Latest / PlanetGeo: The Geology Podcast / Cure the Winter Blues - The Black Hills (ReRelease)

Transcript

Cure the Winter Blues - The Black Hills (ReRelease) · PlanetGeo: The Geology Podcast

0:02
SPEAKER_02: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it worked, and why it matters to you.
0:14
SPEAKER_02: Oh, Vacation Chris.
0:16
SPEAKER_02: Vacation mode, Chris.
0:18
SPEAKER_02: What's happening?
0:19
SPEAKER_01: It's the fun Chris, right?
0:20
SPEAKER_00: That's it's the one everybody likes.
0:22
SPEAKER_00: It's the fun Chris.
0:23
SPEAKER_00: Fun Chris is in the house.
0:26
SPEAKER_00: That's right.
0:26
SPEAKER_00: There are a few versions that not so likable, but Vacation Chris is fun.
0:30
SPEAKER_02: Yeah, Vacation Chris is.
0:30
SPEAKER_02: I can attest to that.
0:32
SPEAKER_02: It's very fun, Chris.
0:33
SPEAKER_02: Very fun.
0:33
SPEAKER_02: It's almost as good as Taylor Swift concert, Chris.
0:36
SPEAKER_02: I'd imagine.
0:37
SPEAKER_00: Oh, that's I bet you're right.
0:38
SPEAKER_00: I bet you're right.
0:39
SPEAKER_00: That'd be different levels of entertainment, but both fun Chris.
0:43
SPEAKER_00: Absolutely.
0:44
SPEAKER_02: Yeah, for sure.
0:44
SPEAKER_02: You know, this is part two in our winter re-release series, and the theme here is Cure the Winter Blues by thinking about very fun trips to do.
0:51
SPEAKER_02: And this is number two in our series.
0:53
SPEAKER_02: And again, just to reiterate here, we are going through in sequence.
0:57
SPEAKER_02: You listener are kind of going along with us on the Summer Science Institute geology trip with Chris Bullhuis.
1:03
SPEAKER_02: So last week we started at the Badlands.
1:05
SPEAKER_02: The next real geology stop is the Black Hills, which is the geological bullseye.
1:11
SPEAKER_02: And Chris, this is one of the most special places to me that there is in geology.
1:17
SPEAKER_02: Maybe second only to the place, the field area that I did my PhD in.
1:21
SPEAKER_02: The Black Hills is it's up there for me, man.
1:24
SPEAKER_00: The Black Hills, it is up there and it should be up there.
1:27
SPEAKER_00: I think it's very underrated.
1:28
SPEAKER_00: Black Hills is known for Mount Rushmore, and it's known as a speed bump to your way out to the Tetons or to Yellowstone.
1:34
SPEAKER_00: To both you and I, the Black Hills is so much more than that.
1:38
SPEAKER_00: My favorite memory, we're gonna do this with every place that we're gonna visit here.
1:42
SPEAKER_00: For the Black Hills, it has to be family time.
1:44
SPEAKER_00: When my kids were young and we would spend three weeks just in the Black Hills, and that's where my kids they cut their teeth in the mountains on the Black Hills.
1:53
SPEAKER_00: That's where they became good at doing mountain stuff.
1:57
SPEAKER_00: What about you?
1:57
SPEAKER_00: What's special?
1:58
SPEAKER_02: I I Chris, I has to be rock collecting trips with yours truly.
2:03
SPEAKER_02: You and you and me, and and you know, anybody else who would come along.
2:07
SPEAKER_02: I mean, uh, the Black Hills are just amazing.
2:09
SPEAKER_02: I mean, they're cool for many reasons.
2:12
SPEAKER_02: They've got really interesting geology, they've got old Precambrian rocks, like old rocks that I like to study nowadays.
2:18
SPEAKER_02: I've not actually worked on the Black Hills before, but they've also got this like amazing mining history, gold rush, Deadwood, like they, you know, the TV show Deadwood.
2:26
SPEAKER_02: They've got this whole like evocative history sort of thing.
2:29
SPEAKER_02: And the rocks are spectacular.
2:31
SPEAKER_02: So we're banging around looking at like pegmatites, go to the Eda mine with the I forget, 10, 15 meter long spodgmine crystal in the wall, picking up just huge blocks of pure spodjamine, digging around for micas, finding some beautiful starlight crosses in the schists and stuff.
2:48
SPEAKER_02: I mean, just really fun rock collecting, not to mention the Ungpapa sandstone, which is this amazing, like really finely banded and finely faulted sandstone that's beautifully colored and uh was protected very well by baby rattlesnakes.
3:02
SPEAKER_02: So Chris was terrified.
3:03
SPEAKER_00: But that's oh come on, you were too.
3:05
SPEAKER_00: I know you were.
3:06
SPEAKER_00: We were all terrible.
3:07
SPEAKER_00: That's my memory.
3:08
SPEAKER_02: I mean amazing rocks, just unbelievable rocks.
3:11
SPEAKER_02: So it's a don't miss a good that's a good answer, Jesse.
3:14
SPEAKER_02: Black Hills coming your way.
3:20
SPEAKER_00: Jesse Remick, how are we doing?
3:22
SPEAKER_02: Chris, Chris, the Black Hills.
3:25
SPEAKER_02: This yeah, is a location that I think not a lot of non-Americans know about, but I think for me, it's probably one of my favorite places in the United States.
3:38
SPEAKER_00: It is for me too.
3:40
SPEAKER_00: You brought this up as a topic, and I'm like, well, gosh, it's like uh it's we're in the winter time.
3:45
SPEAKER_00: Is this a good time to talk about the Black Hills?
3:47
SPEAKER_00: But then as I started thinking about it, I started I I caught myself thinking about the Black Hills, you know, like I got the itch.
3:53
SPEAKER_00: Totally.
3:54
SPEAKER_00: You have the itch to go.
3:55
SPEAKER_00: And I figure, don't you think, that some of our listeners have the itch to like, all right, it's we're starting to look forward to summer, right?
4:02
SPEAKER_02: Yes, let's look forward to getting out there in it.
4:05
SPEAKER_02: Yeah, and one place you need to consider when you're thinking about your summer travel plans is the Black Hills of South Dakota.
4:11
SPEAKER_02: They are unbelievable.
4:12
SPEAKER_00: Yeah, my wife and I we used the Black Hills as the mountains to cut our teeth for our kids.
4:20
SPEAKER_00: Oh, yeah.
4:20
SPEAKER_00: That's where we introduced them.
4:22
SPEAKER_00: We spent literally months in the Black Hills when they were young kids.
4:27
SPEAKER_00: And uh, you know, it was just a great way to introduce some skills to them.
4:31
SPEAKER_02: You mean mountaineering skills, climbing, hiking, that kind of stuff?
4:35
SPEAKER_00: Hiking, you know, like toughness and because there's nothing really extreme there, but it was a good way to take really super young kids and have them walk.
4:46
SPEAKER_00: And so we spent the a lot of their really young years in the Black Hills, and we would go there for three weeks at a time and not go anywhere else.
4:55
SPEAKER_02: It's an amazing place.
4:56
SPEAKER_02: And you and I have some pretty good shared experiences in the Black Hills.
5:00
SPEAKER_02: This is one stop, one of the early stops on the field course that you teach in the summers to soon-to-be high school seniors.
5:07
SPEAKER_02: And I was one of those kids, and and I have such fond memories of the Black Hills, and the geology is very cool, and the rocks are spectacular and have been studied for a long time is sort of one of these uh type localities for a lot of the stuff like pegmatites and certain mineral deposits.
5:25
SPEAKER_02: It's been mined for a long time.
5:26
SPEAKER_02: There's a lot of gold mining that used to take place there, and still a little bit.
5:30
SPEAKER_02: There's TV shows written about the gold rush there, you know, super cool place.
5:34
SPEAKER_00: Yeah, it there's a lot there too.
