Natural resources of the Blue Mountains include gold, chromite, timber, and geothermal energy. These lavas, which covered more than 70,000 square miles of Oregon, Washington, and western Idaho, mostly originated as fissures near the eastern edge of the Blue Mountains, although those in the John Day and Prineville areas came from more local sources. Above the Clarno and John Day Formations lie lava flows of the Columbia River Basalt Group (Figures 2 and 3). These rock units also host a world-class trove of mammal and plant fossils that similarly indicate a cooling climate. The Clarno Formation formed from tropical stratovolcanoes in central Oregon, whereas the John Day Formation reflected a climate that cooled from subtropical to temperate and consists largely of ash erupted from volcanoes to the west. The Clarno and John Day Formations, best known from John Day Fossil Beds National Monument, were deposited from about 48 to 22 million years ago and indicate changes in volcanic activity and climate.
Exposures of Cretaceous rocks in the central and western Blue Mountains, for example, point to a river in the east that flowed westward into the nearby ocean. Volcanic and sedimentary rock overlies the accreted and intrusive rocks to present a variable spectacle of Oregon’s history over about the last 100 million years. Hot fluids emanating from these intrusions deposited gold, silver, and a variety of other minerals in the older rock of the accreted terranes. It intrudes both the Wallowa and Baker terranes to indicate that they came together before then. The Bald Mountain Batholith in the Elkhorn Mountains, for example, contains granitic rock as old as 155 million years. The ages of individual stitching plutons place constraints on when two terranes joined. Intruding these accreted terranes are granitic bodies, called “stitching plutons” because they cut across many terrane boundaries and appear to stitch them together. Similarly, from the late Jurassic through Middle Cretaceous Periods, the Blue Mountains terranes were added to the edge of North America through the process of subduction. While this observation may seem implausible, these fossils likely arrived in Oregon via large-scale plate movements. Some limestone within these terranes contains fossils of organisms that appear to have lived only in the ancient Tethys Ocean, which was near the site of today’s Mediterranean Sea. The Wallowa and Olds Ferry terranes originated as island arcs, the Baker Terrane as one or more subduction zone complexes, and the Izee Terrane as a marine basin that developed over the top of the older terranes. These fragments, called terranes, are bound on all sides by faults and have their own geologic histories distinct from adjacent terranes, which originated in a variety of oceanic settings during the late Paleozoic and early Mesozoic eras. The basement complex consists of several fragments of continental and oceanic lithosphere. From oldest to youngest, it consists of three main parts: the basement complex, which lies beneath everything else granitic intrusive rock and younger volcanic and sedimentary rock, which sits on top of the basement and intrusive rock (Figure 2). Glaciers scoured high elevation areas, leaving behind deposits of till in the Wallowa and Greenhorn Mountains. In some places, such as the north side of the Wallowa or Strawberry Mountains, faulting localized the uplift, visible today as abrupt and straight mountain fronts. The Blue Mountains landscape resulted from the erosion of bedrock as it was uplifted during about the last 15 million years. This rugged landscape and its geology have influenced nearly all aspects of human history in the region, from the homelands of Native peoples, to the migration routes of resettlers, to the location and types of natural resources. Elevations range from about 3,000 feet in the valley bottoms to higher than 9,800 feet at Sacajawea Peak in the Wallowa Mountains. The Blue Mountains also include the Greenhorn Range and the Aldrich, Strawberry, Elkhorn, and Wallowa Mountains (Figure 1). The precise boundaries of the Blues, as they are often called, are indistinct, but the western extent roughly coincides with the western edge of the Ochoco and Maury Mountains and the eastern edge with the Snake River in Hells Canyon. Slightly less than a sixth of Oregon's land area, the region occupies about 15,000 square miles.
BLUE MOUNTAIN STATE SERIES
The Blue Mountains, perhaps the most geologically diverse part of Oregon, consist of a series of mountain ranges, rolling uplands, and valleys in the northeastern part of the state and extending into southeastern Washington.
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