Geology, Life and Climate
The John Day Basin is a geologic wonderland that records much of Oregon’s ancient history. The oldest rocks are more than 225 million years old. They once were volcanic islands not far from the Idaho coast. Canyon Mountain, south of John Day, represents part of this ancient island system. A relict of a subduction zone, 225 million years old, can be found about 8 miles north of Mitchell.
About 120 million years ago, the islands collided with North America building the shoreline to this area.The sedimentary rocks in downtown Mitchell and at Goose Rock near the John Day Fossil Beds‘ Cant Ranch are 100-million-year-old remnants of Oregon’s first beach. Fossils of marine reptiles—cousins of the dinosaurs—found here include an ichthyosaur and plesiosaur. (No true dinosaur fossils have yet been confirmed in Oregon.)
The Painted Hills reveal changes from a near tropical to a temperate climate.
The John Day Fossil Beds are most famous for their record of the tropical-to-temperate climate change of the past 50 million years. South of Fossil, remnants of 45-million -year-old volcanoes create a rugged landscape. Ancestral bananas, citrus, and tea grew in a the forest. Animals included saber-toothed, cat-like creodonts, and small three-toed horses. Their fossils are displayed at the Thomas Condon Visitors Center.
By 35 million years ago, the climate was increasingly dry and cool. This climate change is made obvious by the contrast between the multi-colored red and tan layers of the Painted Hills, where the climate cycled between
warm/wet and cool/dry, between 34 and 28 million years ago, and the younger blue-green Blue Basin—deposits of a cooler, temperate climate about 26 million years ago. Oaks, maples, and alder dominated the forest. Entelodonts (“terminator pigs”) prowled the landscape. The Fossil Beds at Wheeler High School in Fossil represent a shallow, muddy, metasequoia-fringed lake that harbored salamanders about 32.5 million years ago.
Beginning 16 million years ago, fluid lavas covered the landscape. Today these rocks, the Columbia River basalt, form dark rims along the John Day River between Picture Gorge, Spray, and Service Creek. Some of these lavas erupted from local vents that can be seen as elongate “dikes” near Monument and along the John Day River east and west of Kimberly.
About 7 million years ago, a powerful eruption of hot ash from a vent near Burns, Oregon, covered much of the John Day River valley. Known as the Rattlesnake Ignimbrite, it is the youngest major formation in the area, and is largely eroded from the landscape between Kimberly and Fossil. During the Ice Age,
Columbia River Basalts cap the rims and lighter-colored tuff above the John Day River near Kimberly.
small glaciers carved the summits of Canyon Mountain and the Strawberry range. However, the huge floods that scoured the Columbia did not effect the John Day Basin. Today, major geologic forces include the active Maupin Fault Zone which produces small quakes of magnitude 2 to 3, and the slow, continuing uplift of the Blue Mountains.
Sheep Rock, at the John Day Fossil Beds National Monument, reveals almost 25 million years of the geologic record, from tropical volcanoes about 40 million years ago, to the 15-million year-old basalts that cap the peak.