5:36
SPEAKER_00: And I think the I think the Black Hills get overlooked a lot because Mount Rushmore is there, Crazy Horse is there.
5:43
SPEAKER_00: And I think people view it as hey, let's go to Mount Rushmore because they're in the Black Hills, which is that's that's good.
5:50
SPEAKER_00: I'm not I'm not knocking that at all.
5:52
SPEAKER_00: I think though the beauty and the what the Black Hills have to offer get overlooked a lot because it's viewed more like a maybe a speed bump on your way to the outwest, you know.
6:05
SPEAKER_00: I gotta I gotta do this before I really get into the west.
6:07
SPEAKER_02: Yeah, yeah, to get into the real rugged Rockies and stuff.
6:10
SPEAKER_02: And in some ways it is that it like you said, you can't get into too much trouble there as far as outdoors activities, you know.
6:16
SPEAKER_02: The the stakes are lower there than they are in the Rockies or in other parts of the mountains there.
6:21
SPEAKER_00: So that's correct.
6:22
SPEAKER_00: It you know, you're if you're talking about like hiking and things like that, it is there's some serious rock climbing to be done there, though, that is it's a mecca.
6:30
SPEAKER_00: It's a it's a great, it's it's great climbing, but there is so much there, you know.
6:36
SPEAKER_00: Not only Rushmore and Crazy Horse, there's Spearfish Canyon, you have Wind Cave National Park, you have Jewel Cave National Park, Custer State Park, Devil's Tower National Monument is right nearby, Badlands National Park is right nearby.
6:51
SPEAKER_00: I mean, it's you look at that list of like, oh my gosh, there's a lot to do there.
6:57
SPEAKER_00: And you know, both of us, we've been a lot of places, and to to hold the Black Hills in such esteem is is it says something.
7:04
SPEAKER_02: Yeah.
7:05
SPEAKER_02: So guide us through where we're gonna go, Chris.
7:08
SPEAKER_02: Like, what's the flow here of this episode?
7:10
SPEAKER_02: What are we gonna cover?
7:11
SPEAKER_02: Because you have taught geology students here in the Black Hills for what uh 25 years, 20 years, something like that.
7:18
SPEAKER_02: You've and you've been there with your family even more than that.
7:23
SPEAKER_02: You know this, like the back of your hand, and you know the geology.
7:25
SPEAKER_02: So, where are we gonna go?
7:26
SPEAKER_02: Where we're gonna cover, where are you taking us?
7:28
SPEAKER_02: You're gonna lead this episode, I think, in many ways.
7:31
SPEAKER_00: I think we're gonna get into, we're gonna dive into, of course, the geology of the Black Hills.
7:36
SPEAKER_00: And in basically, there are three like suites or ages or events of rocks that happened, and we're gonna go through each of those and like we want to paint a picture.
7:47
SPEAKER_02: And if you're gonna go to the Black Hills, you gotta pay attention because the rocks are spectacular.
7:51
SPEAKER_02: The rocks are really, really cool there.
7:54
SPEAKER_02: So you gotta keep your eyes open, keep your eyes on the ground a little bit when you're when you're doing hikes, because the rocks are amazing.
7:59
SPEAKER_00: You know, I'm gonna do my best to paint like this kind of 30,000-foot view of the Black Hills.
8:04
SPEAKER_00: And and I think you are gonna bring in some detail about you know some of the important things, like the the formation of this granite that was really a large part of the formation of the Black Hills and a large part of the exposures and and so on.
8:19
SPEAKER_00: And then also, Jesse, like you said, we have a ton of shared experiences there.
8:23
SPEAKER_01: So there are gonna be some some stories involved in the There might be a trip down Mendry Lane or two.
8:28
SPEAKER_02: We'll we'll try and keep it PG rated for this.
8:31
SPEAKER_02: But yes, all right, so we're gonna work through time, right, Chris?
8:34
SPEAKER_02: We're gonna start old and work young, in part because that's the structure of the Black Hills.
8:39
SPEAKER_02: The jet Black Hills geology, if you look at a geological map, it's this kind of bullseye where in the center the oldest stuff is, and on the edges the youngest stuff occurs.
8:47
SPEAKER_02: And we're gonna kind of work through the geologic history through time, right?
8:50
SPEAKER_00: Right on, yeah.
8:51
SPEAKER_00: I mean, like you said, the it's kind of this bullseye, it's a it's more like an eyeball-shaped bullseye that the eyeball would be oriented north-south, you know.
9:00
SPEAKER_00: So the black hills are about 140-ish miles long and with a maximum width of 75 miles, you know, so it's kind of this oblong bullseye.
9:12
SPEAKER_00: It's not this perf perfect, you know, circular kind of bullseye, but it definitely has that look to it for sure.
9:18
SPEAKER_02: You know, one thing we gotta mention as well, there's a lot of cave systems here.
9:21
SPEAKER_02: I think Wind Cave National Park is there, like you mentioned earlier.
9:24
SPEAKER_02: I'm not the biggest fan of caves, really.
9:26
SPEAKER_02: I I don't particularly like the idea of being under a bunch of rock, but they are cool.
9:32
SPEAKER_00: They are.
9:32
SPEAKER_00: I think into you know, Wind Cave is one of my favorites, and and I think honestly, we should do an episode on it alone because the geologic story of Wind Cave is why I keep going back there with my students.
9:46
SPEAKER_00: It's not the most spectacular cave in the world to see, it doesn't have all these like amazing speleothems and you know things like this that formed after the formation of the cave itself, but the story behind it is the coolest.
10:01
SPEAKER_00: So this is the coolest cave to me.
10:04
SPEAKER_02: Let's orient ourselves really quickly here.
10:06
SPEAKER_02: The Black Hills, for those people who don't know, we have a fair number of international listeners.
10:10
SPEAKER_02: The Black Hills are in the western part of the state of South Dakota, the eastern part of the state of Wyoming, and they're really this very strange landscape feature.
10:20
SPEAKER_02: There's a a bunch of hills that crop up in the middle of the Great Plains.
10:25
SPEAKER_02: So you've got flat, relatively, dare I say, boring prairies, and then you've got this round, like you said, 140 miles, about 75 miles wide, sort of uplift that has a whole bunch of trees on it.
10:38
SPEAKER_02: If you look on the Google Earth for the Black Hills, you can see the darker area that's all the tree-covered things.
10:44
SPEAKER_02: And this is the black part of the hills, right?
10:46
SPEAKER_02: There are these hills that have all these ponderosa pines, give it this amazing smell.
10:51
SPEAKER_02: Like I I remember when I was a student of yours in high school, that was one thing that you were so passionate about was when you step off the bus in the black hills, take a deep breath through your nose because the Ponderosa pines are the most amazing smell, and this is the first time you're gonna get them.
11:08
SPEAKER_02: So I just remember that.
11:09
SPEAKER_02: That like sticks with me all the time thinking about the black hills.
11:12
SPEAKER_00: It never gets old to me.
11:13
SPEAKER_00: Walking up to a tree, sticking your nose in a crack of the bark, you know, and just inhaling, and it is this powerful scent of vanilla.
11:23
SPEAKER_00: It's it's amazing.
11:24
SPEAKER_00: It's I just I never get tired of it.
11:26
SPEAKER_02: It's so good, and the trees are there because it's this hilled province in the middle of the prairies.
11:31
SPEAKER_02: So the Black Hills are describing that the fact that there are a lot of Ponderosa pine trees in this hilled region in the middle of the prairie lack of trees, I suppose, in the Great Plains.
11:41
SPEAKER_02: It's a rather anomalous geologic province for that part of the world, for the South Dakota, eastern Wyoming, eastern Colorado region.
11:49
SPEAKER_00: That's right.
11:50
SPEAKER_00: And it's a perfect example again of how the geology determines the biology, you know, those trees and and everything is, though.
11:58
SPEAKER_01: That's a great point.
