The Farallon Plate took a turn north some 57 million years ago, sweeping much of western coastal Oregon along with it. The Cascades were beginning to uplift and acting as the breakwater for a retreating Pacific Ocean. By the middle Oligocene, the Cascadia Subduction Zone was in full force. The growing pressure fracturing our magma shield and causing volcanic eruptions along the Western Cascades. Lassen Peak erupted twice in fairly recent history, 1914 and 1921. Mount St. Helens has had a long history of minor eruptions but there was a massive eruption as recently as 1980.
We see a fair bit of volcanic action in Oregon right through to the Miocene. We also see lovely marine fossils from this same era. The soft ocean sediments of Oregon contain beautifully preserved gastropods, bivalves, wood, bone and cephalopods that range in age from 15 to 30 million years old.
