Wine's deeper history

Winemakers and scientists alike have known that what makes good wine is a confluence of factors, including topography, climate and soils ?a known as "terroir." But the characteristics that constitute terroir are largely a reflection of today's landscape, Howell says, and the character of a wine has a much deeper history than that: a geologic history. "There is a saying that ??great wine begins with dirt,'" he says; however, in Napa Valley where the soils are young, great wine also begins with bedrock.

Napa Valley lies within the Coast Ranges of California, bounded on the east by the Sierra Nevadas and the Great Valley and the Pacific Ocean on the west. Napa Valley's geology and fertile farmland are a result of a long and complex series of volcanic and subduction events reaching back 140 million years. And because bedrock is at or near the surface of most of Napa Valley's vineyards and the soils are young by geologic standards (perhaps as young as 15,000 years), "it is important to know what type of structure you're working with," Howell says.

Thus, Howell and Swinchatt began to map out the geologic and topographic history of the valley. By overlaying existing soil maps with geologic maps, they created guides that provide a "new framework for examining the relationships between the ground and grape, place and quality," they write in their upcoming book, The Winemaker's Dance. To define characteristics fundamental to grape growing and the wine produced, Howell and Swinchatt devised "earth process units" or EPUs, three distinct provenances of soils.

Beyond the valley itself, most of the land in Napa is hilly, steep and dissected by streams. This land falls into the first EPU category, that of "residual materials," or thin layers of soil overlying bedrock. The soil that formed in place atop the bedrock is not well-developed, nutrients are sparse, and the grapevines are usually stressed. Wines grown on this land tend to be intensely flavored, highly concentrated and tannic, according to the authors.

In contrast, vineyards located on alluvial fans produce berrylike wine that is much softer in character than that from the mountains. So-called alluvial EPUs lie at the bottom of both mountain chains that line the edges of Napa Valley. The soils atop the alluvial fan materials were carried by streams down the mountain slopes, and may be thick or thin and composed of gravel, sand, silt and clay. Much of the rich history of Napa Valley wine has developed on alluvial EPUs, the authors say.

Fluvial EPUs occupy the remainder of the valley floor and are the thickest and richest of Napa sediments, Howell says, often to the detriment of vineyards. The thick vegetation that grows here gives wines a "prominent herbal" character. Few, if any, wines are produced solely from grapes grown on fluvial EPUs, Howell says.

Although Howell and Swinchatt's EPUs are specific to Napa Valley, "you could use this discrimination system anywhere," Howell says, especially in places where the soils are young. In places such as South Africa where the soils are very old, however, the soils would no longer retain much of the character of the bedrock. Thus the characteristics of the soil itself would be more important to analyze.

In any region, Howell says, taking a holistic approach to winemaking is key. Fortunately, many winemakers are coming to realize, he says, that "good wine comes from the earth, not the winery."