A variety of ecosystems in Sonoma County, CA. Mount Saint Helena is a peak in the Mayacmas Mountains with flanks in Napa, Sonoma, and Lake counties of California. Composed of uplifted 2.4-million-year-old volcanic rocks from the Clear Lake Volcanic Field, it is one of the few mountains in the San Francisco Bay Area to receive any snowfall during the winter. The peak is reachable by hiking trails leading from Robert Louis Stevenson State Park.