After the publication in 1998 of James Wilson’s Terroir, the American wine industry became enchanted with this abstract notion of diverse and slippery meaning. It has become a standard part of the wine writer’s and wine marketer’s lexicon despite ongoing controversy over what the term signifies. In the case of its geological components —bedrock, surface material, and their influence on vines, grapes, and wine — the meaning is particularly murky. In his exhaustive treatise, Wilson alludes to the relationship between geology and wine character but never makes a direct or causal connection. In The Winemaker’s Dance, Swinchatt and Howell suggested strongly that geology influences wine character but came no closer than did Wilson to making a direct connection. Other attempts to establish a causal link between geology and wine character have been similarly unsuccessful. Recently, Maltman (2008, 2009) has extensively reviewed the fundamental nature of the relationship between geology and wine and has concluded that any suggested direct link between geology (bedrock composition and soil mineralogy and geochemistry) and wine flavor can be nothing more than romantic myth.

While a direct link between geology and wine appears to remain largely metaphoric, indirect influences are quite a different issue. In at least three cases in the Napa Valley, areas in vineyards that can be defined on geologic criteria correlate well with either winemaker’s taste buds or issues of viticultural management. Something reflected in the geology is also reflected in vines, grapes, and/or wine. While the specific factors are yet unknown, they include drainage, accessibility to water, microbiology (as influenced, for example, by substrate texture and moisture), soil temperature, and trace element chemistry. The experiments that might unravel these links are devilishly complex and, for the most part, prohibitively expensive. As a result. it is likely that the connections between geology and wine will remain elusive for some time to come. Nevertheless, the recognition that such associations exist should encourage further investigation while underscoring the notion that understanding the geology of a vineyard or AVA enhances the entire wine experience.