<P> Oregon's oldest known rock formations are found in the Blue Mountains and the Klamath Mountains . The state's oldest individual rock is a limestone near Suplee dated to nearly 400 million years ago, during the Devonian period of the Paleozoic era . These deposits include condonts as well as extinct corals and brachiopods, indicating a shallow marine environment . Most of Oregon would remain under water until the Cenozoic era . </P> <P> Starting in the Carboniferous period, a series of volcanic archipelagos formed in the region . Islands in these chains would have hosted warm, wet terrestrial environments . Fossils in Oregon's oldest floral assemblage, dating to the Late Carboniferous period, implies a lagoon ecosystem . Fossils in the assemblage include horsetails, ferns, scale trees, and conifer tree seeds . Formations of similar age also include shallow water invertebrates indicating that Oregon's volcanic islands were surrounded by coral reefs, which is consistent with Charles Darwin's theory of reef formation . </P> <P> Island chains continued to form over Oregon through the Permian period . Fossil assemblages from that time are therefore similar to those from the Carboniferous, although none have been found that include any Permian plant life . One species of Permian snail found in Oregon, Acteonina permiana, lends credence to the theory of plate tectonics because of its resemblance to contemporary Eurasian species . Fragmentary remains of Permian trilobites, including the endemic species Cummingella oregonensis, have been found in the state's Coyote Butte Formation . </P> <P> Oregon remained covered by shallow seaways throughout the Mesozoic era . Rising temperatures throughout the era led to rising sea levels . Oregon's fossil flora and fauna track these environmental changes with the addition of species adapted to deeper water or more tropical terrestrial conditions . </P>

Most rocks in oregon come from which geologic era