The Olympic range is an accretionary wedge: ocean-floor sediment and basalt scraped off the descending Juan de Fuca plate at the Cascadia subduction zone and piled into mountains. The process has continued for tens of millions of years and remains active today, with the offshore plate boundary capable of producing major subduction earthquakes.
Here's how these mountains got made, and it's still happening right now offshore. For tens of millions of years a slab of the Pacific floor, the Juan de Fuca plate, has been diving under North America just off this coast. As it goes down, the soft ocean sediment and seafloor lava on top of it get peeled off the way snow rolls off the front of a plow. That scraped-off pile, packed and folded and shoved upward, is the very ridge you're driving through. You're inside a slow-motion collision that started before there were people to watch it.
Photo: Dllu · CC BY-SA 4.0