11:59
SPEAKER_00: Um, you know, you have this uplift that creates different weather patterns, so you have different moisture content within the hills that you don't get in the surrounding prairie, and that's why the trees are there.
12:09
SPEAKER_00: And it's so again, the geology determines the biology.
12:12
SPEAKER_00: Um, much, yeah.
12:14
SPEAKER_02: We keep blaming that point because it's maybe the most important thing you can take away from Plan G is that the geology determines the biology.
12:20
SPEAKER_02: So we're gonna start out with three basic rock suite formations for the most part.
12:25
SPEAKER_02: Let's start at the oldest, and this is exposed in the core, and this is where actually Mount Rushmore, this you know, the famous monument of four US presidents carved into the side of a mountain.
12:36
SPEAKER_02: It's carved into the oldest rocks around in the Black Hills.
12:40
SPEAKER_00: Yeah, this igneous and metamorphic core.
12:43
SPEAKER_00: Okay, and as you said, uh Mount Rushmore's in this famous landmarks like Black Elk Peak, which is formerly called Harney Peak, and so on, they're within this granitic core to the Black Hills.
12:54
SPEAKER_00: You also have some uh intrusive and some extrusive igneous activity in the northern part of the Black Hills, things like Devil's Tower, Bear Butte, the Missouri Buttes, that's another suite of rocks.
13:07
SPEAKER_00: And then you have these sedimentary rocks that rim and they kind of surround the entire hilled area, forms famous things like the racetrack, and we'll talk about that in a little bit more detail on what exactly that is because it literally, you and I have stood right in the middle of the racetrack contemplating where we're gonna take our next fantastically rippled sandstone, you know.
13:29
SPEAKER_02: Yes, absolutely.
13:31
SPEAKER_00: Um and then these sedimentary rocks is where Wind Cave lies and Jewel Cave.
13:37
SPEAKER_00: Look, there are so many caves that surround the Black Hills, and we want to touch upon that too a little bit later on in this episode.
13:46
SPEAKER_00: So, you know, from a 30,000-foot perspective, you look down, everything looks like this north-south-shaped eyeball.
13:55
SPEAKER_00: It's this eyeball kind of bullseye pattern.
13:58
SPEAKER_00: Because, you know, when you take these rocks and you shove them up and then you throw erosion into the mix, it forms this pattern because all the way around the hills are the same rocks, the same layers expose themselves all the way around.
14:14
SPEAKER_00: So, Jesse, let's go ahead, let's just start.
14:17
SPEAKER_00: We're gonna start old and work our way to the youngest, and uh that's what we do in geology.
14:21
SPEAKER_00: So we're gonna start with the granitic core.
14:24
SPEAKER_00: The metamorphic rocks are actually the oldest, but they're they got metamorphosed by this granite.
14:28
SPEAKER_00: So the core has this really uh awesome pegmatite, and you and I have like we've collected tons of this stuff.
14:36
SPEAKER_00: We have great examples of this pegmatite all over the place in our yards and stuff, that formed between like two billion and 1.8 billion years ago as a part of what's called the trans-Hudson erogeny.
14:50
SPEAKER_00: Jesse, I like I feel like I need to throw this to you because you know you're into these super old rocks, and this is like your thing.
14:58
SPEAKER_00: So what can you tell us about this?
15:01
SPEAKER_02: Yeah, so this is kind of home turf a little bit.
15:03
SPEAKER_02: So you you threw out a couple terms there that I think we need to define really quickly.
15:06
SPEAKER_02: First of all, it's granite, is it's what Mount Rushmore was carved into.
15:10
SPEAKER_02: It is the intrusive rock.
15:12
SPEAKER_02: So it's a magma that crystallized in the earth.
15:14
SPEAKER_02: It has these big crystals in it.
15:16
SPEAKER_02: Pegmatite was another term you used, and that is a rock that has super large crystals.
15:22
SPEAKER_02: So these are crystals that are larger than a centimeter or two, and all of the minerals in that rock are very, very large.
15:29
SPEAKER_02: That's the definition of a pegmatite.
15:31
SPEAKER_00: And we we do have an earlier uh, I think it was a geo short Jesse on pegmatites and so on, but really like a really quick rundown in terms of how pegmatites form is it's this it forms in the late stages of the cooling of a massive granitic body.
15:50
SPEAKER_00: And what you end up with then is this watery solution.
15:54
SPEAKER_00: Okay, it's not really even magma, it's mostly water that is super, super salty.
16:00
SPEAKER_00: And why that's important is because these crystals then that can grow out of this water, because it's watery, the ions can fly through it and grow really large in a very short period of time.
16:14
SPEAKER_00: So it kind of breaks the rule of of igneous rocks, you know, where minerals they need time to grow, and the slower it cools, the bigger they get.
16:22
SPEAKER_00: Pegmatites they form late stage, but the crystals actually get big really fast.
16:28
SPEAKER_02: Yeah, so you can think of uh a granite or an igneous rock forming from a sticky magma, and a pegmatite forms from this really fluid-rich, very easy-to-flow, low viscosity fluid that is a lot more, a lot of water in it, and so it can flow really easily.
16:42
SPEAKER_02: So these things are intimately related, though.
16:44
SPEAKER_02: So the granite here that is in the core of the Black Hills is about 1.7 billion years old.
16:50
SPEAKER_02: There's some older pieces, some younger pieces.
16:53
SPEAKER_02: The pegmatites formed late stage, and the pegmatites are really important because they contain a ton of very valuable mineral deposits.
17:01
SPEAKER_02: So things like tin or aluminum or lithium or gold.
17:06
SPEAKER_02: There was a big gold rush in the Black Hills, and there was a you know a movie made or um and there was a series made about that called Deadwood.
17:12
SPEAKER_02: So these pegmatites are economically and societally really, really important.
17:16
SPEAKER_00: By the way, that series, Deadwood, is absolutely amazing.
17:21
SPEAKER_00: It's a little older now, but man, it is really good.
17:24
SPEAKER_00: But yeah, you and I, Jesse, we collected lithium in the form of spodjamin.
17:31
SPEAKER_00: So you know, we would you remember this?
17:34
SPEAKER_00: This was the Edda mine, I think, right?
17:36
SPEAKER_00: Yes.
17:36
SPEAKER_00: We go to the Edamine, okay.
17:38
SPEAKER_00: We found it.
17:39
SPEAKER_00: We get there, and there is a sign, there's a gate across.
17:43
SPEAKER_02: Let me interrupt you real quick, Chris, because this is another cool part about the Black Hills, is that there are all these old mines that are no longer actively mined, most of them, but they're old mine shafts.
17:52
SPEAKER_02: You can find all sorts of historical mine, you know, mine shafts or actually big pits in the ground.
17:58
SPEAKER_02: So the Eda mine is one of these abandoned mine sites.
18:01
SPEAKER_00: That's right.
18:02
SPEAKER_00: And there was a there was a fence across the entrance because you had to go through this like blasted out tunnel to get through it.
18:08
SPEAKER_00: And that the owner of the mine is just this privately owned thing, right?
18:12
SPEAKER_00: It like you said, it's not active anymore.
18:14
SPEAKER_00: And he put a gate across it with a sign that said, if you do not have permission, and then he got really descriptive with what was gonna happen to you if he found you there.
18:23
unknown: You remember that?
18:24
SPEAKER_01: Yeah, it was crazy.
18:25
SPEAKER_00: We're like, oh my gosh, this is really bad.
18:28
SPEAKER_00: Dismember, man.
18:29
SPEAKER_00: And then it was there was a bad.
18:32
SPEAKER_00: And there's a phone number, and so we call the phone number, and this guy picks up, he's super friendly.
18:38
SPEAKER_00: He's like, Yeah, hey, um, we're like, Well, I'm standing here in front of your mind, I'm looking at the sign, just wondering if we can go in.
18:45
SPEAKER_00: He's like, Well, I'd really like to meet you first.
18:46
SPEAKER_00: So we go to his shop in Keystone, yep, and he was the friendliest guy in the world.
