This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Many hundreds of films have been shot on location in the American West. The rugged, inhospitable landscapes are an integral part of what gives so many American Westerns their distinctive character. Although the region is vast, stretching from the Rockies in Wyoming and Colorado to Utah, New Mexico, Nevada, Arizona and California, a few locations have captured the lion's share of the film industry's attention. What is it about these landscapes that makes them so photogenic, and how did they get that way? In this episode, Craig Jones casts light on the answers to these questions. He is a professor in the Department of Geological Sciences at Colorado University at Boulder. His research aims to uncover the origin of mountains and elevated topography in continents, primarily in the western United States. His main tool for doing this is seismology, but he also incorporates observations from other disciplines, such as gravity measurements, paleoaltimetry and petrology. He's written a book on the Sierra Nevada and leads field trips to the rugged terrains we're discussing here. Craig Jones, welcome to Geology Bites.

Craig Jones: Thank you very much for having me.

Oliver Strimpel: Before we start talking about the specific locations that became Hollywood favorites, how should we approach the question of understanding how the salient features of today's landscape came to be?

Craig Jones: The simplest way to think of it is in three parts. One is basically the intrinsic characteristics of the rocks themselves: how they're actually constructed, what materials were used. Then the physical and chemical processes that affect the rocks after they're created; typically, we're talking about erosional characteristics. And then, to view these kinds of landscapes, the present-day climate is an important component for being able to understand why we get these kinds of landscapes.

Oliver Strimpel: Okay. We're going to focus on just two very distinct areas in the American West that have been favorites of film directors: the Colorado Plateau, which covers parts of Colorado, Utah, New Mexico and Arizona, and the Alabama Hills, which lie 50 kilometers west of Death Valley, in the Sierra Nevada. Let's start with the Colorado Plateau, concentrating on Monument Valley, where over 60 major films were shot, starting with Stagecoach in 1939 and including the classic John Ford movies with John Wayne. We'll also touch on a few other locations in the Colorado Plateau, such as the Grand Canyon, Zion and Bryce Canyon. What is the origin of the rocks in the Colorado Plateau?

Craig Jones: The rocks that are present in all of these places are sedimentary rocks, rocks that are made up of what was originally mud and sand, or little limey creatures. It's varied a little bit over the history of the Colorado Plateau. Most of these deposits have been made within about the last 500 million years. One of the important characteristics of these sedimentary rocks is their variation over time, and it's that combination of seas rising and falling, shorelines moving, and mountains rising and falling that gives you the opportunity to have this diverse arrangement of sedimentary rocks, which is what leads to these dramatic landscapes.

Oliver Strimpel: So you mentioned shorelines. Do we know what the depositional context was for the materials that make up these rocks?

Craig Jones: Yes, it's varied quite a bit. So for instance, if you're at the Grand Canyon and you stand on the rim of the Grand Canyon, you're standing on the Kaibab Limestone, which is a marine deposit. But you go to Monument Valley, and you look at the wonderful spires that are there, and you're looking at the De Chelly Sandstone, which is sand dunes, kind of like what you would have now in the Sahara. And then there are fluvial deposits, river deposits, for instance the Organ Rock Formation, sitting at the base of those spires in Monument Valley. So we've had quite a variation, and really it's the fact that it's come and gone with these different environments that allows for all this diversity.

Oliver Strimpel: Hmm, interesting. We have marine, fluvial and aeolian deposition in different places. What sort of timescale are we talking about? When did this deposition get going, and how long did it last?

Craig Jones: Well, of course, it varies from place to place. For instance, at the Grand Canyon, most of what you're looking at at the bottom is the Tapeats Sandstone, which is a little over 500 million years old, and at the top, that Kaibab Limestone is a little over 300 million years old. So there you're talking about 200 million years of time. Of course, for a significant amount of that time, no rocks were deposited at all. You move over to Monument Valley, and you're looking at somewhat younger rocks, which overlap in part with the Kaibab Limestone, but they were closer to an ancestral Rocky Mountain range that existed here in Colorado, with Monument Valley to the west, in Utah and Arizona. So that time period was actually much shorter: for everything you see in Monument Valley, probably maybe 50 million years. And then if you went to something much younger, let's talk about Bryce Canyon National Park, the rocks there were pretty much deposited within about 10 million years. So different places are sampling different parts of the geologic history of the Colorado Plateau and are representing different durations of time.

Oliver Strimpel: What was the larger tectonic context at the time? Was this before or after the creation of Pangaea, and did North America already exist as an intact piece of continent at that time?

