A Brief-ish History of Our Geology

Summary

Ancient landscapes built up rock layers of sand and mud. Rain and snow wore down the softer layers on the surface of the land. During the same time, the entire area was being pushed up from underneath. These competing directions of movement formed our canyons, which are still changing with every storm. Scroll down for more details.

 

Upper Ute Canyon at sunset

NPS Photo / N Scarborough

 

Rocks Before Life

Let’s start at the bottom of the canyons, which is where we find the oldest rock layer, and work our way upwards. A long time ago, magma (volcanic lava before it surfaces) cooled slowly inside the earth’s crust, forming hard rocks like granite (“igneous rocks”). Across the eons, movement of the crust pushed these rocks around. Intense heat and pressure inside the crust changed their chemical structure, which made these rocks even harder (“metamorphic rocks”). These changes gave the rocks the swirls and stripes that we can still see today.

Surface Changes

As these rocks made it to the surface, other landforms slowly washed away (“erosion”), and their debris (like dirt and mud) built up on top of this layer. Think about how creeks get muddy after a rainstorm because they're carrying away dirt from upstream. A lot more movement, land-building, and erosion happened, until this layer was basically wiped clean, leaving just the craggy ancestral rocks. Eventually, layers of sand and mud built up on top of this layer again (“deposition”) from other ancient landscapes nearby. Over millions of years of this happening, these layers would solidify and become layers of rock (“lithification”).

Dirt, Rocks, & Dirty Rocks

“Ground level” was always at the top – just as it is today. As long as there wasn’t too much erosion, sediments (like dirt, sand, and mud) could keep piling up. This added more weight and pressure onto underlying layers and made more rock layers (“sedimentary rocks”). Don’t forget that during all this, plants were growing in the dirt and mud, and prehistoric animals were living their best lives. Everything happening with the rocks happened beneath the roots and feet of generations of living things.

The Land Does A Push-Up

The land in this area was pushed up from beneath (“uplift”) over a long period of time. Movement of tectonic plates, the pieces of Earth's crust, caused the uplift. Yes, this also caused regular earthquakes. Rock layers of softer materials (like sand and mud) could often bend with the very slow movement. Harder rock layers (like ancient volcanic rocks that had recrystallized) ended up breaking. Nowadays, you can see the effects of this bending in the layers that plunge downwards along the eastern edge of the Monument. In many places along this same boundary, you can also see the edge of the broken layer, sometimes called a shelf or bench, in the harder rock layer.

Walls Shaped By Ice

Like today, weather never stopped shaping the earth’s surface. As the land was pushed up, water continued to run downhill in storms and carry away dirt, rocks, and other debris. And, in winters just like today, snow melted into cracks in the rocks during warm winter days. During cold nights, this water expanded as it froze, which pushed open the cracks and made them larger. With cycles every winter of thawing during the daytime and refreezing at nights (“freeze-thaw”), cracks got larger and larger, until even bigger chunks of rock could fall. So, the canyons started forming even as the land was getting uplifted. Although, at the time, the early “canyons” looked more like streambeds, arroyos, or washes.

Canyons Get Wider

Since the softer rock layers can also wash away faster than the harder rock layers, the canyons became wide along the direction of the flow of water. As the widening edges of next-door canyons met up, the walls between them broke down. In places that took longer to wear down along the eroding walls, fins of rock and stone towers (“monoliths”) stood until erosion took them away as well. There were probably more monoliths in these canyons before people came along. Ancient animals would have had a more complicated network of rocks to navigate.

Life...Finds a Way

Trees and shrubs, insects and mammals would still be interacting through all this. Biological soil crust would grow and provide a stable landing place for forgotten seeds. Lizards would hunt bugs, and then get eaten by snakes or larger carnivores. Species would come and go. The timescale of geology often – but not always – stretches beyond the lives of a single generation. Ancient peoples came through these canyons, the same canyons we walk today. Perhaps they felt a similar sense of awe and wonder at the canyons' immensity.

Constant Change

Most of these processes are still happening today, as you know. Rainstorms wash rocks and dirt downhill. This debris eventually flows into the Colorado River and builds up layers of sand and mud elsewhere. Winter storms cause the freeze-thaw cycle every year. As far as we can tell, the land is no longer being pushed upward. Nowadays, humans add to the erosion, but we can help make sure that natural processes stay in command of the canyons. Walk (without pets) only on the main trails. Leave rocks where you find them. Talk with park rangers about any important updates in the landscape.

Geology's Newsfeed

Some people like to think about geology as pages in a book, where some pages – rock layers – are missing. They say that if you learn to read the language, you can understand what the rocks in the layers mean. I think of geology more like a newsfeed, where the most recent news is almost always at the top. As you scroll down, you’ll find older posts and sometimes broken links. As you scroll the canyons, what will you like, comment on, and share?