The Petrified Forest west of Calistoga is a collection of giant Pliocene-era redwoods, knocked down and buried by a volcanic eruption from Mount St. Helena about 3.4 million years ago and slowly turned to silica. A short interpretive trail loops through the forest, with the largest logs over twenty metres long and up to two metres in diameter.
The site was first publicised in the 1870s by Robert Louis Stevenson, who visited during his Napa Valley honeymoon and described it in The Silverado Squatters. It's a quiet, well-run, family-owned attraction; allow about an hour. Easy combine with Old Faithful and Calistoga.