Darcy: Welcome to Canyon Conversations, a podcast from Zion National Park. In these podcasts, we hope to dig into some of the different aspects of Zion National Park to help you, the listener, get a better appreciation for the resources we protect. I’m ranger Darcy, and I will be hosting today’s podcast with Ranger Ben.
Ben: That’s me!
D: So, if you’ve ever been to Zion, or seen a picture of Zion, or heard of Zion, you know it’s the rocks that draw us to this place. Majestic cliffs soaring two thousand feet above the canyon floor colors that range from rich red to delicate white with orange and purple streaks, dotted with verdant green hanging gardens…sometimes you almost feel like you’ve traveled to another planet. Below the striking cliffs are more humble rock layers. Those layers slope gently to the canyon floor, providing homes for all kinds of plant and animal life. But the thing is, those rocks are more than just beautiful—they also tell stories. And that’s why we have Ranger Ben with us today. He is a geologist, and he knows how to tell the stories of the rocks.
B: This sort of story-telling geology we call historical geology.
D: Yes, so why don’t you tell us about that and what you mean by “historical” geology.
B: Well, the word geology simply means the study of the earth. Historical geology uses clues to craft a story of the geologic past.
D: Geologic past – is that different from normal history?
B: Hmm…sort of. Geologic time is so enormous that I’d say the average person doesn’t really stop to consider it. We exist in minutes, hours, days, years. Geologists may talk about the passing of millions of years in the same way that others talk about things they did last weekend. Think of a history book—each rock layer could be a chapter describing tens of millions of years!
D: In the grand scheme of things, humans are incredibly short-lived creatures. We have a very hard time conceptualizing geologic time scales. To help with that, geologists have made a sort of geologic calendar where they organize time into eras and periods. Those time periods help us organize the stories in the rock.
B: Many of us have probably heard some of the terms used in describing geologic time. If you’ve ever seen or heard about the Jurassic Park films, then you’re at least a little familiar with historical geology. The Jurassic Period was a time when dinosaurs and other reptiles dominated the food chain. It also happens to be the time during which many of material for the rocks in Zion were first laid down.
D: What do you mean?
B: The Jurassic Period is a specific time from the geologic calendar, from around 200 million to 150 million years ago. We know that there were dinosaurs at that time because of the fossils – of bones and footprints, for example – that are in the Jurassic rocks. In fact, some dino fossils can be found right here in Zion!
D: Fascinating! Are all of the rocks from the Jurassic Period?
B: Not all of them. For many of the rocks that people see in Zion Canyon, yes, but there are other areas of the park as well. The Kolob Canyons area, to the Northwest off of Interstate 15, preserves the most complete rock sequence visible in the park. At the bottom you can see the Kaibab formation. The Kaibab formed during the Permian Period, around 260 million years ago. The Carmel Formation caps Kolob Canyons. The Carmel formed around 160 million years ago during the Middle Jurassic. What’s more, the rocks of Zion hold just a small slice of the overall story of the greater Colorado Plateau.
D: What do you mean?
B: The Colorado Plateau spans across much of Utah, Colorado, Arizona and New Mexico. Many of the rocks in Zion may be found in other locations on the Colorado Plateau, sandwiched in between either older or younger layers of rock.
D: Could you give us some examples of Zion rock layers in other places?
B: Sure! My favorite example is Kaibab Limestone, Zion’s oldest rock unit and the youngest rock at the Grand Canyon. The Chinle Formation, particularly the colorful Petrified Forest member, is the main rock unit seen in Petrified Forest National Park. The Navajo Sandstone can be seen from—
D: Whoa, whoa, whoa. Sounds like there are quite a few different rocks out here. Maybe we should start with the simple things. What kinds of rock do we have in Zion?
B: Sure – a quick geo-refresher. There are three main types of rocks: Sedimentary, metamorphic, and igneous. The vast majority of the rocks in this park, as well as many other places on the Colorado Plateau, are sedimentary. These are rocks that have formed from an accumulation of sediment – rock and mineral material that was formed one way, broken down into tiny pieces, and then deposited to form a different rock. Often, we can trace the source of sedimentary rock – that is, determine where the sediment came from. Sedimentary rocks are by far the most common on the Colorado Plateau. Another type of rock that we have in our park is igneous rock. These are rocks that have formed either from volcanic activity at the Earth’s surface or from the cooling of magma material in the Earth’s crust. The third general type of rock is metamorphic; these are rocks that have been changed from intense heat and pressure. There aren’t many metamorphic rocks visible on the Colorado Plateau.
