A Geology Bullseye: The Black Hills of South Dakota
The Black Hills happens to be one of our favorite places. We thought it was the right time to talk about them as people are getting the itch for summer to arrive so we can play in the mountains. The Black Hills is the perfect area to orient a young family to a life of adventure and respect for the outdoors. You can't get into too much trouble here and these mountains and the surrounding area offers so much to do. The Black Hills of South Dakota are a destination - not a speed bump on your way out West. Sit back and enjoy!
Jesse Reimink (00:03):
Welcome to PlanetGeo the podcast where we talk about our amazing planet, how it works and why it matters to you.
Chris Bolhuis (00:15):
Jesse Reimink. How we doing?
Jesse Reimink (00:17):
Chris! I'm okay. I've had a few bad days.
Chris Bolhuis (00:22):
Oh, that right? I don't think I, you know what, I we're just going to stop you there because I don't really don't want, hear about your bad day, but I do have a question for you. I got, I have to know, a few episodes back. I don't remember how far back it was, but you were publishing a paper and you were talking about like the process of maybe it wouldn't get published and all that. And I, I promised everyone that I would check back. So have we heard anything back on this paper?
Jesse Reimink (00:49):
Yeah, we did let's put it this way. The people who reviewed this paper, my colleagues who reviewed this paper did not really enjoy it so much. So
Chris Bolhuis (00:59):
Why, what was it? The content or the way it was written?
Jesse Reimink (01:02):
Well, the peer review process is very, it can be very disheartening. You have to not take it personal because you know, it's their job to be critical. And when I review papers, I, I lean a little bit. I try not be super critical and overly critical. I try not give reviews. Like I received.
Chris Bolhuis (01:23):
I want a few details on that.
Jesse Reimink (01:26):
You have to try and, not take it personal, I guess. So yeah, this paper
Chris Bolhuis (01:31):
Don't think you're doing a very good job of not taking it personal.
Jesse Reimink (01:34):
I'm trying. I'm trying really hard. No, I, you know, I always what I do and a lot of, a lot of people I know do this. When you get a review back, you read it really quickly read what the people had to say, get the decision. And if it's a negative one, you just put it away for a week. Like you just don't look at it again because it, it can't help, but feel a little hurtful, but you put it away for a week and then you come back to it and you can come back to it with a bit more tempered view. So this is one of those scenarios. So now we're back to the, working on the paper and revising it based on the comments. So maybe it'll get accepted, but it did get rejected with the invitation to be resubmitted to the journal. So,
Chris Bolhuis (02:14):
But it was rather harsh. Huh?
Jesse Reimink (02:16):
One of the three reviewers was a little bit harsh and one was moderate and one was kind of positive about it. So, you know,
Chris Bolhuis (02:24):
Was this turf protection maybe or not?
Jesse Reimink (02:26):
Uh,
Chris Bolhuis (02:27):
You can't say that. Can you?
Jesse Reimink (02:28):
Well, I don't know. The harsh people were anonymous. Two people were anonymous and one was wrote their name. So, you know,
Chris Bolhuis (02:34):
Wait a minute. That's, that's kind of BS. If you're going to rip somebody, you can't hide behind that. Can you really like that? I don't. That's not right.
Jesse Reimink (02:41):
Yeah. It's a conversation actively in our community. Like, should you sign your name all the time or not? I try and sign my name all the time. Unless I'm reviewing a paper from somebody very senior and I'm going to review it negatively that I might stay anonymous. But for the most part, I try and sign my name because I think it tempers my views or it tempers what I say. Okay. Which is a good thing.
Chris Bolhuis (03:03):
Okay. So everyone that got rejected, Jesse got rejected.
Jesse Reimink (03:07):
One of many, Maybe we'll have better news further on down the line, but the process continues.
Chris Bolhuis (03:12):
I'm not happy that you got rejected, but did make me smile a little bit. I don't know why
Jesse Reimink (03:17):
You're not happy but not super sad either. Huh? Wow. You're just really such a great friend, Chris. I really appreciate having you in my life. It's so rewarding. Yes.
Chris Bolhuis (03:27):
Everybody's have to be good at something I'm good at being your friend.
Chris Bolhuis (03:32):
Hey, let's
Jesse Reimink (03:32):
Go onto more exciting news, Chris, the Black Hills, this yeah. Is a location that I think not a lot of not Americans know about, but I think for me, it's probably one of my favorite places in the United States.
Chris Bolhuis (03:49):
it is for me too, you brought this up as a topic and I'm like, well, gosh, it's like, it's we're in the winter time, is this a good time to talk about the Black Hills? 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, totally have the itch to go. And I figured don't you think that some of our listeners have the itch to like right. It's we're starting to look forward to summer, right?
Jesse Reimink (04:13):
Yes. Let's look forward to getting out there in it. Yeah. And one place you need to consider when you're thinking about your summer travel plan is the Black Hills of South Dakota. They are unbelievable.
