Petrified Forest National Park - Geology

Arizona > Petrified Forest National Park > Geology

Grey badlands and petrified logs
Petrified wood was formed 225 million years ago during the triassic period and is 4 times as hard as granite and very colorful, due to the effect of impurities such as iron, manganese, copper and lithium present in the wood during the fossilising process. The area of Arizona now contained within the Petrified Forest National Park was once part of a large forest that extended from Texas into Utah - the trees were similar to modern-day conifers and existed at the same time as the dinosaurs, fossils of which are often found in the park. The preservation process began during occasional flooding, when some of the trees were buried by a great depth of water and sediment quickly enough to prevent aerobic decay. Over a long period, water containing dissolved minerals seeped into the wood and replaced the organic cells with stone. Much later, the whole area was uplifted and eroded to give the landscape seen today.


The usual color of petrified wood is red, with yellow, black and white bands although other shades such as blue are often found. The stone has a high silica content, and is generally similar to agate in composition and appearance. Only the trunks of trees seem to be preserved but these may be almost complete, up to 10 m in length and 2 m thick at the base although they tend to be broken up into sections. The wood may be collected over a large area of east-central Arizona, but outside of the National Park, most of the best sites are privately owned. Petrified wood may be also found in Utah, in various locations around the Escalante River, and in the Coyote Buttes region near the Paria River. Sometimes the wood is much less colorful - for example, the specimens that occur in the Bisti Wilderness in New Mexico - but just as interesting.

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