D: Lots of sedimentary rock, then. So, you said that the oldest rock in Zion is the Kaibab Limestone and the youngest was-?
B: The Carmel Formation.
D: Right. If there are more rocks on the Colorado Plateau, then where does Zion’s story fit in?
B: Great question! The Colorado Plateau is a geologic wonderland of mesas and plateaus of these sedimentary rocks. The oldest rocks – some over a billion years old! – are exposed at the very bottom of the Grand Canyon. The youngest sedimentary rocks are perched at the highest elevations, in and around areas just north and east of Zion, such as at Cedar Breaks National Monument and Bryce Canyon National Park. The rocks found in Zion basically represent the middle chapters in the Colorado Plateau’s story.
D: What can you tell us about the rocks we see in the park?
B: Well, as with all good stories, let’s start at the beginning, which, for the rocks of Zion, means starting from the bottom layers and working our way to the top. The oldest rock formation, as we’ve said, is the Kaibab limestone. It’s around 260 million years old. The Kaibab limestone was formed during the time that Pangea, the supercontinent, was coming together.
D: This area must have quite different than it does today.
SFX: Ocean waves
B: Yeah! You see, the area that would eventually become Zion not nearly as far inland as it is today—in fact it was still on the coastal margin then, and much closer to the equator. The general environment at that time, long ago – the paleo-environment – would have been similar to what we might find around the Caribbean Islands of today.
D: Are you saying this area was once under water?
B: Somewhat, yes. Limestones are generally produced in shallow marine environment. These areas are often teeming with ocean lifeforms, like modern-day clams and corals. The Kaibab limestone formed in a place that may have looked similar to a modern day coral reef.
D: Like the Great Barrier Reef in Australia?
B: Yes. If you had a time machine to go visit the west coast of the North American continent, then you’d want to bring your SCUBA gear! The seas near to the shore would be crawling with sea life. Upon close examination, though, many of the critters seen in those reefs would have been different from what we see today. Instead of the clams and mussels that we often find in modern oceans, there would have been mollusks – shelled animals – called brachiopods. Corals hadn’t really made a big appearance on the ocean landscape, yet, but there was an abundance of similar-looking filter-feeders called bryozoans. Other mollusks, called ammonites, would have been around, and their shells would have looked similar to the present-day Chambered Nautilus. Of course, there were some fish present in these oceans, as well. There were likely even trilobites wandering around the sea floors! Anyway, as these critters die and leave their shells behind, that material would eventually become limestone. Some of these shells are even preserved as fossils!
D: Wait, so what happened to all these other animals, like the ammonites and the trilobites? Why are they different from what we see in oceans today?
B: Well, toward the end of the Permian Period, shortly after the Kaibab formation was deposited, there was a pretty big mass extinction.
D: A mass extinction? You mean like what happened to the dinosaurs when that meteor crashed into the Earth?
B: Yes, but worse. The event associated with that meteor resulted in the extinction of about 75% of life on the planet. By contrast, the extinction that took place between the Permian and Triassic periods resulted in the loss of about 90% of life from the rock record. Not all of the species fossilized in the Kaibab limestone died off, but a lot of them did.
D: But there are no books from that time period, no first-hand accounts or easily read history. How can you tell that such a terrible event occurred?
B: We can say this because fossils of various lifeforms from before that time disappear from the rock record; we can find no record of those fossils in rocks younger than the Permian-Triassic boundary.
D: That’s terrible! What could have happened?!
B: It’s hard to say for certain, but there are a few ideas. One is that there was a lot of volcanic activity at the time. The toxic fumes from continuous eruptions polluted the atmosphere and basically made it almost impossible to breathe. Another idea is that there was a catastrophic meteor impact, worse than what would eventually kill the dinosaurs. The problem with verifying that story is that it’s most likely that a meteor would have struck the ocean during an impact, as water covers about three quarters of our planet. The Earth tends to recycle ocean rocks, and there aren’t too many places where we can find Permian-aged ocean crust.
D: Neat. So, Zion opens with this amazing, life-filled ocean that ends in tragedy and mass extinction. We’re off to an exciting start. What happens next? What do we have on top of the Kaibab rocks?
