Geology Bit: the formation of the Grand Canyon: erosion, Vishnu Basement Rocks & Zoroaster granite, the Grand Canyon Supergroup, and the Great Unconformity
The San Francisco Peaks disappearing in the rearview mirror, I traveled north to see the greatest single volume that has ever been written about Earth’s history which lay there like an open book inviting everybody to read as much as they could: the Grand Canyon. The land changed as miles and miles of landscape ran by and before I knew it, I was presenting my National Parks Pass and heading toward one of the most hyped-up places in the United States.
I knew the place was going to be busy; after all, thanks to my morning shenanigans on Elden Mountain, I didn’t have the time to go hiking to some beautiful secluded place in or near the Canyon. But I didn’t expect the South Rim (the North Rim was still closed at that time) to be as full of people as it was—and, pardon the language, stupid people at that! One would think it common sense not to climb over the railing at the edge of a several-hundred-feet-long drop but alas, there were a few who thought it a good idea while they flailed their phones around and dropped rubbish from their pockets.
I took a glance or two at the geological wonder, missing any of the eye-widening emotions my friends described they felt when they first laid their eyes on the canyon. Sure, it was quite impressive but I was supposed to be excited beyond telling; this is a geologist’s paradise! All I wanted was to get as far away as I could from there. So I hopped back in my car and drove to a place one volunteer recommended. And here’s a bit of wisdom: always ask, “what’s your favourite place?” instead of asking for the “best” places to visit. At the volunteer’s recommendation, I ended up at a place with way fewer people and way more seclusion than the main access points.
It wasn’t solitude; that, I had enjoyed in the morning when I climbed toward the sky above Flagstaff. But it was much better than my first stop, and that’s all I could have really asked. I felt I wasn’t being fair to this incredible and magical place because it was overrun with people; it wasn’t its fault, it was ours! So I found a spot to sit down, made myself comfortable on a rock, and let the breeze coming from below lift my spirits just like it lifted the bird of prey sailing on its streams.
The giant, gorgeous scar on the face of our beautiful Earth opened up inside me more than before me; to see and grasp its dimensions with my eyes proved futile but the wind that ran the span of it and visited every nook and cranny could tell all there was about the canyon.
I drove back through time as I descended toward Vermillion Cliffs National Monument where I’d eventually start climbing the Grand Staircase. I was getting too excited, however, there was one more stop near Page that I had to make: the Horseshoe Bend.
I spent the drive getting ready for the inevitable: throngs of people. It’s a popular place for a reason; it’s another incredibly interesting geological place. Oh, and I guess the social media people like it too, because it, indeed, is quite photogenic. Thanks to recent reconstructions, it’s also accessible to strollers and wheelchairs so everyone can enjoy this iconic view.
I didn’t plan it but ended up visiting the place at sunset. The people there, although many, were a bit different than at the canyon. Sure, many were still quite close to the edge but there was a moment, just as the sun touched the horizon, when everyone got incredibly silent. It felt as if we all collectively held our breath until the sun disappeared behind the horizon, creating a strange sense of co-belonging.
I stopped in Page after that, refilling on water and brushing my teeth. I planned to drive for about two more hours to a dispersed camping spot somewhere in Utah but ended up stopping just outside the town in a small gravel pull-out where a van and a truck were already stopped and ready for the night. The next day, I’d learn what a great decision that ended up being.
A Little Geology
By arriving at Grand Canyon, I’ve essentially crested a wave of geologic history that I’d surf down, mesmerized, before starting the climb up what’s called The Grand Staircase. But I’m getting a bit ahead of myself; let’s read the Sparknotes version of the great history book that’s the Grand Canyon first.
If you hike down into the canyon, you’ll be walking down the memory lane of our planet. At the very bottom, you can see the oldest rocks the canyon has revealed so far: the Vishnu Basement Rocks, aged some 1.7 billion years. They are mainly metamorphic (originally sedimentary or igneous rocks that were changed by pressure and/or temperature) with igneous intrusions (magma was pushed upwards through cracks in the rock layer or deposited on top). These intrusions are called Zoroaster granite.
As you go forward in time (and higher up the canyon walls), you’ll arrive at the middle rock set, called the Grand Canyon Supergroup1. The type of rock changes to sedimentary, with mudstone and sandstone being the main rocks. These layers don’t contain many fossils like their younger sedimentary causing because this layer was formed even before any more complex life evolved on Earth. At most, you can find stromatolites, columns of cyanobacteria sediments1.
If you observe the direction of the layers in this rock set, you can see that they’re tilted. Sedimentary layers always form horizontally; tilting occurs when they’re pushed up or dropped down through a few possible forces. In the image below, you can also see that the tilted layers abruptly end in what’s called angular unconformity; this is where a chapter in the history of the land is missing. Erosion took place, erasing full paragraphs before more words were written over the now-empty pages. This particular unconformity is called the Great Unconformity1.
USGS, public domain
Above the Great Unconformity, the geological record begins again. These rocks are much younger than those below them and contain many fossils. Combined with the fact that those layers are mostly sandstone, this fact tells us that those layers were formed in a warm, shallow sea brimming with life such as brachiopods, bryozoans, coral, and crinoids1. The youngest layers, at the very top of the canyon, are only about 300 million years old.
The one reason we have the canyon today is erosion. After the whole area was forced up during an uplifting event, water got the opportunity to flow and, eventually, start cutting deep into the land. The river, known today as Colorado River, patiently carved the canyon over the span of several million years, and will continue to do so. The processes that take place on Earth are on-going; it’s aways a work in progress, never done. We may think that the canyon lies unchanged, however, we exist at a tiny point where the geologic time and our lives intersect. The river might continue revealing older and older rocks and changing the canyon long after humanity ceases to exist.
Read more from this little road trip and learn about the geology around Flagstaff in the previous blog post!
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