3.5 million years ago to present
Geology of Napa Valley
THE VOLCANO
Sometime around 3.5 million years ago, an explosive volcano, situated northeast of Calistoga, erupted, spewing ash and lava as high as 30,000 feet into the air. The light-weight ash was separated from the plume by the wind and blown eastward, landing as far away as Nevada and Utah. The remainder, a mixture of gas, volcanic crystals, molten rock and water, fell back to Earth and flowed out across the flat terrain until it ponded in a topographic depression.
The land was covered by a thick layer of volcanic rock. The top and bottom of this hot mass cooled quickly, forming a glassy crust. The rest cooled slowly, contracting as it cooled and cracking in vertical columnar joints we see in the Palisades cliffs.
Following this and many other eruptions, the low-relief topography underlain by hardened volcanic material began to morph into today’s mountains and valleys as the Pacific tectonic plate, sliding northward, rammed its eastern shoulder into North American plate.
Unlike many valleys carved out by streams and rivers, the Napa Valley was formed by this compression of the colliding tectonic plates. The force of the collision buckled the earth’s crust, first forming the Vaca Mountains and later the Mayacamas Mountains and the ranges to the west in Sonoma County. The two mountain ranges in Napa County cradled and gave birth to Napa Valley itself about one million years ago. The uplift is slow but steady, about one half to one millimeter per year.
The core of the Mayacamas is pre-volcanic rock. That rock has pushed upwards along with the volcanic material lying above it. Along the core of the Mayacamas, vineyards such as Progeny, Promontory, Cain and Pride are all on residual soils made from pre-volcanic basement rock that is 80 to 140 million years old. As the Mayacamas rose, weathering forces eroded the capping volcanic rock, exposing the older material below.
The Vaca range, running on the east side of the Valley, rose first, beginning approximately three million years ago. As it rose, running water from rain and the constant tug of gravity eroded the face of cliffs, shedding debris. The layers of volcanic bedrock we see today above our Palisades Canyon vineyard are what remains from numerous volcanic eruptions, including the one huge eruption that formed the prominent Palisades cliffs.
VINEYARD SOILS
The soils in Napa Valley vary with the geological history and topology of each specific area. They are classified by geologists in three distinct types: alluvial, fluvial and residual. The soils in a particular vineyard can be of a single type, or any combination of the three.
Course-grained alluvial soils are formed from the debris washed down from upland areas following catastrophic flooding events, unlike anything we have witnessed to date. Alluvial soils are concentrated along creeks and aprons (alluvial fans) draped on the mountain front. These sandy soils are mixed with cobbles also washed down during torrential storms.
Alluvial soils strike a good balance between water retention and drainage which is crucial for grapevines that need consistent, but not excessive, moisture. They are generally rich in minerals and organic matter. Some of Napa Valley’s most storied vineyards, like those of the Rutherford Bench and the historic To-Kalon, are planted atop alluvial fans.
Fluvial soils are composed of fine-grained material carried by the trunk stream on the valley floor. In Napa Valley, they are generally found in the center of the valley close to the Napa River. Because the components are finer, fluvial soils are thick and rich, a good farm soil. But for grapes, the soils are prone to too much vigor and thus require some form of mitigation.
Residual soils are formed in situ on the bedrock as it decomposes. In the volcanic bedrock at our Palisades Canyon vineyard, joints in the bedrock form avenues for water to penetrate. The water combines with carbon dioxide to form a dilute carbonic acid that attacks minerals in the bedrock. An onion-skin weathering process leaves behind round “core stones.” Over time, the decay process causes core stones to get smaller and smaller. The resulting soil is composed of the debris left behind from the weathered rock.
Vineyards planted on residual soils tend to be well drained and are less likely to have the variability found in transported soils. Because residual soils reflect the mineral content of their parent material, they can contribute to the terroir of their sites.
PALISADES CANYON
During the early stage of development of our vineyard blocks, we “ripped” the ground with large 3-foot-long tines attached to the blade of a bulldozer. The tines moved easily through alluvial soils and the decayed portions of the residual bedrock, but grabbed hold of the harder core stones and brought them to the surface. We piled these core stones at the periphery of the vineyard where they remain.
Close to Horns Creek and in the areas where the creek meandered in the past, our vineyard soils are primarily alluvial, full of fist-sized tumbled cobbles. The soils are dark with organic matter. Closer to the sides of our canyon, where the vineyard slopes rise along the margins of Horns Creek, the alluvial soils feather out and transitions to the red residual soils from the decayed bedrock. Close to the mouth of the canyon, in the old alluvial fan of Horns Creek, the soils are primarily alluvial. This variation between our alluvial and residual soils—both being volcanic in composition—makes for subtle differences in the flavors of our grapes depending on where they are grown on the vineyard.
Agricultural soil scientists have their own classification of soils that further differentiates alluvial, fluvial and residual based on physical characteristics including color, texture, structure, rock content, plasticity and chemistry. Their classifications are used to assist farmers in preparing vineyards for planting, amending the soil to improve its chemistry for grape cultivation, selecting root stock, and designing irrigation systems.
In 2024 we commissioned an extensive soils analysis in preparation for replanting select vineyard blocks at Palisades Canyon. While the report totaled 37 pages of detailed scientific analysis, the conclusion confirmed what our experience has shown: The soils of the Palisades Vineyard in Calistoga, CA, are well suited for excellent winegrape production.
DAVID HOWELL VIDEOS ON THE GEOLOGY OF PALISADES CANYON
Below are three videos we made in 2020 with David Howell, retired USGS geologist and author of The Winemaker’s Dance: Exploring Terroir in the Napa Valley. David’s book is a deep dive into the geology of the Valley and its relationship to wine terroir. The videos cover much of the material above. Feel free to link to our Geology story and share the videos. We use them in our programs with school students. To share the videos, use the URL www.palisadescanyonwines.com/stories/geology.