Had you been swimming with the marine fossils that were laid down in the Eocene Epoch in Oregon, some 55 to 38 million years ago, you'd be treading water right up to where the Cascade Mountains are today.
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 were fast becoming the breakwater for a retreating Pacific Ocean.
By the middle Oligocene, the Cascadia Subduction Zone was in full force with growing pressure erupting volcanoes along the Western Cascades, a pattern that was to continue well into the Miocene. The soft ocean sediments of Oregon contain beautifully preserved gastropods, bivalves and cephalopods.