Earth Science and Natural Science Article - Fossil Forests of Eastern Washington

Fossil Forests of Eastern Washington - Page 4 of 10
By Thomas A Dillhoff, Pasco, Washington

Approximately 15.4 million years ago, a fresh series of eruptions issued from the vents in southeast Washington, and once again fluid lavas poured across the landscape. These lavas filled the lake at Vantage and since the logs were saturated with water, they were entombed in the lava flows rather than being burned. Lulls in volcanic activity with accompanying forest growth and subsequent burials in lava flows were repeated several times over the next 8 million years, resulting in the preservation of trees from several different intervals during the Miocene. After the lava eruptions ceased approximately six million years ago, the Cascade Range again began to rise, cutting off moisture-laden air from the Pacific Ocean and leading to the end of the vast forests in southeastern Washington.

Based on the species of trees found in the Vantage forests, scientists believe the Miocene climate was similar to the current climate of northern California (S. Reidel, personal interview, April 11, 2000). Towards the end of the Miocene, the global climate started to cool towards the ice ages, which caused further extinctions among eastern Washington plant species. Following the ice ages the area settled into a weather pattern of cold winters, hot summers, and little rainfall, ultimately resulting in the ecosystem seen in the region today.

Formation of the Fossils

For a fossil to form, dead plants or animals must consist of a material resistant to decomposition, such as bone or wood, or must be either buried quickly in an oxygen-free environment to prevent the process of bacterial decomposition from occurring. The majority of plant fossils are found in deposits of ancient lake sediments. This is because lake bottoms are typically low in oxygen and sedimentation can occur quickly, especially during periods of volcanic activity with accompanying ash fall which results in rapid burial of organic materials on the lake bottom. The mechanism for preservation of some of the Washington petrified wood fossils is different than that for most fossils, because the logs were rapidly entombed directly in lava flows rather than in sediments (Beck, 1935). The wood was not consumed by the hot lava because it was waterlogged. In addition to protecting the wood, the water quickly quenched the surrounding lava, creating an insulating blanket of solidified basalt around the logs.

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