About 15 million years ago basaltic lava released from fissures in northeast Oregon and southwest Washington poured through the Columbia River basin, traveling across the Pacific Northwest. Collectively these flows are known as the Columbia River Basalts.
What is perhaps most intriguing is just how far some Columbia River Basalts traveled. Flows can be found in locations as far afield as Silver Falls State Park, for example. Other flows traveled hundreds of miles from their origin through the Coast Range mountains to the Pacific Ocean.
Seal Rock State Park is the site of one such flow—making it a premier location for geology enthusiasts.
So, when I reached out to Sheila Alfsen from the Geological Society of Oregon Country for a hike and interview and she suggested we visit Seal Rock, it was met with a resounding “yes! “
Circuitous routes
I met Sheila in Philomath so we could drive to the coast together and talk geology along the way. As we headed out, she told me a bit about her background.
Sheila’s path to geology was a circuitous one.
She started out as a volunteer and teacher’s assistant at her own children’s schools where she realized she had an interest in and a knack for teaching.
Then, when state requirements insisted she go back to school for her job, her mind and life path were changed.
“My first class was oceanography,” Sheila gushed, “and the first thing we talked about was plate tectonics…This was everything I wanted to know. I was hooked on geology after that.”
Soon enough, Sheila had earned an associate degree, and later a Bachelor’s in Geology and Spanish, and a Master of Arts in Teaching (MAT).
She started teaching high school science and eventually moved on to teaching college courses, some with her mentor, Bill (William) Orr.
Sheila found her passion—teaching geology.
“In Geology, you aren’t just talking about the rocks, but what they tell us about the history, and therefore, future of the planet. In Earth Science, you also talk about the oceans and atmosphere,” Sheila explained—It is all the Earth Systems.
“I can teach basic principles of physical science within the context of earth science.” Everything has a geology connection.
Highway 20
Our first stop on the way to the coast was Ellmaker State Wayside off Highway 20. Here, Sheila laid out a plan for the day and gave a bit of background on the road we would be following to reach Newport.
Several decades ago, the State Department of Transportation attempted to reroute the highway. Back then, the highway was routed through Eddyville where it followed the Yaquina River on windy roads that not only made the drive to the coast longer but more hazardous.
So, the State hired a construction company to cut a new route through the coast range. But problems ensued. The land was unstable, and landslides became a huge issue.
“Basically, they didn’t consider or understand the geology until they already had a lot of problems,” Sheila explained.
Their oversight came at a high cost. By that time, the first company hired had gone broke and a new construction company was brought in with more geological expertise.
“It took 10 years later and over double the budget to get it done,” said Sheila.
Structure
Sheila and I hit the road again to see just what exactly had thwarted the project. Turns out you can see the problem in the rocks.
As we drove up the highway, Sheila pointed out roadcuts, as we passed. The rocks in the roadcuts were light colored and dipped to the east as we headed up the pass. Later, a bit further up the road, the layers were arranged nearly horizontally. Then, we reached a spot where the rock layers had turned—dipping westward toward the ocean.
Here we pulled over to take a closer look.
Sheila explained that the reason that the highway road project didn’t succeed is that from the start they didn’t pay attention to the geology—specifically, the structure of the rocks.
“When we say structure in geology,” Sheila explained, “we are talking about how the rocks are folded and how they are positioned.”
She went on “Geological structure is how the rocks are put together. It makes a big difference.”
The structure we were observing as we came over the Coast Range on highway 20 is what is called an anticline.
“An anticline is an arch,” said Sheila “and this is one limb of the anticline,” she pointed westward, “and the other way is the other arm.”
Sheila went on to explain that this giant arch was also plunging—dipping to the north.
“Pressure from this direction,” she pointed west again, “from the Juan de Fuca plate, creates the anticline.”
The Juan de Fuca plate is the current tectonic plate that is subducting (going under) the North American Plate just off Oregon’s coast. However, according to Sheila, there is also pressure from the Klamath Mountains to the south that has resulted in a “rotation of the whole coast range”—this is what makes the anticline tilt to the North. This is why pieces of rock were breaking off and sliding onto the road, inhibiting the progress of the construction.
Tyee Formation
We got out of the car to get a closer look at the rock layers themselves.
As we stood there talking, a police car pulled up to see if we were okay.
