The Blue Mountains are a great chain of highlands stretching from central Oregon near the Cascade Range northeastward to extreme southeastern Washington. A place of supremely varied geology, these old mashed-together volcanoes and seafloor sediments now provide Oregon with some of its most substantial high country and wildest terrain.
The Blue Mountains actually consist of a series of mountain uplifts. From their southwest end near Prineville, Oregon, to the vicinity of Walla Walla, Washington, they include the Ochoco Mountains, the Aldrich Mountains, the Strawberry Range, the Elkhorn Range, the Greenhorn Mountains and then the incised plateaus of the Northern Blue Mountains. Along with the Cascade Range, the Oregon Coast Range, the Klamath Mountains and the isolated uplifts of the Great Basin, they are among Oregon’s chief mountain ranges.
The origins of the Blue Mountains begin some 200 million years ago, when the Pacific tectonic plate began colliding with and sloughing underneath the North American plate along its western coast, then located east of its present configuration. Landforms common to such subduction zones, including volcanic chains and coastal ranges composed of heaped-up seafloor sediments and island arcs, all left geologic record in the jumbled rocks of the Blues. Rocks of similar age and composition are found in the Klamath Mountains of southwestern Oregon and northwestern California, suggesting these were once part of a continuous coastal range with the Blues. Today, the Blue Mountains are well inland, and the Oregon Coast Range, along with the volcanic Cascades around 100 miles from the Pacific, are the modern equivalent.
Broadly, rock types in the Blue Mountains include sedimentary formations derived from marine sources, granite stemming from intrusions of molten rock and basalt layers from more recent lava flows. These include striking Paleozoic greenstones in the aptly named Greenhorn Mountains, great fields of granite in the Elkhorn Range and thick layers of Columbia River Basalt in the Northern Blues and adjacent areas.
Columbia River Basalts
Between about six and 17 million years ago, vast quantities of lava issued from volcanic vents in the vicinity of northeastern Oregon and southeastern Washington, the exact origin of which is still a little hazy. These outpourings of what are called flood basalts occurred in distinct phases with intervening periods of calm, so they are exposed as parallel bands of basalt interbedded by sediment layers. The flood basalts cover an enormous area — not just the Columbia Plateau of eastern Washington and north-central and northeastern Oregon, but also what is now the Columbia River Gorge, the northern Willamette Valley and portions of the northern Oregon Coast Range and Pacific Coast. The Northern Blue Mountains are essentially entirely flooded by these basalts; their relief comes from incised canyons of streams and rivers, so they are at heart a dissected tableland. The Wallowa Mountains just east of the Blues in far northeastern Oregon have uplifted bands of Columbia Basalt in their high country, and the great gorges of Hells Canyon and its tributaries along the Oregon-Idaho border in this vicinity dramatically display the many flood-basalt layers.