B: Zion’s next chapter is uncovered in Moenkopi Formation, often referred to as the Chocolate Cliffs because of its deep red and brown coloration. The Moenkopi Formation formed around 240 million years ago during a time when the oceans were receding westward. This area was essentially a massive tidal flat with some rivers flowing through from the southeast. Imagine a broad flat, soggy, muddy area many, many miles across that gets periodically flooded and drained as tides rise and recede. Modern-day examples may be found on the west coast of South Korea or the Wadden Sea along the coasts of the Netherlands, Germany, and Denmark.
D: What about the rock? Is it anything like the Kaibab limestone?
B: The Moenkopi rocks are very muddy from the fine mud and silt of the tidal flats. You can find fossils in these rocks, although they’re a little different from the Kaibab fossils. There may be a few trace fossils – from critter footprints or worm burrows – but what we see a lot of are preserved fossilized ripple marks.
D: Ripples? How can ripples be fossilized?
B: Think of a tidal zone of a beach. The tide will rise a few feet and then recede again in a cycle. As the water washes over the beach it will leave ripples on the beach. Now if there’s a river that flows across this beach, and if there’s a bit of a flood so that the river turns brown from the mud getting mixed into the water, some of that mud can cover the ripples that were there before. That’s how all fossils are made – something has to be covered to have any chance of being turned to stone.
D: Okay. Now, you mentioned critters – what sorts of things would have been wandering around on land at that time?
B: During that time in earth’s history, much of life on land was restricted to areas around water. There were plenty of insects and other arthropods on land, as well as some amphibians. Reptiles, some of which could venture farther away from water because they had evolved hard-shelled eggs, were really starting to make a strong appearance on the scene.
D: And where can we find the Moenkopi formation in Zion?
B: Within the park, only up around the road cuts in Kolob Canyons. The crumbling slopes of the Moenkopi can be seen from State route 9 around the towns of Rockville and Virgin to the west of the park, as well.
D: Okay, so Kaibab then Moenkopi. What comes next?
B: Next up is the Chinle formation, which is around 215 million years old. This rock unit is best known for the colorful cliffs of the Petrified Forest Member, which, as you might have guessed, contains quite a bit of petrified wood. And the Chinle formation isn’t limited to Zion. It dominates the visible landscape at Petrified Forest National Park in AZ. Around the Grand Canyon to the south of us, the Chinle formation is referred to as the Painted Desert because it’s so vibrantly colorful.
D: What makes this rock so colorful? And why is there so much petrified wood in this particular rock?
B: Iron is responsible for many of the colors in the rock out here. Iron can produce many colors – not just the rusty/orange-red we’re familiar with! Different iron minerals, some of which form only in swampy conditions, can produce colors other than the reddish hues we’re used to. There was also volcanic activity at the time, which contributed other colorful minerals, as well.
D: There were volcanoes?
B: Yes! Similar to the western coast of Canada and Alaska today, there were tall mountains with volcanoes to the south of where we are today. These volcanoes erupted from time to time, adding more material to what would become the Chinle Formation. Material from the volcanic ash that covered larger log jams in the streams moved through the soils and into spaces and weaknesses in the decomposing organic matter, eventually producing petrified wood!
D: Fascinating! Is that how other fossils are made, too? Like bones?
B: Generally, yes. Again, the thing that is to be fossilized has to be buried. In the case of the petrified wood, the conditions were right to get lots of dead and downed woody material clumped up into big groups – probably from periodic floods during those times. Bones aren’t always so easily buried naturally, but it’s the same sort of process as petrifying wood – as the organic material breaks down while it’s buried, harder, more stable rock minerals fill in the gaps over time, leaving us with a rock that looks remarkably similar to the original thing.
D: Neat! So, what about life during that time. Had life changed much from the time of the Moenkopi?
B: The story of life during the time of the Chinle formation is one of diversification. Just as the Kaibab represents a time of diverse ocean life, the Chinle reveals a time of diversifying life on land. While modern-day flowers weren’t yet around, ferns, horsetails, and tall plants similar to modern palm trees were in their heyday. The climate was semi-arid, likely with a significant monsoon – similar, in a way, to what we have in Zion today. Imagine a place that goes much of the year without rain, but for a few months there would have been load of rain falling in the higher mountains to the south. Water would flow through this area, and as the rains let up, swamps and marshes would form. Animal life included an abundance of giant reptiles, including early dinosaurs, amphibians, fish, and many invertebrates.