Chris Bolhuis (04:23):
Yeah. My wife and I, we used the Black Hills as the mountains to cut our teeth for our kids. Oh yeah. That's where we introduced them. We spent literally months in the Black Hills when they were young kids and you know, it was just a great way to introduce some skills to them.
Jesse Reimink (04:42):
You mean mountaineering skills, climbing, hiking, that kind of stuff,
Chris Bolhuis (04:45):
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. And so we spent 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.
Jesse Reimink (05:06):
It's an amazing place. And you and I have some pretty good shared experiences in the Black Hills. 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. And I was one of those kids 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, type localities for a lot of the stuff like pegmatites and certain mineral deposits. It's been mine for a long time. There's a lot of gold mining that used to take place there. And still a little bit there's TV shows written about the gold rush there, you know, super cool place.
Chris Bolhuis (05:45):
Yeah. It there's a lot there too. And I think, I think the Black Hills get overlooked a lot because Mount Rushmore is there Crazy Horse is there. 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. I'm not, I'm not knocking that at all. I think though that 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 out west. You know, I have to, I have to do this before. I really get into the west.
Jesse Reimink (06:18):
Yeah, yeah. To get into the real rugged Rockies and stuff. And in some ways it is that, like you said, you can't get into too much trouble there as far as outdoors activities. You know, the, the stakes are lower there than they are in the Rockies or in other parts of the mountains there. So
Chris Bolhuis (06:32):
That's correct. It, it, you know, 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. It's a, it's a great, it's, it's great climbing, but there is so much there, you know, 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. I mean, it, you look at that list of like, oh my gosh, there's a lot to do there. And you know, both of us, we've been a lot of places and to, to hold the Black Hills in such a steam, is, is it says something. Yeah.
Jesse Reimink (07:15):
So guide us through where we're going to to go, Chris, like, what's the flow here of this episode? What are we going to cover? Because you have taught geology students here in the Black Hills for what, 25 years, 20 years, something like that. Yeah. You've and you've been there with your family even more than that, you know, this like the back of your hand and you know, the geology. So where are we going to go? Where are we going to cover? Where are you taking us? You're going to lead this episode. I think in many ways,
Chris Bolhuis (07:42):
I think we're going to get into, we're going to dive into, of course the geology of the Black Hills. And, and basically there are three like suites or ages or events of rocks that happened. And we're going to go through each of those. And you're like, we want to paint a picture
Jesse Reimink (07:58):
And if you're going to go Black Hills, you have to pay attention. Because the rocks are spectacular. The rocks are really, really cool there. So you have to 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.
Chris Bolhuis (08:10):
You know, I'm going to do my best to paint like this kind of 30,000 foot view of the Black Hills. And, and, and I think you are going to bring in some detail about 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. And then also Jesse, like you said, we have a ton of shared experiences there. So there are going to be some, some stories involved in there
Jesse Reimink (08:37):
Might be a trip down memory later too. We'll try and keep that's PG rated for this, but yes. All right. So we're going to work through time, right? Chris, we're going to start old and work young in part because that's the structure of the Black Hills, the Black Hills geology. If you look at a geological map, it's this kind of bullseye where in the center of the oldest stuff is, and on the edges, the youngest stuff occurs and we're going to kind of work through the geologic history through time. Right,
Chris Bolhuis (09:01):
Right on. Yeah. I mean, like you said, the it's kind of this bullseye, it's a, it's more like a eyeball shaped bullseye that the eyeball would be oriented north south, you know? 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 odd long bullseye. It's not this per perfect, you know, circular kind of bullseye, but it definitely has that look to it for sure.
Jesse Reimink (09:29):
You know, one thing we have to mention as well, there there's a lot of cave systems here. I think win cave. National park. Is there, like you mentioned earlier, I'm not the biggest fan of caves. Really. I, I don't particularly like the idea of being on under a bunch of rock, but they are cool.
Chris Bolhuis (09:42):
They are, I think, and two, you know, win cave is one of my favorites. And, and I think honestly, we should do an episode on it alone because the geologic story of win cave is why I going back there with my students. It's not the most spectacular cave in the world to see it. Doesn't have all these like amazing spial thumbs and you know, things like this that formed after the formation of the cave itself. But the story behind it is the coolest. So this is the coolest cave I, to me,
Jesse Reimink (10:14):
Let's orient ourselves really. It quickly here, the Black Hills for those people who don't know, we have a fair number of international listeners. 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. There's a, a bunch of Hills that crop up in the middle of the great planes. So you've got flat relatively dare I say, boring prairies. And then you've got this round, like you said, 140 miles by 75 miles wide sort of uplift that has a whole bunch of trees on it. If you look on the Google earth for the Black Hills, you can see the darker area, that's all the tree covered things. And this is the black part of the hill, right? There are these Hills that have these Ponderosa Pines give it this amazing smell. 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 ponder Rosa Pines are the most amazing smell and this is the first time you're going to get them. So I just remember that that like sticks with me all the time, thinking about the Black Hills,
Chris Bolhuis (11:22):
It never gets old to me walking up to a tree, sticking your nose in a crack at the bark, you know, and just inhaling. And it is this powerful scent of vanilla. It's, it's amazing. It's I just, I never get tired of it.