18:52
SPEAKER_00: I yeah, he's like, Oh, yeah, sure, no problem.
18:55
SPEAKER_00: Just go in, take whatever you want, you know.
18:57
SPEAKER_00: And so we did it the right way, but man, had me scared for a while.
19:01
SPEAKER_02: And we really listened the most to the take whatever you want part because spodamine is this it's a lithium amphibol, is what it is.
19:08
SPEAKER_02: It's a mineral that has lithium as a major constituent phase.
19:11
SPEAKER_02: It's not a big lithium source right now because it's extremely hard to get the lithium out of the spodumine, but I'll never forget, Chris.
19:18
SPEAKER_02: We walk into this mine thing, and there's like the usual pit with the water, like a little lake right in the middle of this mine pit, this ancient mine shaft.
19:25
SPEAKER_02: And on the wall is a massive spodgamine crystal.
19:29
SPEAKER_02: I mean, this is the size of a car.
19:31
SPEAKER_02: It's probably eight feet in diameter, and you know, it pokes out of the wall, but it goes into the wall, who knows how long, but it's 30 feet long or something.
19:40
SPEAKER_02: This big white crystal of spodgamine.
19:43
SPEAKER_02: That was so cool.
19:44
SPEAKER_02: We took some big chunks of spodgamine.
19:46
SPEAKER_02: I still have some at my house right now.
19:47
SPEAKER_02: It's they're they're totally cool.
19:49
SPEAKER_00: Do you remember that perfect one that you spent like two hours trying to get?
19:52
SPEAKER_00: It was maybe a foot in height.
19:56
SPEAKER_00: It was right in the wall, and you gave up, you couldn't do it.
19:59
SPEAKER_02: Never did get it.
20:00
SPEAKER_02: I wasn't uh wasn't gritty enough.
20:02
SPEAKER_02: Anyway, back on point.
20:04
SPEAKER_02: Back on point.
20:04
SPEAKER_02: So the pegmatites are related to the granites, they're the late stage of the fluids of the granite.
20:09
SPEAKER_02: So as the granite crystallizes, the stuff that doesn't fit into the granite minerals gets pushed into the pegmatites, and that's why all these valuable elements are concentrated in pegmatites.
20:19
SPEAKER_02: So the granite, how did the granite form?
20:22
SPEAKER_02: Because the granite formed and then the pegmatites are directly related to the granite.
20:26
SPEAKER_02: The granite is part of this 1.7 billion year old suite of igneous intrusions, which are actually very prevalent around the globe.
20:37
SPEAKER_02: There's a huge pulse of granite formation between 1.9 and 1.7 billion years ago.
20:45
SPEAKER_02: And it's not super well understood why there are so many of these rocks of this age around.
20:52
SPEAKER_00: Okay.
20:52
SPEAKER_00: I feel the need right now to call you Dr.
20:54
SPEAKER_00: Rymank because you're acting very doctor-ish right now.
20:57
SPEAKER_00: Can you get to the point, please?
20:59
SPEAKER_02: Let's keep my finger up in there a little bit.
21:02
SPEAKER_02: So anyway, this granite is formed.
21:03
SPEAKER_02: It's it's formed by melting of stuff.
21:05
SPEAKER_02: It's melt partially melting the mantle, it's partially melting ancient continental crust, but stuff's melting, it's forming the granite, and the granite is intruding.
21:13
SPEAKER_02: But really, the granite represents what's underneath of all of our feet in North America, in most parts of North America, especially in the Midwestern plain states.
21:23
SPEAKER_02: If you would drill down, you would eventually hit what we call basement gneiss or basement rocks.
21:28
SPEAKER_02: That is this granite, this type of granite, or different forms of this granite, and metamorphosed rocks.
21:34
SPEAKER_02: Most of it is this age or older.
21:37
SPEAKER_02: So basically anywhere from where I am sitting in Pennsylvania, all the way out, probably to Eastern California, maybe not quite that far, maybe into Nevada or Montana.
21:50
SPEAKER_02: If you would drill down, you would hit rocks kind of like this this age, granites and metamorphosed rocks.
21:57
SPEAKER_00: Wow, that's amazing.
21:58
SPEAKER_00: I do want to take this opportunity.
22:00
SPEAKER_00: Though I rip on you a lot for getting in the weeds and so on, okay, and being doctor-ish and so on.
22:05
SPEAKER_02: But well, I do get into weeds a lot.
22:07
SPEAKER_00: I am gonna take this opportunity though to say that like you are an expert in this.
22:12
SPEAKER_00: I mean, this is your thing, and I I have a deep respect.
22:16
SPEAKER_00: I mean, like, you know so much about this kind of stuff.
22:19
SPEAKER_00: It's gotta be hard to like distill that down and pick and choose what you want to talk about with this because you have this like just this deep understanding of these kinds of rocks.
22:32
SPEAKER_00: There you go.
22:33
SPEAKER_00: There's your compliment.
22:34
SPEAKER_02: It's really hard to be me.
22:35
SPEAKER_02: I think the take-home point is it's really hard to be me.
22:38
SPEAKER_02: Yeah, I agree.
22:38
SPEAKER_02: It's hard to be me, definitely.
22:40
SPEAKER_00: I I bet it is.
22:41
SPEAKER_00: I bet it is.
22:42
SPEAKER_00: All right, well, let's go into.
22:45
SPEAKER_00: Can we can we move on?
22:46
SPEAKER_00: Are we good?
22:46
SPEAKER_00: Yeah, yeah, absolutely.
22:47
SPEAKER_00: Let's do it.
22:47
SPEAKER_00: All right.
22:48
SPEAKER_00: Anyway, you have this around the granite, kind of like so.
22:52
SPEAKER_00: The granite makes the core.
22:54
SPEAKER_00: This, like the inside of the of the mountain range, right?
22:57
SPEAKER_00: The very central part of it.
22:59
SPEAKER_00: Surrounding this is a very diverse set of beautiful metamorphic rocks.
23:03
SPEAKER_00: These things are amazing, lots of schists, um, some gneiss, not not a whole lot of gneiss, but a lot of schist and and a lot of phyllites and slates and things like this, with this really diverse host of minerals that go along with it.
23:15
SPEAKER_02: Yeah, we uh we collected some amazing starlights.
23:18
SPEAKER_02: Probably the best rock I have.
23:19
SPEAKER_02: One of the best rocks I have in my collection is starlights.
23:22
SPEAKER_02: These little, you know, maybe one centimeter crosses.
23:25
SPEAKER_02: They're beautiful.
23:25
SPEAKER_02: With these metamorphic rocks, we can tell really precisely the pressures and temperatures at which those metamorphic rocks formed.
23:32
SPEAKER_02: So starlight plus the other minerals in the rock, we can calculate very clearly the pressure and temperature, the depth and temperature at which those metamorphic rocks were metamorphosed at.
23:42
SPEAKER_00: Yeah, that's a very good point.
23:44
SPEAKER_00: I also do want to say this um that all of the rocks that we are talking about here were collected legally.
23:50
SPEAKER_00: We had either permission or they were taken on national forest property, not national park, national forest, and it is legal to collect there.
24:00
SPEAKER_00: So we didn't break any laws in doing this or anything like that.
24:04
SPEAKER_02: Excellent point.
24:04
SPEAKER_00: You know, but also I live in a log home and I have two fireplaces, and the walls behind my fireplace are made out of my rocks, and a lot of them, a lot of them were taken from the Black Hills.
24:19
SPEAKER_02: You have an amazing fireplace, and you built those, those, um, the the what that the columns.
24:26
SPEAKER_02: You built the columns outside of your driveway with your rocks.
24:29
SPEAKER_02: Super, super spectacular.
24:30
SPEAKER_02: So the metamorphic rocks and the granite, the two things combined form the what we call the Precambrian core.
24:37
SPEAKER_02: This is rocks older than about 650 million years old.