Craig Jones: Well, North America definitely existed at that point. Before Pangaea assembled, we call it Laurentia, because parts of what are today North America were at the time on other continents. But this area, in the southwestern part of the United States, was definitely part of Laurentia, and part of North America. Parts of that geology predate Pangaea. So for instance, the section you see in the Grand Canyon is, for the most part, older. The stuff you see in Monument Valley was accumulating pretty much at the time that Pangaea was finally being assembled, and so those Ancestral Rocky Mountains were a product, in part, of the final closure of an ocean between South America and North America. And then a number of the rocks that are present are younger and actually date to when North America had separated once again from pieces of Pangaea. So Zion National Park, the big sandstone cliffs there, and Bryce Canyon National Park: both of those postdate Pangaea.

Oliver Strimpel: So as you mentioned earlier, the depositional environments are very different from place to place: some of them were fluvial, some marine, some aeolian. But even in a given spot, you can see, from the way that formations change as you go up and down across an exposure, for example, that there is a huge variation. What accounts for that in a given place, and how does that wind up affecting the rocks, how they appear and how strong they are?

Craig Jones: One of the big factors in the Colorado Plateau, again, is the Ancestral Rockies. Prior to that time period, things were relatively quiet, and the plateau was fairly close to sea level. Further west, you were in a little bit deeper water, and so you have things like the Muav Limestone, which is a prominent cliff former in the Grand Canyon, and you go a little further east and you find some fairly thin shoreline deposits and some coastal deposits. As global sea level changes, of course, you can get these little variations. But then you throw in something like the Ancestral Rockies, and they start to erode, and that material has to go somewhere, and one of the places it went was the Colorado Plateau. And so the vast pile of red rock that you find in Monument Valley, which you also find in the Grand Canyon, the Supai Group sedimentary rocks, is basically there because you now had this big source of sand and gravel and mud that was flooding across the landscape. As those mountains were finally worn down, roughly 250 million years ago, you started to see the return of the oceans, coming back into parts of the plateau. Similarly, much later on, the modern Rocky Mountains were created, starting roughly 75 million years ago, and as part of that, the river systems were disrupted, and we ended up with some very large lakes. One of those large lakes is actually the source of the sedimentary rock that we have in Bryce Canyon. So it's been a mix of broad-scale global changes and some local changes because of tectonic activity.

Oliver Strimpel: Okay, so let's talk about the second phase, then: what's happened since these sediments were originally laid down.

Craig Jones: In order to get erosion, the very first thing you need is elevation. If the rock is sitting down near or below sea level, not much is going to happen. Once you've managed to raise the rock up, then erosion is going to be able to act on it. There are two different aspects of that. The main one, really, for our purposes, is physical erosion: basically the weathering, breaking down the rock, and then transporting it away as erosion. And different kinds of rock will behave differently. So the fluvial sandstones and mudstones typically are very thin-bedded; when you look at them, they kind of look like rubbly slopes. Then the aeolian sandstones tend to be quite massive. You see what's called cross-bedding in them, which comes from frozen sand dunes, basically, that have been preserved, and that material tends to form large cliffs. The limestones, and some other things like conglomerates, which are much coarser than the sandstones, tend to be far more resistant. They'll be ledge formers, but they're typically not very thick. So the physical erosion of this material results in different-looking things. The other thing that's important, I think, for a lot of people is really the diversity of colors. So for instance, we have what's called the Painted Desert, which is just to the east of the Grand Canyon. This is actually the Chinle Formation, and it's so colorful, in the case of the Chinle, because of volcanic ash deposits, which have lent colors not only from iron, which is the most common rock-coloring agent, but from manganese and copper, and occasionally some other metals, which will give you the pinks and maroons and some greens, and even some bluish-looking rock. Most of the red rock that you see, though, is iron, and that iron has been oxidized; it's been rusted. The iron might have originally been in the rock as little grains of magnetite; it could have been deposited as hematite; there are some other mineral grains that are iron-rich. The key thing, though, is that you need these deposits to have been made in an environment where you have oxidation, which is typically going to be at or above sea level. So in particular, the really red rocks, say at the base of the pinnacles in Monument Valley, tend to be fluvial sediments; they were deposited by rivers. So the muds, before they became rock, were being exposed to the atmosphere, and they weathered into these very, very red colors, which are now exposed. And so most of the red rock that we have in the Colorado Plateau has been produced in that manner.

Oliver Strimpel: But there are places where the rock is kind of whitish. Is that a different environment?

