The upright fossil trees of the Gallatin Petrified Forest in the Gallatin Range and the Yellowstone Petrified Forest at Amethyst Mountain and Specimen Ridge in Yellowstone National Park, occur buried within the lahars and other volcanic deposits comprising the Eocene Lamar River Formation as the result of periods of rapid sedimentation associated with explosive volcanism. This type of volcanism generates and deposits large quantities of loose volcanic material as a blanket over the slope of a volcano, as happened during the 1991 eruption of Mount Pinatubo. Both during and for years after a period of volcanism, lahars and normal stream activity wash this loose volcanic material downslope. These processes result in the rapid burial of large areas of the surrounding countryside beneath several meters of sediment, as directly observed during the 1991 eruption of Mount Pinatubo. As with modern lahar deposits, the sedimentary layers containing upright trees of the Yellowstone petrified forest are discontinuous and very limited in areal extent. Individual layers containing upright trees and individual buried forests occupy only a very small fraction of the total area of Yellowstone National Park.