24:41
SPEAKER_02: We also use this term crystalline basement.
24:44
SPEAKER_02: So it's crystalline rocks, metamorphic and igneous rocks.
24:47
SPEAKER_00: Well, the metamorphic rocks are there because there were rocks that existed there when this granite intruded, the trans-Hudson erogeny, this granite that happened, that cooked and baked and metamorphosed these rocks that we're talking about.
25:01
SPEAKER_00: And that's why they kind of surround this core.
25:04
SPEAKER_02: Exactly.
25:04
SPEAKER_02: And that's that's the interior core to the black hills.
25:07
SPEAKER_02: So that's the old part of the bullseye, the center of the bullseye.
25:10
SPEAKER_02: And now we're gonna kind of work our way out from the bullseye, and we're gonna move into much younger rocks because we have old rocks, granite and gneisses.
25:18
SPEAKER_02: Do you want to interject something?
25:19
SPEAKER_00: I do actually, but go ahead, finish your thought.
25:21
SPEAKER_00: Finish your thought.
25:22
SPEAKER_00: Yeah.
25:23
SPEAKER_02: Because we have old igneous and metamorphic rocks, and there are sediments sitting right on top of there.
25:27
SPEAKER_02: So that's what we call an unconformity, a gap in time.
25:30
SPEAKER_00: I want to just ask, though, you know, I'm asking the listener to think about this a second.
25:36
SPEAKER_00: Where do granites and these kinds of metamorphic rocks form?
25:43
SPEAKER_00: Right?
25:43
SPEAKER_00: If you think about that for a second, that's an important question, right?
25:48
SPEAKER_00: They form really deep inside the earth.
25:52
SPEAKER_02: Let me interrupt there and put some numbers on this, Chris.
25:54
SPEAKER_02: Yeah.
25:55
SPEAKER_02: The estimates for the metamorphic pressures and temperatures that these rocks experience, the depth and temperatures that they were metamorphosed at, are 800 degrees centigrade and six kilobars, which is basically like 18 or 20 kilometers deep in the crust.
26:08
SPEAKER_02: So pretty damn deep and pretty hot.
26:11
SPEAKER_00: Yeah, that's that's important because you look at this and you you think, oh, here they are at the surface.
26:17
SPEAKER_00: But as a geologist, you need to look at this differently and say, well, okay, here they are.
26:22
SPEAKER_00: They're 7,000 feet above sea level right now.
26:24
SPEAKER_00: These things formed deep under immense pressure, immense temperatures.
26:30
SPEAKER_00: So how did they get where they are, right?
26:33
SPEAKER_00: I mean, that's a hugely important part of the story.
26:36
SPEAKER_00: Those rocks that we just got done talking about, the igneous core, the metamorphic rocks that surround the igneous core formed really deep.
26:47
SPEAKER_00: Okay, this is a part of a mountain-building event.
26:50
SPEAKER_00: You know, the mountains were uplifting and then erosion happens, and more uplift and more erosion.
26:56
SPEAKER_00: Well, if you keep doing this, eventually these rocks get closer and closer and closer to the surface.
27:03
SPEAKER_00: And eventually they're at the surface of the earth.
27:08
SPEAKER_00: First of all, that's amazing to me because you the amount of time involved in this, this deep time that we get into in geology all the time, that's that's crazy, right?
27:19
SPEAKER_00: The amount of that it would take to do this, to to shed off the mountains, uplift, shed more off, uplift, and just keep doing this until they're at the surface, right?
27:28
SPEAKER_00: Now, what you're talking about then is here they are now, exposed at the surface, eroded relatively flat.
27:37
SPEAKER_00: Okay.
27:38
SPEAKER_00: And then what we know happened, and we know this happened, is that a sea invaded.
27:47
SPEAKER_00: Okay, a shallow sea, and it was depositing rocks like sandstones, limestones, breches, conglomerates in some cases, shale, the common sedimentary rocks, right?
28:02
SPEAKER_00: On top of these ancient roots, right?
28:08
SPEAKER_00: Now, how do we know that that's what had to happen?
28:12
SPEAKER_00: How do we know that the granite, and I'm throwing this to you and I want you to take it, but how do we know that the granite didn't just intrude those sedimentary rocks?
28:24
SPEAKER_00: How do we know that the granite formed much, much older, got brought to the surface through this really time-intensive process, and then a sea invaded and began depositing cold sedimentary rocks on top?
28:37
SPEAKER_00: How do we know that?
28:38
SPEAKER_02: Yeah, well, that's a great question and a really kind of fundamental one.
28:41
SPEAKER_02: There are some very clear tests that either of these hypotheses would predict.
28:46
SPEAKER_02: So if the granite intruded into the sedimentary rocks, that would predict that the granite, which is much hotter, would have metamorphosed the sedimentary rocks.
28:54
SPEAKER_02: So it would have heated up the sedimentary rocks around it, it would have baked them, and the granite should be younger than the sedimentary rocks around it.
29:02
SPEAKER_02: If the granite instead was there originally and sediments were deposited on top of it, we would expect no baked contact.
29:09
SPEAKER_02: We would expect no metamorphism of the sediments.
29:12
SPEAKER_02: And actually, instead, we would expect the granites to be weathered in some areas.
29:16
SPEAKER_02: They would be a little bit weathered and then sediments directly on top of it.
29:19
SPEAKER_02: And also the granites would be much older than the sediments.
29:22
SPEAKER_02: So we can use both geochronology and field observations to test both of these situations.
29:28
SPEAKER_02: And the result is really clear.
29:30
SPEAKER_00: It is, and that's exactly what we see.
29:32
SPEAKER_00: We see a what we call in geology a lot, at least I refer to it this way.
29:36
SPEAKER_00: I don't know if we is the proper term.
29:38
SPEAKER_00: I call it a knife-like contact.
29:40
SPEAKER_00: Do you use that term at all?
29:41
SPEAKER_02: Yeah, yeah, for sure.
29:42
SPEAKER_02: It's super sharp.
29:43
SPEAKER_02: It just represents how this is a really gradational boundary.
29:47
SPEAKER_02: This is a super sharp boundary.
29:49
SPEAKER_02: If you walk on a beach, like go to the beach of Maine, think of the beach in Maine, rocky shoreline, and you have sediments being deposited right on top of there.
29:56
SPEAKER_02: That's a knife-like boundary.
29:58
SPEAKER_02: And you can see these forming in the modern earth, and that's what we see in the ancient rock record.
30:02
SPEAKER_00: That's right.
30:03
SPEAKER_00: It is there's no gradual change.
30:07
SPEAKER_00: You have igneous and intensely metamorphosed rocks, and then right above them, no gradation at all, sandstones, shales, limestones, unmetamorphosed, unaltered, unheated, on everything.
30:23
SPEAKER_00: We call that in geology an unconformity.
30:26
SPEAKER_00: We've talked about this before in our geologic time episode.
30:29
SPEAKER_00: It's what's called a non-conformity, and it is that never gets old for me to go there, put your finger on that knife-like contact, and like it makes the hair stand up on the back of my neck because you're looking at a billion years of missing time.
30:47
SPEAKER_02: And that's just pretty freaking amazing.
30:50
SPEAKER_00: It is, it absolutely is.
30:52
SPEAKER_00: That's how we know that's what happened.
30:55
SPEAKER_00: By the way, this sea was called the Western Interior Seaway, and this was something that go ahead.
31:02
SPEAKER_02: And it deposited rocks all over the middle of the North American continent, all the way up into Canada, all the way down into Texas.
31:12
SPEAKER_02: There are the same basic sequence of sediments all over this whole entire province because this sea invaded into the continent and started laying down a lot of different types of sediments, and we can trace these laterally for a long time.
31:25
SPEAKER_00: And so you have these sedimentary rocks getting deposited like layer upon layer upon layer of sandstones, shales, limestones, brecchas, all these different things, right?
31:39
SPEAKER_00: Up to 7,500 feet of vertically accumulated sedimentary rocks.
