The Oregon Coast -

Exotic Terrane, Geologically Speaking

Oregon’s coastal mountains plunge into the Pacific Ocean, creating wet temperate rain forests along the northern coastline. There you can walk the beaches observing ever-changing scenery as ocean fog rolls in off the Pacific, and coastal mist flows oceanward from the mountains. This back and forth opens up stunning views and just as quickly closes them again.

e Highway 26, aka Sunset Highway, is a popular route for a summer day at the beach. But take a jacket since 80 degrees F in Portland can often be 50 degrees on the coast. Slightly northwest of the city, you will find yourself on the winding roads of the coastal mountains, perhaps stuck behind a slow-moving truck. It’s a rugged, densely forested landscape, and you could be forgiven for thinking these mountains are part of the original North American Continent. However, they are not. Instead, they form what geologists describe as an exotic terrane, and they are, in essence, foreign land glued onto the edge of North America.

The core of the continent is a massive chunk of the earth’s crust referred to as the North American Craton. Craton is the terminology used to designate the geological core or nucleus of a continent. It constitutes a large, stable segment of the earth’s crust around which the continent forms. The North American Craton, for example, encompasses much of the eastern, midcontinent, and western portions of the USA. It then extends northward to underly most of Canada and even farther north to take in Greenland. But Oregon, Washington, and California are not part of the North American Craton. They formed as new land, which attached to the craton long after it originally formed.

Oregon and the North American Craton are now tightly glued together, but this bonding was not always the case. The craton contains some of the oldest rocks on earth. The Nuvvuagittuq greenstone belt in Canada has rocks dating back to 4.3 billion years ago. For comparison, the earth is only about 4.5 billion years old. Some 4.25 billion years after the greenstone belt formed, the Coastal Range arrived. Clearly, this mountainous area was a latecomer to the continent.

Some 50 million years ago, during the Eocene, a large group of subsea volcanoes collided with the edge of North America. This large province of volcanic seamounts is called the Siletzia Terrane. The geological term “terrane” denotes a large chunk of the earth’s crust bound by faults and having a geological history different from the regions next to it. Siletzia was originally riding on top of the Juan de Fuca Plate. When this tectonic plate subducted below North America, its top (Siletzia) peeled off and stuck itself to North America in a process called “accretion.”

Over the next 50 million years, the Siletzia rocks were squished and uplifted into the Coastal Mountain Range. The Sunset Highway lets you drive over the top of these uplifted rocks. The whole area is rugged because as the rocks move up, erosion works to wear them down. The softest bits wash away first, forming the valleys, leaving the hard bits standing tall. The whole process is very dynamic and is still going on today.

Portland Basin

Portland itself owes its unique landscaping to movement of the Coastal Range. Portland occupies part of a geologic feature aptly named the Portland Basin. Geologists like to call an area that is sinking relative to its neighbors a “basin.” This deformation creates a low-lying geographic area where sediments collect. Essentially, a basin is a broad, shallow hole in the ground. Rivers and streams carry sediments into the basin forming new layers of clay, sand, and gravel. Over time, as the down warping continues, thick deposits of these sediments accumulate. In the case of Portland, the Columbia River is the great benefactor, delivering uncounted tons of sediments to the Portland Basin each year.

Gradually the Columbia River has filled in the basin. The thickest area is near Vancouver, on the Washington side of the river, where 1,800 feet of sediments have accumulated. These sediments are the foundation upon which much of the Portland-Vancouver metropolitan infrastructure rests.

The Portland Basin started forming about twenty million years ago in response to large-scale tectonics caused by the movement of the Coastal Range. This movement forced the Portland Hills on the southwest side of the basin upward and the floor of the basin downward. So, on our trips to the beach, we pass from a basin at sea level, over the uplifted mountains, and back down to sea level along the coast.

The Coast

No one questions the natural beauty of Oregon’s beaches. But behind the scenes, the exquisite views afforded to beachcombers result from a unique interface between the ocean, air, and land. Another way to think of the coast is a perfect blending of the geosphere, hydrosphere, atmosphere, and biosphere.

Rugged mountains from the exotic and uplifted Siletzia terrane plunge into the Pacific Ocean, creating a coastal wall. Fog and moist winds roll in off the cool Pacific waters, and they are partially trapped against this wall of mountains, keeping the temperatures low and the coastal forests wet — perfect conditions for lush temperate rain forests. Moisture rising from evapotranspiration saturates the forest air, creating a mountain mist that rolls back through the valleys and out onto the ocean. As shorelines and mountains fade into these mists, they produce some of the iconic beach views Oregon is famous for.

Over time, Pacific storms beat against the mountains, eroding away solid rock and forcing the shoreline to retreat eastward. But the retreat is uneven, with softer rock eroding more quickly, leaving the harder bits as promontories and stranded rock stacks. Then, the isolated coves formed during this process fill with sandy beaches.

When we leave the heat of Portland for a cooler oceanside, these coves are often our destination, and they contain the beaches we love to walk on. There, rock, sand, ocean, and mist mingle together into a relaxing and contemplative landscape. An exotic geological terrane, mixing with forests, ocean, sky, fog, and mist, creates a great location to pass the day by the sea.

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