Geology


The Chuckanut Formation

Fifty million years ago, during the Eocene period, in a time before the Cascades were formed, a giant swampy flood plain and river system covered most of the Pacific Northwest, from the southern part of Canada to where Everett sits today. As time progressed, layers of sediment collected in this expansive riverbank basin and formed the Chuckanut Formation.

The Chuckanut Formation consists of 6,000 meters of sedimentary sandstone deposit, the largest formation of its kind in North America. Over the last fifty million years, the movement of tectonic plates broke up and scattered this rock across an area that covers from Puget Sound to the foothills of the North Cascades, including Sehome Hill and the Western Washington campus.

Chuckanut Geologic Formation

The movement of this massive sandstone deposit is an awesome geological transformation. The fragmentation and dispersal of all this rock occurred through the motion of tectonic plates. The rock is now in pieces that tilt and point in directions different from the horizontal layers of sediment that existed when the rock was created by the riverbank. For instance, the sandstone beds of Sehome Hill dip westward at an incline ranging from 25 degrees to 40 degrees. Western Washington University campus sits in a valley between two sandstone beds of the Chuckanut Formation.

Fifty Million Years Ago

What did this area look like fifty million years ago when the Chuckanut Formation formed? During the Eocene, the dinosaurs were already gone, but other animals like the Dawn Horse and ancient relatives of the Tapir roamed a huge flood plain and riverbank system that was home also to an array of swamp-dwelling vegetation. Many types of this vegetation, including seeds, nuts, and leaves, became trapped underneath river sediment, forming fossils.

Fossils in the Chuckanut Formation

The Chuckanut Formation is an incredible resource for studying fossils of plants, including ferns and palms that grew when the Chuckanut Formation was a swampy river system millions of years ago. The swampy conditions were perfect for the creation of fossils. Dead plants were easily buried, and sometimes fossils of entire tree trunks are discovered.

Fossil of Alder Leaf
Fossil of Alder Leaf
Fossils of Alder Leaves

One of the most plentiful examples is fossils of Alder leaves from trees whose relatives still grow on the hill today. Fossils of palm fronds are also frequently discovered, a reminder that this area was a tropical environment in the past.

Animal footprints have also been found in the Chuckanut Formation, though not on Sehome Hill.

Finding the Old Quarry
  1. Start at the entrance to the arboretum, across the street from the environmental sciences building.
  2. Turn to the right.
  3. Walk south until the trail merges with the larger Huntoon Trail.
  4. Turn left and walk to the north along Huntoon Trail (the old road on the western edge of the arboretum).
  5. Notice the exposed rock face of the old quarry on the right hand side of the trail.