Arches National Park contains the largest concentration of natural stone arches in the
world.
More than 2,000 documented arches dot this landscape, along with an assortment of balanced
rocks, fins, spires, and spectacular red rock formations.
From the photogenic isolation of Delicate Arch, to the seemingly impossible span of
Landscape Arch, from the precarious solidity of Balanced Rock to the towering duality of
Double Arch, it is yet another outstanding addition to the already mesmerizing Utah canyon
country.
And all of this in a park of just 76,000 acres.
It really is spectacular and wonderful to look at - there’s a reason millions of people
visit this park every year.
But today, I want to share with you HOW it all got that way.
Hello, and welcome to National Park Diaries.
My name is Cameron and this is a channel dedicated entirely to telling stories from the world’s
protected places.
I tell educational, informative stories about National Parks, public lands, and protected
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Alright, let’s figure out how Arches got all those…
arches.
So, like with most other unique geological phenomena, you need a certain combination
of “ingredients.”
In this case, our first ingredient is… salt?
Probably not what you were expecting, but let me explain.
About 300 million years ago, this area of what would become Utah, was subject to nearly
30 different cycles of the ocean advancing and retreating.
And each time the ocean advanced and retreated, salt water became concentrated in what was
known as the Paradox Basin.
And because it was a basin, water had no outlet to leave - it eventually just evaporated away.
But, because it was salty ocean water, when the WATER evaporated away, it left behind
a massive deposit of… salt.
5,000 ft worth of it.
Enough to fill a YouTube comment section, for sure.
Then, you fast forward a few million more years and you’ve got some mountain building
activity in this area.
And with mountains comes erosion.
Because, as rain falls on those mountains, it carries sediment away and that sediment
has to be deposited somewhere.
This is known as DEPOSITION.
So, deposition occurred in the Paradox Basin ON TOP of all that salt that was deposited
millions of years earlier.
Eventually, that sediment is going to solidify to form rocks.
Sedimentary rocks.
So now you’ve got rocks on top of salt.
2 ingredients.
Now, here’s the thing with salt.
When you’ve collected enough of it, like what happened in the Paradox Basin millions
of years ago, it doesn’t really behave like you’d expect when subjected to immense geological
forces - like piling a few million years worth of rock on top of it.
When that happens, the pressures and forces acting on the salt actually cause it to “flow.”
It’s a solid, yes, but it has these properties that cause it act all weird and, over insanely
long geologic time scales, yeah it “flows.”
Think of, like, a glacier.
Same relative concept.
Anyway, so yeah, the salt is flowing underground.
And then it runs into these deep lying faults in the Earth’s crust.
It can’t go through the faults, so instead this mass of salt is pushed upward, causing
a salt dome.
Now, as the salt moves upward, again, it has this flow property.
But the rock above it does NOT have this flow property.
Rocks don’t flow, they break.
And that’s exactly what happened.
As the salt pushed upward, it caused the rock above it to break and fracture in these parallel
vertical fissures.
So, now we’ve got this salt dome with these vertically fissured rocks above it.
Eventually, the rock on top of THOSE rocks (this is a lot of rocks, I know), but THOSE
rocks, the top layers, are going to erode until…
eventually…
the rock layer with the vertical fissures is exposed to the surface.
You can imagine what happens next.
Water infiltrates those vertical fissures allll the way down to the big ol’ pile of
salt below.
And because salt dissolves in water, the big ol’ pile of salt begins to disappear.
But now, it’s no longer holding up those vertically fissured rocks above it, and everything
begins to collapse, and this is how we ended up with what is today known as…
Salt Valley.
When you’re driving on the main road through Arches National Park, you’re driving through
Salt Valley.
And, it’s on the edges of Salt Valley where we see most of the arches in the park.
You can see this really well on Google Earth, actually.
The reason you’ll find most of the arches on the edge of this Valley is because that’s
actually where you’ll find the remnants of those vertically fissured rocks.
Down in the middle of the Valley, that’s kind of where everything collapsed, so you
don’t really see anything down there.
But yeah, out on the edges, the last vestiges of those vertical fissures can still be seen.
Today, you’ll notice them as these massive deeply-red-colored fins.
What happened was those fissures, again being exposed to the surface, allowed slightly acidic
rainwater to infiltrate into the rock, which widened the fissures and the cracks and ultimately
created the fins.
And, the type of rock here is important too.
That’s an ingredient we haven’t really talked about yet.
Most of the arches in the park are found in a rock layer known as the Entrada Sandstone,
which has these nice, spherical granules of sand and which is very porous - it has a lot
of tiny holes in it.
Lots of pores make it really easy for water to infiltrate the rock and erode it and continue
opening up those cracks and fissures.
Now, let’s finish this thing off by forming some actual arches.
Because, there’s ANOTHER type of rock that’s important here as well.
Below the Entrada Sandstone, where all the arches form because the rock is really porous
and erodable, lies a rock layer known as the Carmel Formation.
It has some sand in it too, but it also has CLAY.
Clay grains are much smaller than sand grains, and thus do NOT allow as much water to infiltrate
into the rock and erode it away.
As a result, water ends up just kind of pooling on top of the Carmel Formation, where it can
further eat away at and erode the Entrada Sandstone above it.
You can start to see how these arches come together.
That water sits and pools, infiltrates the pores of the Entrada Sandstone, begins to
form a little opening, exposing more rock to erosion, the opening becomes wider, rinse
and repeat, until….
you get a window.
A little window in the rock that will become an arch.
Now, after that opening gets big enough, it’s not just the acidic water wearing away at
that sandstone anymore.
You’ve got some wind action thrown in there as it carries sand grains off the ground and
blasts the rocks, chipping away further at the opening.
Basically like a sand blaster.
Once things get far enough off the ground too, gravity can even take over, as the weight
of the rock becomes too much to support itself and these massive chunks can fall off and
enlarge the opening even further.
This was actually captured on film at Landscape Arch in 1991.
And, that’s how all these arches formed.
Some familiar geologic processes, but a unique combination of ingredients.
Now, 2 important things to remember here.
As with basically every other geological thing I’ve talked about on this channel before,
this isn’t happening over night.
Geologic wonderlands like Arches take millions of years to form, and we happen to be lucky
enough to exist at just the right time in Earth’s history to witness these magnificent
natural creations.
In that same vein, these arches are not permanent.
As sturdy and strong as they look today, the same forces that shaped them will eventually
tear them down.
See the collapse of Wall Arch in 2008.
Time waits for no… arch and erosion WILL eventually bring down the arches of Arches
National Park, no matter how much we like putting them on Instagram.
And, I hate to end on a sour note, but that is everything I have for you today about the
formation of Arches National Park.
Have you ever been?
Do you have a favorite arch?
Let me know down in the comments below.
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Thanks so much for watching, I’ll see you next time, goodbye.