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

Fossil Forests of Eastern Washington - Page 3 of 10
By Thomas A Dillhoff, Pasco, Washington

According to Dr. Reidel, approximately 85% of the basalt that fills the current Columbia Basin was laid down during the first two million years of the eruptions. Following this initial major eruptive event, the lava flows ceased for a period of about 200,000 years. Also by this time, the Cascade Range had eroded to the point where it was no longer blocking the weather systems from providing rainfall to eastern Washington. Rich forests of alpine conifers and lowland deciduous trees grew on newly formed volcanic soils (S. Reidel, personal interview, April 11, 2000).

The Miocene forest was significantly different from the current forests in the Pacific Northwest. Many of the conifers were similar to those seen in the region today, including true firs, Douglas fir, spruce, and pine, but the forests also included conifer families that currently exist in different parts of North America, such as redwoods and swamp cypress. Logs of these trees have been found which range up to 5 feet in diameter and 90 feet long. The ancient ginkgo tree also occurred in the Miocene forests, although wood of this species is rarely found.

The deciduous tree families were much more diverse than those currently found in the Pacific Northwest. Familiar types that existed in the Miocene and still exist today in western North America include poplar, birch, and maple. The Miocene forests also contained several species of trees that are more commonly found in eastern North America today, such as hickory, oak, walnut, beech, elm, and ash, plus some species that are now only found in eastern Asia, such as katsura and Chinese walnut. When the fossils from eastern Washington are correlated with fossil deposits from other parts of the world, it is clear that many of the tree families had a much wider distribution in the Miocene than they do today (Prakash, 1968).

In the area that is currently occupied by Vantage, Washington, it is believed that a large lake formed where the ancestral Columbia River was blocked by either a lava flow or a large mudflow from an erupting Cascade volcano. Logs from the forests fell into the river system, and were carried down to the lake where they formed large rafts on the surface of the lake and the lake bottom. This is believed to be the reason why some petrified wood deposits contain remnants of trees from diverse habitats, such as the alpine firs and more temperate lowland hardwoods found at Vantage.

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