Craig Jones: Yeah. In particular, the case of the Navajo is kind of interesting. Again, the Navajo is what forms the tremendous cliffs in Zion National Park, and you'll see both red and white colors in the Navajo, pretty much with the white above the red. And for a long time, geologists argued about just exactly whether the Navajo started white and was stained red, or whether it started red and was bleached white. I think starting red and being bleached white has got the best support. And the interesting thing is that what would do this is if you had a reducing environment, one which is the opposite of oxidizing. And it turns out that if you have petrochemicals, petroleum and gas and other sorts of organics, that move up in the rock... The Navajo Sandstone is actually a very porous sandstone; it's what we would consider an aquifer. So as petroleum is released from some underlying limestones, say, it rises up, and it maybe gets trapped in the Navajo Sandstone. Above it is something called the Temple Cap Limestone, which then prevents any further upward motion of these fluids, and the fluids, being reducing, change the color of the rock from red to white. So it's been suggested, I think with good evidence, that a lot of the white Navajo Sandstone that we find in the southwestern US used to be a big oil reservoir that subsequently was breached by erosion. All the oil and gas went into the atmosphere, went into the surface waters, and we've now been left with these different colors.

Oliver Strimpel: Was there any particular confluence of circumstances that led to the formation of the incredibly dramatic rock towers in Monument Valley?

Craig Jones: The big thing that is true in Monument Valley is, first, at the very bottom, you have a fairly resistant rock unit that provides a base, and sitting on top of that are some fluvial deposits, which is basically the Organ Rock Formation, and then you have the De Chelly Sandstone. And the trick, of course, with this sort of cliff former... the way I sometimes think about it: if you went to the beach, and you had sand, and you had, for some reason, a pile of dominoes, and you set them up on end, kind of like you're trying to knock over a whole series of dominoes, but you actually bring them close together, so that they're right up against one another, they'll sit there just fine, so long as the sand is sitting underneath them. But if you start digging away at the sand at the edges, the dominoes basically start to fall in, as the base they sit on starts to become unstable. And that's kind of what happens with these big cliff formers: as long as you have something firm underneath, they'll support these tremendous cliffs, but the moment you start to eat away at that material underneath, they fail, sometimes quite dramatically, and still preserve a cliff. So as the cliff retreats, it stays this big vertical cliff. And that variation in the style of the rock that was deposited, going from the much more muddy fluvial rocks of the Organ Rock, which sits below the De Chelly Sandstone, transitioning to the big, thick, pure aeolian sandstones of the De Chelly, gives you that wonderful contrast that gives you this incredibly dramatic and iconic topography.

Oliver Strimpel: And they stick out because all around them, the sandy base became unstable, and the rocks just fell away and have been washed away by rivers. Is that right?

Craig Jones: Yes, that would be accurate.

Oliver Strimpel: And what about the massive cliffs in Zion, or the pinnacles in Bryce?

Craig Jones: So Zion is very much the same as Monument Valley. It's younger again, in terms of... the Navajo Sandstone is younger than the stuff in Monument Valley, and in fact it was probably one of the largest sand seas on Earth. Zion has about the thickest of the Navajo Sandstone. The case there is that you have the Kayenta Formation underneath, which is almost exactly like the Organ Rock, in terms of it being a fluvial deposit that can be eroded away. The neat thing about Zion is that as you walk upstream, if you go upstream in the main canyon, eventually the walls come together and you can walk up the Narrows of the Virgin River (the Virgin River is the river draining Zion), and you can reach places where you can reach across and touch both sides of the canyon from one spot. And when you're doing that, you're now standing on the Navajo Sandstone; you're no longer standing on any of the underlying sedimentary rocks. And so that's a place where the strength of these sandstones, their resistance to collapsing, really stands out. You know, there are places in the Narrows where you look up and you cannot see the sky, because it's basically this slot, and these kinds of slot canyons are relatively common in the Colorado Plateau. Bryce is a much younger story, and it's different, because in Bryce you have what are called hoodoos. So the thing that stands out about Bryce, aside from the colors, which are these bright oranges, is these pinnacles. And in this case, you're dealing with a differential resistance to weathering that's present in some of the layers. Some of the layers are lake deposits. They're really, to a geologist, sort of ugly; they're actually kind of hard to interpret, because they were altered a fair bit after they were deposited. But part of that was that some areas are cemented and some areas aren't, and there's a bunch of cracks. We call them joints, to indicate there's no real motion across them. These cracks act as places where weathering proceeds very rapidly, and in between the cracks, weathering is much slower. And as a result, if you have crosscutting sets of these joints, you end up with isolated pinnacles. And depending on how efficacious the erosion is (in the case of Bryce, it's basically a steep-sided amphitheater, where erosion works very, very well), you flush away all the material that's between these pinnacles, and you end up with these dramatic hoodoos.

