The trees which are petrified now were alive 200 millions years ago. Some were huge, up to 10 ft in diameter. When they died and fell, some were covered in sediments before they decomposed. Eventually organic matter was substituted by minerals preserving the original structure.
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1875

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The logs were often carried in stream beds and their position was preserved when they surfaced.

1880

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1906 + 1907 panorama

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1855

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1916

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