31:45
SPEAKER_00: That's pretty impressive.
31:46
SPEAKER_00: You know, on any given day, you know, you step outside and you look up, and you know, maybe the clouds are what a mile high.
31:52
SPEAKER_00: Yeah.
31:52
SPEAKER_00: You know, and that's 5,200 feet, something like that, right?
31:56
SPEAKER_00: These rocks are one and a half times that thickness deposited by this sea.
32:02
SPEAKER_00: Okay.
32:04
SPEAKER_00: So that's what we have.
32:07
SPEAKER_00: Where we're at right now is just kind of well, it's kind of boring, right?
32:12
SPEAKER_00: We have granite, these basement roots and metamorphic rocks that are covered by about a mile and a half of sedimentary rocks that are, by the way, awesome sedimentary rocks.
32:22
SPEAKER_00: Oh yeah.
32:23
SPEAKER_02: So we get we got to give a brief little tour into some of the sedimentary rocks because you know, we have a lot of these sedimentary rocks from this this area, from the Black Hills, in our collections, Chris.
32:32
SPEAKER_02: So what's your go-to?
32:34
SPEAKER_02: I have two go-to's in my collection that I think we should highlight here for how cool these sediments are.
32:40
SPEAKER_00: Yeah, I I know what you're gonna say.
32:42
SPEAKER_00: And in when we were out there on one of our trips, I don't remember which one, you were super into the unk Papa sandstone.
32:49
SPEAKER_00: Okay, that is and it's uh so I know that's what your go-to is gonna be.
32:53
SPEAKER_00: It's an amazing sandstone.
32:55
SPEAKER_00: We first saw it, like we first saw it firsthand in the museum.
32:58
SPEAKER_02: Yeah, and we're like exactly okay.
33:01
SPEAKER_00: It's so we're like, wait a minute, that's called the Unk Papa Sandstone.
33:04
SPEAKER_00: We started doing research on the fly about like, all right, well, where can we find this?
33:09
SPEAKER_00: You know, we knew the geology of the Black Hills, so we spent, I don't know, Jesse, maybe I'm gonna say two full days looking for this sandstone.
33:17
SPEAKER_02: Yeah, like just driving.
33:19
SPEAKER_02: We put 800 miles on my truck.
33:21
SPEAKER_02: But let me describe it here because we walked into the museum and there's this huge cut slab.
33:26
SPEAKER_02: It looks like a countertop size cut slab.
33:28
SPEAKER_02: And this rock is pink and really finely layered with pink and purple and white and red and orange layers, like 10 layers per inch, basically.
33:39
SPEAKER_02: So really finely layered, and it has all these faults running through it.
33:43
SPEAKER_02: So the rock is faulted, it is just spectacular.
33:45
SPEAKER_02: Actually, there's a big slab of it in the Smithsonian Museum in Washington, D.C., too.
33:49
SPEAKER_02: The Unk Papa Sanchina.
33:50
SPEAKER_02: That's how does it really?
33:52
SPEAKER_02: So that's how spectacular this rock is.
33:54
SPEAKER_02: I mean, it is amazing, and we actually found it.
33:57
SPEAKER_02: We found an amazing outcrop, and you survived the rattlesnakes despite your massive fear of snakes.
34:03
SPEAKER_02: Oh man.
34:03
SPEAKER_00: We have a very unhealthy fear of snakes.
34:05
SPEAKER_00: It's so unrealistic.
34:06
SPEAKER_00: I understand it.
34:07
SPEAKER_02: I have to tell the story because we jumped out the truck.
34:10
SPEAKER_02: We pulled up in, you know, up behind this little hill in this ranch land, and we jump off the truck, and within two seconds, Chris is back in the bed of the truck, cowering down.
34:20
SPEAKER_02: I heard a snake.
34:21
SPEAKER_02: Did you guys hear a snake?
34:22
SPEAKER_02: Did you hear a snake?
34:23
SPEAKER_02: I think I heard a snake.
34:24
SPEAKER_02: There was a rattle over there, and we're just like, what are you doing, Chris?
34:28
SPEAKER_01: However, come on, man.
34:29
SPEAKER_00: To be fair, we saw probably 10 rattlers though.
34:33
SPEAKER_02: Yeah, it's at the popcorn.
34:34
SPEAKER_02: They're definitely sandstone.
34:35
SPEAKER_02: Like you didn't hear them right then, but they were definitely around.
34:39
unknown: They were.
34:40
SPEAKER_01: Oh my gosh.
34:41
SPEAKER_01: Including in your head.
34:42
SPEAKER_00: Uh I did overcome my fear, and we got some unk Papa Sandstone.
34:49
SPEAKER_01: We did.
34:49
SPEAKER_01: Yeah.
34:49
SPEAKER_00: Which is just very careful when we started flipping over rocks.
34:52
SPEAKER_02: Yes, very, very careful.
34:54
SPEAKER_00: Okay, so that's gotta be your go-to.
34:56
SPEAKER_00: My go-to, though, is the ripple marked sandstone.
34:59
SPEAKER_00: Oh yeah.
35:00
SPEAKER_00: I mean so cool.
35:02
SPEAKER_00: It's it's incredible.
35:04
SPEAKER_00: It's beautifully colored.
35:05
SPEAKER_00: It's reds and browns and tans and and the ripples are just spectacular.
35:12
SPEAKER_00: Yeah.
35:12
SPEAKER_02: So ripples are like waves on a beach, you know, the ripple marks you see in a beach where you walk in the water and it feels there's this undulation under your feet, right?
35:21
SPEAKER_02: That is it is that except preserved in the rock record, and they're really stunning.
35:25
SPEAKER_02: They're very cool to see in outcrops.
35:27
SPEAKER_02: So, Chris, is there anything else?
35:29
SPEAKER_02: We're we're running a little bit long here.
35:30
SPEAKER_02: Is there anything else you want to touch on for the sedimentary record before we move into the actual uplift?
35:36
SPEAKER_00: Right on top of the igneous and metamorphic rocks is a layer called, or it's a formation of rocks, actually called the Deadwood Formation, and that's what the or it's named after the town of Deadwood in the northern Black Hills.
35:49
SPEAKER_00: Um, this is where all the gold was, you know, that's what led to the gold rush there in the Black Hills and so on.
35:55
SPEAKER_00: Above that lies a very infamous layer of rock in the Black Hills that is locally called the Paha Sapa limestone.
36:04
SPEAKER_00: And paha sapa is Lakota for the Native Americans, that means hills that are black.
36:10
SPEAKER_00: And this limestone layer is why there are so many caves that surround the Black Hills.
36:18
SPEAKER_00: They are all over the place, and they're in this layer.
36:22
SPEAKER_00: This is a very famous rock layer, though, in the Western U.S.
36:25
SPEAKER_00: that outside of the Black Hills is called the Madison limestone, and literally it's everywhere.
36:31
SPEAKER_00: Okay, so it's a very broad blanket-like layer, very thick layer, like 400 to 600 feet thick layer of limestone in the western U.S.
36:43
SPEAKER_00: So locally in the Black Hills it's called Paha Sapa, but otherwise it's the Madison.
36:47
SPEAKER_00: Other notable formations, there's the spearfish formation.
36:50
SPEAKER_00: This is that really red shale layer.
36:53
SPEAKER_00: It's bright red, it's brilliant that surrounds the Black Hill.
36:58
SPEAKER_00: It forms what's called the Red Valley, which is this bullseye, literally, like it goes all the way around the Black Hills.
37:07
SPEAKER_00: It's called the Red Valley, or it's often referred to as the Racetrack because it's oval and it goes all the way around, so it's called the Racetrack.
37:15
SPEAKER_00: So those are like famous, notable layers that I think we'd be remiss if we didn't mention those.
37:20
SPEAKER_00: I grabbed um a big, huge chunk of the spearfish formation that forms the uh the Red Valley and I brought it home.
