The Geology of Remarkable Terroir

In early March of 2020, I found myself living in a cabin nestled along a rushing mountain stream with soaring redwoods as a backdrop. Little did I know at the time that this would become my home for the next eight months. Luckily it was located on the Peter Michael Winery estate in Knights Valley, Sonoma County, which afforded ample space for long hikes up into the vineyards to explore one of the world’s most fascinating collections of terroirs.

When I was growing up, my parents were infinitely patient on family vacations when I requested stops along mountain highways so I could chip away at rock outcrops with my geologic hammer and bring home new specimens for an ever-growing rock collection. In college, I put that same hammer to use during six weeks of field mapping at a geologic field camp in central Utah. And this year I used it for smashing open rocks here at the Knights Valley, Oakville, and beautiful Seaview Estates. All three locations have distinctly different terroirs.

The terroir in both the Oakville Estate and the Knights Valley Estate vineyards has dominantly volcanic origins. The Knights Valley vineyards are underlain by rhyolite, the most explosive form of volcanic rock, while Oakville features andesite, a less explosive volcanic rock that is generally deposited without geological drama as a continuous lava flow.

What makes one volcanic rock more explosive than another? In this case, the amount of silica controls the viscosity of the lava. Higher silica content means higher viscosity and therefore less opportunity for gases to escape. Those pent-up gases build pressure, like carbon dioxide in a bottle of Champagne, and eventually cause the volcano to violently explode. This is the phenomenon mainly responsible for the character of the ground beneath the Knights Valley Estate vineyards. Much of the Belle Côte, La Carrière, Ma Belle-Fille, and Mon Plaisir vineyards feature rocks deposited approximately 3 million years ago when a gaseous cloud of pumice and ash came to rest, forming a layer of white rock known as “rhyolitic ash-flow tuff.” The volcanic material in Oakville, on the other hand, had a lower silica content, which made the lava less viscous. Gases did not build up since they could escape, and so no major explosion occurred. Rather, the andesite flowed out of a volcanic vent roughly 4.3 million years ago, making it over a million years older than the Knights Valley Estate rocks—keeping in mind that both are quite young in geologic history.

For context, Seaview is not simply a stone’s throw from the winery at the Knights Valley Estate. A full ninety-minute drive is required, over the Mayacamas Mountains and then down into the Russian River Valley, following the twists of the river west through ancient coastal redwood forests and eventually to the cliffs overlooking the ever-changing Pacific Ocean. After merging onto the Pacific Coast Highway in Jenner, the journey continues north, navigating dramatic switchbacks while climbing and dropping with the topography of the land. This is one of the most picturesque sections of the highway due to an interesting geologic oddity: while most oceanfront highways would have a series of downward-stepping terraces between the road and the ocean, this section is adjacent to the San Andreas Fault, which has carried away the terraces to the north, resulting in sheer drops from cliffs down to the ocean.

Leaving the ocean, the route climbs northeast another six miles along a ridge before turning onto the Seaview property. A small descent and a sweeping curve lead to the first view of this stunning Pinot Noir vineyard. On this particularly misty, cool morning, with the ocean making its nearby presence felt, I immediately realized why Morlet wanted to personally introduce me to this extraordinary place. Reminiscent of my introduction to La Carrière by our vineyard manager, Javier Aviña, I stood in amazement gazing out over Seaview. The vines are immaculate, sculpted with precision into symmetric bearers of world-class Pinot Noir fruit. The first buds of the season had just burst, a special moment in the vineyard when the new vintage is born. We were in the blocks that produce ‘Le Caprice’ Pinot Noir. Two of the blocks were the steepest I had ever encountered for Pinot Noir. A rush of thoughts flowed through my mind. This terroir must produce amazing fruit. How on earth do Javier and his team farm this land? I will certainly need to be in top hiking shape for grape maturity sampling this fall! In Burgundy, I had trekked up and down the slopes of Volnay, the hill of Corton, the upper reaches of Beaune Premier Cru hillside climats (named plots with their unique geologic setting and microclimates), and up into the slopes above Gevrey-Chambertin, but I had never come across Pinot Noir on such a dramatic terrain. The Clos du Ciel and Ma Danseuse blocks were also on impressive slopes. Touring the property revealed beautiful, pristine forest streams and towering groves of redwoods—a gorgeous Sonoma Coast backdrop for growing grand cru–level vines. The uppermost blocks reach an elevation almost 1,500 feet above the level of the ocean, which lies just over 2 miles away.

