées from several sites in an AVA, such as Beaulieu Vineyard's Rutherford cuvée from the mix of mineral-rich volcanic rocks and river cobbles that make up the Rutherford Bench.
How soil type influences a wine's flavor profile is cause for much debate among winemakers worldwide. In France, the term terroir has long been used to define the unique taste of the place where a wine is grown. Most French winemakers and vineyardists have absolute faith in the idea that soil is a determining factor in the taste of a wine. Their American counterparts were not always so sure.
For many years, the conventional wisdom, perpetuated by UC-Davis teachings, was that a soil's drainage capacity, rather than some ineffable nutrient, had the biggest influence on the resulting quality of a wine. But that view is changing (although some still insist that it's all in the drainage, they are a dying breed).
Across California, growers and winemakers intuitively believe that the make-up of the soil will influence the taste of the wine. Napa Valley is a compact and well-defined laboratory to test this belief.
"Distinctive soils of different vineyards and regions of the Napa Valley produce Cabernet Sauvignon wines that are distinguishable," says Deborah Elliott-Fisk, a UC-Davis scientist writing in the Journal of Wine Research. "Soils have been shown to affect wine chemistry, quality, flavors and aromas. Cabernet Sauvignon wines can be discriminated by flavors and aromas as a function of soil type, overriding the viticultural and enological variables."
Her conclusions are important to understand because soil is, after all, a remnant of the entire geological history of a vineyard. So significant, in fact, that the Geological Society of America is holding a symposium in Seattle, Washington, November 1 to 4, on "Terroir, Geology, and Wine"; John Livingston, a geologist and owner of Livingston Moffett Winery in Rutherford, where he produces about 5,000 cases of mostly Cabernet and Cab blends, will be in attendance.
On a brisk spring morning, Livingston sat on his front porch and gave me a capsule history of the geology of the Napa Valley. We began with his own property, which is tucked up against the Mayacamas Mountains on the western edge of the Rutherford Bench - an alluvial fan formed from debris carried down from the mountains over geological time and deposited onto the floor of the valley. This alluvial debris drains rapidly, a condition that leads to rich, concentrated wines. Volcanic rock in the soil gives his wines a complex minerality, while silt and clay particles hold organic nutrients that enhance fruit flavors.
Livingston says the Napa Valley is a "syncline." That's geologist-speak for a U-shaped fold in layered rock. There are four distinct layers of rock in this syncline. At the surface, the youngest layer, which is thin around the edges of the valley and hundreds of feet thick in the center, and which covers the central valley floor, is a mixture of cobbles, sand, silt and organic matter deposited by the south-flowing Napa River and lateral streams. This is prime agricultural soil, well-drained and rich in nutrients - so rich, in fact, that winegrowers have to carefully manage the vigor of their vines on these soils, lest they produce large, watery berries instead of the small, intensely-flavored grapes that make the best wines.
Under the river deposits is a thick sequence of volcanic deposits called the Sonoma Volcanics, which, in places, are hundreds of feet thick.
Beginning about four or five million years ago, the Napa Valley was a region of active volcanism. This ancient volcanic rock comes to the surface today on the lower slopes on either side of the valley. Livingston has dug his winery's caves into just such a lower slope on his Rutherford property. The excavation revealed compacted volcanic ash called tuff. Livingston explains that this rock "was once molten and charged with gas like rhyolite and andesite."
Rhyolite is mostly silica dioxide and can take many forms, including the volcanic glass called obsidian. On the east side of the valley along the Silverado Trail not far from Duckhorn Vineyards in St. Helena, Glass Mountain - chock-full of sharp-edged pieces of obsidian - rises. Andesite, named for the Andes Mountains of South America, possesses less silica and more of a variety of other minerals than rhyolite. South of Glass Mountain, on the lower eastern slopes of the Stags Leap District, Stags' Leap Winery's caves are dug back into a stratum of volcanic rock so hard that dynamite had to be used to break it up. Stags' Leap's sister winery, Beringer Vineyards, has no less a complex mix to choose from, ranging from Knights Valley's well-draining alluvial, gravelly loam, to Chabot Vineyard's chunky obsidian, and Howell Mountain's Bancroft Ranch with its gritty, volcanic white tuff.