37:29
SPEAKER_00: It did not survive three Michigan winters.
37:33
SPEAKER_00: It absolutely turned into a puddle of clay.
37:36
SPEAKER_02: That's funny.
37:37
SPEAKER_02: So I mean, this is why it forms the racetrack, right?
37:39
SPEAKER_02: Because it is easily eroded and easily weathered and forms this valley that wraps all the way around.
37:44
SPEAKER_02: So this really nicely ties into the uplift, right?
37:48
SPEAKER_02: So the fact that this single rock unit that's buried in the earth can be exposed at the surface and form a big oval valley.
37:58
SPEAKER_02: That's really key to describing the uplift here and also the bullseye.
38:03
SPEAKER_02: It's really points us to the bullseye that is the Black Hills.
38:07
SPEAKER_00: So let's try to paint a picture, right, Jesse?
38:10
SPEAKER_00: And I want you to jump in because this is I want I want the listeners, our listeners to envision this, right?
38:15
SPEAKER_00: Okay, so we have this basement rock, the trans Hudson erogeny, granite, and metamorphic.
38:19
SPEAKER_00: We've got a mile and a half of sedimentary rocks.
38:22
SPEAKER_00: And then something happens that pushes all of this stuff up from the bottom.
38:31
SPEAKER_00: Okay, and that forms in geology, we call that a structural dome.
38:35
SPEAKER_00: What happened is a very famous mountain building event.
38:39
SPEAKER_00: It's called the Laramitarogeny, and that's not important at all.
38:43
SPEAKER_00: We don't have to know the names of it, but that's what it's called.
38:45
SPEAKER_00: After like Laramie, Wyoming, and Laramiderogeny.
38:48
SPEAKER_00: That mountain building event is what formed the Rocky Mountains.
38:51
SPEAKER_00: But what happened is for whatever reason, a stray blob of magma ascended from really deep, not straight up, it made its way east.
39:04
SPEAKER_00: Because the Black Hills are they're pretty far east from the Rocky Mountains.
39:08
SPEAKER_00: And it it rose up kind of to this, you know, it went up, it went east, it went up, it went east, and so on.
39:13
SPEAKER_00: And it encountered that lid of basement rocks.
39:19
SPEAKER_00: The granite that you talked about, those basement granites and metamorphic rocks, and it shoved them up.
39:25
SPEAKER_00: Okay, now remember, they also have a mile and a half of sedimentary rocks on top of them.
39:29
SPEAKER_00: So this blob, think about like kind of like a piston.
39:33
SPEAKER_00: I don't know, does that work?
39:34
SPEAKER_00: Can I call it a piston?
39:35
SPEAKER_02: I think absolutely like a piston.
39:37
SPEAKER_02: And I think there are many ways to do uplift like this.
39:42
SPEAKER_02: So we get we have to envision this bullseye a minute to kind of get the picture of this piston.
39:47
SPEAKER_02: So imagine you have a whole bunch of layered rocks.
39:50
SPEAKER_02: So you have got this igneous and metamorphic rocks at below it.
39:53
SPEAKER_02: We've got a mile and a half of sediment on top of it.
39:56
SPEAKER_02: How do you get an oval structure?
39:58
SPEAKER_02: Well, you take your fist and you just punch up through that, you push up from a central point.
40:03
SPEAKER_02: That means that all those rocks form this dome.
40:06
SPEAKER_02: And then you erode those rocks down, and what you're left with is this bullseye pattern because uh it used to be a really high uplifted region that got weathered down, and now we see the cross section of that.
40:18
SPEAKER_02: We're looking into the bullseye pattern.
40:20
SPEAKER_02: And so there's many ways to do uplift.
40:23
SPEAKER_02: One of the ways is to take a big igneous intrusion and punch it up through there.
40:27
SPEAKER_02: The igneous intrusion would have to be very large to do this amount of uplift, and it was certainly possible.
40:32
SPEAKER_02: There's all sorts of magmatism associated with the layer miterogeny all throughout the western United States and uh in Canada.
40:40
SPEAKER_02: And so a stray piece of magma could have come up, but it had to be a big one though, to punch this stuff up and create this big dome pattern that then create a dome that then gets cut off, eroded down, and has this bullseye pattern that we see today.
40:55
SPEAKER_00: So if you can envision a like a blanket of shale sandwiched in between two other rocks, and those aren't important, right?
41:06
SPEAKER_00: But if you have a blanket of shale that covers the entire area, and then you shove up this granite and metamorphic rocks and the shale that's on top of it.
41:18
SPEAKER_00: Well, that shale then is gonna show up as you throw erosion into the mix.
41:22
SPEAKER_00: We're gonna start scrubbing away the rocks and the highest parts, right?
41:27
SPEAKER_00: And so on the flank of that highest part is gonna be that shale that's gonna go all the way around the black hills.
41:38
SPEAKER_00: That's what you get with a dome, is the same rocks show up all the way around the black hills.
41:46
SPEAKER_02: And those rocks, Chris, are tilted and they're tilted to reflect this dome.
41:51
SPEAKER_02: So on the eastern side, the rocks dive to the east, down into the ground.
41:55
SPEAKER_02: On the northern side, they dive down into the ground to the north.
41:58
SPEAKER_02: On the western side, they dive to the west, and the south, they dive to the south.
42:01
SPEAKER_02: And that tells you that all the rock layers that used to be flat, their sediments laid down flat, they were tipped up, and they were tipped up in this concentric zoning ring.
42:10
SPEAKER_02: And it's just a beautiful structural feature.
42:12
SPEAKER_02: It is a type locality for a dome that is cored by these deep igneous and metamorphic rocks.
42:18
SPEAKER_00: And then knowing a little bit about what we just said, the geology of the Black Hills, you always know where you are.
42:25
SPEAKER_00: You're looking at this pegmatitic granite, you're like, well, I'm in the middle of the Black Hills.
42:29
SPEAKER_00: Okay.
42:30
SPEAKER_00: If you go a little bit further out, you got these metamorphic rocks.
42:32
SPEAKER_00: You're like, well, I know where I am.
42:34
SPEAKER_00: If you go further away, you know that you're not near the core because you're in unaltered sedimentary rocks.
42:41
SPEAKER_00: But by looking at the direction that they're tilted, you know what side of the black hills you're on.
42:46
SPEAKER_00: It's a beautiful thing.
42:48
SPEAKER_00: The way this geology just very cool, it's very cool.
42:53
SPEAKER_02: And I we want to point out one sort of misconception or sort of common misconception here.
42:58
SPEAKER_02: The granite that is in the core, that is not the granite that did the uplifting.
43:02
SPEAKER_02: So we kind of have to give a little bit of review of the geology here to sort of rectify this common misconception.
43:08
SPEAKER_02: Remember that we have these really old sedimentary rocks that were metamorphosed 1.7 billion years ago.
43:14
SPEAKER_02: So now they're a metamorphic rock.
43:16
SPEAKER_02: We have a granite that did the metamorphosing 1.72 billion years old, sitting in the middle.
43:22
SPEAKER_02: That stuff was buried down beneath the earth.
43:26
SPEAKER_02: Then this inland sea came in and deposited a mile and a half of sediments on top of it.
43:31
SPEAKER_02: That was all flat lying.
43:32
SPEAKER_02: That granite was there.
43:34
SPEAKER_02: So that granite did not do the uplifting.
43:36
SPEAKER_02: We have to have another intrusion, another igneous event that actually does the uplifting.
43:42
SPEAKER_02: And this the piston, right?
43:43
SPEAKER_02: The piston, exactly.
43:44
SPEAKER_02: So this is a younger event, about 70, 60 million year old event.
43:49
SPEAKER_02: 65, yep.
43:50
SPEAKER_02: Nothing to do with the granite that is exposed in the middle of the Black Hills.
43:56
SPEAKER_00: Now we do see some of that igneous.
44:00
SPEAKER_00: Rock though on the northern part of the Black Hills.
