Petrified Forest National Park, 93,533 acres (37,881 hectares), E Ariz.; est. as a national monument 1906, designated a national park 1962. A part of the Painted Desert, it contains the largest known display of petrified wood in the world. There are six separate "forests," with great logs of jasper and agate lying on the ground surrounded by the varied colors of endless fragments and small chips. Dating from the Triassic period, these "stone trees" were killed by natural processes, such as fire, insect attacks, and fungus (or rot). The trees were deeply buried in mud and sand that contained silica-rich volcanic ash. The logs became petrified as the mineral, carried into the wood by groundwater, replaced the wood cells. As the surrounding material was eroded away, the petrified trees were exposed on the surface. Prehistoric Native Americans lived among the stone trees; ruins of their dwellings and their petroglyphs (ancient rock art) remain. Although the first known report of the petrified forests was made by Lt. Lorenzo Sitgreaves, an army officer who explored the area in 1851, they were virtually unknown until the late 1870s. See National Parks and Monuments (table).