The impressive terroir that is Seaview Estate was created from processes dating back over 40 million years. It happens to lie in an exceptionally complex area in terms of the historical movement of four different tectonic plates. The large oceanic Pacific Plate has been moving along in a northwest direction for the last 40 million years while two smaller oceanic plates, the Farallon and Juan de Fuca, have been moving east and have been dragged (subducted) underneath the less dense continental North American plate. During this process, rocks from the oceanic plates were scraped off as they disappeared below the rest of California and accumulated onshore—a bit like ice from an ice scraper stacking up while clearing a windshield. This heap of material, known as the Franciscan Complex, underlies Seaview. Whereas rock layers in nature are normally deposited in regular, horizontal layers, here the rock layers have been tilted up on their edges, like a collection of books on a bookshelf.

But the tectonic story is not over. For the last 10 million years, the San Andreas Fault system has been actively moving the western edge of California northward. This movement is along what is known as a strike-slip fault. Instead of rocks dropping downward on the other side of the fault, the rocks on either side pass each other along the direction of the fault. The San Andreas Fault passes within a mile west of the vineyard. The rocks on the western side of the fault originated in the very southern part of California and have been transported up to Sonoma County over the last 10 million years. The Seaview side of the fault, meanwhile, came from the material associated with the oceanic plates that have long since disappeared. In short, Seaview is—geologically speaking—a highly unique area.

My second trip to Seaview was as memorable as the first. Early April 2020, in the midst of the global pandemic, was not conducive to vacation travel. Instead, we spent the week of my son Sylvester’s spring break hiking about the Knights Valley and Oakville Estates, an adventure that culminated at Seaview, with camping gear in tow. It was three weeks after my initial visit, and now budbreak was widespread, with the beautiful rose-tinted hue on the tips of the tiny leaves as they first began to open and start the growing season. In some blocks, the vines had awakened earlier and the shoots were already 2 to 4 inches in length. While I was busily examining the vines, Sylvester (a second-grader at the time) suggested we look for a campsite—not a trivial task at Seaview. We visited all sections of the vineyard but were foiled by the splendid slopes, superb for growing excellent wine grapes but lacking any areas flat enough to pitch a tent. The top of a Clos du Ciel block was one candidate, but a bit damp. We finally settled on the base of a Ma Danseuse block designed for turning around vineyard equipment. The only noise was the occasional gobble from a rafter of wild turkeys, common residents on the Sonoma Coast.

The next morning we stepped out of the tent to vineyards enshrouded in fog, a serene and enchanting scene. We continued our hiking expedition with the goal of visiting each vineyard block. Along the way, we met some orange-bellied California newts, bright yellow banana slugs, gray-brown snails, and those ever-present wild turkeys. We also collected rocks from the different areas: tan, green, black, gray, yellow, white…truly a mélange. Most of the rocks, likely dating back from about 30 to almost 100 million years ago, were originally deposited in the ocean along the coast and then shoved onshore when slabs of tectonic plates were scraped onto the continent.

Another memorable visit to Seaview included a different vantage point, from an approach made by air. In early July, Morlet made a weeklong consulting visit. Important goals included working on Chardonnay blends and visiting all the vineyards to check on the crop loads and health of the 2020 vintage on the vine. We had spent time at the Knights Valley and Oakville Estates but then, with just one day remaining, we needed to visit both the Pisoni Vineyard down in the Santa Lucia Highlands (a drive of over three and a half hours) and the Seaview property.

To accomplish this, we headed to the Sonoma County Airport by 6:00 a.m. and were in the air by 7:00 a.m. What better mission for my first trip by helicopter than to visit all the Pinot Noir vineyards of Peter Michael Winery in one day? The flight south offered aerial views of the vineyards of Sonoma County, then the San Pablo Bay with views of Mount Tamalpais in the distance before following the east side of the San Francisco Bay, with Angel Island in the foreground and the San Francisco skyline as the backdrop.

