Eastern Oregon is divided between two large features. The southern part is in the Basin and Range province, where the continent has stretched in the east-west direction, breaking up into great blocks with intervening valleys, like the rocks of Nevada. This high lonesome place is known as the Oregon Outback. The northern part is a vast expanse of lava, the Columbia River Basalt. These rocks were emplaced in fearsome fissure eruptions as the continent overrode the Yellowstone hotspot, during Miocene time some 15 million years ago. The hotspot has torched its way across southern Idaho and now sits at the corner of Wyoming and Montana beneath the geysers of Yellowstone National Park, far from dead. At the same time, another trend of volcanism led westward (the darkest red) and now sits at Newberry Caldera, south of Bend at the center of Oregon.
This is a scanned copy of U.S. Geological Survey Map I-595 by George Walker and Philip B. King, published in 1969. I've created two more versions: the 1200x1550 pixel version (1.1 MB) includes the explanation of the map units, and all of it is legible. The 2000x2600 pixel version (2.6 MB) is suitable for printing.
Visit the Oregon Department of Geology and Mineral Industries to find more information and published products. Its new section, "Oregon: A Geologic History," is an excellent place to learn more detail.
More Oregon resources on About.com:
About Portland, Oregon
Oregon Geography, State Symbols & Facts
Oregon National Parks
Oregon State Parks
Oregon Nude Beaches and Resorts
Oregon Saltwater Fishing
Oregon Bed & Breakfasts