Oliver Strimpel: Okay, so turning now to the third and final phase that you mentioned: the reason the rocks are in plain sight from horizon to horizon is because this area is so extremely arid, and nothing can grow. Why is it so dry?

Craig Jones: There are two pieces to that. One is that we're kind of on the edge of the global desert belt that you find near the Tropic of Cancer, so you're not quite up into the big wet part of the higher latitudes. But a huge part of it is that the Colorado Plateau is surrounded by mountains that are significantly higher than it. Storms at this latitude tend to come from west to east through the winter, and what those storms encounter to the west is the Sierra Nevada, and the Sierra Nevada does a perfectly fine job of earning its name by scraping off a lot of the moisture. So very little is actually left, and even what little is left then encounters what are called the High Plateaus, at the very west side of the Colorado Plateau, and the High Plateaus scrape off a little bit more. So when you get into the interior of the plateau, there's very little moisture making its way there. Now, there are times of the year when you do get storms moving from east to west. Those of us who live here on the east side of the Rockies see this typically in early spring, with moisture coming, maybe from the Gulf of Mexico, that's trying to work its way west. And again, it encounters mountains; it encounters the Rocky Mountains. The Rocky Mountains again scrape off all the moisture; as the air goes down the other side, it dries out, and again the Colorado Plateau kind of gets cut out of any significant moisture. The last direction you might get moisture from is the south, during the summer monsoon season. There you run into what's called the Mogollon Rim, which is not as high as the Sierra Nevada or the Rocky Mountains, but it's still higher than the bulk of the Colorado Plateau, and so once again you tend to scrape off the moisture that might be coming in from the summer monsoon. So a little bit of it is the global environment that the Colorado Plateau finds itself in, but an awful lot of it is that, despite the fact that it's well over a mile in elevation in large parts of the plateau, it's surrounded by higher mountains that basically prevent any of that moisture from sneaking in.

Oliver Strimpel: Okay, let's turn to our second area of moviemaking fame, the misleadingly named Alabama Hills, in the southern California Sierra Nevada mountains. Its proximity to Hollywood made it extremely popular with film directors, and over 300 films were shot there, such as High Sierra, with Humphrey Bogart, Django Unchained and many episodes of The Lone Ranger series, not to mention tons of truck and SUV ads that were shown on US television. This must be a completely different geological story.

Craig Jones: Yeah, and indeed it is. Almost all the rocks that you find in the Alabama Hills, and in the Sierra Nevada that sits behind it, are granites, or granitoids.

Oliver Strimpel: Where do these granites come from?

Craig Jones: Typically, what you're looking at is subduction of an oceanic plate underneath the edge of the continent. In that water-saturated oceanic plate, the water is bound in minerals, and it becomes unbound as you get to certain pressures and temperatures. That water then rises off the subducting slab and interacts with the overriding mantle, and it melts the mantle. Much as throwing salt onto ice will cause the ice to melt, throwing water into the mantle causes the mantle to melt, yielding magmas that rise up into the crust, and you end up creating volcanoes, such as Mount St. Helens and Mount Rainier in the Pacific Northwest today, and underneath them you produce these granites. So we're basically looking at one of the longest-lived volcanic arcs in the world, at least over the past 500 million years: the volcanic arc that existed in the Sierra Nevada from something like 320 million years ago to about 80 million years ago.

Oliver Strimpel: Okay. So what's happened to this granite since it was emplaced?

Craig Jones: So these granites crystallized at depth. The way that you get the nice big crystals that you can see in a granite is by having a lot of time for the minerals to get organized, and so you need to be at some significant depth. Some of the shallowest of the granites may have been just five kilometers below the surface; some, as deep as almost 30 kilometers below the surface. Well, they're not at great depth anymore, so you had to actually remove a lot of the material on top, and for the most part that was done by erosion. On top were the volcanoes, so the lavas and tephras and other volcanic materials. Those were far more easily eroded, and most of that material was removed by about 60 or 65 million years ago. Subsequent to that, you start seeing in the sedimentary rocks the transition from the volcanic materials being eroded to the granites being eroded. Again, in order to have erosion, you have to have elevation. Certainly in the Late Cretaceous, say 70 to 75 million years ago, the Sierra Nevada had to be high. The big question since then has been exactly what's happened. This actually leads us into a little bit of controversy. There are two camps, basically: one that thinks the Sierra Nevada has stayed high, much like it is today, pretty much since 70 million years ago, and the other camp, which feels that the range was lower: it eroded down and eroded down, ended up significantly lower than it is today, and has subsequently risen up in about the last 10 million years.