Volcanic rock differs in its affect on the final product depending upon, among other things, the size of the rock. It can impart minerality and encourage balancing acidity, sugar and phenolics due to its superior soil-structuring qualities.
"Over time, the volcanism has shifted north to the Cascades in Oregon and Washington," Livingston says, recalling the Mount St. Helens eruption on May 18, 1980.
But the ancient California volcanism left more than volcanic rocks behind. In Calistoga, situated in the northern part of the Napa Valley, natural hot springs have spawned a spa industry, and there's a geyser among the vineyards just north of town. Where Opus One now stands in Oakville (approximately mid-valley), the story goes, cattle grazed on the lush grass that grew there all winter. Locals speculated that the phenomenon was caused by heat from underground thermal springs. When the foundation for the winery was dug, it is even rumored that workers hit hot water. Opus One, however, claims the story is apocryphal.
Beneath the Sonoma Volcanics is a layer of sedimentary rock formed from ancient seabeds during the Jurassic and Cretaceous periods (from 190 to 65 million years ago). This band of rock is called the Great Valley Sequence. As the edges of the syncline were elevated by the folding and uplift of the Napa Valley, these sedimentary rocks became exposed farther up the slopes and formed the parent material for the soils there. Under the sedimentary rock is oceanic crust, which surfaces on the peaks of the mountains on either side of the valley. This is igneous rock formed from magma under the earth's mantle and is the base of the seabed off the California coast under thousands of feet of sediment. It rarely surfaces, but does so on the mountain ranges that flank the Napa Valley because of the syncline. "I have some weird rocks on the back of my property," Livingston says, gesturing up the hill and referring to chunks of oceanic crust.
"These rocks are ultramafic, which means they contain high levels of magnesium and iron. They can also be high in boron and other nasty stuff," he notes. An excess of boron in the soil will poison most plants. Weathering has washed all of these rock types down the gullies between the high mountains on both the east and west sides of the valley, forming alluvial fans, or benchlands.
"I tend to think the two best soil types for growing grapes in the Napa Valley are the volcanic rock soils along the valley's sides and the alluvial fans," Livingston says. "Drainage is very important for growing high-quality grapes, and the volcanic soils and benchlands generally have good drainage. The river deposits in the middle of the valley are well-drained, but they can be too fertile."
The Rutherford appellation's western flank is situated on just such an alluvial fan, with spectacular results for the wines from grapes grown there.
Dominus and its new label Napanook are located to the south on a similar alluvial fan that forms another bench and extends into the Oakville AVA. Dominus Winemaker Boris Champy says, "Over time, water flowing through the alluvial fan has washed away nutrients in the soil, creating 'starving' conditions for the vines. This condition allows for a deep rooting system for the vines and the ability to dry farm."
In other words, the poor soil causes the vine's roots to strike deeply into the earth in search of nutrients. At these depths, they are apt to find enough moisture to sustain themselves. Vines more shallowly rooted would need supplemental irrigation to get them through California's typically sustained summer drought. The deep roots mean that the vines get just the right amount of nourishment and water to develop complexity and concentrated, deep flavors.
Champy cites two more vineyards on alluvial fans: Stag's Leap Wine Cellars on the alluvium from the volcanic heights of Stags Leap District's slice of the eastern mountains, and Chateau Montelena on the alluvial fan of volcanic soil east of Calistoga. Although there is hot water roiling under nearby Calistoga (water that is very corrosive and contains boron and hydrogen sulfide), the alluvial soil beneath Montelena protects its vineyards from the thermal springs.
"These three estate wineries well represent the notion of terroir in California," Champy says. "The wines they produce are, in my opinion, great classics of the Napa Valley. They share great similarities: All three vineyards are comprised of poor, gravelly loam, resulting in very intense wine, and also clay loam, yielding less intense but fruitier and more elegant wine, ideal to blend with the (gravelly loam) soil for balance." During the summer, Champy explains that the vines slowly get water stressed. "This gentle stress makes the greatness of the grapes," he says. "All of this is a complicated equilibrium: too much water equals too much vigor, so bad grapes; not enough water equals too much stress, so bad grapes." The same holds true for nutrients, he says. In the Napa Valley and beyond, quality results from a balance of the right amount of nutrients, water and gentle stress on the vines.