Jesse Reimink (11:37):
It's so good. And the trees are there because it's this hilled province in the middle of the prairies. So the Black Hills are describing that the fact that there are a lot of pond Rosa pine trees in this hilled region in the middle of the Prairie lack of trees, I suppose in the great Plains, it's a rather anomalous geologic province for that part of the world, for the South Dakota, Eastern Wyoming, Eastern Colorado region.
Chris Bolhuis (12:00):
That's right. And it's a perfect example again, of how the geology determines the biology, you know, those trees and, and everything. That's exactly right. It is though. That's a great point. you know, you have this uplift, it 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. And it's, so again, the geology determines the biology, which yeah,
Jesse Reimink (12:25):
We keep levering that point. Cause it's maybe the most important thing you can take away from plant G is that the geology determines the biology. So we're going to start out with three basic rock suite formations for the most part, 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, it's carved into the oldest rocks around in the Black Hills.
Chris Bolhuis (12:51):
Yeah. This igneous and metamorphic core. Okay. And as you said, Mount Rushmore’s in this famous landmarks, like black elk peak, which is formerly called Harney peak. And so on they're within this, this core to the Black Hills, you also have some, intrusive and, 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. And then you have these sedimentary rocks that rim and they kind of surround the entire hill area forms, famous things like the racetrack. And we'll talk about that in a little bit more detail and what exactly that is, because it, it literally, you and I have stood right in the middle of the race, a track contemplating where we're going to take our next fantastically rippled sandstone, you know?
Jesse Reimink (13:40):
Yes, absolutely.
Chris Bolhuis (13:43):
And then these sedimentary rocks is where wind cave lies and jewel cave. Look, there are so many caves that surround the Black Hills and we want to touch upon that to, a little bit later on in this episode. So, you know, from a 30,000 foot perspective, you look down, everything looks like this north, south shaped eyeball, it's this eyeball kind of bullseye pattern 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, expos themselves all the way around. So, so Jesse, let's go ahead. Let's just start. We're going to start old and work our way to the youngest. And that's what we do in geology. So we're going to start with the GRT core. The metamorphic rocks are actually the oldest, but they're, they got metamorphoses by this granite. So the core has this really awesome pegmatite and you and I have like, we've collected tons of this stuff. We have great examples of this. Pegmatite all over the place in our yards and stuff that formed between like 2000000001.8 billion years ago as a part of what's called the trance Hudson androgyny, 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. So what can you tell us about this?
Jesse Reimink (15:11):
Yeah. So this is kind of home turf a little bit. So you, you threw out a couple terms there that I think we need to define really quickly. First of all, is granite is it's what Mount Rushmore was carved into. It is the intrusive rock. So it's a magma that crystallized in the earth that has these big crystals in it pegmatite was another term you used. And that is a rock has super large crystals. So these are crystals that are larger than a centimeter or two in all of the minerals in that rock are very, very large. That's the definition of AEG type.
Chris Bolhuis (15:42):
And we, we do have an earlier, I think it was a geo short Jesse on pegmatites and so on, but really like a really quick rundown. I, in terms of how pegmatites form is, it's this, it forms in the late stages of the cooling of a massive granites body. And what you end up with then is this watery solution. Okay. It's not really even magma. It's mostly water. That is super, super salty. Then 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. 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 pegmatites they form late stage, but the crystals actually get big really fast.
Jesse Reimink (16:38):
Yeah. So you can think of, 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. So these things are intimately related though. So the granite here that is in the core of the Black Hills is about 1.7 billion years old. There's some older pieces, some younger pieces, the pegmatites formed late stage and the pegmatites are really important because they contain a ton of very valuable mineral deposits. So things like 10 or aluminum or lithium or gold, there was a big gold rush in the Black Hills. And there was a, you know, a movie made or, and there was a series made about that called Deadwood. So these rocks are economically and societally really, really important
Chris Bolhuis (17:26):
By the way that series Deadwood is absolutely amazing.
Jesse Reimink (17:32):
It's
Chris Bolhuis (17:32):
Very, it's a, a little older now, but man, it is really good, but yeah, you and I, Jesse, we collected lithium in the form of spading. So I'll yes. You know, we, do you remember
Jesse Reimink (17:44):
This? This was the Edine I think, right?
Chris Bolhuis (17:46):
Yes. We go to the Edine. Okay. We found it. We get there and there is a sign there's a gate across.
Jesse Reimink (17:53):
Let me interrupt you real quick, Chris, because this is another cool part about the Black Hills is that there are all these old minds that are no longer actively mine, most of them, but they're old mind shafts. You can find all sorts of historical mind, you know, mind shafts or actually big pit in the ground. So the Edine is one of these abandoned mines.
