Olympic Mountains - Geology

Geology

The Olympics are made up of an obducted clastic wedge material and oceanic crust. They are primarily Eocene sandstones, turbidites, and basaltic oceanic crust. Unlike the Cascades, the Olympic Mountains are not volcanic.

Millions of years ago, vents and fissures opened under the Pacific ocean and lava flowed forth, creating huge underwater mountains and ranges called seamounts. The plates that formed the ocean floor inched toward North America about 35 million years ago and most of the sea floor went beneath the continental land mass. Some of the sea floor, however, was scraped off and jammed against the mainland, creating the dome that was the forerunner of today's Olympics. Powerful forces fractured, folded, and over-turned rock formations, which helps explain the jumbled appearance of the Olympics.

In the Pleistocene era, a vast continental ice sheet descended from Alaska south through British Columbia to the Olympics. The ice split into the Juan de Fuca and Puget ice lobes, as they encountered the resistant Olympic Mountains. A glacial outwash stream surged around the southern end of the peninsula to the Pacific Ocean. This isolated the Olympic Peninsula from the nearby Cascade Mountains and limited species from entering and exiting the peninsula. When the ice sheet reached the Peninsula, large areas of the continental shelf were also exposed by the lower sea levels since so much water was trapped as ice. This created a coastal refuge. The distance from Mount Olympus to the Pacific Ocean may have been double that of today.