D: Interesting that the climate of today could be comparable to the climate of around 200 million years or so in the past!
B: Indeed. And all that moisture of those times wouldn’t stick around forever. The majority of the rocks seen in Zion Canyon tell a story of much drier times.
D: So what’s the next chapter? What other rock layers can we see in Zion Canyon?
B: Above the Chinle are the Moenave and Kayenta Formations. We’ll talk about them together here because they both formed in similar environments, but the larger setting on the Colorado Plateau was a little different enough between the two. In the late Triassic Period, around 200 million years ago, toward the end of the Chinle times, things were really starting to dry up.
D: How do we know? What’s in the rock that tells us this?
B: Although both the Moenave and Kayenta Formations have some stream and river deposits (think silt and clay, really fine sediments), there is much more wind-blown sand in these rocks. Wind-blown sand – like with sand dunes – indicates that there’s not much moisture in the soils, otherwise these sand grains would be weighted down on the surface. What we had during these times was an environment roughly similar to what we see in the lower Nile River valley in Egypt, or the Tigris and Euphrates valleys in the Middle East – arid, desert environments that still had persistent rivers flowing through.
D: If the area was so dry, were there any plants or animals that lived in this region? Or was it pretty barren?
B: The plants that were around would have been similar to what was found during the time of the Chinle Formation, though perhaps not as abundant, but the really interesting stuff was in the Animal Kingdom. Around these times, we start seeing evidence of more, larger dinosaurs in the area. There are a few places in the park where you can find tracks from species of Coelophysus and Dilophosaurus, which were true dinosaurs. These rocks were deposited around the start of the Jurassic Period, so we literally had a Jurassic Park where Zion National Park is today!
D: Fossils in the park! Are they easy to get to?
B: Generally no. There aren’t any on the major trails.
D: Oh well. So tell me this - if the Moenave and Kayenta rocks are so similar, why are they not grouped together?
B: They’re similar here at Zion, but not across the whole Colorado Plateau. Whereas the Kayenta Formation can be found generally throughout the Colorado Plateau region, the Moenave is much less extensive. In fact, if you were to visit Capitol Reef, Canyonlands, or Arches National Parks, you wouldn’t find any Moenave rocks at all!
D: Why is that?
B: While rivers were present in the southwest portion of the area, a large erg – or sand dune desert – dominated the landscape to the east. So while the Moenave formation was being deposited here, that erg would eventually form the Wingate sandstone found in those parks to the east.
D: Interesting! Where in the park can we look to see these rocks?
B: In Zion Canyon near the Visitor Center, Human History Museum, as you drive up the Zion-Mt Carmel Highway toward the tunnel, and along the Emerald Pools trails, for starters. These rocks are the vegetated slopes leading up to the base of the massive cliffs of Zion Canyon. The separation between the Moenave and Kayenta is marked by a thin red sandstone cliff – the Springdale Sandstone.
D: The big cliffs – you must mean the Navajo Sandstone!
B: Yes, the Navajo Sandstone. This Jurassic rock unit, about 180 million years old, is at its thickest here in Zion. It’s present across the Colorado Plateau, as well as some other regions even further away, but it is most impressive here. Many of its cliffs are over 2000 feet tall in Zion Canyon, but in other areas it may only be as thick as a mere few hundred feet.
D: So what was this area like as the Navajo Sandstone formed?
B: The material for the Navajo Sandstone was deposited in one of the largest erg deserts ever to have existed on the planet. These sand dunes would have covered much of modern day Utah, and of Colorado, Wyoming, Idaho, Nevada, Arizona, and New Mexico—a total area of around 850,000 square miles, by some estimations. Dune fields cover around 495,000 to 660,000 square miles of the modern-day Sahara Desert. We should be careful with this comparison, though, because although the area covered by the sand dunes was larger in producing the Navajo Sandstone, the Sahara Desert covers an area larger than the lower 48 United States.
D: That’s enormous! So I noticed that as you drive through the East side of the park, the sandstone there often has these diagonal lines through them, almost like hash marks. What’s the story there—are those the remnants of the old sand dunes?
