Petrified Forest

Petrified Forest National Park lies in north-eastern Arizona, on the Colorado Plateau, and protects some 895 km² of badlands and semi-arid plains. It was declared a national monument in 1906, in the first batch under the Antiquities Act, and raised to national park status in 1962; a major extension of its boundaries was approved in 2004.

Its value is palaeontological. Some 220 million years ago, in the Late Triassic, the region was a tropical river plain covered in giant conifers. The fallen trunks were carried along by floods, buried by sediment and volcanic ash, and dissolved silica replaced the organic matter cell by cell, preserving the internal structure of the wood and tinting it with the colours of mineral oxides. The result is fields of large fossilised trunks, one of the most important deposits in the world, exposed by the erosion of the Chinle Formation.

Those same strata contain fossils of phytosaurs, giant amphibians and some of the earliest dinosaurs, and the park is a reference site for the study of the Triassic. The Painted Desert, in its northern sector, offers badlands coloured in bands by their different mineral contents. The park also preserves petroglyphs and archaeological sites, among them Puerco Pueblo, and a stretch of the historic Route 66.