The heaviest concentration of petrified wood in the national park lies at the top of Blue Mesa in a layer of consolidated gravel, essentially a type of conglomerate, called the Sonsela member of the much-thicker Chinle Formation. Beneath the Sonsela are dull-colored layers of soft mudstone easily eroded into sloping hillsides like the one in this photo. North of Blue Mesa, the Sonsela has been washed away and with it most of the petrified wood in that part of the park.
It's likely the wood was originally deposited as log jams beside a flooded river, then quickly covered by sediments that slowed decay. Subsequent layers of volcanic ash leached silica that slowly replaced the wood in minute detail.
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Dimitri Vasileiou • Editor