Monday, 2 February 2009

MEANDERING THROUGH THE EOCENE: CHUCKANUT DRIVE


by Heidi Henderson


Chuckanut Drive, in northwestern Washington provides a visual feast from sea to sky.

An amazing array of plants and animals call this coastline home. For the fossil enthusiast, it is a chance to slip back in time and have a bird’s eye view to a more tropical time with a visit to the Chuckanut Formation. Snug up against the Pacific Ocean, this 6000m thick exposure yields a vast number of tropical and flowering plants that you might see in Mexico today. Easily accessible by car, this rich natural playground makes for an enjoyable daytrip just one hour south of the US Border.

Shaping our World

Over vast expanses of time, powerful tectonic forces have massaged the western edge of the continent, smashing together a seemingly endless number of islands to produce what we now know as North America and the Pacific Northwest.

Intuition tells us that the earth’s crust is a permanent, fixed outer shell – terra firma. Aside from the rare event of an earthquake or the eruption of Mount St. Helen’s, our world seems unchanging, the landscape constant. In fact, it has been on the move for billions of years and continues to shift each day. As the earth’s core began cooling, some 4.5 billion years ago, plates, small bits of continental crust, have become larger and smaller as they are swept up in or swept under their neighboring plates. Large chunks of the ocean floor have been uplifted, shifted and now find themselves thousands of miles in the air, part of mountain chains far from the ocean today or carved by glacial ice into valleys and basins.

Washington is Born

Two hundred million years ago, Washington was two large islands, bits of continent on the move westward, eventually bumping up against the North American continent and calling it home. Even with their new fixed address, the shifting continues; the more extreme movement has subsided laterally and continues vertically. The upthrusting of plates continues to move our mountain ranges skyward – the path of least resistance.

This dynamic movement has created the landscape we see today and helped form the fossil record that tells much of Washington’s relatively recent history – the past 50 million years.

Chuckanut Formation

The area we will be visiting along Chuckanut Drive is much younger than other parts of Washington. The fossils we will visit lived and died some 40-55 million years ago, very close to where they are now, but in a much warmer, swampy setting.

The exposures of the Chuckanut Formation were once part of a vast river delta; imagine, if you will, the bayou country of the Lower Mississippi. The siltstones, sandstones, mudstones and conglomerates of the Chuckanut Formation were laid down about 40-54 million years ago during the Eocene epoch, a time of luxuriant plant growth in the subtropical flood plain that covered much of the Pacific Northwest. This ancient wetland provided ideal conditions to preserve the many trees, shrubs & plants that thrived here.

Plants are important in the fossil record because they are more abundant and can give us a lot of information about climate, temperature, the water cycle and humidity of the region. The Chuckanut flora is made up predominantly of plants whose modern relatives live in tropical areas such as Mexico and Central America. If you are interesting in viewing a tropical paradise in your own backyard, look no further than the Chuckanut.

Images and tag lines: Glyptostrobus, the Chinese swamp cypress, is perhaps the most common plant found here. Also abundant are fossilized remains of the North American bald cypress, Taxodium; Metasequoia (dawn redwood), Lygodium (climbing fern), large Sabal (palm) and leaves from a variety of broad leaf angiosperm plants such as (witch hazel), Laurus (laurel), Ficus (fig) and Platanus (sycamore), and several other forms.

Inset: Mammal Fossils in Washington

While less abundant, evidence of the animals that called this ancient swamp home are also found here. Rare bird, reptile, and mammal tracks have been immortalized in the outcrops of the Chuckanut Formation.

Tracks of a type of archaic mammal of the Orders Pantodonta or Dinocerata (blunt foot herbivores), footprints from a small shorebird, and tracks from an early equid or webbed bird track give evidence to the vertebrates that inhabited the swamps, lakes and river ways of the Pacific Northwest 50 million years ago. The movement of these celebrity vertebrates was captured in the soft mud on the banks of a river, one of the only depositional environments favorable for track preservation.

Sidebar: Fossils Must be Dinosaurs…

We can thank Mr. Spielberg and popular culture for the fact that most people think of dinosaurs when they think of fossils. The bone record is actually far less abundant that the plant record. While calcium rich bones and teeth fossilize well, they often do not get laid down in a situation that makes this possible.

Look around at the site today and the abundance of plants and lack of visible animal life. They are far fewer animals than plants and consequently in a setting such as this far fewer animals in the fossil record. It is the reverse at some sites, i.e. the Gobi desert and Alberta, but in the Chuckanut, this is the way it plays out.

In Alberta, most of what we find are small bone fragments from vertebrates. This colors our notions of what the world must have looked like. It shows us only one small piece of the puzzle as to what life must have been like in an area when part of the fossil record is missing.

1 comment:

Anonymous said...

1. I wonder if the coal deposits in Bellingham are part of the Chuckanut Formation.
2. I wonder if there's a possibility that we may find a diatryma footprint, or traces of some other large animals. A big floodplain would seem to be a feasible habitat for bigger animals.
3. I note some strata of the Chuckanut Formation contain rounded quartz-granite type pebbles. Did these wash here from the Rocky Mountains before the Cascades existed?
Thanks for your interesting article.