Geological Time

Note: This transcript was generated by Otter.ai with light human correction

MILLS KELLY: Hello and welcome to Season Three of The Green Tunnel, a podcast on the history of the Appalachian Trail. My name is Mills Kelly, and I’m your host. This season the history we’re starting with is, well, really, really old. As in so old it’s almost impossible for me to wrap my mind around it. If you’ve listened to early episodes of our show, you know that the men and women who created the Appalachian Trail 100 years ago routed the trail so that it was challenging, but not too challenging. They also wanted it to offer hikers beautiful views as a sort of reward for all that uphill walking. What you probably don’t know is that the old trail guides often extolled the geological landscape as well as the beautiful views. Hikers were informed that various trail sections included some of the “finest geological sections exposed in the entire East” or in the step-by-step directions, the guides often mentioned specific formations to be aware of. To the average hiker – me included – all those rocks look like, well, rocks. But not to a geologist. To a geologist, the rocks that are so much a part of the Appalachian Trail experience tell much more than the history of continental drift and the uplift of massive mountains. They tell the entire story of the mountains from their formation hundreds of millions of years ago to today. And it’s a history a geologist can read for us. Here’s Jamie Levine, associate professor of geology at Appalachian State University in Boone, North Carolina.

JAMIE LEVINE: So these mountains have a very long history. They are more than a billion years old. And they've basically formed through a series of orogenies. Or mountain-building events. And the first one that we talked about was about 1 billion to 1.2 billion years ago. And that was the Grenville orogeny. 

KELLY: That’s billion. With a B.

LEVINE: And after the Grenville Orogeny, there was a period of not a lot going on. And there was a period where the North American continent tried to break apart but was unsuccessful. And we call this a failed rifting event. And then, about 250 million years later, or 550 million years ago, we had successful rifting, so North America broke apart. And then we had sort of the same thing happening over and over again. So we had three more orogenies, sort of in quick geologic succession, from about 450 to 250 million years ago. And sometimes we had things like islands colliding with North America, and then it ended with Africa colliding with North America 250 million years ago.

KELLY: I hope you’re noticing that for a geologist “quick” means something entirely different from what it means for the rest of us. Unless maybe you’re an astrophysicist. To Jamie, “quick” means 200 million years. The one hundred years that the Appalachian Trail has been around? That’s not even an eye blink to a geologist.

KELLY: And here’s another thing about mountains. They move around. A lot. And sometimes they move a long way. Sometimes that movement occurs because of a collision between two land masses. Sometimes it happens because of rifts appearing in the earth’s crust. And sometimes it happens because something beneath the crust moves around causing the mountains on the surface to move to get out of the way. I asked Jamie if it was possible that the Southern Appalachians might have been west of where they are today, but it seems that instead, they moved east for a while.

LEVINE: Yeah, they were moving the other direction. They could have been further, like in Raleigh or something, and then they've moved over westward.

KELLY: As Jamie told us just a minute ago, the Appalachian Mountains we hike in today formed when Africa slammed into North America somewhat recently – meaning about 250 million years ago. And the mountains formed by that collision were tall. Very, very tall.

LEVINE: I think that they were very high. And I don't know that anybody has a really good constraint on how high is really high. They were a substantial mountain range similar to the Himalayas or the Andes today, but it's pretty difficult to constrain actually their exact height.

KELLY: So if you think hiking on the AT today can be a challenge, just imagine if the mountains we love were topping out above 20,000 feet. Just like the Southern Appalachians, the Green Mountains of Vermont are also the product of one of those events where one land mass crashes into another. And they were just as forbiddingly tall. 

KEITH KLEPEIS: So at its root, the Green Mountains are the product of a collision between two masses, two big giant, geological masses. And if you want to really understand the geology of Vermont, you’ve got to know about this feature called Iapetus. 

KELLY: That’s Keith Klepeis, professor of geology at the University of Vermont.