B: Sort of. Those diagonal lines are what we call cross-bedding. Cross-beds essentially record the movement of ancient sand dunes. And from these cross-beds, we can learn a lot about the area. For example, we can interpret from which direction the prevailing winds were blowing! And we can extrapolate the height of the cross-beds to figure out how tall the sand dunes were!
D: So, how tall WERE the sand dunes?
B: Most likely up to a few hundred feet tall!
D: That’s incredible! I bet it would have been really fun to slide down those dunes. On a different note, why does the color of the cliffs seem so different from top to bottom? Why do the cliff bottoms tend to be really, really red, almost vermillion, while the tops of the cliffs are almost white?
B: As for the color difference, we’ve already pointed out iron as a source of color for other rocks, and they provide the red color at the base of the cliff. The upper portion of the Navajo Sandstone, however, lacks that iron which is why they’re relatively white.
D: Interesting – Whatever the cause, the colors on the Navajo Sandstone are really quite breathtaking. There can’t be that many more, though, right? Are there more rock layers above the Navajo Sandstone?
B: Just a few more! On top of the Navajo Sandstone is the Temple Cap Formation. This particular rock unit is only really found around Zion National Park.
D: There are quite a few features in Zion that are named “Temple” – Is that a coincidence?
B: Not at all! The Temple Cap formation may be found on top of the West Temple, behind the Human History Museum, as well as on the East Temple, which looms over the Zion-Mt. Carmel hwy. This rock unit creates the colorful reddish to yellowish blocks found at the summits of these iconic features of Zion.
D: And what sorts of stories can we learn from this rock formation?
B: Well, unlike the other rocks we’ve discussed, the Temple Cap is only found in the immediate area of Zion National Park. Because its exposure is so limited, it’s somewhat more difficult to interpret. Based on the type of rock, though, which is a mix of sand, mud, and limestone, we can guess that the area was getting more moisture introduced – from somewhere.
D: But it’s hard to say because you can’t see the evidence over a large area?
B: You’ve got it!
D: Is there anything on top of the Temple Cap?
B: One more. The youngest rock seen throughout the area of Zion is the Carmel Formation. It’s around 160 million years old, and in the area of the park, the Carmel formation is a fossiliferous limestone.
D: Oooh – so it formed in a shallow marine environment, like with the Kaibab Formation?
B: It certainly did! However, the area wasn’t exactly an ocean. Instead, there had been an inland sea that had intruded onto the continent from the northwest, and the area of Zion was around the southernmost extent of that sea, so we got limestone from those times. Other areas east of that sea, though, were still quite dry and sandy, so while we were getting this limestone deposition, places that are now Canyonlands and Arches National Parks were getting their Entrada Sandstone.
D: So that’s it? End of story?
B: Of course not! I mean, that’s a quick summary of the history told by the sedimentary rocks in Zion, but there was a lot of time recorded in other rocks across the Colorado Plateau, both older and younger. The rocks of Zion are just one snapshot within the far larger geologic context of the Colorado Plateau. To understand more about those rocks in those other places, we’d have to do many more episodes! It might be better to just get out there and explore all the parks and monuments of the Desert Southwest! But at least now, hopefully, we’ve helped describe some of what folks will see when they visit Zion National Park.
D: Well there you have it! From the Kaibab to the Carmel, we’ve gone through the various sedimentary rocks you’re bound to see and contemplate here in Zion National Park. The beautiful slickrock areas on the East Side of the park, the sheer cliffs of Zion Canyon, and the colorful slopes found throughout this region are lovely in their own right. But, we’ve now taken a closer look, describing a fascinating range of environments that have produced these rocks- from oceans, to mudflats and swamps, river basins and sandy deserts, to inland seas. We’ve also noted how life has changed through time, and what sorts of fossil evidence we can look for in the rocks. When you visit Zion and observe its undeniable beauty, try to think about times long ago, when dinosaurs roamed the earth, or topical waves lapped against sandy beaches and lifeforms alien to what we know of today. Imagine, and you might feel an even deeper connection to this special place.
B: We thank you for listening to this episode of Canyon Conversations. My name is Ben Gibson.
D: And I’m Darcy McKinley Lester. For more information about Zion National Park, you can check our website at www.nps.gov/zion. If you have any questions about this podcast or about the park in general, feel free to send an email to email@example.com.