One of the terroir components in the Pisoni Vineyard that make the ‘Le Moulin Rouge’ Pinot Noir unique is the fog pulled down the Salinas Valley from the ocean with the daily winds as the inland temperatures rise. Consequently, we made an unscheduled landing in San Martin (south of San Jose) due to heavy morning fog. Landing at small regional airports by helicopter is far simpler than landing by airplane at a major airport. A few quick radio messages and we were in and out of the airport within forty minutes. The fog lifted sufficiently for a smooth landing at the Pisoni ranch. The view approaching by helicopter reinforced how far above the valley floor Le Moulin Rouge fruit originates. The geology is quite different from Seaview. Here schist and gneiss (rocks metamorphosed by high temperature and pressure) and igneous granites provide the material for the alluvial fans that cover the vineyard. This year, an excellent fruit set and late spring rains led to a larger than normal crop in terms of the number and size of clusters. After discussing necessary adjustments with the vineyard foreman, we were off again.

The higher vantage point revealed additional undulations and vine-growth patterns within the vineyard blocks. Our vineyard team was awaiting the arrival of our helicopter and came over to greet us after our pilot masterfully landed in a narrow spot between the vineyard shop and the reservoir, just past a drop-off of the slope. Here the fruit set was lighter, and Aviña’s team had done an excellent job adjusting the crop and the amount of canopy left to protect the young clusters from the upcoming weeks of the summer sun. After ninety minutes of touring each of the blocks in a small 4x4 vehicle, we climbed back aboard the helicopter and followed the trace of the Russian River back to Santa Rosa. By 5:30 p.m., we were back at the winery and proceeded to taste vertical flights of each of the Peter Michael Winery Pinot Noir wines. Having toured and discussed the terroir of each vineyard block that day enhanced the discussion of each wine and vintage as we tasted into the night.

There is no better way to fully understand a terroir than to explore it by foot while sampling grapes for maturity as harvest approaches. I first found this to be true sampling premier cru vineyards in Burgundy. What at first appears to be a gentle, fairly uniform slope from a vehicle can quickly prove to be much more variable while hiking, tasting fruit, and observing why the taste and chemistry in one section of a vineyard block are quite different from another section. Oftentimes a change in soil affects how much water is stored in the soil or the amount and type of minerals available to the vine, which ultimately control the vigor and flavor of the resulting fruit. Changes in the steepness of the slope and the position on the slope also greatly affect fruit quality. The grand cru sites in Burgundy are found in the midslope section, while the very top of the slope and the bottom are relegated to lesser quality levels (villages or regional). The premier cru parcels generally form bands just above or below the grand cru sections.

At Seaview, nearly the entire vineyard is in the midslope region. The flatter and uppermost parts of the slopes are not planted with vines and the flat base of the slope is well below the vineyard down in the Gualala River Valley. In terms of grape cluster sampling before harvest, this translates to all the samples being taken on significant slopes. To prepare for the harvest push, I like to train with some local hiking, then a rigorous father-son trip to Yosemite with Sylvester in July or early August. This year featured a four-day backpacking adventure in the Grand Canyon of the Tuolumne River in the Yosemite granitic dome high country. The extra weight of the pack and the steep terrain put me in good stead for hiking the eight-plus miles around the slopes of Seaview in a given sampling day.

Six grape-maturity sampling trips to Seaview helped inform me about the earlier and later ripening blocks, about which had stronger or weaker canopies and which had more elegant tasting or more concentrated flavors—and most definitely gave me a close-up look at those with the steepest slopes! In Burgundy, the Clos du Ciel, Ma Danseuse, and Le Caprice blocks would all be separate climats. Tasting the fruit and walking each block reveals those differences: the Clos du Ciel blocks are a bit warmer, and even on the vine, the grapes offer greater concentration. The exceptionally steep Le Caprice blocks produce grapes with more primary fruit flavors, and the Ma Danseuse blocks receive more maritime influences of wind and fog, which contribute to elegant, delicate flavors in the grapes and resulting wine.

In my first year with Peter Michael Winery, I have come to appreciate the remarkable collection of terroirs at the Knights Valley, Oakville, and Seaview Estates. I truly look forward to learning more from them through ongoing studies and observation.