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N. Oregon Coast's Rockaway Beach Has a Geologic, Land-Altering Surprise

Published 03/27/2015
By Andre' Hagestedt



(Rockaway Beach, Oregon) – There truly are some awe-inspiring aspects of the Oregon coast simply hiding in plain sight. People may see a landmark all the time but never see the unusual detail right in front of them because they don't have the correct kind of trained eye. Yet sometimes, even the professional doesn't see the surprise coming. (Photo above: you can see a slight curve beginning here in front of Twin Rocks).

Case in point: those famed Twin Rocks at Rockaway Beach are doing something interesting to the landscape no one has noticed. Until, this week, anyway. Oregon Coast Beach Connection brought a seemingly insignificant detail to the attention of Seaside geologist Tom Horning – of Horning Geosciences – and he saw something he'd never seen before.

It turns out, that pair of sea stacks is creating a structure we can't see, and it's bending the beach around it.

Photo above courtesy ormap.net: the curve of Rockaway's Beach is larger than you can see from the ground, but while on the ground you can a sizable bump in front of the rocks as well.

Head to down to Minehaha Street in Rockaway Beach, and you'll find yourself the closest to Twin Rocks that you can get. The sea stacks are a mere half mile away from you – maybe less. Look closely at the beach directly in front of the stacks. Notice anything different?

The tideline curves outward here, away from the beach. There's a large chunk of sand that's maybe 20 – 30 feet closer to the rocks directly in front of them. This stretch is almost as long as the rocks are as well. Moreover, the curve is much bigger than you can see while standing on the beach.

It's as if Twin Rocks are pulling at the sand. It turns out that's not too far off from the truth.

Horning was surprised by the observation, and confirmed via aerial photographs the sand does indeed project out quite a ways at that spot. It comes down to the rock formation breaking up the power of the waves and then creating some kind of land mass underwater, which in turn builds even more sand in that spot.

“When the waves hit a rock or sea stack like that, they warp around it and they lose energy,” Horning said. “So it dumps sand directly behind the sea stack.”

Then there's one of two things happening here, Horning said, resulting in what is called a tombolo – the term for a tract of land connecting two areas that are in the water. Or, in this case, something is connecting the rocks to the shore.

If the tide was for some reason low enough, you might actually see a bridge of sand between the beach and the formations. Or, it could be made of gravel. Whatever it is, it's underwater.

“It's pretty far out and pretty deep,” Horning said. “Since the bar is high, at low tide it makes the shoreline appear to stick farther out into the ocean.”

If it's made of gravel, this could be erosion of rock coming off the Twins. The structure is about 17 million years old, having been created by a super lava flow that trudged this way from some three hundred miles to the east, from the same crack in the Earth that now fuels Yellowstone National Park. (Tectonic plate movement over those millions of years caused the land to shift well to the west, while the crack at Yellowstone stayed in the same place.)

Twin Rocks was once part of something much bigger, but millions upon millions of years of geologic movement and then wave action whittled it away into its current form.

What's eroding from the rocks may be creating this tombolo.

“It's hard for waves to move gravel,” Horning said. “So it forms a long, skinny ridge of rock maybe a quarter mile to half a mile long that connects the sea stack to the beach. So maybe what we're looking at there is part of the tombolo we can't see. The rest is under water.”

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Whatever is exactly going on there, it has a reach far beyond just this little stretch of beach. And it's been around for a long, long time. Horning noticed the property lines of the oceanfront homes here jut further out to the west than the rest of Rockaway Beach as well. It's created conditions favorable to building construction a bit closer to the beach, another intresting fact on top of the hard-to-see curiosities of this beach's layout.

More on Rockaway Beach below and at the Rockaway Beach, Manzanita Virtual Tour, Map.





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