PETRIFIED FOREST
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
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.