Chris Bolhuis (18:12):
That's right. And there was a, there was that fence across the entrance, because you had to go through this like blasted out tunnel to get through it. And that the owner of the mine is just this privately owned thing, right? Like you said, it's not active anymore. 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 going to happen to you. If he found you there, do you remember that? Yes, it was crazy. We're like, oh my gosh, this is really bad dismemberment. And then it was, there was a
Jesse Reimink (18:41):
All sorts of
Chris Bolhuis (18:41):
Stuff. It was bad and there's a phone number. And so we call the phone number and this guy picks up. He's super friendly. He's like, yeah. Hey, 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. He's like, well I'd really like to meet you first. So we go to his shop in Keystone. Yep. And he was the friendliest guy in the world. I, yeah. It's so fun. He's like, oh yeah, sure, no problem. Just go in, take whatever you want. You know? And so we did it the right way, but had me scared for a while
Jesse Reimink (19:11):
We really listened the most to the take, whatever you want part because spa is this, it's a lithium mineral is what it is. It's a mineral that has lithium as a major constituent phase. It's not a big lithium source right now because it's extremely hard to get the lithium out of the Spodumene, but I'll never forget, Chris. We walk into this mine thing and there's like the U usual pit with the water, like a little lake right in the middle of this mind pit, this ancient mind shaft and on the wall is a massive SP crystal. I mean, this is the size of a car. 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. This big white crystal of spa mean that was so cool. We took some big chunks of spa. I still have some at my house right now. It's they're totally cool.
Chris Bolhuis (19:59):
Do you remember that perfect one that you spent like two hours trying to get, it was maybe a foot in height. It was right in the wall and you gave up, you couldn't do it.
Jesse Reimink (20:10):
Never did get it. I was wasn't gritty enough. Anyway.
Chris Bolhuis (20:13):
Back on point
Jesse Reimink (20:14):
Back on point. So the pegmatites are related to the granites. They're the late stage of the fluids of the granite. 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. So the granite, how did the granite form? Because the granite formed and then the pegmatites are ed directly related to the granite. The granite is part of this 1.7 billion year old suite of igneous intrusions, which are actually very prevalent around the globe. There's a huge pulse of granite formation between 1.9 and 1.7 billion years ago. And it's not super well understood why there are so many of these rocks of this age around,
Chris Bolhuis (21:03):
Okay. I feel the need right now to call you Dr. Reim, because you're at very doctor ish right now. Can you get to the point please?
Jesse Reimink (21:11):
Bit. Okay. So anyway, this granite is formed. It's it's formed by melting of stuff. 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, but really the granite represents what's underneath the, of all of our feet in north America. In most parts in north America, especially in the Midwestern plain states, if you would drill down, you would eventually hit what we call basement, nice or basement rocks that is this granite, this type of granite or different forms of this granite and metamorphoses rocks. Most of it is this age are older. 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, if you would drill down, you would hit rocks kind of like this, this age granites and metamorphose rocks.
Chris Bolhuis (22:08):
Well, that's amazing. I do want to take this opportunity though. I rip on you a lot for getting into weeds and so on. Okay. And being doctor and so on. But
Jesse Reimink (22:17):
Well I do get into weeds a lot.
Chris Bolhuis (22:18):
I am going to take this opportunity though, to say that like you are an expert in this, I mean, this is your thing. And I, I have a, a deep respect. I mean like, you know much about this kind of stuff. It's have to 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. There you go. There's your compliment. It's
Jesse Reimink (22:45):
Really hard to be me. I think the take home point is it's really hard to be me. Yeah. I agree. It's hard to be definitely.
Chris Bolhuis (22:51):
I, I bet it is. I bet it is. All right. Well, let's go into, can we, can we move on? Are
Jesse Reimink (22:57):
We good? Yeah. Yeah, absolutely. Let's do it. All right.
Chris Bolhuis (22:59):
Anyway, you have this around the granite kind of like, so the granite makes the core, this like the inside of the, of the mountain range, right? The very central part of it surrounding this is a very diverse set of beautiful metamorphic rocks. These things are amazing. Lots of shifts. some nice, not, not a whole lot of nice, but a lot of shifts and, and a lot of fill lights and slates and things like this with this really diverse host of minerals that go along with it.
Jesse Reimink (23:26):
Yeah. We, we collected some amazing star lights, probably the best rock. One of the best rocks I have in my collection is Staurolites these little, you know, maybe one centimeter crosses, they're beautiful with these metamorphic rocks. We can tell really precisely the pressures and temperatures at which those metamorphic rocks formed. So Staurolites 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 metamorphose at.
Chris Bolhuis (23:53):
Yeah, that's a very good point. I also do want to say this, that all of the rocks that we are talking about here were collected legally. We had either permission or they were taken on net national forest property, not national park national forest, and it is legal to collect there. So we didn't break any laws in, in doing this or anything like that.
Jesse Reimink (24:15):
Excellent point,
Chris Bolhuis (24:15):
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 rock. And a lot of them, a lot of them were taken from the Black Hills.
Jesse Reimink (24:30):
Like you have an amazing fireplace and you built those, those, the, the, what, the columns, you built the columns outside of your driveway with your rocks, super, super spectacular. So the metamorphic rocks in the granite, the two to two things combined full, the, what we call the PreCambrian core. This is rocks older than about 650 million years old. We also use this term crystalline basement. So it's crystal in rocks, metamorphic and igneous rocks.