KLEPEIS: Iapetus was this ancient ocean that no longer exists. But you can see its remnants, and especially the eastern side of the Green Mountains. And about 460 million years ago, Vermont lay on the shore of that giant ocean, and off to our east an island arc formed. So an island arc is just a volcanic province that forms on oceanic crust, it's like the Aleutians or the Mariana Islands today, and that island arc collided with what is now North America. And that collision created these giant fault slices, that uplifted crust and formed part of the Green Mountains. And then the most amazing thing is, that whole cycle happened again. And what is now southern Maine and parts of New Hampshire, this big giant continental fragments slammed into the margin again, and uplifted the mountains even further. And at that time, the Green Mountains were probably about the size of the Himalayas, you know, just these giant things separating what is now Vermont from the rest in New England. And then because it was, you know, 370 plus million years ago, we had time to erode those down to sort of a reasonable size.

KELLY: Like Jamie, Keith thinks of mountains and their life cycle on a vast time scale.

KLEPEIS: In geology terms, mountain ranges are pretty rapid. They come and they go. So the reference frame for geology is the entire 4.5 billion year history of the Earth, right. So that's the time scale that's beginning, up until now, and so 50 million years, in the context of 4.5 billion is quick.

KELLY: The Grayson Highlands are located in Southern Virginia and are the highest mountains in the state. If you’ve heard of them, it’s probably because of the wild ponies. But when Jamie Levine thinks about the area, it is because of volcanoes.

LEVINE: So the Grayson Highlands area, along with Mount Rogers and White Top mountain are also volcanic centers. So These are areas that are thought to be volcanoes that erupted during actually this failed rifting of North America. So this was about 750 million years ago. So quite a long time ago.

KELLY: And if you stop looking at the ponies for a minute and look at the rocks instead, you can see some pretty amazing things. 

LEVINE: In the Grayson Highlands and Whitetop Mountain, and Mount Rogers, you can see a bunch of these volcanic rocks. And there are some rocks just pretty close to the trail and all throughout the area where you can see these tuffs. And these are rocks that were erupted from the volcanoes, you get pumice and glass and crystals. And then they flowed. And so they've all been welded together. So we call it a welded tuff. And they're about 750 million years old. And it's not something that you can see, anywhere else in the east coast. So those are pretty interesting.

KELLY: We have a photo of some of that welded tuff from Mount Rogers in the show notes.

KELLY: While volcanoes played a role in creating some of the really interesting mountains in the Southern Appalachians, in the New England states it was glaciers that made the mountains into what they are today. And while the volcanoes in Southern Virginia erupted 750 million years ago – give or take a few years – the glaciers shaping the Green and White Mountains happened really, really recently – in geologic terms that is.

KLEPEIS: Glaciers are key in sculpting the landscape. A lot of people make a mistake when they're learning about the geology of Vermont. Where we think that the glaciers created the mountains, right, and that isn't true. The mountains were created by these collisions, these tectonic motions. And there's several of them. But the glaciers were not trivial, right. So they covered at their peak about 14,000 to about 13,500 years ago. They were about a mile high. And so if you put that in perspective, one of our highest peaks Camel’s Hump today, you can hike up there, that was under 1000 feet of ice. So a major event and those glaciers retreated and advanced and scraped, and sculpted most of the mountains. So if you kind of look at the mountains and profile some of them have this teardrop shape, Camel's Hump is a great example. It's got a deep southern face, gentle North Face, and it's the result of that scraping of glaciers. 

KELLY: Camel’s Hump in the Green Mountains is just a little over a dozen miles north of the AT on the Vermont’s Long Trail.The next time you are on the peak of one of the Green or White Mountains, I want you to look up. Imagine that right where you’re standing there is a glacier and that that glacier is 1,000 feet thick. One thousand. That’s a lot of ice.

KELLY: The distinctive notches of the White Mountains are also the product of the advancing glaciers. Crawford Notch, Franconia Notch, these beautiful places are the result of massive glaciers shearing off the sides of mountains as they moved inexorably south. If you’ve hiked on the AT anywhere in the Northeast, you will have seen massive boulders just sitting all by themselves in the forest. I know I’ve looked at those boulders and wondered how, are they on the trail. Just like the notches in the Whites, those drop stones, or erratics, were left there for us by the glaciers.

KLEPEIS: The other thing I look for when I'm hiking on the Trail is the evidence of that glacial history, there are dropped stones. So as the glaciers advanced and retreated, they picked up, house size and bigger boulders and they tend to be rounded. And these seem to be sitting in the forest. And you don't know why often, but they're dropped by glaciers, and they're everywhere.

