Washington: petrified wood (state gem)

The most popular fossil to garner state honors is petrified wood, and Washington has chosen it for its state gem. Most of the petrified wood in Washington grew during the Miocene Epoch, some 5 -12 million years ago, when the state was swampy and mild, and played host to vast forests of cypress, oak, elm, and ginkgo trees.

Petrified wood forms when logs are buried, and their organic matter replaced by minerals in the groundwater, primarily silica. Although much petrified wood is buried in river sediments and is thus found in mudstone or sandstone, the trees in ancient Washington grew next to large volcanoes which spewed tons of ash into the air when they erupted. This volcanic ash settled and buried the trees in place; sometimes they were even engulfed by lava flows. The major petrified wood-bearing unit in Washington is the Columbia Plateau basalts.

The most famous petrified wood site in the state is Ginkgo Petrified Forest State Park near Vantage. Many types of wood are preserved there, but the abundance of ginkgo wood gave the park its name. Ginkgos are gymnosperm trees (non-flowering plants in the same grouping as pines, spruces, and cycads) that were thought to have gone extinct at the end of the Mesozoic Era until living specimens were discovered in China in the last century. Ginkgos are now widely cultivated in North America and the Far East, and are called "maidenhair trees" after the resemblance of their leaves to those of the maidenhair fern.

Despite its abundance, there are few published records of the history of petrified wood in the state. A notable exception is studies conducted by professor George Beck of Washington State College in the 1950s, who noted the occurrence of oak, beech, and elm, among others. Although petrified wood occurs in the western portion of the state, it is rarer than in the east. The western portion of North America is a newcomer by geological standards, consisting of a mèlange of terranes which have been splatted up against the ancient core of the continent by plate tectonic action. Thus the eastern portion of the state has been dry land for a longer period of time, and has had more time than coastal regions to develop forests and fossils.

Because of the abundance and popularity of this wood, the Washington State Legislature named it the official state gem on March 12, 1975.

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Petrified Wood from Western Washington