Volcanic ash
Volcanoes erupted from 3.6 to 2.3 million years ago near what is now Long Valley. Rhyolitic eruptions occurred in and around Glass Mountain in the same area from 2.1 to 0.8 million years ago. Volcanic ash from the massive ( of ejecta) eruption of Long Valley Caldera some 760,000 years ago is preserved in the thick Bishop Tuff that covers much of the region.