MARK WILLIAMSON / SCIENCE PHOTO LIBRARY MARK WILLIAMSON / SCIENCE PHOTO LIBRARY
Agate Bridge at Petrified Forest, Arizona. This 34m long petrified tree was undermined by floodwaters that eroded the less resistant sandstone beneath it. The concrete span was built in 1917 to support it. Millions of years ago fallen trees were buried by a mixture of silt, mud and volcanic ash. The sediment isolated them from the influence of the atmosphere and slowed their decay. Silica-laden groundwater seeped through the logs, infusing the cells with silica and replacing the original tissues. Later the silica crystallised into quartz and the logs were preserved as petrified wood.
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