Sheila laughed, “Just a little geology lesson,” she told them, before inviting them to join us. They declined, but I got the sense that this was not the first time Sheila has made such an invite.
“This rock is the Tyee Formation,” Sheila described as we looked across the highway at the tilted layers.“This layer of rock is famous,” she went on, “It goes all the way down to the Klamath Mountains.”
The Tyee Formation is comprised of sandstone and shale, formed from sediment that was deposited in a large underwater delta some 45 million years ago. There was no Willamette Valley or Coast Range at the time, just a gigantic bay. The Klamath Mountains were already in existence and shedding sediments into the bay to form the delta.
“The delta was huge and went all the way out northward to about Dallas,” described Sheila. I tried to imagine Oregon 45 million years ago, missing a good quarter of its landmass.
Eventually, the delta turned to rock and was folded and lifted into the Coast Range, powered by the subduction of the Juna de Fuca plate—a process that continues even today.
Turbidity Currents
Sheila suggested we walk closer to the roadcut to look at the rocks of the Tyee more closely.
She explained that when the sediments from the Klamath Mountains would fall into the bay, this resulted in “turbidity currents”— a sudden flush of sediment and water rushing off the continental shelf before settling into distinct layers.
These fast flushes of sediment became the layers of rock that make up the shale and sandstone of the Tyee formation. The sandstone layers in the rock formed from quickly settling sand, and turned into thick, light brown colored layers of sandstone. Clay, on the other hand, “takes a long time to settle out.” These clay layers presented themselves as dark gray, incised bands in the roadcut.
“One layer of sand and one layer of clay above it is one event,” Sheila pointed out. “This is what the Coast Range is made of.”
Sheila pointed out the shiny flecks that glittered in the sandstone layers. “Muscovite,” she called them, “from the Idaho batholiths”—a clue that when the Klamath Mountains were first accreted, they were near the Idaho border.
A Closer Look
Sheila soon began to poke around, digging into the roadcut rocks.
“If we are lucky,” said Sheila, as she pulled a rock from the base of the loose shale layer, “we will find little trails of marine organisms.”
You see, between each turbidity current, the organisms that are living and feeding on the sediment before they are wiped out by the thick sequence of sand that suddenly gets dumped on them. Their fossil remains can often be observed as trails in the sandstone and can be used to date the layers.
Sheila and I continued to pick at the roadcut and examine any loose pieces of rock that came away easily. The shale broke off in thin layers, while the sandstone felt gritty and rough.
I held a piece of rock up to my eye with a hand lens to see the shiny flat muscovite mineral amongst the grains of tan-colored quartz and feldspars.
“A geologist sees things. When you learn about the geology you look at the world differently and it is beautiful.”
The Road to Jump-off Joe
Sheila and I hit the road again. We were going to make one more stop before heading to Seal Rock—a place called Jump-off-Joe.
After another 30 minutes of driving through the Coast Range, we reached Newport and the Pacific Ocean. We drove North a bit on Highway 101 before veering off onto a side street and pulling over in front of a roadblock and a parking lot with an oceanfront view.
Just past the cliff edge, you could see an old building foundation in disrepair, as the land around it had subsided and begun the process of crumbling into the sea.
As we stepped out of our cars for a closer look, Sheila laughed at a sign on the adjacent hotel that boasted about its “ocean views.”
“This building was a football field away from the edge,” said Sheila, thinking back to her last visit. “The view is getting more and more exciting,” she snickered.
“Coastlines are unstable,” said Sheila. A lot of the rock on the coast is layered sedimentary rock and “some are inherently unstable.”
The fact that someone tried to build in this location was ludicrous to Sheila.
“Immediately it started slipping,” said Sheila. “Yaquina Head in the north, to the opening of the estuary is all landslide area.”
Time and the elements had really taken a toll on the abandoned structure. Graffiti covered large portions of the dilapidated foundation. Signs warned people to stay back.
It was all a bit ominous. We kept our distance from the edge.
Sandstone Arch
Then, Sheila pointed to the right of the crumbling foundation, a small sandstone mound stood just below on the beach. Another sign of erodibility and instability of the rocks that make up much of this part of the coast.
“Back in the late 1800s or 1920s that was an arch,” said Sheila pointing to the small, but visible sea stack. “It has been eroded.”
