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.
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 Drive is much
younger than other parts of Washington. The fossils found there 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
this 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 and 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.
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.
Fossil mammals from Washington do get most of the press. 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.
The bone record is actually far less abundant that the plant
record, except near shell middens, given the preserving qualities of calcium
and an alkaline environment. While calcium rich bones and teeth fossilize well,
they often do not get laid down in a situation that makes this possible.
Hence the terrestrial paleontological record of Washington State
at sites like Chuckanut is primarily made up of plant material.
