Geology and soil quality in Napa Valley wine production

Geology-Wines

Particularly over the past quarter century, California wines have grown in prestige to become among the world's most celebrated, even challenging more historically established wines from the Bordeaux region of France (Marcus). California's Napa Valley region is considered a bit of a geographic wonder in the wine-making industry for its ability to produce a wide variety of high-quality red wines in a fairly limited space - just 40,000 acres of cultivated, planted land (Livingston). The wines from this California region, about 50 miles north of San Francisco (See Map 1), owe their quality and diversity to some unique geological features of the area. Throughout its rich geological history, Napa Valley has been affected by a wide variety of natural occurrences ranging from volcanic eruptions to plate tectonics. Over the course of millions of years, these occurrences formed a geological foundation that is extremely conducive to the growing of a diverse set of quality wine grapes. According to vintner and geologist Robert Livingston, soil, bedrock, topography and climate and are four critical, inter-related factors that help foster the development of quality wines (Livingston). These factors all have benefited from Napa Valley's unique geological features, resulting in some of the best grape-growing conditions in the world.

Geology and soil

Every wine grower knows that soil conditions are critical to routinely producing hearty grapes. The geological history of Napa Valley has contributed to making its soil ideal for growing a variety of grapes commonly used in the production of red wines. According to Livingston (1998), there are at least 60 different soil types in Napa Valley, which means more types of wine can be produced there than in any other area of comparable size in the world. This variety of soils is the result of the unique geological conditions that are inherent to California's global positioning. It is well-known that California represents the meeting place of the North American and Pacific global plates, which results in friction that causes California's frequent earthquakes (See Map 2). That friction, over the course of millions of years, also once caused a great deal of volcanic activity in California and the Napa Valley, in general. As a result, the soil in Napa Valley frequently contains large deposits of igneous rock and volcanic ash, which helps support the healthy growing of crops in many regions of the world (Napa Valley Vintners).

Of course, volcanic ash is just one common component of Napa Valley soils. Because of plate tectonics -- the constant moving of plates -- Napa Valley soil contains rocks and minerals that are not necessarily indigenous to the region, but were deposited there by the Pacific plate (Livingston). As the Pacific and North American plates scraped together, parts of the Pacific plate rose and formed mountains that contained nutrient-rich oceanic crust, elements of which are found in Napa Valley soil. The Napa River, which flows to the bottom of the valley, also plays a critical role in depositing clays, which hold and allow for the easy transport of water, into the soils at the valley's low points.

In essence, if one was to take a sample of Napa Valley soil, one would find fine clays and sands at the valley's lowest levels, and larger rocks deposited by swift stream currents at slightly higher altitudes (Livingston). Volcanic ash is commonly found in the next highest levels, while the peaks of the hills and mountains in western Napa Valley often feature soils with high levels of iron and magnesium, which are common in rocks found in oceanic crust (Livingston). As one might expect, these different soils support a wide variety of grapes used in several kinds of wine (typically reds). For example, the low-lands, with their rich clay deposits, are perfect for growing large, robust grapes, while the soil in highlands, with features significant levels of oceanic crust and volcanic ash, typically supports smaller grapes with more robust flavors, strong mineral content and good levels of acidity (Napa Valley Vintners and "Napa Valley: An ideal"). But the fact that such a range of grapes can be produced in such a limited space is a testament to Napa Valley's unique geological features and the soils that resulted.

A ography

The topography of Napa Valley is unique and helps foster a variety of different climactic and soil conditions that are conducive to grape growing. Napa Valley is bordered by mountain ranges that each have their own distinctive characteristics. The Mayacamas Mountains border the valley to the west, the result of previously discussed plate friction. These mountains divide the Napa and Sonoma valleys and many peaks fall between 2,000 and 3,000 feet, with the highest point being Cobb Mountain at more than 4,700 feet. The Mayacamas are high enough to maintain cooler temperatures year-round, and many peaks will feature snow during winter months. The Mayacamas Mountains often contain soils with volcanic ash from ancient volcanoes (Napa Vintners Association). To the northeast, in Calistoga, is Mt. St. Helena, which is the highest point in the San Francisco Bay Area watershed. Mt. St. Helena rises to a peak of more than 4,300 feet, allowing it to tower over many of the Mayacamas peaks. It was formed by the rising of volcanic material from the Clear Lake volcanic field, which still features a variety of geysers and geothermal springs (Napa Vintners Association). In fact, this volcanic material also created a series of small hills in other parts of the valley (Napa Vintners Association). The Napa River also originates near Mt. St. Helena and runs down the mountain's southern slope, carrying igneous material and a variety of river sediments, such as clay, to the valley below. Mt. St. Helena can be cold during the winter, often showing snow in its upper sections.