Chris Bolhuis (24:57):
Well, the metamorphic rocks are there because there were rocks that existed there. When this granite intruded, the Trans-Hudson arrived need this granite that happened that cooked and, and baked and metamorphoses, these rocks that we're talking about. And that's why they kind of surround this core.
Jesse Reimink (25:14):
Exactly. And that's, that's the interior core to the Black Hills. So that's the old part of the bullseye, the center of the bullseye. And now we're going to kind of work our way out from the bullseye and we're going to move into much younger rocks because we have old rocks granites, and gneisses do you want to interject something?
Chris Bolhuis (25:30):
I do actually, but go ahead, finish your thought, finish your thought. Yeah.
Jesse Reimink (25:34):
Because we have old igneous and metamorphic rocks and there are sediments sitting right on top of there. So that's what we call unconformity a gap in time.
Chris Bolhuis (25:41):
I want to just ask though, you know, I'm asking the listener to think about this a second, where do granites and these kinds of metamorphic rocks form. Right? If you think about that for a second, that's an important question, right? They form really deep inside the earth.
Jesse Reimink (26:03):
Let me interrupt there and put some numbers on this, Chris. Yeah. The estimates for the metamorphic pressures and temperatures that these rocks experience, the depth and temperatures that they were metamorphosed at that are 800 degrees centigrade and six kilobars, which is basically like 18 or 20 kilometers deep in the cross. So pretty deep and pretty hot.
Chris Bolhuis (26:22):
Yeah. That's, that's important because you look at this and you, you think, oh, here they are at the surface. But as a geologist, you need to look at this differently and say, well, okay, here they are. They're 7,000 feet above sea level. Right now these things formed deep under immense pressure, immense temperatures. So how did they get where they are? Right. I mean, that's a hugely important part of the story, those rocks that we just got done talking about the igneous core, the metamorphic rocks that surround the igneous core formed really deep. Okay. This is a part of a mountain building event. You know, the mountains lifting and then erosion happens and more uplift and more erosion. Well, if you keep doing this, eventually these rocks get closer and closer and closer to the surface. And eventually they're at the surface of the earth. First of all, that's amazing to me because you, the amount of, of time involved in this, this deep time that we get into in geology all the time, that's, that's crazy, right?
Chris Bolhuis (27:30):
The amount that it would take to, to do this, to, to shed off the mountains, uplift, shed more off uplift and, and just keep doing this till they're at the surface right now, what you're talking about than is here, they are now at the surface eroded relatively flat. Okay. And then what we know happened, and we know this happened is that a sea invaded, okay. A shallow sea. And it was depositing rocks like sandstones, limestones, breccias conglomerates, in some cases shale the common sedimentary rocks, right on top of these ancient roots right now, how do we know that that's what had to happen? How do we know that the granite and I'm throwing this to you and I want you to, to take it, but how do we know that the granite didn't just intrude those sedimentary rocks? 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 how do we know
Jesse Reimink (28:48):
That? Yeah. Well that's a great question. And a really kind of fundamental one. There are some very clear tests that either these hypotheses would predict. So if the granite intruded into the sedimentary rocks, that would predict that the granite, which is much hotter would have metamorphoses the sedimentary rocks. So it would've heated up the sedimentary rocks around it, it, it would've baked them and the granite should be younger than the sedimentary rocks around it. If the granite instead was there originally and sediments were deposited on top of it, we would expect no baked contact. We would expect no metamorphism of the sediments. And actually instead we would expect the granites to be weathered in some areas, they would be a little bit weathered and then sediments directly on top of it. And also the granites would be much older than the sediments. So we can use both endocrinology and field observations to test both of these situations. And the result is really clear.
Chris Bolhuis (29:41):
It is. And that's exactly what we see. We see a, what we call in geology a lot. At least I refer to it this way. I don't know if we, as the proper term, I call it a knife-like contact. Do you use that term at all?
Jesse Reimink (29:52):
Yeah. Yeah. For sure. It's super sharp. It just represents how this is a really, this is not a radiational boundary. This is a super sharp boundary. If you walk on a beach, like go to the, the beach of Maine, think of the beach in Maine, Rocky shoreline, and you have sediments being deposited right on top of there, that's a knife like boundary and you can see these forming in the modern earth. And that's what we see in the ancient rock record.
Chris Bolhuis (30:13):
That's right. It is. There's no gradual change. You have igneous and intensely metamorphose rocks. And then right above them, no gradation at all, sandstones Shas limestone UN metamorphose unaltered, unheated, UN everything. We call that in geology. And unconformity, we've talked about this before in our, in our geologic time episode, it's, what's called a nonconform 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 and that's just
Jesse Reimink (30:59):
Pretty fricking amazing. It is.
Chris Bolhuis (31:01):
It absolutely is. That's how we know that's what happened by the way this, this sea was called the Western Interior Seaway. And this was something that go
Jesse Reimink (31:13):
Ahead 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. There are this same basic sequence of sediments all over this whole entire province, because the 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.
