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").

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.
Works Cited

Branson, Anne. "Map." Napa Real Estate Web site. No date. 4 Dec. 2007 http://www.annebranson.com/images/napa_map.gif

Climate of the Napa Valley." Napa Now. No date. 4 Dec. 2007 http://www.napanow.com/climate.html

Geological resources." Watershed Information Center & Conservancy of Napa County. 2005. 4 Dec. 2007 http://www.napawatersheds.org/Content/10247/Geological_Resources.html#2

Howell, David G. "The terroir of the Napa Valley ava." The Geological Society of America Web site. 2003. 5 Dec. 2007 http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_60313.htm

Larson, Ronald. "Napa Valley: The lay of the land." Napa Wedding Source. 2007. 5 Dec. 2007 http://www.napaweddingsource.com/14/the_napa_valley/napa_valley_the_lay_of_the_land.htm

Livingston, John. "The geology of fine wines." California Wild, 51:4. Fall 1998. 4 Dec. 2007 http://www.calacademy.org/calwild/1998fall/stories/wines.html

Marcus, Kim. "California takes on the world." Wine Spectator Online. Dec. 18, 2002. 5 Dec. 2007 http://www.winespectator.com/Wine/Features/0,1197,1572,00.html

Napa Valley: An ideal place to grow grapes." Napa Vintners Web site. No date. 5 Dec. 2007 http://64.233.167.104/search?q=cache:kwgJR0tbCgUJ:napavintners.com/downl oads/napa_valley_seminar.ppt+%22napa+valley%22%2Bbedrock&hl=en&ct=clnk&cd=15&gl=us>

Napa Valley Vintners. "Science of Napa Valley." Napa Valley Vintners Web site. No date. 4 Dec. 2007 http://www.napavintners.com/about/ab_3_geog.aspx

Smith, Rod. "Napa's hidden faces." Private Clubs magazine, March/April 2005. 4 Dec. 2007 http://www.clubs.com/archives/2005-mar- apr/wine_napas_hidden_faces.htm>

Tinney, Mary-Colleen. "Winemakers examine what makes Napa AVAs unique during premiere Napa Valley event." Wine Business. Feb. 2007. 4 Dec. 2007 http://www.winebusiness.com/html/PrinterVersion.cfm?dataId=46781

U.S. Geological Survey. "Probability map." 2002. 5 Dec. 2007 http://earthquake.usgs.gov/regional/nca/wg02/images/percmap-lrg.html

Map 1: Napa Valley region

Branson)

Map 2: Napa County faults and seismic activity

U.S. Geological Survey)