Ever since Gooseberry, I’ve been noticing rocks more. This always happens when I get into something. I research the heck out of it for a day or two, and then whenever I go out, that thing- or that kind of thing- is practically the only thing I see.
City Creek Canyon has some weird rocks I’ve always half-noticed, but never really thought much about. They look almost like giant boulders, cliffs and outcrops made out of dirt, but with gazillions of small, rounded pebbles and rocks embedded. Sound familiar? Yeah. Sunday when I climbed through the little notch between the outcrops I think of as “Scylla & Charybdis”, the rocks I’d pedaled by a hundred times clicked “conglomerate.”
One of the interesting things about the Wasatch canyons feeding into Salt Lake Valley* is that they’re pretty varied geologically. While botanically they follow a pretty consistent pattern in line with aspect and elevation, the formations exposed in each canyon differ quite a bit.
*Specifically City Creek, Emigration, Parley, Mill Creek, Big Cottonwood and Little Cottonwood Canyons. Red Butte I’m leaving out as I still haven’t gotten around to trespassing yet, and I’m also omitting numerous minor draws, including Dry Creek, George’s Hollow, Neff’s and Bell’s.
Conglomerate, like the Shinarump we rode over down on Gooseberry and Little Creek, is composed of generally smooth pebbles and smaller rocks embedded in a matrix, usually sand. The matrix in a conglomerate isn’t always sand. It might be volcanic debris. Volcanic conglomerate is conglomerate in which the matrix is at least 50% volcanic material. Just to be clear- or maybe more confusing- the volcanic material in volcanic conglomerate is always water-deposited, as opposed to just flowed out of the ground as lava and hardened or whatever. The water action is what rounded the pebbles embedded in the conglomerate, and so when you spot conglomerate, you always know that it’s the result of some ancient water-related action, whether streams, lakes or seas. We’ll follow up on non-water-deposited volcanic debris mix-ins in just a bit…
Tangent: One of the big, unresolved mysteries in geology is why the Earth has so much water. This isn’t as dopy a question as it sounds. Things were way hot when the Earth formed out of the solar nebula, too hot for any pure water to hand around. Geologists generally agree that it came from 2 sources, but they can’t agree how much came from which source. The first is from rock. A family of granite-y rocks called amphiboles are composed of crystal lattices of silicon, calcium, magnesium and other elements held together by hydroxyl ions. (Hydroxyl = OH, one oxygen and one hydrogen atom.) Under sufficient heat and pressure, amphiboles break down, and when they do, they release their hydroxyl ions, which are then free to regroup into water molecules. About 60 miles below the Earth’s surface, things are hot and stressful enough to breakdown amphiboles.
But it doesn’t seem that amphibole break-down could have produced enough water to account for all that we have. The other source is thought to be comets. Many comets are partly or largely composed of water ice, and back in the early days of the solar system, it’s thought that there were a lot more comets (and other stuff) flying around in eccentric orbits, and crashing into planets.
The conglomerate rocks on the sides of City Creek canyon look like dirt, but they’re not. They’re real rocks, “rock-hard” to the touch. They form little bluffs and cliffs with overhangs and caves up above the trail. For years I’ve heard rumors of a Mountain Lion living near the mouth of City Creek, and when I climb Shoreline trail alone in the early morning, I find myself casting nervous glances toward the dark openings up above. These conglomerate caves would be the perfect lair, the stone an almost perfect color match for a tawny cat.
But the really interesting thing about this conglomerate is how new it is. It is, in geology-speak, Tertiary Conglomerate, tertiary referring to the geologic period running from 65 million to 1.8 million years ago. Specifically, the tertiary conglomerate in City Creek is thought to have formed just 35 – 40 million years ago.
This is relatively recent. 2 weeks ago, when we were down in Gooseberry, we were riding on and by rocks that were around 200 million years old, rocks laid down before T. Rex or flowering plants. But by the time the tertiary conglomerates of City Creek were laid down, the dinosaurs had been extinct for close to 30 million years. These rocks were formed during the latter part of the Eocene epoch, which is both one of the most fascinating times in the planet’s history, and yet largely unknown to the general public.
When most of us think of the ancient world, we have a broad outline of the history of life and climate on Earth. We know that life originated in the sea as little microscope-y-type things, and then eventually evolved into bigger things, and that some of these things eventually evolved into bigger things, like trees and dinosaurs, and there were big jungles and T. Rexes, and that’s pretty much how things were until 65 million years ago when the asteroid hit, and then mammals took over and the world was more or less like it is now until a group of monkeys started walking around on their hind legs, lighting fires and building strip malls.
What’s sad about this standard view of the history of the world is that it skips over so many exciting things that were at least as amazing and spectacular as the dinosaurs. Really ancient things like the Permian extinction and Snowball Earth fall into this category, but it also includes far more recent things, like the amazing stuff going on right here when the tertiary conglomerates were laid down. The Eocene, from about 56 million to 34 million years ago was one of the weirdest times ever.
