The Priest Rapids Member (Wanapum Basalt) and the Pomona Member (Saddle Mountains Basalt) of the Columbia River Basalt Group crossed the Miocene Cascade Range into western Oregon and Washington as intracanyon flows about 14 m.y. and 12 m.y. ago, respectively. A thick, allogenic, bedded palagonite complex underlying the Priest Rapids intracanyon flow originated when the first Priest Rapids flow (Rosalia chemical type) interacted with a shallow lake on the Columbia Plateau, displacing water that flushed hyaloclastic debris into an incipient ancestral Columbia River channel. The Priest Rapids flow then overfilled the canyon, forcing the river northward, where it established the Bridal Veil channel. The Bridal Veil channel, part of a river system that extended across the Columbia Plateau, was only partly filled by the Pomona flow, allowing the river to remain in this channel through the Cascade Range.

The Troutdale Formation is made up of (1) older alluvial gravels deposited below and above the Pomona intracanyon flow while the ancestral Columbia River occupied the Bridal Veil channel and (2) younger, more varied alluvial deposits characterized by clastic and hyaloclastic debris from Boring and High Cascade volcanism, spread over a much wider area, when aggradation overfilled the Bridal Veil channel less than 6 m.y. ago. The shift of the lower Columbia River to its present course took place during this time of maximum alluviation, aided by scattered eruptions of younger volcanoes of the Boring and High Cascade Lavas. The present Columbia River Gorge was formed in post-Troutdale time by entrenchment of the Columbia River during rapid uplift of the Cascade Range of northern Oregon during the past 1 to 2 m.y.

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