For the best glimpse of rarely exposed oceanic crust, turn east onto Highway 128 and drive along Conn Creek below Lake Hennessey. After passing the spillway on your left, look to the right side of the road at the jumbled boulders gleaming green from the roadcut. Here are an array of ultramafic rocks, which formed and cooled at great depth beneath the seafloor before being squeezed upward like toothpaste. Over time, the rocks have been uplifted and transported eastward thousands of miles aboard the Pacific plate. Parallel striations run across the jade-green surface veneer of a boulder or cobble a testament to tectonic friction.
Gray pines grow from the meager soil.
Back on Silverado Trail heading north, near Zinfandel Lane you'll pass plenty of cream-colored volcanic tuffs, ash compressed into rock. The ash fell a few million years ago in the series of powerful eruptions known as the Sonoma Volcanics. You're standing atop an ancient alluvial fan, silts and sands mixed with rock cobbles carried by long-gone streams which flowed from mountains that have since eroded. You can almost see the flow of water around and over these rocks, layered in beds inclined toward the valley floor. Soils from alluvial deposits on either side of Napa Valley underlie some of the region's most renowned vineyards.