One of the most spectacularly scenic places on Earth for seeing an abundance of fossilized wood is eastern Arizona's Petrified Forest.  Intensely colored, ~uncompressed fossil trees and tree trunks literally litter the ground.  The fossils here are weathering out from the Chinle Formation (Carnian to Norian Stages, Upper Triassic).  The member-level stratigraphy of Chinle is moderately complex (see summaries in Parker et al., 2006).


Despite the name, this locality does not preserve a fossil forest.  The park could be more accurately named “Petrified Logjam National Park”.  The fossil wood at this locality represents trees that grew in an ancient forest on a swampy floodplain in a paleotropical setting.  The fossil logs are not preserved in their upright, growing positions.  They were removed from their original living sites by flood events, washed downstream, and ultimately buried by stream & river sediments.  Note how many modern rivers have floating trees or logs being washed downstream, especially during storms & floods.  Extensive logjams were once a common phenomenon in eastern American rivers, and were significant barriers to riverine transportation.




Agate Bridge consists of a single permineralized fossil log of Araucarioxylon arizonicum that spans a watercourse (usually dry).  The tree is weathering out from a fluvial sandstone/pebbly sandstone unit in the Sonsela Member of the Chinle Formation.


One of the vintage photos below (from a National Park Service public display) shows the original, unsupported fossil log spanning the dry creek.  Natural fractures in the fossil prompted concern about its eventual collapse.  In the early 1910s, pedestals were erected underneath the fossil log.  These were replaced in the late 1910s by a concrete support spanning the entire free length of the fossil.  Modern NPS policies don't permit additional preventative measures.  The NPS now prefers to let nature run its course.  The modern NPS attitude: if it collapses due to natural weathering & erosion, so be it.





Agate Bridge (Araucarioxylon arizonicum) - vintage NPS photo showing the original, unsupported fossil log that is Agate Bridge.


Agate Bridge (Araucarioxylon arizonicum) - the pedestals were built in the early 1910s to support the fossil log. (NPS vintage photo)




  Agate Bridge (5 pics)


  Jasper Forest (xx pics)


  Crystal Forest


  Giant Logs


  Rainbow Forest Museum



Parker, W.G., S.R. Ash & R.B. Irmis.  2006.  A century of research at Petrified Forest National Park, 1906-2006, geology and paleontology.  Museum of Northern Arizona Bulletin 62.  187 pp.



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