Chuckanut Drive fossils reveal tropical past

It may seem hard to fathom now, as we trudge through our typical cold, wet fall days, but 55 million years ago, the Pacific Northwest was a warm-weather paradise. The fossils revealed by last week’s rockslide on Chuckanut Drive (SR 11) in Bellingham are proof.


In the Eocene epoch (pronounced ee-oh-scene ee-pock), the Chuckanut Drive area looked dramatically different than the steep cliffs and rugged sandstone rock outcroppings we see today. More than 55 million years ago, the area was a lush floodplain filled with winding rivers and side channels. Large, palm-like trees, ferns and other warm-weather vegetation filled the floodplain and riverbanks.

Over time, river sediments accumulated, trapping various plant leaves and tree roots, and turned to rock. The area gradually cooled for several million years, ushering in the Pleistocene ice ages. Today, finding fossils of ferns, roots and tree leaves is a pretty common occurrence for maintenance crews cleaning up rockslides along the road.

If you’ve driven along Chuckanut Drive, you may have also noticed the various stripes and layers in the rock. Typical layers found in the Chuckanut area include sandstone, siltstone, shale, conglomerates and coal. Many of these layers are tilted and folded – an indication that the area was susceptible to tectonic activity from several faults in the area.

The makeup of the rock formations along Chuckanut Drive is also one of the main reasons we have so many rockslides during the fall and spring. Sandstone, siltstone and shale are porous, brittle rocks. Over time, when water seeps into cracks in the rock, it can dissolve and remove material holding the rock together. During the fall and spring, this is a regular occurrence along the rock outcroppings that border the highway. When enough material washes away, the rocks become unstable and can slide onto the road.

Our maintenance crews have years of experience dealing with the rocks along Chuckanut Drive. In fact, they lovingly refer to the hillside as rotten rock because of how brittle some of the rock is. You can break it apart with your fingers. As you might imagine, this presents a challenge when it comes to trying to stabilize the hillside. We’ve stabilized the slope many times during the past 20 years, and will likely continue to do so in the coming years.

The Chuckanut area is a fascinating example of the environmental forces that helped shape Puget Sound. So the next time you’re winding your way along Chuckanut Drive, try and imagine that nice, warm, tropical atmosphere from 55 million years ago.