From the guidebook:
The GINKGO PETRIFIED FOREST STATE PARK (R), 140.1 m., is an area of fossils, discovered in 1932 by George F. Beck, a geologist of Ellensburg. 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. Descendants of the ginkgo still flourish in the Orient, where they are known as temple trees and sometimes reach a height of 80 to 100 feet. 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 forest of the Miocene Age of the Tertiary Period, it is, nevertheless, at least 10,000,000 years old. Most pertified (sp) 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 or 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 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 form 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.
The Administration Building (R) contains polished specimens, microphotographs of the grains of petrified wood, samples of crystal, and shells and pieces of rock specimens from many parts of the world. Trails lead (R) from the building to spots where fossils of fir and cedar have been unearthed. (Chipping and appropriation of specimens, and removal of shrubs, flowers, or rocks prohibited).
---Washington: A guide to the Evergreen State
Today this is just a small roadside interpretive site and trail and the main State Park facilities are located about two miles to the east overlooking the Columbia River.
State highway workers began finding petrified wood in the area as early as 1927. The State purchased this key 10-acre parcel in 1935 and established the Ginkgo Petrified Forest State Park. Crews from the Civilian Conservation Corps began excavations in 1936 and by 1938 had completed the excavations, trails, a small museum and caretaker’s cottage. The State Park opened to the public in 1938. In 1953, the original museum (Admin. Building) was replaced by the current museum about 2 miles to the east. The park was designated a National Natural Landmark in 1965 and by 1975, the park had been expanded to 7,470 acres. Ginkgo is the rarest variety of petrified wood found in the park. This summary of the history of the State Park comes from http://www.historylink.org/index.cfm?DisplayPage=output.cfm&File_Id=7396
The Ginkgo Petrified Forest is the only known petrified forest that has been preserved in a lava flow. The typical petrified forest is buried in volcanic ash or covered by sediments. The concentration of petrified logs at the Vantage area is greater than any other known deposit. In addition, nowhere has a greater number of different types of petrified woods been found in a single area. More than 200 types have been recognized and the genera and species of 75 established. In general at most other petrified forests around the world no more than 10 to 20 different types of petrified wood have been identified. This is the most diverse collection of species from different ecological setting found at any petrified forest in the world. There are representative species from uplands, lowlands, swamps, drylands, temperate and warm temperate settings. This summary of the significance of the Ginkgo Petrified Forest is based on information found at http://www.efsec.wa.gov/oplarchive/oplpft/Parks%20and%20Rec/epk-t.pdf