Geological Outings Around the Bay: Napa Glass Mountain

A nodule of Napa Glass Mountain obsidian, etched by hydration during its long burial, reveals signs of its origin as a thick, sticky lava. Photos by Andrew Alden

A nodule of Napa Glass Mountain obsidian, etched by hydration during its long burial, reveals signs of its origin as a thick, sticky lava. Photos by Andrew Alden

The Napa Valley is an intriguing place even if wine doesn't interest you. Its very geography has an intangible personality. Running north-south, the valley is lit differently throughout the course of the day. It has the flat valley floor along the Napa River that you'd expect in a major agricultural region, but odd rocky hills are scattered about the valley with the irregularity of tossed pebbles. On behalf of Napa Valley winegrowers, geologists have studied the underlying soils and rocks, tracing from them a history of tectonic upheaval, recent volcanism and gigantic landslides.

The western side of the valley is the Mayacamas Mountains, here made up largely of marine sedimentary rocks of the Franciscan and Great Valley complexes (100 to 150 million years old). The eastern side is the Vaca Mountains, which consists mostly of much younger lavas of the Sonoma Volcanics (less than 5 m.y.). Here it is in a simplified geologic map.

The valley floor consists of recent sediments (Q) while young volcanics (T) and older sedimentary rocks line its sides. From USGS Scientific Investigations Map 2956.

Each set of rocks responds its own way to California climate and yields its own set of soils, and the floodplain between them mixes the two in different blends. Its wide range of soils and settings is part of what makes the Napa Valley such an endless playground for winegrowers. The same was true in prehistoric times, when the valley was a rich habitat for the Wappo and Patwin tribes and other peoples before them.

And this brings us to Napa Glass Mountain.

The natives made their cutting and scraping tools as needed, chipping them from stone of a few select kinds. The best toolstone is obsidian—a glasslike lava without crystals or bubbles that would mar the sharp edges and flat faces of an effective tool.

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Obsidian is uncommon. To make it, a lava must have both a very high silica content and a low water content; it also helps to cool quickly. High-silica lava is classified as rhyolite and the Sonoma Volcanics field has plenty, but few rhyolites yield obsidian. Notable sources for the natives in this part of California were Annadel near Santa Rosa, Borax Lake near Clear Lake, and Napa Glass Mountain, which is just north of the town of St. Helena. Toolstone was an important trade item, and archaeologists map its distribution in ancient sites as clues to prehistoric commerce.

The Napa obsidian is a stone of a luscious pure black, and you can see it in a large roadcut on the Silverado Trail north of St. Helena at the narrowest point of the Napa River's floodplain. Here's a closeup geologic map.

Pinkish units are part of the Sonoma Volcanics (Tsa, Tsr, etc.), gray units are Franciscan rocks, units with "Q" are recent sediments. From USGS Scientific Investigations Map 2956.

The roadcut is just north of the intersection with Lodi Lane; you can park on either side of the road but be careful of traffic. The exposure is tall and shows the crude bedding of the mixed ash and lava beds, now highly weathered.

Obsidian occurs scattered in a whitish matrix in fractured lumps up to a meter long. The matrix may be altered material that was once obsidian, and/or it may consist of fine rhyolite ash—I have not studied it closely enough to hazard a guess. Parts of it may actually be perlite, a lightweight stone that forms where rhyolite reacts with internal water. Obsidian does not last long in the geologic record because its crystal-free substance is prone to attack by chemical weathering.

You don't need to dig into the cliffs; let other people who actually use obsidian do that risky operation. Instead, poke around your feet and look for bits like these.

The outsides are rough and coated with white hydration rinds, but inside they display their creamy texture and lustrous conchoidal fracture, the properties that make obsidian the ideal toolstone. Don't be greedy. And be ready to tell passers-by, like the curious police officer I spoke to during my last visit, what this cool place is all about.

More:
Obsidian photo gallery
Perlite
Rhyolite
Stone Tools