The Blue Mountains physiographic province is located south of the Snake River in the southeast corner of Washington. The major portion of the Blue Mountains is situated to the south in Oregon. Current authors tend to call the Blue Mountains a subprovince of the Columbia Basin.
The Blue Mountains are characterized by a broad uplift, reaching elevations of more than 6,000 feet above sea level. Windows of Paleozoic or Mesozoic metamorphic rocks are exposed at four locations where streams and rivers have incised deep canyons through the overlying rocks of the Columbia River Basalt Group. The basement rocks consist of Jurassic-Triassic(?) limestone lenses, amphibole-quartz schist, greenstone, graywacke, sandstones, cherty dark argillite, and diorite.
In the Blue Mountains, Grande Ronde Basalt lavas of the Columbia River Basalt Group were extruded from northwest-trending fissures. Dikes now represent the locations of these vents. As thick sequences of basalt were erupted in the Blue Mountains during the lower Miocene, adjacent basins formed--for example, in the vicinity of Troy, at the Oregon-Washington border. Ancestral valleys were blocked by basalt flows, causing the Troy Basin to be infilled with thick sequences of sediments and peat. Since the Miocene, the peat has turned to lignite. The Grouse Creek unit of the Grande Ronde Basalt contains lignite interbeds up to 40 feet thick. Diatomite beds are also associated with interbeds in the Troy Basin.
Arching of the Blue Mountains was initiated 12 to 10 Ma. This uplift, which has continued through the Cenozoic, caused drainage reversals and numerous unconformities between the Columbia River Basalt Group flows and sediments. Younger Miocene flows did not cover the emerging Blue Mountains; they only lapped up against the flanks of the uplift. Finally, folding and faulting accompanied the uplift to complete the geological setting of the Blue Mountains.
The above text is modified from the following article: Lasmanis, Raymond, 1991, The geology of Washington: Rocks and Minerals, v. 66, no. 4, p. 262-277. © Copyright Heldref Publications (Helen Dwight Reid Educational Foundation). Used with permission.