Erosion’s Mark - A Horseshoe Bend Story

[Transcript]

Hey, folks! Welcome to the famous Horseshoe Bend. Earlier, I told you a little about how this geological marvel was formed. But now, let's really dig into its background.

Let's start with a question. If fast-moving rivers tend to flow straight, and slow-moving rivers tend to curve, then how did the famously fast Colorado River create this giant bend in the ground? Horseshoe Bend is one of Arizona's most famous natural landmarks, attracting visitors from around the globe. And its origin story began 300 million years ago.

Back then, this whole area was under a vast, shallow ocean called Western Interior Seaway. Over millions of years, sediment accumulated on the seafloor. Eventually, this sediment compacted into sedimentary rocks like limestone and sandstone.

And on top of that lies the Navajo Sandstone. Fast forward to just 70 million years ago, and the inland sea was gone. In its place were a bunch of volcanoes.

You see, we're currently on what's called the Colorado Plateau. It's an area of land about 130,000 square miles, spread across the Four Corners region of Colorado, Utah, New Mexico, and Arizona. And back then, the plateau was going through a bit of an upheaval.

Actually, the correct word is uplift. Tectonic activity caused the Colorado Plateau to rise up 5,000 feet, or nearly a mile in elevation. The same tectonic activity also created the Rocky Mountains, where the Colorado River headwaters are located.

That uplift took millions of years. So for most of the Colorado River's existence, it was a slow, meandering river across a relatively flat plain. Definitely not the fast Whitewater Rapids River we know today.

This allowed the river to establish much of its path. However, the plateau kept rising, and it reached its full uplift height only about 5 million years ago. That placed the river's headwaters in the Rocky Mountains 4,000 feet higher than where we are now.

And what happens to water when it flows down a steep slope? Well, it picks up speed, of course. The faster water moves over rock, the quicker it erodes that rock. Now, because the river's path was already established when it was moving a lot slower, it continued to carve down instead of straight ahead.

And what we're left with is something called an entrenched meander. That's when a river bend is established when water moves slowly, but then deepens when the water increases speed. However, because the Colorado River moves fast, horseshoe bend won't last forever.

Eventually, the river will probably cut right through the rock in the middle and bypass the bend altogether. So enjoy those views today, because in just a few million years, it'll look a lot different.