Chris Bolhuis (31:36):
And so you have these sediments rocks getting deposited. It like layer upon layer upon lay of sandstones, shales, limestones, breccias all these different things right up to 7,500 feet of vertically accumulated sediment rocks. That's pretty impressive. 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. Yeah. You know, and that's 5,000, 200 feet. Something like that. Right? These rocks are one and a half times that thickness deposited by this sea. Okay. So that's what we have where we're at right now is just kind of, what's kind of boring, right? We have granite these basement and metamorphic rocks that are covered by about a mile and a half of sedimentary rocks that are by the way. Awesome sedimentary rocks. Oh man.
Jesse Reimink (32:33):
We have, we get, we have to give a brief little tour into some of the sedimentary rocks because you know, we have a lot of these sedimentary rocks from these, this area from the Black Hills in our collections, Chris. So what's your go-to. I have two go-tos in my, in, in, in my collection that I think we should highlight here for how cool these sediments are.
Chris Bolhuis (32:51):
Yeah. I, I know what you're going to say. And, and when we were out there on one of our trips, I don't remember which one you were super in into the UNC Papa sand. Oh,
Jesse Reimink (33:00):
Okay.
Chris Bolhuis (33:00):
That is, and it's so I know that's what your go-to's going to be. It's an amazing sandstone. We first saw it. Like we first saw it firsthand in the museum.
Jesse Reimink (33:09):
Yeah. And we're like, exactly.
Chris Bolhuis (33:11):
Okay. It's so we're like, wait a minute. That's called the Unkpapa sandstone. And we started doing research on the fly about like, all right, well we, we find this, you know, we knew the geology of the Black Hills. So we spent, I don't know if Jesse, maybe I'm going to say two full days looking for this sandstone. Yeah. Like just driving. We put 800 miles on my truck,
Jesse Reimink (33:32):
But let me describe it here because we walked into the museum and there's this huge cut slab. It looks like a countertop size cut slab rock is pink and really finely layered with pink and purple and white and red in orange layers. Like 10 layers per inch, basically. So really fine layered. And it has all these faults running through it. So the rock is faulted. It is just spectacular. Actually. There's a big slab of it in the Smithsonian museum in Washington, DC too. The Unkpapa sandstone.
Chris Bolhuis (34:01):
That house has one as well
Jesse Reimink (34:02):
Does it really? So that's how spectacular this rock is. I mean, it is amazing. And we actually found it. We found an amazing outro and you survived the rattlesnakes despite your massive fear of snakes. Oh man,
Chris Bolhuis (34:14):
I have a very unhealthy fear of snakes. It's so unrealistic. I understand it.
Jesse Reimink (34:18):
I have to tell the story. Because we jumped out the truck. We pulled up, 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. crouching down. I heard a snake. Did you guys hear a snake? Did you hear a snake? I think I heard a snake. There was a rattle over there and we're just like, what are you doing, Chris? However on man,
Chris Bolhuis (34:40):
To be fair. We saw probably 10 Rattlers though. Yeah.
Jesse Reimink (34:44):
It's true. Pop there
Chris Bolhuis (34:45):
Definitely sandstone. Like
Jesse Reimink (34:46):
You didn't hear them right then, but they were definitely around.
Chris Bolhuis (34:50):
They were, oh my gosh.
Jesse Reimink (34:51):
Including in your head. Uh,
Chris Bolhuis (34:54):
I did overcome my fear and we got some Unkpapa sandstone we
Jesse Reimink (35:00):
Did. Yeah. Which is
Chris Bolhuis (35:01):
Very careful when we started flipping over rocks.
Jesse Reimink (35:03):
Yes. Very, very careful.
Chris Bolhuis (35:05):
Okay. So that's have to be your go-to my go-to though is the ripple marked sandstone. Oh yeah. I mean so
Jesse Reimink (35:12):
Cool.
Chris Bolhuis (35:12):
It's it's incredible. It's beautifully colored it's reds and browns and tans and, and the ripples are just spectacular. They are
Jesse Reimink (35:23):
So, 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? That is it. Is that except preserved in the rock record. And they're really stunning. They're very cool to see in out crops. So Chris, is there anything else we're, we're running a little bit long here. Is there anything else you want to touch on for the sedimentary record before we move into the actual uplift?
Chris Bolhuis (35:46):
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, the town of Deadwood and the Northern Black Hills. 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 above that lies a very infamous layer of rock in the Black Hills that is locally called Pahasapa limestone. And Pahasapa is Dakota for the native Americans. That means Hills that are black. And this limestone layer is why there are so many caves that surround the Black Hills. They are all over the place. And they're in this layer. This is a very famous rock layer though in the Western us, that outside of the black hill is called the Madison limestone and literally it's everywhere.
Chris Bolhuis (36:42):
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 us. So locally in the Black Hills is called Pahasapa, but otherwise it's the Madison other notable formations. There's the Spearfish formation. This is that really red shale layer. It's bright red. It's brilliant that surrounds the black hill. It forms what's called the red valley, which is this bullseye literally like it goes all the way around the Black Hills. It, 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. So those are like famous, notable layers that I think we'd be remiss if we didn't mention those. I grabbed, a big, huge chunk of the Spearfish formation that forms the, the red valley. And I brought it home. It did not survive three Michigan winters. It absolutely. It turned into a puddle of clay.