44:04
SPEAKER_00: This went on for a long time, right?
44:06
SPEAKER_00: I mean, this this is this was not like a quick event.
44:09
SPEAKER_00: Okay.
44:10
SPEAKER_00: At the latter part of this like mountain building domal uplift, it caused cracking on the northern end of the Black Hills.
44:18
SPEAKER_00: And those cracks, they kind of extend east-west.
44:22
SPEAKER_00: And so you have this line of extrusive igneous rocks or shallow intrusive igneous rocks that form really famous landmarks like Bear Butte, Devil's Tower, the Missouri Buttes.
44:38
SPEAKER_00: All of that is a part of that dome will uplift igneous material that caused the whole thing.
44:45
SPEAKER_02: There's also other features associated with uplift, which is the needles, Chris, which is this.
44:49
SPEAKER_02: I remember in this trip you take the students on this great hike where basically you have to squeeze your way through really narrow gaps in granite spires.
44:59
SPEAKER_02: So you're kind of walking through these narrow hallways, like really narrow hallways in granite with big cliffs going up, up on either side of you.
45:08
SPEAKER_02: Sometimes you actually have to climb up in the air, you know, 10-15 feet in the air, because it's too narrow to walk through at ground level.
45:15
SPEAKER_02: And this is a weathering and erosion feature.
45:18
SPEAKER_02: It forms the needles part of the Black Hills.
45:20
SPEAKER_02: But this has to do with the fracturing of the granite, this 1.7 billion year old granite.
45:25
SPEAKER_02: It fractures, and when that piston pushes up from below, think of like uh, you know, a layer of mud.
45:31
SPEAKER_02: If you push up on a layer of mud, it's going to crack at the top.
45:34
SPEAKER_02: And the same thing is happening to this granite.
45:37
SPEAKER_02: You get all of these cracks in it, and those cracks get exploited by rain and by weathering and erosion, and they get made bigger, and you kind of end up with this rounded conical spire that is called the needles terrain in the Black Hills.
45:54
SPEAKER_02: It's beautiful, stunning.
45:56
SPEAKER_02: If you go into the Black Hills, go to the Needles, go for a hike.
45:59
SPEAKER_02: It's amazing.
46:00
SPEAKER_00: I think what you're referring to where I where I take these students is actually in the cathedral spires area of the Black Hills.
46:07
SPEAKER_00: It's called that because it it resembles church cathedrals.
46:11
SPEAKER_00: I mean, it's it's these just these beautiful spires of granite sticking up.
46:15
SPEAKER_00: It's pegmatitic granite straight up, right?
46:18
SPEAKER_00: And I just I don't know, found a place that there's no trail leading to it, but you can go pretty much anywhere you want.
46:25
SPEAKER_00: There's no law saying, hey, you can't go here.
46:27
SPEAKER_00: So Jenny and I a number of years ago were just messing around, banging around the Black Hills, and we kind of shimmy through this like 30-foot narrow crack in the granite, and we come upon you come out, and it looks like a Greek courtyard.
46:41
SPEAKER_00: It's just amazing.
46:43
SPEAKER_00: And so we all shimmy through there and go down into this like grassy courtyard area, right?
46:50
SPEAKER_00: Smack dab in the middle of the cathedral spires.
46:53
SPEAKER_00: Nobody knows you're there.
46:54
SPEAKER_00: Like, I mean, you you're gonna be the only people that are there.
46:57
SPEAKER_00: It's just it's amazing.
46:59
SPEAKER_00: It's it's beautiful, and and that's a that's a part of the Black Hills.
47:02
SPEAKER_02: And in some places, you know, the the road is carved out through the cathedral spires, right?
47:07
SPEAKER_02: You had the roads had to be cut out and cut a little tunnel through the granite to put roads into this region.
47:13
SPEAKER_02: Just very, very cool.
47:14
SPEAKER_02: It's an amazing part of the geologic story, and it represents this sort of last geologic event that has occurred in the Black Hills of South Dakota.
47:22
SPEAKER_02: It kind of summarizes and puts a capstone on this amazing geologic history.
47:27
SPEAKER_02: So, Chris, yeah, I think that's a wrap.
47:30
SPEAKER_02: What do you think?
47:31
SPEAKER_00: I do.
47:32
SPEAKER_00: You know, like I I just want to have have I done a good enough job of telling everybody how much I love the Black Hills.
47:39
SPEAKER_00: Have I done it?
47:40
SPEAKER_02: I think I think you I think you have, yeah.
47:42
SPEAKER_02: But you know what?
47:43
SPEAKER_02: You and I need to go back there.
47:44
SPEAKER_02: That's five million.
47:46
SPEAKER_00: It's a dangerous place for us to go though.
47:48
SPEAKER_00: Because like where do you stop?
47:51
SPEAKER_00: You know, you have to look, it has so much there.
47:56
SPEAKER_00: Yeah.
47:57
SPEAKER_00: Because you have the bad lands, and I'm telling you, these caves are amazing.
48:02
SPEAKER_00: We went to a new cave a couple years ago because of COVID.
48:05
SPEAKER_00: We couldn't get into Wind Cave, so I found another cave, and it's just like, oh my gosh, this place just gets better.
48:12
SPEAKER_02: You can spend so much time there, it's very cool.
48:14
SPEAKER_02: And actually, the Black Hills one thing we haven't mentioned is that it's probably most famous for the Sturgis bike rally every year, right?
48:20
SPEAKER_02: Oh, I've yeah, I've been there once.
48:22
SPEAKER_02: Yeah, you've you've gone to it.
48:23
SPEAKER_02: And you can, you know, it is a fun place to go to a casino and you know, have a couple drinks.
48:29
SPEAKER_02: It's just a really fun town.
48:30
SPEAKER_02: There's cool little cultural, you know, bits around, and the geology is amazing.
48:35
SPEAKER_02: So go to the Black Hills.
48:36
SPEAKER_00: I'll take a hard pass on the casinos, but I am gonna throw a shout out to one of my favorite towns.
48:43
SPEAKER_00: It's Hill City.
48:45
SPEAKER_00: It's in the core of the Black Hills.
48:48
SPEAKER_00: It is the coolest little cowboy town.
48:51
SPEAKER_00: The people there are so warm and friendly, and it's just it has such a cool vibe.
48:57
SPEAKER_00: And you know what?
48:58
SPEAKER_00: There are all kinds of these towns sprinkled throughout the Black Hills.
49:02
SPEAKER_00: The people are amazing.
49:04
SPEAKER_00: Uh, I just I I love pretty much I love the geology, I love the culture, I love the people, I love the geology.
49:13
SPEAKER_00: You know, it's just it's great.
49:15
SPEAKER_00: I love it.
49:15
SPEAKER_02: It's an amazing place.
49:17
SPEAKER_00: I'm passionate about the hills.
49:18
SPEAKER_02: So, you know, this is the episode of the Black Hills sponsored by Hill City, South Dakota.
49:23
SPEAKER_02: And um with that, let's wrap it up here, Chris.
49:26
SPEAKER_02: This was a fun episode.
49:28
SPEAKER_02: We got a lot of great memories there.
49:29
SPEAKER_02: Let's go make some more sometime soon.
49:31
SPEAKER_00: Let's do it.
49:31
SPEAKER_00: All right, cheers.
49:33
SPEAKER_02: Cheers.
49:42
SPEAKER_02: Don't forget, please follow us on social media.
49:45
SPEAKER_02: Uh, we are at Planet GeoCast.
49:46
SPEAKER_02: Give us a like and a subscribe and share with your friends.
49:49
SPEAKER_02: Share Planet Geo with your friends.
49:51
SPEAKER_02: We really, really value that.
49:53
SPEAKER_02: And those likes and reviews help the algorithm make us more discoverable.
49:57
SPEAKER_02: Take care.
49:58
SPEAKER_01: Good deal.
49:59
unknown: Um, I think that's a good one.