Much of the far southern end of the Napa Valley, especially the Carneros region where wineries such as Artesa, Domaine Carneros and Saintsbury are located, is former bay bottom and contains a layer of hardpan clay not far beneath the surface. The impenetrable hardpan limits the growth of vine roots and checks their natural vigor. Once again, the stress creates great fruit because it forces the vine to produce small berries. A wine's color and flavor components come mainly from the skins, so the smaller the grape berry, the higher the ratio of skin to juice, and thus more flavor components per unit of juice.
Because most vines in the Napa Valley get some supplemental irrigation from wells drilled into the various kinds of rock, it seems logical to suppose the water has some effect on the vines and on their wines.
Richard Slade, a groundwater geologist from Southern California who frequently works in the Napa Valley, says, "The quality of wine depends in part on the quality of the water those vines absorb, both in terms of volume and mineral content." He notes that the best wells come from areas of Sonoma Volcanics, which is usually porous rock, especially the tuff that's compressed volcanic ash. "Some of these wells will produce between 25 and 100 gallons a minute, and they contain calcium and magnesium, both beneficial minerals for the vines." Yet he says a vineyard manager is lucky to get two or three gallons of water a minute from wells drilled in the older rocks.
Slade offers "a gross analogy" as further explanation: "Go to the beach with a bucket and fill it with sand. Then measure how much water it takes to fill the bucket of sand to the brim with water. Now fill the bucket with cement and let it harden, then smack it with a sledgehammer so it fractures. Now see how much water it takes to fill the bucket of fractured cement to the brim. It will be far less. So, far less water is stored in hard, fractured rock than in sandy, gravelly alluvial soils or in porous volcanic rock."
Bob Burrows, proprietor of Liparita Cellars, recently contracted Slade to do some work. Liparita, founded in 1985, was initially a 70-acre estate planted mostly to cabernet sauvignon on Howell Mountain, but Burrows sold that land to Kendall-Jackson several years ago. Now he has transformed the Liparita label by blending wine from what he calls "a necklace of sites" around the Napa Valley's sub-appellations. He sources grapes, for instance, from Diamond Mountain, Spring Mountain, Howell Mountain and Mount Veeder, as well as sub-appellations on the valley floor.
Slade notes that the mountain areas are very site- specific in terms of rock type. "All the grape varieties show the influence of their soil types, and the geology and its effect on groundwater are the foundation of the soil types. We're learning which vineyards are best for which varieties, and understanding the kind of rock that underlies the vineyard is essential."
Burrows is skeptical about fruit from vineyards on the old rock formations - the Oceanic Crust and Cretaceous-Tertiary rocks found along the very tops of the mountains on both the east and west sides of the valley - "because the water there tends to be loaded with stuff like boron and arsenic." He has closely studied the findings of acclaimed Australian vineyard consultant Richard Smart, who says that grape growers don't have to be at the mercy of conditions if they understand the scientific facts of the vineyards and their soils.
"There's been such an evolution in the past 20 years in understanding the geology, the aspect (the way the vines relate to the sun's path), the types of soils, and the water in the Napa Valley," says Liparita winemaker Kerry Signoracci. "And once we understand these things, we can maximize flavors in the vineyard." He and Burrows strive to craft wines that deeply reflect these environmental factors and display increasing quality of flavors and textures.
The quest to better grasp the valley's composition is time-consuming and expensive, but according to Livingston, the Napa Valley Vintner's Association is underwriting substantial research into the scientific understanding of the region's geology, as well as its microclimates.
He and his peers have already gained a more intimate knowledge of the region's soils, but more data should lead to growing even better, more flavorful wines. In the meantime, it's safe to say that the Napa Valley's reputation as a producer of largely fine wine is based on a foundation of solid rock.
Sonoma-based Contributing Editor Jeff Cox is the author of From Vines to Wines and the upcoming Cellaring Wine (Storey Publishing, Fall 2003).
