TONY CRADDOCK / SCIENCE PHOTO LIBRARY TONY CRADDOCK / SCIENCE PHOTO LIBRARY
Fossilised logs in the Petrified Forest National Park, Arizona, USA. Tall, pine-like trees covered parts of the flood plain that occupied this area in the late Triassic period (225 million years ago). Fallen trees were washed by swollen streams & covered in silt, mud & volcanic ash, which excluded oxygen & slowed their decay. In time, water containing silica minerals seeped through the logs & encased their woody structures in silica. This slow process evolved with the silica crystallising into quartz, and the logs were preserved as petrified wood. Geological uplifting & the more recent effects of wind & water have exposed the logs to the surface.
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