Oliver Strimpel: If the evidence can be interpreted to mean that it was a fairly sudden uplift over a certain period of time, this reminds me a little bit of what Peter Molnar was talking about in his podcast to explain the rapid uplift of Tibet. And I'm wondering if the seismic imaging under the Sierras is good enough to see if there's a piece of mantle that might have dropped off that could be implicated here.

Craig Jones: Yeah. So, I mean, those of us who are inclined to think that the range is young point to an anomaly that long ago I called the Isabella anomaly, and that name has stuck. This is a high-velocity body (high velocity in the sense of the speed of seismic waves through the material) that sits in the upper mantle, going down to depths of around 300 kilometers, underneath the southeastern Great Valley and the southwestern Sierra Nevada. And it turns out, volumetrically, that this is very much like the material that would have sat underneath the high part of the Sierra Nevada 10 million years ago. And unlike the situation in Tibet, our ability to image this is much better in the case of California, in part because it's a smaller area; logistically, it's much easier to access. And actually, the geometry of earthquake sources for being able to do tomography is very good in California. So we have this really excellent image of this body. We also have evidence from volcanic pipes that came up between 8 and 12 million years ago in the Sierra Nevada, which actually brought up pieces of the uppermost mantle and the lowest crust that used to be present in the Sierra Nevada. Some of that material is basically just garnet and pyroxene, so it's a garnet pyroxenite. This material is very, very dense; it's denser than the mantle underneath it. And it's gone today: when we do imaging under the high part of the range, there is none of this material present. But the characteristics of the material in the Isabella anomaly are perfectly compatible with it containing a lot of this garnet pyroxenite. So it does seem like a way to get the mountain range to rise up is that you have removed this very dense material that sat at the bottom of the crust, and now you've just kind of poured it down this hole, so it's on its way down into the mantle.

Oliver Strimpel: Let's talk a bit about the shapes of the landscape. In contrast to Monument Valley and the Colorado Plateau, we don't see tabletop mountains, stepped cliffs and the towers that we see there. It looks really different.

Craig Jones: The first thing is, of course, that the granite is far more uniform in its behavior. You're not going to have a rubbly slope and then a big cliff and some capping thing that is more resistant; it's all granite. But what really affects the way the granite erodes has a lot to do with the climate. So if we go back to the Alabama Hills, when you look at the Alabama Hills, it's these rounded knobs. The Chamber of Commerce in Lone Pine likes to tell people they're the oldest hills in the world, which they're not; they're not particularly old, but they give you that kind of sense that they've been there forever, and they've been sort of rounded and ground down. Whereas the exact same rock that sits up on the crest of the Sierra Nevada, which is in the background of those movies, is forming these tremendous spires, arêtes, horns, all kinds of wonderful, very sharp alpine kinds of geometry. It's still the granite, but there it's being affected by ice: plucking, glacial erosion, glacial polishing. So the glacial processes acting on the granite give you these very different shapes from what you're going to see down low, in the desert, where the Alabama Hills are. By and large, that uniformity of the rock tends to give you these nice, beautiful U-shaped valleys, which is the classic shape that you'll see in the Alps. You see them in much of the Sierra Nevada. But the funny part about the Sierra Nevada is that in some places the granite is a little bit different. So in the Tuolumne drainage, which is just north of Yosemite Valley, the rock is extraordinarily uniform, and it has very, very few joints, very few of these little cracks, and so the glaciers were not able to pluck any of the rocks out. So when you look at what's called the Grand Canyon of the Tuolumne, it's V-shaped, even though it had the thickest glacier in the Sierra Nevada. There was nothing for the glacier to do but just polish it. In contrast, Yosemite Valley has this amazing diversity of lots of different kinds of granite, and as a result, it has lots and lots of different fractures in different places, so that parts of it are extremely fractured and parts of it are not, like El Capitan. And that combination has allowed glaciers to excavate to an extreme degree, so much so that there are 2,000 feet of sediment sitting in the bottom of Yosemite Valley. So the original U-shaped valley has been filled in with sediment, and so you have these nice flat meadows practically going up to the base of these tremendous cliffs.

