How Was the Petrified Forest Made? During the Triassic Period (200 - 250 million years ago) the Colorado Plateau area of northeastern Arizona was located near the equator and on the southwestern edge of the landmass known as Pangea.

The Pangea land mass separated to create the continents we have today. Pangea had a tropical climate very different from our climate today. Fossil evidence of this ancient land lies in the sediments called the Chinle Formation that is now exposed in Petrified Forest National Park.

Over time floods and other weather conditions knocked down trees and some were buried in the soil. Most of the trees decomposed but some didn’t. The ones that didn’t were petrified and became the fossils seen at the Petrified national Park today. Most of the fossilized logs are from a tree called Araucarioxylon arizonicum. Two others, Woodworthia and Schilderia, occur in small quantities in the northern part of the park. All three species are now extinct.

How Did the Redwood Get Petrified? Tree logs were buried in sediment and ash from volcanoes was dissolved in the ground water. The dissolved ash and layers of sediment became silica and traveled through the buried logs. This silica filled, or replaced cell walls, crystallizing as the mineral quartz. The process was often so exact that replacement left a fossil that shows every detail of the logs’ original surfaces and, occasionally, the internal cell structures. Iron rich minerals combined with quartz during the petrification process, creating the brilliant rainbow of colors.

About 60 million years ago, the buried Chinle Formation was uplifted as part of the massive Colorado Plateau. As time passed, many rivers and storms eroded the land, removing the layers of rock until, again, the Chinle Formation was exposed. Now fossilized logs lie strewn across the clay hills and are exposed in cliff faces. Most logs are broken into segments. Humans did not cut the logs. Because the sections are still in order, we know that the logs fractured after they were buried and the petrification process was complete. Since petrified logs are composed of quartz, they are hard and brittle and break easily when subjected to stress. Earthquakes or the gradual lifting of the Colorado Plateau may have produced such stress.

*Learn more about the Petrified National Forest at: www.nps.gov/pefo

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