Brief history of Washington's geology -- Okanogan Highlands : ancient rock of Northeastern Washington -- Columbia Plateau : massive basalt flows ravaged by catastrophic Ice Age floods -- Cascade Range: Washington's volcanic backbone -- Puget Sound: Glacial ice in a tectonic trough -- San Juan Islands: stacked thrust sheets scraped by ice -- Olympic Mountains: a youthful coast range -- Willapa Hills: sands of the Southwestern Coast.