Jesse Reimink (37:47):
That's funny. So I mean, this is why it forms the racetrack, right? Because it is easily eroded and easily weathered informs this valley that wraps all the way around. So this really nicely ties into the uplift, right? So the fact this single rock unit that's buried in the earth can be exposed at the surface and form a big oval valley. That's really key to describing the uplift here. And also the bullseye it's really points us to the bullseye. That is the Black Hills.
Chris Bolhuis (38:18):
So let's try to paint a picture, right? Just scene. I want you to jump in because this is, I want, I want the listeners, our listeners to envision this, right. Okay. So we have this basement rock, the trans Hudson androgyny, granite and metamorphic. We got a mile and a half of sedimentary rocks. And then something happens that pushes all of this stuff up from the bottom. Okay. And that forms in geology, we call that a structural dome. Well, what happened is a very famous mountain building event. It's called the Laramide and that's not important at all. We don't have to know the of it, but that's what it's called after like lame Wyoming and Laramide, that mountain building event is what formed the Rocky mountains. But what happened is for whatever reason, a stray blob of magma ascended from really deep, not straight up, it made its way east because the Black Hills are they're pretty far east from the Rocky mountains. And it, it rose up kind of to this, you know, goat went up, it went east, went up, it went east and so on. And it encountered that lid of basement rocks, the granite that you talked about, those basement grants, memorable for rocks and it shoved them up. Okay. Now remember they also have a mile and a half a sediment rocks on top of them. So this blob think about like kind of like a piston, I don't know, does that work? Can I call it a piston?
Jesse Reimink (39:46):
I think absolutely like a piston. And I think there are many ways to do uplift like this. So we get, we have to envision this bullseye a minute to, to kind of get the picture of this piston. So imagine you have a whole bunch of layered rocks. So you have got this igneous and metamorphic rocks at below it. We've got a mile and a half of sediment on top of it. How do you get an oval structure? Well, you take your fist and you just punch up through that. You push up from a central point. That means that all those rocks form, this dome you, and then you erode those rocks down in what you're left with is this bullseye pattern because it used to be a really high uplifted region that got weathered down. And now we see the, the cross section of that.
Jesse Reimink (40:29):
We're looking into the bullseye pattern. And so there's many ways to do uplift. One of the ways is to take a big igneous intrusion and punch it up through there. The, the IRU would have to be very large to do this amount of uplift. And it is certainly possible. There is all sorts of magnetism associated with the layer Laramide all throughout the Western United States and in Canada. And so a straight piece of magma could have come up, had to be a big one though, to punch this stuff up and create this big pattern that then create a dome that then gets cut off, eroded down and has this bullseye pattern that we see today.
Chris Bolhuis (41:06):
So if you can envision a, like a blanket of shale sandwiched in between two other rocks and those aren't important, right? But if you have a blanket of shale that cut the entire area, and then you shove up this granite and metamorphic rocks and the shale that's on top of it. Well, that shale then is going to show up. As you throw erosion into the mix, we're going to start scrubbing away the rocks and the highest parts, right? And so on the flank of that highest part is going to be that shale. That's going to go all the way around the Black Hills. That's what you get with the dome is the same rocks show up all the way around the Black Hills
Jesse Reimink (41:57):
And those rocks because are tilted and they're tilted to reflect this dome. So on the Eastern side, the rocks dive to the east, down into the ground. On the Northern side, they dive down into the ground to the north. On the Western side, they dive to the west and the south, they dive to the south and thatt you that all the rock layers they used to be flat. Their sediments laid down flat. They were tipped up and they were tipped up in this concentric zoning ring. And it's just a beautiful structural feature. It is a type locality for a dome that is corded by these deep igneous and metamorphic rocks.
Chris Bolhuis (42:29):
And then knowing a little bit about what we just said, the geology of the Black Hills. So you always know where you are. You're looking at this Pema granite. You're like, well, I'm in the middle of the Black Hills. Okay. And if you go a little bit further out, you got these metamorphic rocks. You're like, well, I know where I am. If you go further away, you know that you're not near the core because you're in unaltered sedimentary rocks. But by looking at the direction that they're tilted, you know what side of the Black Hills you're on, it's a beautiful thing. The way this geology just
Jesse Reimink (43:02):
It's very cool. It's very cool. And we want to point out one sort of misconception or, or sort of common misconception here. The granite that is in the core, that is not the granite that did the uplifting. So we kind of have to give a little bit of review of the geology here to sort of rectify this common misconception. Remember that we have these really old sedentary rocks that were metamorphose 1.7 billion years ago. So now they're a metamorphic rock. We have a granite that did the metamorphism 1.7 and, 2 billion years old sitting in the middle, that stuff was buried down beneath the earth. Then this inland sea came in and deposited a mile and a half of sediments on top of it. That was all flat line that granite was there. So that granite did not do the uplifting. We have to have another intrusion, another igneous event that actually does the uplifting and this, the
Chris Bolhuis (43:53):
Piston,
Jesse Reimink (43:54):
Right? The piston. Exactly. So the, this is a younger event about 70, 60 million year old event, 65. Yep. Nothing to do with the granite that is exposed in the middle of the Black Hills.
