Thanks to Subaru for supporting this SciShow mini series.

The all new 2020 Subaru Outback helped us stay comfortable during filming. [♪INTRO].

Stefan: Welcome back to Washington! This is the last episode of our special, three-part mini-series on the geology of Olympic National Park!

Alexis: So far, we've talked about Mt. Olympus and an ancient structure called Siletzia, and now, it's time for one last story. Stefan, let's hit the beach!

Stefan: Oh yeah!

Stefan: Last week when we were at Rialto Beach, you might have noticed a bunch of cool rock towers in the background. Those are called sea stacks, and these beaches in western Washington are famous for them.

Alexis: But these rocks don't just look cool. They also tell a fascinating story about how this landscape has changed over the last few centuries.

Alexis: Welcome to Ruby Beach!

Stefan: While we were in Washington, we figured we'd at least check out two place with sea stacks. Because, you know, when in Rome…

Alexis: Something like that. Anyway!

Alexis: So, behind us, you can see some of Ruby Beach's famous sea stacks. Sea stacks can be made of all kinds of rock, but the ones here are generally made of the sedimentary rock breccia and mostly-volcanic conglomerate. Basically, that means they're made of a bunch of rocks that got cemented together.

Stefan: Meanwhile, the ones at Rialto Beach are made of things like volcanic basalt, sedimentary sandstone, and conglomerate, depending on where you look.

Alexis: Exactly what the stacks are made of doesn't matter that much, but the important thing to know is that these sea stacks are made of harder rock than the stuff you find on the mainland. And that's important because of how sea stacks are made in the first place.

Stefan: Sea stacks form because of something called preferential erosion. You start off with a headland, or a piece of the coast that juts out into the ocean. As the ocean waves come in, they get bent around the headland and end up hitting the coast in specific spots. And that's where the trouble starts.

Or the fun. If the area where the waves hit is made of softer rock, or made of a rock that contains some kind of fracture, the waves will erode it. That leads to small indents in the headland, and then small caves, and then eventually, the water cuts straight through.

And then you're left with a sea arch!

Stefan: You can find sea arches all over the place, but it's not like erosion just stops once that arch is formed. The waves continue to erode the rock, and over time the arch can get bigger until it eventually collapses to form sea stacks.

Alexis: But the rock composition isn't the only variable here, though. According to a cool model published in 2014, the beach in front of the sea stack is important, too. The model found that really wide beaches often prevent sea stacks from forming, because they act as a protective barrier. Basically, the waves crash against the beach and don't actually hit the headland.

Meanwhile, really narrow beaches can encourage sea stacks to form. In that case, the water picks up all the sand and other sediment, and it rubs against the rock like sandpaper. So erosion happens faster.

Over all, this means that, if Ruby Beach were a lot wider, we might not have these rocks to look at!

Stefan: So, the sea stacks at Olympic National Park are really cool, but they're not the only ones in the world. You can find structures like this all over the place, from Scotland to Australia. And when you do find them, there are some things you can learn about the area you're visiting.

Alexis: For one, at the most basic level, you can learn where the coastline used to be. Because anywhere there's a sea stack — well, that used to be part of the mainland!

Stefan: Sea stacks also tell you that waves are mainly approaching the coast head-on, rather than on an angle. When the waves come in sideways, they erode mostly one side of a headland, and that makes it a lot harder sea stacks to form.

Alexis: And finally, sea stacks tell you that there's a sort of balanced system happening here. Kind of like we talked about in the Mt. Olympus episode earlier on this trip! If you find a sea stack, that means there's a good amount of sediment being carried by the water and eroding the cliffs, but there's not enough to actually form a beach wide enough to protect against erosion.

So, generally, wherever you find sea stacks, there won't be some kind of huge, super-wide beach stretching way out into the water.

Stefan: Instead, you're more likely to see something like this!

Stefan: So while sea stacks are cool, but they won't last forever — probably not more than a few hundred years, depending on their size and what they're made of.

Alexis: They're a reminder that the land around you is constantly changing, and that there's a lot you can learn if you know what to look for.

Stefan: But… there's probably one more thing we should mention about sea stacks. Because while they are really nice to look at, they are also kind of dangerous.

Alexis: It's true. Like, in the late 1800s, one ship near Rialto Beach got caught in some rough waves and foggy weather. Someone on the board saw a sea stack through the fog and thought it was another boat, so they decided to steer toward it… and ended up crashing.

Stefan: There have actually been more than a hundred shipwrecks off the Washington coast, thanks to sea stacks, rough waters, lots of fog, and plenty of reefs hidden just under the waves. So even though these things are great to look at in the daylight….

Alexis: We're going to stick to dry land. And on that note, we should probably head back to the car and let Hank know we're about done here!

Stefan: That sounds great! So until next time, thanks for joining us on our road trip!

Alexis: Thanks to Subaru for partnering with us to make this three-part road trip series, and for lending the support of an all-new 2020. Subaru Outback.

Stefan: We drove a lot to make these episodes, both inside Olympic National Park and out, and all of the Outback's features made our trip a lot easier. My favorite feature was the roomy cargo space! We fit our luggage and even film gear easily. And with the hands-free power rear gate, I didn't have to put my bags down on the wet ground to open the door.

Which is always a struggle, right?

Alexis: If you want to learn more about the 2020 Subaru. Outback, you can check out the link in the description. [ ♪OUTRO ].