Petrified National Forest is located near the southern edge of the Colorado Plateau in eastern Arizona.  The forest encompasses an area of 93,533 acres or 146 square miles.  It was proclaimed a national monument on December 8, 1906 and was established as a national park on December 9, 1962.   

Petrified Forest National Park is dominated by the same erosional features that are characteristic of most of the arid Colorado Plateau.  The main feature that sets the Petrified Forest apart from other parks is the abundance of petrified wood for which it got its’ name. 

            There are thousands of petrified logs, branches and segments of tree trunks that are scattered throughout the park.  A vast majority of the petrified wood has been eroded from the Triassic age Upper and Lower Petrified Forest Members of the Chinle Formation. 

            The paleoenvironment in which the forest was deposited was much different than today.  At the beginning of the Triassic, the Colorado Plateau was located near the equator.  The sandy, muddy beds that covered the plateau accumulated to depths up to 600 feet.  The sea continued to regress westward leaving behind immense alluvial plains that covered the plateau. During this time, the climate became more humid allowing marshes and tropical grasslands to develop. 

            Forty different species of fossil plants have been identified within the Chinle Formation; most common of which are large conifers that originated in the highlands.  There is little evidence of any post depositional decomposition which suggests that the trees were transported and deposited rapidly in the low lying marshlands.  Scientists believe that these large trees where carried to the lowlands by floods in the area.

(http://www.petrified.forest.national-park.com/map.htm)

Towards the end of the Triassic, volcanoes that formed in the westward sea started erupting large quantities of silica rich ash.  The ash was carried eastward by winds and deposited on the basins, highlands and plains of what is now the Colorado Plateau.  This ash provided a good source of silica for the process of petrifaction.

Petrifaction is a type of fossil preservation in which the original organic material is replaced by silica.  The process by which the wood became petrified is not completely understood because of the vast degree of variation between different samples.  Some of the logs in the Petrified Forest, for example, have preserved the internal structure of the wood with great detail.  Others, called pseudomorphs have retained the external shape of the log but lost the internal structure during the replacement process.  Some of the logs still contain original organic material that has been preserved through a process called permineralization.  

Permineralization is a type of fossil preservation in which minerals such as silica are deposited in and around the original cell matter without disturbing it.  All of these different processes eventually led to what we see today exposed in the national park.

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