The location of the arch was once referred to as “Jump-off Joe,” apparently because the cliff down to it was steep. It was quite the site to see back in the day, as evidenced by a quick google search.
Now, it was hardly an attraction, having been weathered down to a remnant of its former self.
Of course, not all the rocks on the coast are as suspectable to erosion and weathering as much of Newport Bay. Yaquina head, for example, just visible to the north is made of basalt—a much more resistant rock.
“That is why those are points out there,” reasoned Sheila. In fact, basalt rocks make up much of the Oregon Coasts’ headlands.
But where did all this basalt come from?
I was about to find out.
Sea Stacks
Sheila and I took off again for our final destination—Seal Rock State Recreation Site.
We arrived around lunchtime and stopped for a quick picnic lunch at a table just behind the bathrooms.
After lunch, we followed the paved trail back up through the twisted shore pines that led out to the Seal Rock viewpoint. From here, sea stacks of various sizes jet out of the ocean in a curved line.
“We call this a ringed dike because it forms a ring shape,” said Sheila. “What used to be a low space fill with lava, and the stuff around it erodes away,” she explained.
Elephant Rock
The largest of the rocks—a massive rock towering structure—is known as elephant rock.
“Elephant rock is what we call a sill,” said Sheila, “in igneous geology, a layer of lava that squeezes between two layers of rock.”
“In this case, the lava didn’t intrude between the layers, it just fell into the soft sediments of the coast and re-erupted,” Sheila backtracked, So, “not technically an igneous sill…but it is basalt.”
Basalt—a hard and resistant rock. Waves “eat away at sandstone,” but basalt, not so easily.
“You can see the cave under the rock, to the right,” said Sheila as we started further down the trail that leads to the beach. “It is sandstone. It is easier to eat away.” A small cave carved into sandstone cliffs to our right.
Just like at Jump-off Joe there are signs that warn people not to walk on these cliffs. Just like Jump-off Joe, the area is unstable.
Cobbles
The trail eventually petered out as we neared the beach. We carefully clambered over rounded rock cobbles that had been turned by the waves.
“This is nicely polished basalt,” said Sheila as she picked her way down.
Basalt, Sheila explained has cracks in it that develop when the lava cools. The columns of elephant rock are a great example.
“It is easy for the waves to break it up,” remarked Sheila.
Magnetite
After some careful maneuvering, we reached the beach and headed south, following the ocean’s edge where the sand is firm. Soft gray-colored sand lay underfoot, but Sheila was on the hunt for something darker.
“If you look at the beach, have you seen areas with dark sand?” asked Sheila. “That is magnetite.”
Magnetite, she explained comes from weathered basalt. Magnetite is a dark-colored mineral made of iron and magnesium—making it magnetic. It is heavy and often accumulates in areas.
“Near stream you see it,” Sheila advised. She had seen a thick layer of it on previous visits to the beach and was curious to see it again.
“Here is magnetite,” said Sheila a few moments later—though not the band of magnetite she was hoping to find. Black sand lay in a rippled pattern on the otherwise pale-colored sand.
Dynamic
“Here we are watching the pattern that develops in the sediments,” said Sheila.
She went on to explain how sediments are pushed up on the beach at an angle by the surf and then fall straight back down the beach so that they constantly are moving along the shoreline.
“A coast is a dynamic place, always changing,” she affirmed.
The magnetite pattern was just one sign of constant coastal change.
A Lava Story
Sills, dikes, cobbles, and magnetite… we headed toward the far shore and crossed a small creek. It was time to get to the main event. Where did the lava come from?
“This is the southernmost extent of the Columbia River Basalt,” said Sheila.
The Columbia River Basalt, as mentioned earlier, are lava flows that originated from fissures in eastern Oregon and Washington some 15 million years ago.
“They made their way through the Cascades, down the Willamette Valley, and as far south as Salem Hills,” said Sheila.
In fact, the Salem Hills are Columbia River Basalts—“they are just coved with vegetation,” explained Sheila.
“A typical flow was 100 ft thick,” Sheila described. “Imagine a wall of lava that is one hundred feet thick and flows like syrup.”
