On a cold and rainy Saturday in March, wine lovers gathered on a hill in Coombsville, eager to hear what geologist David Howell was about to say. Howell is the leading local expert on terroir and a coauthor of The Winemaker's Dance: Exploring Terroir in the Napa Valley—a book that he says “should be required reading for anybody who is an ambassador or in a hospitality room in Napa.”

Howell, who, with his tan felt hat, distinguished beard, and muddy rubber shoes, could be a Dictionary’s definition of a geologist, spent much of his life studying the rocks and earth of California. After retiring from an illustrious career with the US Geological Survey, he turned his attention to his other passion, wine. Now Howell instructs a wine and geology course at Stanford and leads wine-centric geology tours in Napa and France.

One of the first things Howell teaches his disciples is the importance of terroir. “Without knowing terroir, why are you drinking the wine?” He asks. “It's just a beverage. Whereas if you know about terroir, you know from whence it came.” A talented artist who sometimes shares his cheerful depictions of the New Hampshire countryside on Instagram, Howell likens terroir to the provenance of a painting.

“If you just look at the painting itself, you're worrying about the genre of the painting, the color, the size, the person who painted it,” he explains. “But if you look at the provenance of a painting, what has happened since that painting was painted? Was it stolen by the Nazis and ended up in a museum tucked away in some neighborhood in Tokyo, then eventually discovered and brought back, and the rightful owner was given back the painting. Doesn't that change a little bit, the painting, how you look at it?” It’s hard to argue Howell’s perspective. It’s why the group is here: to learn the origin story of the earth in Coombsville. 

Vintner John Caldwell has long wondered about its almost perfect grape-growing grounds. Thus, today’s tour starts at Caldwell Vineyard. The 42-year-old winery is where Caldwell produces Bordeaux varietals from French clones he smuggled into the valley in 1983. Caldwell is excited to finally understand the earliest known history of the place where he built his life.

A Coombsville pioneer, Caldwell was the first to analyze the region's temperature. His findings demonstrated that the area would be fantastic for growing Cabernet Sauvignon, which the AVA is now known for. Interpreting the conditions is crucial to Caldwell’s success, so he was ecstatic to receive Howell’s geological insight. 

There is 10 million years of earth history in every glass of wine from Coombsville; that’s how old the land is, and the boundaries of the AVA were created by mother nature. “It's a circular AVA roughly five miles by five miles. And that is entirely the base of what was once a huge volcano,” Howell says. “It probably started to erupt eight or nine million years ago.”

The land surrounding the volcano was flat—none of the current mountain ranges existed. As volcanos do, “the whole thing blew up, and an enormous amount of material from the volcano and the magma chamber below it shot up into the air.”

The ash blew off to the east, and any other material flowed down, creating a massive deposit outside Coombsville. A caldera similar to Crater Lake was left in the volcano’s place. “We know there was a lake here, too, because there are deposits of diatomaceous earth. Diatoms are small little organisms made up of silica, SiO2, and quartz,” Howell says.

Layers of volcanic rock began to form, and at the same time, around three million years ago, the mountain ranges started developing. The thrust from the formation of the Vaca Mountains eventually uplifted the caldera until part of it collapsed into what would become the city of Napa. In the millions of years between now and then, no other rocks were deposited in the area. 

Coombsville’s geology is unique because it’s the only place in Napa (other than Calistoga on the other side of the valley) where the soil is volcanic without anything else mixed in. Volcanic soil is ideal for growing grapes; its lack of nutrients strains them.

“You want to stress the grapes because you want the grape to put all of its energy into its fruit because that's the next generation, not into its leaves and vines. You want it into the fruit,” Howell says. “Soils that would not be good for almost anything else might be perfect for grapes.” With grapes in their prime, making outstanding wine is not difficult. As John Caldwell says, “If you got good grapes, you just blend it. Shit turns out to be pretty good.”

Does the combination of Coombsville’s perfect soil and ideal climate make it the best AVA in Napa? “You can't say one is better than the other. They're just different,” Howell says. “The winemakers have a whole bunch of tools they can use to apply. And from that, they can produce really good wine. Now, are there some places better than others? There absolutely are.”

Before the tour moves inside for a much-needed lunch respite from the rain, Howell addresses the crowd one last time. “To summarize, in this part of the world, John Caldwell has only volcanic rocks. The Coombsville AVA is the caldera. It is one of the best AVAs because it is clearly defined. There's no doubt about it. This is the Coombsville caldera, and it's a fabulous AVA.” 

Cover photography by Elan Villamor.