Oliver Strimpel: And what about the final phase, the present-day climate? Are the rocks there in the Alabama Hills exposed, as in the Colorado Plateau, because it's a desert where nothing can grow to cover them up?

Craig Jones: Yeah, certainly. In the Alabama Hills, you are in one of the biggest rain shadows on Earth. The high peaks of the Sierra Nevada to the west are over 4,000 meters; the passes are nearly 4,000 meters. So all the precipitation that would be coming from the west gets scraped off, and the Alabama Hills are very much desert; it's extremely dry. Mary Austin's book, The Land of Little Rain, was about Owens Valley, and just the fact that she chose that title for the book tells you what the situation is. In contrast, the exposure in the high part of the range is because of actually having all of that moisture, and having been in an ice age: a lot of soil got scraped off, and you just have lots and lots of bare rock, and there's only so much vegetation that can claw its way into these bare granite cliffs.

Oliver Strimpel: What do you consider to be some of the most important geological problems that have yet to be solved in the American West?

Craig Jones: You know, I think it comes down to paleoelevation. For geophysicists, elevation is what we would call the zeroth-order observable. It is the most important basic piece of information, and we kind of take it for granted today. You look around and you say, well, yes, we can measure it; we can measure it lots and lots of different ways. But as soon as you get into the geologic past, if you're much above sea level, it's really hard to know how high. So for instance, take the argument about the Sierra Nevada. If the Sierra Nevada is young, and we've created a young mountain range by removing dense material at the base of the crust, that's important to know, because it's telling us about a process that we only see hints of in other places, that could be very important. In contrast, if the range is really old, if it was really high, say, 40 million years ago, then we have a problem: we need to understand how the range could stay that high while things were changing so dramatically in the upper mantle and lower crust underneath. And if we look further afield, there are arguments about the Colorado Plateau, there are arguments about the Rocky Mountains, and, amazingly, there is an argument about the High Plains, which are here, east of where I'm sitting in Boulder, Colorado. Were the High Plains high 60 million years ago, or have they just gone to their present elevation, making Denver the Mile High City, 5 million years ago? And again, knowing which of those is really the case, or something in between, is telling us about very large-scale Earth processes that are not contradicting plate tectonics, but expanding upon it. We're learning about processes that we don't really have a grip on at present.

Oliver Strimpel: It's really intriguing, because when you think of large flat areas, at least naively, I would think of a floodplain, which would be very low, because it's got water flooding into it and depositing, or perhaps something like the Tibetan Plateau, which is very high because of something like some of the processes you were just talking about, making that region anomalously high, although perhaps not on the same scale as the High Plains. So are you thinking that there might be yet some completely new process at work here?

Craig Jones: It certainly looks like there's got to be something very unusual. The kinds of processes we've seen, for instance dripping off of the lower crust and upper mantle, like we see under the Sierra Nevada: the scales of the High Plains are too large. That's not a process that works at the scales we see in the High Plains. We have hundreds of kilometers of gentle gradient, going down from the Rocky Mountains towards the Mississippi River. So it's requiring something novel. And ideas range from large-scale flow in the mantle, having either pulled everything down and piled up sediment and then pushed it back up, to ideas about changing the character of the mantle over very long wavelengths by adding some water and creating less dense minerals in the mantle, to a kind of nutty idea that we put forward a few years ago, where the water coming off the old Laramide slab came up through the mantle and made it into the lower crust. Where the lower crust previously had been rich in garnet, that garnet would retrograde under those conditions into less dense minerals and produce a very broad uplift. So, you know, these are the hooks that I think those of us doing research really look for: things that you can't really explain easily with the current paradigm. And I think the High Plains are a really stupendously dramatic example of that. Because a big difference from, say, the Tibetan Plateau is that in the Tibetan Plateau, there's a tremendous amount of deformation and crustal thickening. There are thrust faults all the way across the Tibetan Plateau. The High Plains are undeformed. There's no crustal shortening by thrust faulting; there's been no physical deformation of it. For the oil geologists who work across it, it's practically a layer cake of sediments that have accumulated over the past 400 or 500 million years. So it's the kind of place where we learn new things about how a planet can actually operate.

Oliver Strimpel: Craig Jones, thank you very much.

Craig Jones: Thank you very much again for having me.

Oliver Strimpel: For more about Geology Bites, as well as pictures and illustrations that support this podcast, you can go to geologybites.com.