Remarkably the flow stayed liquid as it traveled all the way to the coast. This is different than one might expect especially if you have seen a Hawaiian eruption. Sheila described seeing a lava flow in Hawaii cool right before her eyes.
In the case of the Columbia River Basalts, there is “so much lava, the outside will crust over, and it will break through its own crust and keep going,” Sheila described. “It could advance 3-4 miles per day.”
According to Sheila, the basalt rocks we were seeing were Wanapum basalt, the youngest of the Columbia River Basalts, specifically the Gingko flow.
Final Contact
By now we had made our way over to the sandstone and basalt cliffs opposite the ocean. Here, we passed by what looked like a small black stone wall jetting out of lighter-colored sandstone.
“It was probably soft sand when the dark lava intruded but now it is sandstone,” explained Sheila.
“This is part of the ring dike,” said Sheila, “a crack that is filled with lava.”
We saw more cobbles of polished rock before reaching the far end of the ring dike.
“Basalt is here,” said Sheila pointing up at some heavily fractured black rock overhead. “And the contact between the basalt and the soft sediments,” she pointed to a deeply eroded area below the rocks where thin ribbons of rock layered together.
“Looks as fresh as it did when it cooled 15 million years ago,” she exclaimed with a smile.
Tracking Flood Basalts
At this point, Sheila and I turned to retrace our steps. But before we made it back very far, we stopped for a quick geology lesson and big-picture discussion on the basalt flows.
“Coastal provinces are kind of a collage of everything that has happened inland,” said Sheila, as she traced a sketch of Oregon into the sand.
She began pointing out important landmarks… “the Columbia River, Cape Blanco…”
“Cracks opened over here and issued lava,” she pointed up to the northeastern part of the state. “Most of it came down the Columbia River.”
The Columbia River used to be further south in what is now known as the Columbia Plateau, she explained, but it got pushed up north as the lava flowed through.
“Then when it comes to Portland and the Willamette Valley,” we moved further down the map, “it makes up the Amity Hills, Eola hills, and Salem Hills.” Again, these would have been low points, or depressions at the time.
“We find it in the Molalla River in what used to be river valleys,” she continued, and in places like “Silver Falls State Park.”
“Then we see it out here and in the Capes all the way as far south as Seal Rock,” she concluded.
A Gap
But there is a problem—a gap if you will. There is not a clear sign of Columbia River Basalt flows through the Coast Range Mountains. How did they make it all the way to Coast near Newport?
This is where Sheila comes in. She has made it her mission to find Columbia River Basalts in the Coast Range Mountains—to trace its path to the Ocean.
Now there is a lot of basalt in the Coast Range Mountains, but the problem is “the chemistry doesn’t match up.”
“A lot of it is Siletz River Basalt,” Sheila said as we restarted our walk back.
Siletz River Basalts are part of a massive igneous province that formed in the Pacific Ocean before accreting to North America beginning about 50 million years ago known as Siletzia or the Siletzia Terrane. This exotic terrane became the foundation for the Coast Range but is also visible in various locations in the Coast Range.
According to Sheila, Columbia River Basalts have “higher silica than most basalt” and each flow or unit has a specific chemistry. She has collected samples at various promising locations in the Coast Range but has yet to find a match.
Perpetual Teaching and Learning
Sheila and I soon recrossed the creek we had waded over earlier.
After we crossed, I asked Sheila to tell me about one of her favorite places on the Oregon Coast. She had mentioned Cape Perpetua earlier and I wanted to know the story.
“Cape Perpetua was a personal thing,” started Shiela. “ I was studying oceanography and looking out at the ocean.”
She could see the waves breaking below her and she realized she could calculate how far apart each wave from another using known distances, like the road. The distance of one wave to another where they start to break tells you the depth of the water at that location.
“It came to me,” she went on. “I really love this. I want to do this.”
Sheila paused.
“That was 25 years ago. I haven’t tired of it.”
We continued our conversation passing through the creek, back up the basalt cobble, and up the paved path to our cars—and Sheila never tired.
And you know what? Neither did I.
Sheila Alfsen is a geology instructor at Chemeketa Community College, Linn-Benton Community College, and Portland State University. She is also a past president and program director of the Geological Society of Oregon Country in Portland. Sheila earned d Bachelor of Arts from Western Oregon University for Geology and Spanish before going on to get an MAT from Western Oregon University.