Chris Bolhuis (44:07):
Now we do see some of that igneous rock though on the Northern part of the Black Hills, this went on for a long time, right? I mean this, this, this was not like a quick event. Okay. At the latter part of this like mountain building, domal uplift, it caused cracking on the Northern end of the Black Hills and those cracks, they kind of extend east west. 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 Butte, all of that is part of that. Dom will uplift igneous material that caused the whole thing.
Jesse Reimink (44:56):
There's also other features associated with uplift, which is the needles, Chris, which is this. I remember in, 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. So you're of walking through these narrow hallways, like really narrow hallways in granite with big cliffs going up up on either side of you. 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. And this is a weathering and erosion feature. It forms the needles part of the Black Hills. But this has to do with the fracturing of the granite, this 1.7 billion year old granite it fractures. And when that piston pushes up from below, think of like, you know, a layer of mud. If you push up on a layer of mud, it's going to crack at the top. And the same thing is happening to this granite. You get all of these cracks in it. And those cracks get exploited by I rain and by weathering and erosion and they get made bigger. And you kind of end up with this rounded Chronicle Spire that is called the needles terrain in the Black Hills. It's beautiful. Stunning. If you go to the Black Hills, go to the needles, go for a hike. It's amazing.
Chris Bolhuis (46:11):
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. It's called that because it resembles church cathedrals. I mean, it's, it's these just beautiful spires of granite sticking up its tic granite straight up. Right. And I just, I don't know, found a place that there's no trail leading to it, but she can go pretty much anywhere you want. There's no law saying, Hey, you can't go here. 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. It's just amazing. And so we all shimmy through there and go down into this like grassy courtyard area, right? Smack dab in the middle of the cathedral spires, nobody knows you're there. Like, I mean you you're going to be the only people that are there. It's just, it's amazing. It's, it's beautiful. And, and that's a, that's a part of the Black Hills.
Jesse Reimink (47:13):
And in some places the, you know, the, the road is carved out through the cathedral spires, right? You had the roads had to be cut out and cut a little tunnel through the granite to put roads into this region. Just very, very cool. It's a 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 and kind of summarizes and puts a capstone on this amazing geologic history. So Chris,
Chris Bolhuis (47:38):
Yeah, it
Jesse Reimink (47:39):
Does. I think that's a wrap. What do you think
Chris Bolhuis (47:42):
I do, you know, like I, I just want to have, have I done a good enough job of tell and everybody how much I love the Black Hills? Have I done that?
Jesse Reimink (47:51):
I think, I think you, I think you have. Yeah, but you know what? You and I need to go back there. That's I know
Chris Bolhuis (47:56):
By, I know it's a dangerous place for us to go though, because like where do you stop? You know, you have to look, it has so much there. Yeah. Because you have the bad lands and, and I'm telling you, these caves are amazing. We went to a new cave a couple years ago because of COVID 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.
Jesse Reimink (48:22):
You can spend so much time there. It's very cool. And actually the Black Hills with one thing we haven't mentioned is that it's probably most famous for the Sturgis bike rally every year. Right? Oh
Chris Bolhuis (48:31):
I've yeah. I've been there once.
Jesse Reimink (48:32):
Yeah. You've you've gone to it and you can, you know, it is a fun place to go to a casino and you know, have a couple drinks. It's just a really fun town. There's cool. Little cultural, you know, bits around and the geology's amazing. So go to the Black Hills,
Chris Bolhuis (48:47):
I'll take a hard pass on the casinos, but I am going to throw a shout out to one of my favorite towns. It's hill city it's in the core of the Black Hills. It is the coolest little cowboy town. The people there are so warm and friendly and it's just, it has such a cool vibe. And you know what? There are all kinds of these town sprinkled throughout the Black Hills. The people are amazing. I just, I love pretty much. I love the geology. I love the culture. I love the people. I love the geology, you know, it's just, it's great. I
Jesse Reimink (49:26):
Love it. It's an amazing place.
Chris Bolhuis (49:27):
I'm passionate about the Hills.
Jesse Reimink (49:29):
So you know, this is the episode of the Black Hills sponsored by hill city, South Dakota. And with that, let's wrap it up here, Chris, this was a fun episode. We got a lot of great memories there. Let's go make some more, sometimes
Chris Bolhuis (49:41):
Soon. Let's do it. All right. All right. Cheers.
Jesse Reimink (49:43):
Cheers. Don't forget. Please follow us on social media. we are at planet geo cast. Give us a like and a subscribe and share with your friends, share planet geo with your friends. We really, really value that and those likes and reviews help the algorithm make us more discoverable, take care. Good
Chris Bolhuis (50:09):
Deal.