On the east side of the Napa Valley are the Vaca Mountains, which are geologically distinct from the Mayacamas and Mt. St. Helena. Because the Mayacamas block the cool, moist air blowing in from the Pacific, the Vaca Mountains tend to be dryer and hotter (Larson). Where plants that require significant levels of water, such as conifers, thrive in the Mayacamas, the Vaca Mountains are characterized by low-moisture oaks and grasses (Larson). At their highest point, the Vaca Mountains rise to more than 2,800 feet above sea level, although many of the range's peaks are at significantly lower altitudes. According to the Web site for Rudd Winery, located in the foothills of the Vaca range, the low moisture and the red soil, which contains significant volcanic ash, are conducive to growing small, flavorful grapes.

The valley floor itself can experience a variety of topographical changes. As was discussed, the Napa River and other streams deposit clays, shales, and a variety of other river sediments in the valley lowlands, while a historical narrowing of the river has resulted in large concentrations of sedimentary rocks. The Napa River also can flood from late fall to early spring, depositing river sediments far past its typical banks (Larson).

The elevation of the valley also changes as one travels south to north. At the southernmost point of Napa Valley, the elevation is essentially at sea level, but it rises to 362 feet at the northern end, near Calistoga (Larson). The valley also narrows as one moves north, from a width of five miles in Napa to just one mile in Calistoga, where the Mayacamas, Mt. St. Helena and the Vacas close in around it (Larson). The topographical changes across Napa Valley result in a variety of different grapes and wines. For example, vintners claim that the Mt. St. Helena region has a gravelly, clay soil that produces rich reds with deep tannins, while the Vacas region's dry, warmer conditions permit wines with hints of spice and peppercorn, although the porous rocks make irrigation critical (Tinney). The Mayacamas grapes are known for having concentrated, mineral-rich flavor, a result of soil conditions presented by the uplifted oceanic crust (Tinney).

Climate

Naturally, the topography of Napa Valley has a significant influence on the region's climate, which can vary significantly by location, season, and time of day. As mentioned earlier, the coastal Mayacamas mountains affect the climate in other areas of Napa Valley. The Mayacamas can be cool and wet, receiving the chilling winds and moisture from the Pacific. On the other hand, they block cool air and humidity from reaching the Vacas range, which is warmer and dryer as a result. In essence, Napa Valley serves as a buffer between the cool coastal regions of Northern California and the hotter Vacas range. It can grow high- and low-humidity grapes within miles of each other.

The southern part of Napa Valley, near San Pablo Bay -- the northern tip of San Francisco Bay -- tends to be several degrees cooler than the rest of the valley, as it has unfettered access to ocean winds ("Climate of"). As Napa Valley stretches north, it takes a sharp turn west, and the cooling ocean winds do not make the turn ("Climate of"). As a result, some northern valley communities, such as St. Helena and Calistoga, tend to be much warmer than the southern towns ("Climate of").

A ography results in temperature and precipitation variances that have a significant effect on which grapes are grown in certain parts of Napa Valley. For example, in the Howell Mountain region, in the northeast part of the valley, wine growers can expect as much as 50 inches a year of rain, as well as warm, Mediterranean-type temperatures and fairly cool evenings ("Napa Valley: An ideal"). This permit's the growing of excellent cabernet sauvignon, merlot and zinfandel grapes; chardonnay and viognier grapes are grown as well, but they are not as fruity as those grown at lower elevations ("Napa Valley: An ideal"). By contrast, the Rutherford region in southern Napa Valley sees a maximum annual rainfall of just 35 inches, and cooler temperatures that rarely exceed 90 degrees Fahrenheit, even in the summer ("Napa Valley: An ideal"). This climate is conducive to growing cabernet sauvignon and sangiovese grapes, but also allows for a crisp and aromatic chardonnay ("Napa Valley: An ideal"). The cool temperatures of Oak Valley in southern Napa Valley also allow for a strong Riesling grape, although that regional benefits from alluvial deposits in the soil ("Napa Valley: An ideal").

Conclusion

When it comes to wine-making, Napa Valley is nothing less than a natural wonder. Although considerably smaller than other regions renowned for wine, Napa Valley is able to produce a great variety of grapes for various wines, and some of the top red wines in the world. Napa Valley owes its success to the unique geological features of the region. Millions of years of friction between the Pacific and North American plates have transported non-native rocks and minerals to the valley, while also raising oceanic crust above sea level, causing volcanic eruptions, and creating mountain ranges. The end result has been ideal soil, bedrock, topographical and climate conditions for growing wine grapes. Soils have benefited from deposits of oceanic crust and volcanic ash, as well s sediments from valley waterways. The bedrock also is a unique mix of volcanic rock, oceanic crust, shale, limestone and other rock varieties, which help nurture various grape species. Napa Valley also is a unique topographical mix of mountain ranges and lowlands that shield some areas from moisture and ocean breezes, while allowing other parts of the valley to remain wet and comparatively cool. This results in climactic variations that allow Napa Valley vintners to grow grapes that require ample water supplies, while also accommodating low-moisture grape vines that can produce small, strongly flavored grapes. In conclusion, a variety of geological conditions have come together in a near-perfect fashion in Napa Valley to create a setting that is ideally suited for the production of quality wines.