The basic geologic framework of the Columbia River Gorge has been known for over a century. In the western gorge, the package of Columbia River Basalt Group (CRBG) flood-basalt flows unconformably overlies volcanogenic rocks of ancestral Cascade volcanic arc. Vigorous and widespread volcanism characterized the arc from its inception 40 Ma until ca. 18 Ma, when activity greatly declines. The arc must have been relatively quiescent during emplacement of the most voluminous CRBG flows, because interflow volcanic sediments are sparse. The larger flows passed through a 50-km-wide ancestral Columbia River valley on their way to the ocean. Owing to late Cenozoic uplift of the Cascade Range and resultant incision by the Columbia River, CRBG flows are now spectacularly exposed in the cliffs and waterfalls of the Columbia River Gorge. The modern gorge roughly follows the northern margin of the broad Miocene valley. Grande Ronde flows clearly abut the northern paleovalley wall formed by early Miocene volcaniclastic rocks of the 19 Ma Eagle Creek Formation. ...

The slight southward dip of the Columbia River Basalt Group (CRBG) section and the underlying Eagle Creek Formation gives the western gorge an asymmetric physiographic cross section. In Washington, failure of weakly lithified Eagle Creek strata that dip toward the river under the load of superincumbent basalt has produced huge landslide complexes composed largely of CRBG debris. In Oregon, where strata dip away from the river, undercutting of the Eagle Creek Formation instead creates towering cliffs. As a result, the CRBG section south of the river consists of continuous cliffs, whereas to the north the CRBG forms scattered peaks (Greenleaf Peak, Table Mountain, Hamilton Mountain, and Archer Mountain) separated by low-lying terrain underlain by the Eagle Creek Formation or landslide debris. Each of these peaks is actually the southern end of a N-S ridge of CRBG, marking sites where basalt flows backfilled south-flowing tributaries of ancestral Columbia River."


Source:    Wells, R.E., Niem, A.R., Evarts, R.C., and Hagstrum, J.T., 2010, "The Columbia River Basalt Group -- From the gorge to the sea", IN: Geologic Society of America Field Guild 15, 2009.