We think of the times of the dinosaurs as being hot and jungly, and then the times after the dinosaurs being either kind of like today, or else cold and Ice Age-y, but the Eocene couldn’t have been more different. It was a “Hothouse Earth” period, with tropical and semi-tropical forests spanning huge parts of the globe and mean global temps far higher than today. In the early Eocene, palm trees grew in Alaska and swamp cypresses and redwoods grew on Ellesmere Island. There were no polar ice-caps- Antarctica was ice-free- and sea levels were hundreds of feet higher than today.
The warmer temperatures impacted animal life as well. We think of the age of reptiles as having ended with the dinosaurs, but maybe that’s not quite fair. Paleontologists a few years back unearthed the fossil remains of an Eocene snake in Colombia that measured 40 feet in length and weighed ~1.5 tons! The Eocene in North America featured a succession of fantastic faunas, including such other fearsome characters as 6 foot tall flightless predatory birds and giant carnivorous pigs. The Eocene was also the time when the ancestors of New World Monkeys and Porcupines likely rafted across the Atlantic. There were a series of widespread extinctions during the Eocene, probably related- at least in part- to swings in climate.
The hottest part of the Eocene was right at the beginning. Over just 20,000 years, mean global temps increased by 11F. Think about that. The difference in average annual temperature between Salt Lake City and St. George is just 9F. Though there were periodic “hyperthermal” events throughout the Eocene, overall there was a gradual cooling trend toward the end of the epoch, with continental interiors drying and forest cover thinning. About ½ way through, Australia broke away from Antarctica , completing the break-up of Gondwanaland, and setting up the circumpolar currents that have cooled Antarctica ever since.
Tangent: It’s worth noting that the causes of the warming are still unresolved. Methane emissions from under the ocean floor are one hypothesis, but no one knows. What we do know is that the Earth’s climate has changed way dramatically multiple times, long before there were people or factories or automobiles around, and that these climate-shifts were catastrophic for living things at the time.
Some “climate change skeptics” point to these ancient happenings as evidence that the Earth’s climate has always been variable, will continue to be so, and will do so regardless of the influence of mankind. But that’s obviously the wrong takeaway. The right takeaway is that relatively small changes in atmospheric chemistry appear to be capable of creating positive feedback loops which lead to dramatic and catastrophic climate changes in remarkably short periods of times, and that maybe- just maybe- we ought to slow down and seek to understand these mechanisms a bit better before we continue to fill the atmosphere with more CO2 than at any time during the last 20 million years.
Nested Tangent: I wonder, way back when people were starting to live in permanent settlements, and had to figure out basic sanitation engineering, were there Sanitation Skeptics? Imagine…
PALEO-ENVIRONMENTALIST (PE): Hey, there are a lot of us living in the same place now. Maybe we should start to dig separate latrines, instead of just pooping in the well.
CAVEMAN-SANITATION-SKEPTIC (CSS): There’s no evidence that pooping in the well is causing any harm.
PE: Yes, but there’s more poop in our drinking water than ever before. And some people are starting to get sick. Maybe we should try pooping somewhere else while we figure out if more poop in the well is going to cause problems.
CSS: Show me the evidence! People have always occasionally gotten sick from drinking water. There’s no evidence that drinking-water illnesses are poop- I mean human- caused.
PE: Yes, but we know that ingesting poop makes people sick, and we know there’s more poop in our water than ever before. How about we just be a little conservative and try pooping a little less in the well.
CSS: But think of the economic impact! Effort spent digging latrines and walking farther to poop will impede our critical economic development of cutting down forests and slaughtering Pleistocene megafauna!
PE: Well, maybe not eating poop is worth expending just a little more effort and having a little less stuff…
CSS: Socialist! The last thing we need is more government regulation…
Side Note: This cooling/drying didn’t result in grasslands though. Modern-day grasslands and savannahs, which today cover something like a third of dry land, didn’t really exist much before about 15 million years ago. Though grasses have been around for something like 70 million years, the vast majority of that time their range was more localized, near streams and other wet areas. Then fairly recently, they expanded into all sorts of drier areas and essentially took over much of the world. It’s kind of weird when you think about it. It’s as if someday horsetails or ferns suddenly became capable of colonizing huge, dry, sunny expanses across the continental interiors and started spreading like crazy…
The end of the Eocene (and beginning of the Oligocene), not long after the City Creek conglomerates were laid down, was marked by a mass extinction called the Grande Coupure*, in which existing European fauna was replaced by Asian fauna. Like all mass extinctions, there are multiple hypotheses as to its causes. One explanation is that the circumpolar-induced-chilling-icecap-formation-drop-in-sea-levels deal was the culprit. Another, more dramatic, possibility is one or more large meteor impacts. Likely suspects include ~34 million year-old impact craters in the Chesapeake Bay and just off the New Jersey coast by Atlantic City.
*Which by the way, would be an excellent name for a luxury automobile.
