Petrified Forest National Park was first established as a National
Monument on December 8, 1906, when Teddy Roosevelt signed the proclamation. Congress
passed a bill that elevated the monument into a national park in 1962.
The ''Petrified forest" is a spectacular region of fossil logs in the Painted Desert in Arizona. The famous petrified forests occur in the colorfully banded sequence of rocks called the Chinle Formation. The transformation from wood to stone occurred about 200 million years ago during the end of the Triassic Period of the Mesozoic Era. These trees lived in a region which had a moist and probably warm climate. Washed from upland areas where they grew, the logs accumulated on delta flats where they were buried by sands and silts, and by layers of volcanic ash, derived from volcanoes further to the west. The volcanic ash was the source of the silica which helped to mineralize the logs after they were covered in sediment. The logs were replaced by silica, colored with oxides of iron and manganese and are generally very well preserved. The trees belong generally to three species of conifers of the family Araucariaceae. The family is still around today, with modern representatives such as the rather strangely named, because it is extremely prickly, "Monkey Puzzle" tree. The most common of the fossil trees in the Petrified Forest is Araucarioxylon arizonicum. Some of the logs, although broken into segments, represent substantial trees which would be over 50 meters tall when they were alive. Some trees have been replaced so perfectly that the annual growth rings and cell walls show clearly, as seen in the samples below.
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