David Howell is one of the authorities on Napa geology and he explains how Napa Valley is a very young wine valley, with its geological formation only happening in the last few million years and creating three distinctive soil types.
In this exclusive 80 Harvests interview, the geologist and author of ‘The Winemaker’s Dance’ talks about the geological origin of Napa, how it was formed and what major soil types it has. He also discusses the different climatic influences in the valley, the main grape varieties found in Napa, and how climate challenges and climate change might affect the future of Napa Valley and surrounding wine valleys.
Beyond geology and climate, Howell also discusses the evolution of winemaking in the valley from the impact of phylloxera and the Judgement of Paris, through to the era of experimentation that Napa finds itself in today with an exciting winemaking scene. Amanda Barnes also interviews him on the further potential of Napa valley for more vineyard plantations and which are the riskiest areas being developed in the valley, as well as the threat of urbanisation.
There are some clips available below for all readers, however the full in-depth interview is for subscribers only:
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Shorter clips on Napa geology, soils and climate change are posted below, including David Howell’s opinion on what makes Napa Valley unique.
Napa geology and the formation of the wine valley
“So Napa Valley’s history really goes back 140 million years, that’s the oldest rocks that we have. And it was a time when it wasn’t a valley at all. The valley as we see geographically is really quite young, probably only maybe 2 million years old. But California margins was starting to grow 140 million years ago. So as the Pacific was coming in, slamming in and causing all kinds of disruption, earthquakes, lots and lots of earthquakes! Building up out of the ocean and forming land masses, and then about 9 million years ago, the San Andreas fault started moving in this direction and volcanos started popping off. So we had volcanoes all along that ridge over there, and probably here as well volcanoes. So for about 6 million years, volcanic material built up, so we had the old rocks and a gap, then the volcanic rocks, and then forces started to squeeze the land. And the mountains off to the east, the Vaca Mountains are squeezed, it was like the wrinkling of a rug, building up and then the valley sunk down and then this mountain over here was squeezed more and rose. So we have the Mayacamas mountains here, the Vaca mountains there, and the valley in between.”
The main soils of Napa Valley
What are the main soil types that we’ll find here in Napa?
“So there are really what I refer to as three broad categories of soils. One is the soils that form on the bed rock, those are residual soils. And the nature of those soils varies from place to place depending on the bed rock that they are basically forming on. It’s the decayed part, so decayed sandstone, decayed shale, decayed metamorphic rocks, decayed volcanic rocks, all have different residual soil components. Because of the hills we periodically have catastrophic rainfalls, or rainfalls where we get an inch an hour for maybe ten or twenty hours and that will cause the material in the canyons to swosh out and be deposited and create what are called alluvial fans. And so we have a whole series of alluvial fans along the west side of the valley and then we have some smaller alluvial fans on the east side. There were some very big ones in the past because we have these big drainage basins but those fans are gone. And this is a little hard to imagine, but in this continent for the last five million years we have had continental glaciation, roughly every 200,000 years, the sea level falls about 120 meters, and so when the sea level falls, the drainage of the Napa River suddenly gets steeper, as it gets steeper material is eroded away. So you downcut during low stands of sealevel. So it removed all that debris. So we have old debris flows, excuse me – we have old alluvial fans, dissected old alluvial fans on the east, on the west side here we have actively growing alluvial fans. And then the third kind of soil, which would be the soils that form in the centre of the valley, when the river overflows and it is the most fine-grained material. So decayed bedrock, alluvial fans (alot of gravel and sand) and then clay and loam in the centre of the valley due to the flooding of the river.”
The effect of climate change on Napa Valley
“It’s not clear exactly what’s going to happen, but let’s just look at one of the elements: which is the increased amount of temperature. And that creates an interesting situation because what happens in California, when you go across several mountain ranges, you get far away from the marine influence and you end up in Sacramento Valley and it can get really, really hot. If climate change causes it to get even hotter, it will get even greater heat. And what happens is that the hot air rises, and as it rises, it sucks in air from the west which is the marine air. So in fact, there is a possibility that as California overall warms up, parts of southern Napa will actually cool down because there will be a greater amount of the marine air coming in.”
What makes Napa Valley unique?
“I’m reticent to use the word unique because you can always find different things place to place. What makes Napa kind of special however is that it is politically all in one domain, all of the water and drainage into Napa River is all within Napa County. It is a single valley, with two mountains on either side. There is a very characteristic thermal gradient where it is cooler in the south as it opens up to San Pablo Bay and the San Francisco Bay region overall, hotter in the north where the valley narrows and the mountains come together. Occasionally fog coming in from the Pacific over the mountains over here. So, it’s… I would say it is not a unique spot, because I’m sure there are other places in the world that have valleys with rivers, and on both sides, but it is a pretty darn special spot!”