Here’s the video again. Like before, I suggest you open it in a separate window if you want to check out the times/locations I describe below. (And again, you can switch it to HD…)
In the video, I pass right by tertiary conglomerate at 2:36, and again at 3:00. At 3:09 you can see high cliffs of it ahead and up above on the left. In more recent times, the exposed conglomerate has weathered and eroded down-canyon, and we’ll encounter it further below.
According to the geologic maps, in stretches along this long descent, the slope is overlaid with wind-blown alluvium- gravel, silt, sand and clay originally deposited in stream, floodplains, or more often here in Salt Lake Valley- shoreline terraces. My geo-eye isn’t yet good enough to eyeball such areas, but I suspect the trail crosses such alluvium patches in at least 2 spots. The first is the first ~30 seconds of video, coming off the saddle. The soil here has a high clay content* and there seem to be few larger pebbles/rocks mixed in. The second is at the Blue Flax switchback, at 3:42 (pic right). The soil here appears to be mainly silt, and if you look at the earth-cut on the uphill (descender’s right) side of the switchback, you’ll notice the soil is completely rock and pebble-free.
*This stretch was in fact the problem area highlighted in the Clay post.
As we drop into the Green Tunnel section, the geology doesn’t really change; though it’s hard to see in the video, there are still small (now partly moss-covered) conglomerate outcrops off the trail in the woods.
But when we exit the Tunnel and turn sharply right to parallel the canyon bottom at 5:16, the geology changes. According to the geologic maps, this section of trail crosses a formation type known as Volcanic Breccia. Remember a moment ago I described conglomerate as always having a matrix-base that was water-deposited? Well, when lava or magma flows and then stops and hardens around smaller pebbles and rocks- in other words not water-deposited- it’s called volcanic breccia. Breccia is analogous to conglomerate, but the embedded rocks/pebbles are sharp-edged and angular, not rounded, as there was no water-action in its formation. Unfortunately, this stretch has few exposed rock outcrops, but you’ll notice small ones at 6:38, 6:48 and 6:55.
On closer examination, these rocks are definitely different from the conglomerate higher up; the matrix is gray, suggesting volcanic debris, and the embedded components are more angular, suggesting breccia vs. conglomerate, but not so much so that I can be certain…
Side Note: Back down at the trailhead however, a number of large boulders have been placed by the side of the lot and adjacent road. The boulders are clearly 1 one of 2 types. The first are tertiary conglomerate, same as up above. But the second (pic right) look like a real good match for the photos I’ve seen of volcanic breccia. The conglomerate boulders are surely local in origin, and it seems likely that the breccia boulders are as well.
BTW, the green pipe we roll over just after at 5:26 is a spring. Winter or summer, that spring is always running. It’s interesting to think how these different geologic layers route the subterranean flows that feed springs like this one*.
*I haven’t done a proper post on the hydrology of springs, but touched upon it back in the hot pots post.
Rolling on down, somewhere around 7:30*, we actually transition back to conglomerate-based soils, now washed/eroded down from the slopes higher above. Finally, right around 8:55, we’re on the verge of crossing another transition point, this time onto the soils deposited by ancient Lake Bonneville. Later this week I rolled down the paved road a couple hundred yards toward the state capitol, and the a road-cut soon exposed the Lake Bonneville sediments.
*Not clear. Guess based on geologic map and hints of shape/color of trailside stones
Tangent: Right before the bottom, at 8:32, you’ll see the camera swing twice, quickly to the right. Though the camera-view didn’t catch it, I was glancing at the semi-permanent tent camp in the clearing about 20 feet off to the right. Something like a ½ dozen+ homeless people live here, almost directly below the huge monster trophy-homes we rode past back 2:00 – 2:30. Kind of weird, eh? You’ll often come across them walking down/out the trail around 8:00 or so in the morning, or coming home around 6:00 in the evening, as they “commute” to and from whatever they do downtown during the day.
I like plants in part because of the stories they tell. As I’ve started to pay attention to rocks, I’m seeing that they tell stories too. Geology improves your view of the real world in the same way that color improves your vision; it doesn’t make your vision any sharper or clearer, but it adds another axis, another dimension, that makes the view your vision brings you more complete. Rocks are way cool.
Know what’s even cooler? Dirt- where rock and life come together. But that’s a whole other post.
Extra Info: City Creek is by no means the only, best, or even easiest place to view tertiary conglomerate around Salt Lake Valley. The Northeast corner of the valley is practically rotten with it. Specifically, you pass a good outcrop or 2 in road-cuts in upper Emigration Canyon, and on the climb up to Big Mountain Pass switchbacks #2 and #4 (out of 5) are road-cut out of it. Finally, if you ever bike the “Brink” loop, up behind/North of Affleck Park and Big Mountain, you’ll pass by, and actually ride over, lots of it around the North end of the loop.
My recent passion for geology has been largely inspired by the best geology book I’ve ever read: Reading the Rocks, by Marcia Bjornerud, a geology professor at Lawrence University. The first third is slow, but stick with it, because the rest is fantastic. It also has a linked prologue/epilogue, which while a bit corny, totally works and really moved me. Bjornerud’s book was the source for the water-tangent in this post.