KELLY: In our interview, Jamie pointed out that the glaciers that carved the Green and White Mountains were just the most recent glaciers to work their magic. Much, much older ice ages left much smaller drop stones as well. 

LEVINE: Just north of Grayson Highlands, you can also see this unit, which is called the Konnarock Formation. And it is thought to be a result of something called snowball Earth. And that was when the earth was primarily covered with glaciers. And so it's a little bit younger, maybe 720 million years ago, it was thought that the earth was entirely or almost entirely covered by glaciers. And you can see in this kind of rock unit, you can see evidence of the glacial rocks, and you can see things called drop stones, which are these really big, sort of 10s of centimeters to 50 centimeter large rocks that were deposited in a lake by the glaciers moving.

KELLY: At various points in their long, long history, the Appalachian Mountains were also on the coast of very ancient oceans. As Keith Klepeis told us earlier, Southern Vermont was right on the coast. And if you look carefully at the limestone formations along the trail, you can see fossils that are as much as 550 million years old. 

KLEPEIS: Vermont was in the southern hemisphere. Kind of like a tropical environment, like the Bahamas are now and so you get these thick, fossiliferous limestones. And you can see them and sometimes you get these erosion of material off the continent, and you get court sites. And you can see those and as you walk north from, say, south of Bennington, and you see on your eastern side, your right side, the core of the Green Mountains, off to your left towards the west, are the sediments that were deposited on this ancient shoreline of the Iapetus right, so White Rocks National Recreation Area, which is northwest of Wilder mountain, you can see you're actually walking on that shoreline.

KELLY: I asked our two geologists if they had any favorite spots along the Appalachian Trail where hikers can see amazing things. Here’s what they told me.

LEVINE: I take my students to Laurel Falls and we do a mapping project. They're sort of their first project where they're learning how to map the rocks. And it's an old railroad grade, that the AT goes along in that section. And so it's nice and flat for part of it, and the rocks are really well exposed because it was blasted. So that area is really nice, because it sort of puts together some history and some geology. There's a beautiful waterfall and those rocks are recording the successful rifting of North America. And that was 540 million years ago. And so what we see is basically a bunch of beach deposits. So it's as if we were looking at the coast of North America at that time. And so you can see sort of white sandstone which you can think of like a white sand beach, and you also get some muddy areas in there. It's a beautiful waterfall, and it represents an interesting period in the earth's history. It's a fantastic place to learn about the geology.

KLEPEIS: One of my favorite places is up near Rutland airport. So just east of Clarendon, where the Appalachian trail takes a bend to the east, there's a State Forest called Clarendon Gorge. And it's pretty close to the Appalachian Trail. And it's a great spot because it's one of the best places you kind of walk in the gorge might take you half a day. And you can see the result of those forces, those collisional forces that uplifted the mountains, and you can see those sediments. Sediments are deposited more or less horizontally, while these are vertical. And they're twisted into these giant structures of folds and different material. And you can see the result of those forces. And let's see as you go farther east on the trail, and you kind of paralleling route four this place where route four takes a dive south, it turns to the south. And it parallels the audit Koichi River. And it's right there. You're kind of on the eastern slope. So the green mountains at that point you're walking on the ancient remnants the floor of that old ancient disappeared ocean called the Lapetus that one in which that island arc formed and slammed into the end of the continents and created the mountains you can see slivers of that.

KELLY: My personal favorite is the summit of Blackrock Mountain in the Southern district of Shenandoah National Park. Blackrock Summit looks a bit like the child of giants dumped his bag of building blocks on top of the mountain. Those angular blocks of stone actually are quartzite that formed on the bottom of that ancient lapetus Ocean that Keith mentioned. Then as the mountains rose, the seabed fractured into hundreds and hundreds of big rocks with almost perfectly rectangular edges. Those rocks sit on the summit in what’s called a talus pile. It’s a really spectacular geologic feature and it’s easily reached from a parking area on Skyline Drive. Plus, the sunrise views are some of the best on the entire AT. But if you go, remember, those rocks are at least 500 million years old. Just saying.

