The Ginkgo Petrified Forest State Park is an area of fossils discovered in 1932 by George F. Beck, a geologist of Ellensburg. Because it contains one of the finest natural collections of petrified wood in the country, the Ginkgo Petrified Forest State Park was designated a National Natural Landmark in October 1965.
The fossils of the ginkgo tree were a find of much interest, since they provided the last examples of the fossilized wood of this prehistoric gymnospermous tree. Since the first discoveries, fossils of many other kinds of trees have been found here over a wide area; they include elms, oaks 6 feet in diameter, maples, walnuts, and sequoias 10 feet in diameter, a spruce 100 feet in length, and a maple of 50 feet. About 75 prehistoric species have been found; further explorations will probably add several more to the list. Though this forest perished millions of years ago, descendants of about 80 per cent of the 75 species are still alive.
Within the boundaries of the park are about 7,000 acres, but the Ginkgo Forest itself actually covers about 3,000 square miles, roughly centered around the park. Although the youngest of all the known petrified forests of the Miocene Age of the Tertiary Period, it is, nevertheless, at least 10,000,000 years old. Most petrified forests are found buried in mud or volcanic ash, but this one is embedded in basalt. To explain the fact that the trees were not charred and consumed, a theory is advanced that at the time of the flow they were probably submerged in the waters of some prehistoric lake. The petrified trees are embedded in from 6 to 15 layers of soil and rock; some lie separately, some in close rows. Occasional specimens lie on the surface, in whole or half sections; others occur in peat bogs, amid tangles of roots, stumps, empty tree molds, and now and then an erect trunk. It is estimated that there are between 5,000 and 10,000 logs in the bed. The fossils are of an opal formation and in many instances are clearly and beautifully grained and reflect the color of the original wood. This opal formation represents a distinct point of difference from the agate formation of fossils of Arizona and the calcite formations of Scotland.
Fossilized remains of prehistoric mammals have also been discovered in clay pits of the vicinity, among them camels of various sizes and kinds, mastodon, deer, antelope, rhinoceros, three-toed horses, cougar-sized cats, wild pigs, and rodents of various types.