Editing Polystrate fossil (section)

====Volcanic deposits====
At this time, the best documented occurrences of unfossilized buried upright trees occur within the historic and late-Holocene volcanic deposits of Mount St. Helens (Skamania County, Washington) and of Mount Pinatubo in the Philippines. At Mount St. Helens, both unfossilized and partially fossilized trees have occurred in many outcrops of volcanic debris and mud flows (lahars) and pyroclastic flow deposits, which date from 1885 to over 30,000 BP., along the South Toutle and other rivers. Late Holocene forests of upright trees also occur within the volcanic deposits of other [[Cascade Range]] volcanoes.<ref name="KaroweOthers1987a)"/><ref name="KaroweOthers1995a">Yamaguchi, D.K., and R.P. Hoblitt, 1995, ''Tree-ring dating of pre-1980 volcanic flowage deposits at Mount St. Helens, Washington'', Geological Society of America Bulletin 107(9):1077-1093.</ref> In the space of a few years after the eruption of [[Mount Pinatubo]] in 1991, the erosion of loose pyroclastic deposits covering the slopes of the mountain generated a series of volcanic lahars, which ultimately buried large parts of the countryside along major streams draining these slopes beneath several meters of volcanic sediments. The repeated deposition of sediments by volcanic lahars and by sediment-filled rivers not only created innumerable polystrate trees, but also "polystrate" telephone-poles, churches, and houses, over a period a few years.<ref name=newhall /> The volcanic deposits enclosing modern upright trees are often virtually identical in their sedimentary structures, external and internal layering, texture, buried soils, and other general character to the volcanic deposits containing the Yellowstone buried forests. As in case of modern forests buried by lahars, the individual buried forests of the Yellowstone Petrified Forest and the layers containing them are very limited in their areal extent.<ref name="Amidon, L. 1997"/><ref name="Retallack1981"/>