KELLY: Since you’re listening to this podcast, you probably know that the Appalachian Trail is almost 2,200 miles long and that it extends from Georgia to Maine. But did you know that there is also an International Appalachian Trail that begins at Mount Katahdin? From there it goes north to the northeasternmost tip of Newfoundland, Canada almost 1,600 miles away. And the IAT, as it’s known, continues beyond that! There are bits of the IAT in southern Greenland, in Iceland, in Ireland, and from the northern tip of Scotland down to the south coast of England. Beyond that the IAT crosses into France, Spain, and eventually into the Atlas Mountains of Morocco. I’m not sure anyone knows exactly how long the IAT is at the moment. But it’s really really long. The natural question, of course, is why anyone would think it made sense to extend the AT north to the tip of Newfoundland, and then even further across the North Atlantic and eventually into North Africa. The answer is simple. Geology. 

LEVINE: We've got the Appalachian Mountains, they have different names in different places, but we've got some of the same mountains in Europe and the Atlas Mountains in Morocco, are the same. And so all of these areas were together at one time when they were colliding with each other. But then when the plates moved apart, they went back to their respective locations, probably they were actually in a different location than they were originally. But we see sort of the other half of the mountains in England, in the Alps. The Alps are slightly different, but we see some of those same rocks, and then also in Morocco, the Atlas Mountains. It's pretty difficult to sort of wrap your mind around the fact that they're so far away now, but that they were all together. But yeah, we're just we're on one side and they're on the other side of the Atlantic and the Atlantic is what was formed as this rifting of Pangea occurred.

KELLY: In other words, back in the day, you know, 250 million years or so ago, the supercontinent broke apart and the Appalachian Mountains ended up on both sides of the Atlantic. So it only makes sense that the AT should really go from Georgia to Morocco. Geologically, that is.

KELLY: AT hikers often return to their favorite stretches of the trail over and over. Those of us of a certain age sometimes feel like those mountains we climbed a couple of decades ago seem to have gotten just a bit taller over time. According to Jamie Levine, those feelings are not entirely the product of aging or an overactive imagination. 

LEVINE: There's actually some evidence that we've actually had additional sort of topographic uplift more recently. More recently, geologically, but also, sort of, it could be happening right now.  Nothing has happened of interest geologically here for about 250 million years. So since erosion is always occurring, we would expect that this area would be quite flat, you can think of the Midwest, it's very flat, it should not be so high now in this area. And so that has led people to believe that we are, there's something that's causing the crust to be uplifted even now. But we have a lot more topography, basically, than we should considering that nothing has happened in 250 million years.

KELLY: In case you were wondering if I somehow encouraged Jamie to say the mountains were getting taller just so I’d feel better about finding them harder to climb, I most certainly did not. Jamie Levine takes her science seriously. 

LEVINE: The southern Appalachians are changing a bit through uplift. But a lot of people are looking at stream channels that go from higher elevations in the Appalachians down into the Piedmont, particularly in North Carolina, and Virginia. And they've noticed that it seems like there is uplift that is continuing in this area. And so this is again something that people don't fully understand why it's happening. But it seems like there's something that is happening that's causing the area to continue to be uplifted, it's often called topographic rejuvenation, because we're getting more topography in an area that again, isn't having a lot that's happening geologically. And then the other thing that's always happening is erosion. So we're always breaking down the rocks. And so that's sort of why it's strange that we are having this topographic uplift, because we would expect that the mountains would just be getting lower and lower over time.

KELLY: Just once I wish that “over time” to a geologist meant decades, not millions of years. Then I’d be vindicated in my sense that the mountains are getting taller. Because as Jamie says, they are getting taller. Unfortunately for me, the time scale here is once again geological, not historical. 

KELLY: I guess that means I’m just getting older. In the historical sense.

KELLY: The Green Tunnel is a production of R2 Studios at the Roy Rosenzweig Center for History and New Media at George Mason University. Today’s episode was produced by me. Jeanette Patrick and Jim Ambuske are the executive producers. We want to offer a big thank you to Jamie Levine and Keith Klepeis for their insights into the long geologic history of the Appalachian mountains. Original music for The Green Tunnel is performed by Scott Miller of Swope, Virginia, and Andrew Small and Ashley Watkins of Floyd, Virginia. To help us keep making the world’s best podcast about the Appalachian Trail, please go to our website, R2Studios.org and click on the Support Us link. From there you can make a donation of any amount to help us keep doing this work. Thanks so much for listening and we’ll see you soon!