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If you look at a map of Scotland, you’ll notice an almost perfectly straight line through the Highlands that cuts from coast to coast. This region is home to plenty of valleys and peaks, but this valley is eerily straight.

Some interesting geology had to happen there at some point. And it wasn’t just one event. About 520 million years ago, most of the Earth’s landmass was split between two big continents: Laurentia and Gondwana.

This meant the modern-day island of Great Britain was separated, with the north of. Scotland sitting on Laurentia, and the southern half of the island on Gondwana. Then, the two landmasses collided around 430 million years ago during a period known as the Caledonian Orogeny, which joined the two pieces through the collision forming the island we now know as Great Britain.

This process resulted in the crumple and buckle of the Earth’s crust which also formed new mountains and fault lines all around the world. One of those new faults was the Great Glen, which is a strike-slip fault. This kind of fault happens when two tectonic plates shear, or move horizontally, past each other.

Other types of faults typically move one of the plates vertically, which creates mountains and all kinds of elevated terrain above the fault. But because of how these two plates collided with each other during its formation, the pieces of the Great Glen Fault move horizontally. And they’ve actually moved a few times since the formation of the fault.

The Great Glen Fault has occasionally reactivated and the two landmasses,. Laurentia and Gondwana, moved anywhere from 8 to 29 kilometers each time. This is a thing that faults do from time to time to dissipate built-up stress.

When force is applied by the two landmasses punching against each other, stress is created and when it reaches a tipping point, the plates shear and move in opposite directions, to relieve that built-up stress. The biggest reactivation happened relatively recently, sometime in the last 66 million years, possibly prompted by other parts of. Earth’s crust spreading apart nearby.

But why don’t we see straight lines like this more often? Well, these days, the Great Glen fault line is even more visible due to a string of lakes, or lochs. That’s because most of the islands of Great Britain and Ireland were covered in enormous ice sheets during a handful of Ice Ages over the last few hundred thousand years.

The glaciers around the Great Glen started receding over 10,000 years ago, carving a deep valley along the fault line that actually goes below sea level, making that straight line through Scotland even more visible. So, this straight fault line is the product of half a billion years of time and geology and it’s evidence of the large-scale events that formed Earth as we know it today! Thank you for watching this episode of SciShow!

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