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GRAPEGROWING William A. Fuchs A thin section of basaltic andesite with crystals of feldspar laths and colorful pyroxene from Trefethen are viewed under a microscope at 50 times magnification. mapping rocks, though not necessarily unusual for a soil scientist. Contour interval is important as well. This gives you the vertical component on a map, and a good geologist can look at a topographic map and see the ground in three dimensions. The topographic map is the base upon which the mapped geology is laid. A good contour interval at this scale is 2 or 5 feet, and it had better be a custom-made topographic map. Sonoma Volcanics Napa has a variety of geologic terranes (areas with distinctive rock types and geological history), but the hillside vineyards I mapped are all in the Sonoma Volcanics, as is much of the valley. I like to imagine what the place would have been like at the time the rock was formed, though in this spot you would likely have been dead within minutes or seconds. The volcanism at the time was as spectacular as anything we have ever seen. The Sonoma Volcanics were formed between 2 million and 7 million years ago. Human evolution began about 7 million years ago, so these rocks span most of human evolution (except the past 2 million years.) It was a busy time for humans, but nobody was here in California (although there���s some uncertainty about that point.) Most of the valley rocks span a more restricted interval of 2.7 million to 5.5 million years. It sounds like long ago, but to a geologist that was only yesterday���not that old at all. 88 W in es & V i ne s JANUARY 20 13 Since about 2 million years ago, Napa has mostly been eroding away, but the area is seismically active as well. Tectonically, the valley is known as a pull-apart basin, a geologic feature that is well known to petroleum exploration. But there���s no oil here; the rocks are wrong. Complex interactions along faults, some still potentially active, expanded the area and down-dropped the rocks to form the basin that is Napa Valley. There are also horizontal motions along the faults. Some uplift in the mountains is probably still ongoing, and there is a battle between erosion, uplift and down-dropping that is being orchestrated by the plate tectonics of California. Nature is messy, and there is much geologic complexity in this area. There is some folding, but the mapping I did shows that block faulting with tilted (but unfolded) blocks bounded by faults is the norm on these three vineyards. Traversing the vineyards that I mapped is a very large fault that was previously unrecognized. I named it the Trefethen fault and determined that it has hundreds of feet of vertical slip (potentially more than a thousand feet) with a sense of movement that is down to the east. The Trefethen fault has had a major influence on the distribution of rock types and appears to be a branch of the more famous West Napa fault located just east of my map and those pictured here. One important aspect of geology is stratigraphy, the sequence of rocks from older to younger in a layer-cake succession. Knowing the stratigraphy is helpful in determining the structure��� the latter dealing more with faults, folds and dynamic deformation. Some rocks have no internal stratigraphy, such as the granites in the Sierra Nevada Range. Sedimentary rocks have stratigraphy, and so do volcanics. The most important initial step in mapping is to select mappable units. I have eight stratigraphic units mapped on these properties, where before there was only one: Sonoma Volcanics undifferentiated. I found the units I defined to be mappable, and each is represented by a different color on the map. Importantly, some of these stratigraphic units were found to correlate with good wines, so I will focus on those units. Whole-rock analyses of samples What helped to refine and define the various units mapped was obtaining whole-rock chemical analyses for many rock samples collected from the site. Whole-rock refers to the main chemical constituents in a rock such that they add up to 100% of the rock (in contrast to trace elements, which are present in very small amounts.) Volcanic rocks can be classified on the basis of whole-rock analyses (see Rock Classifications of Samples from Three Vineyards on page 90), mineralogically or based on field classification. Because whole-rock analyses are expensive, most mapping is based on field classification, but I found that whole-rock analyses were essential for the classification of these particular rocks. Microscopic analyses also were important in determining rock type, though somewhat less so than the chemical analyses. I wrestled a little with how to define wine quality for the purposes of this project. It appeared to me that it was best to confine vineyard plot comparisons to within each of the three entities (Trefethen, Darms Lane and Shifflett), rather than comparisons between the three entities or even Napa vineyards in general. In this way, I could rely on the individual owners/managers to make the determinations. After all, who knows the relative quality of the vineyard plot wines better than those closest to them? The subject of wine quality still would seem difficult when you consider the different variables (vine ages, cultivars, rootstocks and more within a single operation), but the managers of all three of these operations seemed