The Rocky Mountains are a dazzling example of what our planet can build.

And they’re still in their  youth, geologically speaking. They’re like 80 million years old, tops.

But as it turns out, Earth had some practice. Because it’s on its second  round of Rocky Mountains. That’s right, an entire mountain range grew and then vanished more than  a hundred million years ago.

If that’s the case, how do  we even know it was there? Let’s take a look at the geologic  clues that tell the tale of what geologists call the  Ancestral Rocky Mountains. [♪INTRO] So our story is going to start in Colorado, not far from the famous Red Rocks Amphitheatre. Here, along the side of a path, you’ll encounter a boundary between  two different types of rock.

There’s even a handy plaque pointing it out, if you don’t stare at rocks  as much as geologists do. Alright, so there’s two rocks  touching. What’s the big deal?

Well, according to some slowly decaying radioactive atoms in the rocks on one side, they are over 1.7 billion years old. And according to trace fossils on the other side, they’re a mere 300 million years old. In other words, roughly a quarter of our planet’s entire history lies between these two rocks, completely absent from this geologic record!

Now, the rocks on the older side of the boundary are sometimes called the Idaho Springs Formation. They began as sediments and lava on an ancient supercontinent called Columbia. Despite the world map looking  completely different than today, Columbia contained the same continental cores, called cratons, as our modern continents.

But 1.7 or so billion years ago,  these cratons were still forming. In fact, an arc of islands collided  with the early core of North America. That collision smushed a bunch of rocks together in an event called the Colorado Orogeny.

An orogeny is what geologists call  an event that builds mountains. It’s basically Greek for “mountain creation”, so they weren’t particularly creative about it. This collision buried the original rocks of the Idaho Springs Formation deep under the crust, where heat and pressure changed them into metamorphic rocks like gneiss and schist.

And thanks to some later geologic  activity that brought them back to the surface, those are the  rocks you can go touch, today. We still haven’t have not  gotten to the Ancestral Rockies, so let’s move forward in the timeline to that 1.4 billion years of missing rock. Anytime you have a gap like that,  it’s called an unconformity.

These form when one rock erodes  down over a long period of time, erasing all that geologic history,  and then another forms on top of it. But why was there so much erosion here? Well, one hypothesis is a global cold snap, aptly named Snowball Earth.

This was a period of two  globe-spanning glaciations that took place between 717 and 635 million years ago. The longer of the two could have covered the entire planet in ice for nearly 60 million years! And during this time, the  ice sheets scraped across the continents which had just broken apart from another supercontinent, called Rodinia.

One study estimates the ice could have removed as much as five kilometers of rock. That’s about 12% of the average continent! Now, this is just one hypothesis that explains this particular unconformity.

There’s a lively debate amongst  geologists around what erased such an enormous chapter from  Earth’s proverbial history textbook. But whatever the cause,  eventually that erosion ceased, and the Fountain Formation appeared. This is the iconic red sandstone  that Red Rocks Park is so famous for.

In contrast to the Idaho Spring  Formation’s metamorphosed rocks, sandstone is a sedimentary rock made  of tiny grains that first eroded from another rock, then got  squished into something new. Specifically, we’re dealing  with an arkose sandstone, which means it has a high  concentration of the mineral feldspar. Feldspar is found in granite and  gneiss, so that’s a clue to where the Fountain Formation’s ingredients  came from in the first place.

There are also larger chunks of granite and gneiss mixed in amongst the tiny grains. That tells geologists all  the ingredients to make the Fountain Formation were deposited  at the end of a fast flowing river: one that had enough energy to push  those larger chunks of rock along. If a mountain range existed in  this spot 300 million years ago, that could have supplied both the  right rocks, granite and gneiss, as well as/ the steep landscape  for rivers to whisk it away and drop it in a fan of  sediment at the mountain’s base.

But 300 million years is still  more than 200 million years before the modern Rocky Mountains started growing. So we need another mountain range to explain the Fountain Formation. Enter, the Ancestral Rocky Mountains.

Actually, stay there, Ancestral Rockies, I have to show them an everybody a quick ad first. This SciShow video is supported by DeleteMe, the hands-free subscription service that removes your personal information from hundreds of data broker  and people search sites. SciShow’s executive producer, Nicole, signed up this year and said it took less than 10 minutes to enter her information  and get the process going.

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To sign up, use the QR code on screen or go to joindeleteme.com/SCISHOW and use promo code SCISHOW at checkout. You’ll get 20% off DeleteMe US consumer plans. The Ancestral Rockies stood mostly in what is today Colorado and New Mexico.

Of course, neither Colorado nor New Mexico existed 323 million years ago,  when they started to form. This was just as the continents were coming together to make the supercontinent Pangea. By this point in time, North America’s craton was in the middle of the continent.

So both the Ancestral and Modern Rockies formed 1500 kilometers away from  the nearest plate boundary… which is, admittedly, a bit weird. Normally, mountain ranges form at the boundaries between tectonic plates, when one plate slips under or collides into another and pushes it up. Geologists don’t even know for sure why the Modern Rockies formed where they did.

Which feels like a thing that we should know but I guess we’re still working on it. But most geologists think  that the Ancestral Rockies formed because of that  collision that created Pangea. Smooshing what is now Africa  and South America into the southeast of North America  certainly built mountains along the borders where  they did all the smooshing.

But the idea is that ripples also  formed far beyond those borders, like folds appearing in the middle of a  carpet when you push on its edge. Can you believe plate tectonics sometimes– I’m like really? That’s how this happened?

Something similar happened in  Asia when India crashed into it. This built the Himalayas  right at the collision point, but also deformed the Asian continent  3000 kilometers further inland. But just like there are multiple ideas for how 1.4 billion years of geologic history were erased between the Idaho Springs and Fountain Formations, there are multiple ideas for how  the Ancestral Rockies formed.

Another is that the collisions which formed Pangea reactivated an old system of  faults, or fractures between separate blocks of rock,  called the Wichita Megashear. Which in my humble opinion is a very good name for a very hard rock band. They could open for Kansas!

The collision caused blocks  of rock to start sliding along all these old faults  again, piling them up over time. And if you get a big enough  pile… That’s a mountain. So the Ancestral Rockies probably started growing more than 300 million years ago.

But sometime before 275 million years ago, the forces pushing up mountains in the middle of North America abruptly stopped. And without that assist, the  Ancestral Rockies were at the agonizingly slow mercy  of erosion, which ground them down to nothing over the  next hundred million years. Eventually, a new set of  mountains would rise in its place.

But the legacy of the Ancestral  Rockies isn’t completely lost to time. The sediment from their erosion  flowed into basins across North America, creating the right  conditions to trap oil and gas. We humans eventually came along  and found this very useful, to make a bit of an understatement, honestly.

And of course, there's the  striking Fountain Formation, which is just the right age to have been involved in building up the Ancestral Rockies. So if you go to Red Rocks Park, and touch the famous red rocks, remember you’re actually touching the leftover bits of a long lost mountain range. [♪OUTRO]