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GRAPEGROWING Viticultural Geology A larger geologic map of all three vineyards can be found in the online OR edition of this issue at winesandvines.com. How what���s below the surface affects wine quality By William A. Fuchs CALIFORNIA T his is a tale of three hillside vineyards in Napa, Calif. I am a geologist who has lived in Napa County for eight years. I know rocks���especially the rocks of a small area of west Napa Valley, where I mapped the geology of the vineyards of Trefethen, Darms Lane and Shifflett Estate in great detail. A few years later, the managers of these vineyards told me that they saw correlations between the geology I mapped and the origins of their better wines. Yes, this is an article about terroir, but it emphasizes the mapping of rocks. I mapped soil also, but not from the perspective of a soil scientist, rather from the perspective of geomorphology, the study of landforms. Soils generally have a rock component in them, and it is a general truth that the rock component typically becomes dominant the closer you are to rock outcrops���and, in the case of Napa, on the hillsides. There, the rock component is not only dominant but also more closely tied to the immediate rock geology. In the valleys, the rock component of soils may or may not be dominant, as compared to organics and clays produced by weathering, but it is much less tied to the local rock outcroppings. Valley soils are to my mind more complex and homogenized, and some of them are more mature. My point here (and one of the main themes of this article) is that by mapping the rocks on a detailed scale on the hillsides, you have in your hands maybe 60%-80% of the important information you need about the soils. Important to what? Well to wine, of course! Starting at Trefethen I started my project at the Trefethens��� Hillspring Vineyard (Trefethen Family Vineyards), roughly 1.5 miles north of the city of Napa, where the owners, Janet and John Trefethen, were good enough to give me access to their ground and support the 86 W in e s & V i ne s JANUARY 20 13 Mendocino Healdsburg Trefethen Paci���c Ocean Napa Darms Lane Shifflett Estate cost of some analytical work. After Trefethen, I continued mapping southward, where I had the cooperation of Larry Bump and his daughter, Tricia Bump Davis, of Darms Lane (formerly Crichton Hall Vineyard), and then on to Jeffrey Shifflett���s ground (Shifflett Estate.) The results of all this, in somewhat simplified and reduced form, are the three separate maps in this issue (pages 87 and 89), portions of the original full-size map. Having mapped during my career in places such as New Guinea, Turkey, Mexico, Alaska and the Canadian Arctic, I expected Napa to be pretty tame. For the most part it was, but I met up with rattlesnakes, poison oak, ticks (one of which nailed me in the belly button, creating an infection that sent me to the emergency room), coyotes and mountain lions���OK maybe not the last one on this list, but it seemed as though they should be there. The experience was wilder than I expected. The geology was very interesting, partly because of the surprisingly large number of rock units. Most people are not aware that much of west Napa Valley is incompletely mapped. I work for a wine cave builder and perform initial site investigations (we like some rocks for cave building and others not so much.) The three NV mapped vineyards San Francisco The state survey mapping is more complete further west in Sonoma County, in the Mayacamas Mountains east of the cities of Sonoma and Santa Rosa. I mapped at a scale of 1:2,400 (or 1 inch=200 feet.) In the field of geologic mapping, scale is not an abstract concept; it is of premier importance. You must choose the right scale to accomplish a certain objective. To get information that can be correlated with vineyard/wine quality, I believe a scale of 1:2,400 is the right choice. It is a somewhat unusually detailed scale for Highlights ��� eologist William A. Fuchs ran numerG ous chemical analyses of rock types in three adjacent Napa Valley properties. ��� e mapped the sites in detail to explore H the connection between rock types underlying vineyards and wine quality. ��� is data uncovered correlations between H rock types and vineyard blocks that the owners rate most highly for wine quality.