Geology of the National Parks

Video Lecture


The Unit 9 lecture features Dr. Richard Alley and is 52 minutes long.

Unit 9 Lecture
Click for a transcript of the Unit 9 lecture.

Hello. We are going to switch to something different now. You have built mountains with Sridhar. You've torn down mountains with Sridhar, and then you've seen where they go down to the coast with me. And now, we're going to start becoming historians.

A lot of the job of a geologist is to read the past, to find out what it tells us and what we can do with it. As you know, a lot of people make you take history when you're going to school because they believe that, if you know where you came from, that it helps. If it happened, it has to be possible. And so, learning about history is a useful thing.

And so, now we're going to start looking back into history and see if we can tell time, see if we can tell stories, see if we can find out how the environment works. We're going to do a lot of this by looking at sedimentary rocks. We're going to look at what happens to all that sediment that's washed down the river before it gets subducted and erupted again. And we're going to do this by trying to tell time. We're going to try to sort out time.

We're going to start looking at some pretty pictures because we've got to look at pretty pictures. We've got to go to national parks and have fun. And so, we're going to start out with the CAUSE class at Arches National Park.

Arches sits just outside of Moab, Utah. It's a great town, if you're into mountain biking. If you're into a whole bunch of things, you can go to Moab and have a good time. There are national parks just scattered all around the place. And Arches is certainly one of the prettiest ones.

Here's Sridhar, as you'd certainly recognize. And behind Sridhar are the La Sal Mountains. Way up in here are the La Sals.

And La Sal is salt, the salt. And down deep underneath this there are salt deposits from an old sea that dried up. And salt, when you squeeze it, gets soft and it moves.

And so, the rocks in this area have done a little bit of this over time. And by doing a little bit of this, they've given us some very interesting things. And in particular, they've given us the arches.

If you move underneath a really hard rock-- these are hard sandstone rocks from a giant dune field that looked like the Sahara way back when-- if you move underneath really strong rocks, they tend to break. And so, whether you're looking diagonal in the bottom picture or you're looking end-on in the top picture, you can see that, as the salt underneath moved and the rocks flexed, they broke.

Whenever there's a crack, the world can get in. It can go beat up the rocks. It can try to change the rocks.

And in particular, we're now underneath looking up, and you can see the blue sky way up there in the corner. And you can see where a rock has been cracked. The yellow arrow is pointing at it. And I'm going to draw you a red arrow that points at it too. You can see where the rock has been cracked.

And then these funny-looking streaks coming down the side of the rock, water is dripping through the crack. And as it does, it carries sand away. It causes chemical changes. And eventually, the rocks are going to fall off there. And as the rocks fall off, they make these beautiful structures that we know as the arches.

Here you can see a couple of places that arches have started. They haven't broken all the way through, but they've started. You'll see, on the right where Raya is sitting in front of a big stone arch that's arching up over the top of him up there, and the rocks have fallen out of that hole and have fallen down the hill here. These big chunks behind Raya came out of that arch. They fell out.

That one is in Glen Canyon. And on the left is one at Canyon de Chelly where the same thing has happened, except the stream has washed the pieces away. And so, you get this action.

You get cracks. You get weather working down the cracks. You get rocks falling out. And eventually, they break through. And you get glorious, beautiful things.

And you can see Sridhar in the very far, lower left of the left-hand picture sitting down there. And he is looking at Delicate Arch. It's one of the best icons of the National Park Service, sitting up there with the mountains framed behind it, just a glorious place.

If you look closely at the rocks of Delicate Arch, you will see, A, that they're sand, they're sandstone. They're sand that's glued together by hard water deposits. And B, you'll see that they have layers in them. In fact, they look just like a sand dune that's had the sand glued together by hard water deposits. And that is part of the story that we're going to be telling here in a little bit, is how sediment turns into sedimentary rock and then what we can do with it.

This is Landscape Arch. This is the longest natural stone arch in the world. Landscape Arch is probably not that long with us. You'll notice it's getting very, very thin in places such as that one.

You would not want to be under this one if things were shaking and baking just a little bit because, if you look underneath Landscape Arch, you'll see a huge number of bigger rocks that have fallen off. And Landscape Arch is slowly caving in. A very large one fell off of fairly recently, the last couple of decades.

And the scar is shown in the lower right-hand picture. So, down here, on the very far right-hand side, is where a bunch of rocks fell off. And you can see the rocks that fell off sitting down underneath it. And so, the thing is slowly changing. The parks really are active. Geology really does happen.

Now, this is a picture in Hidden Canyon in Zion National Park. It's one of the most beautiful places that you can ever imagine. It's a little bit south and west of Arches.

And you go up in this canyon. And it's just glorious. And you walk back there just a long distance.

And there's sand on the bottom. And the sand has little ripple marks in it from when water flows in the occasional rainstorms. And so, you see things like this. It's loose sand sitting on the bottom with these ripples.

This is sandstone. And all this is is loose sand that's been glued together by hard water deposits. And so, you go from sand to sandstone.

When does it change? Well, there isn't any set line. In just about everywhere, you get a little bit of hard water deposit put down.

At some point, you say, this one's hard enough that I'm going to change the name. But there's no real strong line. We find all sorts of gradations between sediment, sand or mud or something like that, and sedimentary rock. And so, let's go and look at that just a little bit.

So, we will switch over and we'll write a few things. And so, we are going to talk now about sedimentary rocks. You've met metamorphic rocks in Rocky Mountain with Sridhar. You've met igneous rocks belching out of volcanoes. And now, we're going to look at sedimentary rocks.

We tore apart the igneous. We tore apart the metamorphic. We made mud. We made sand and all sorts of things like that. And now, we're going to see what happens to them.

So, you saw that weathering happens. And when weathering happens, which is just weather beating up on rocks, it gives rise to a couple of things. It gave rise to clasts, pieces, chunks. So, clast is a fancy geological word, but chunk will do fine.

And we also saw that it gives rise to dissolved salts, stuff that you can't see that breaks down into individual ions and washes away to make the ocean salty. So, it gave rise also to dissolved salts. And so, these are the two things that come out of weather beating up on rocks.

And so, you can imagine that they don't always go immediately to the ocean, to the subduction zone, to the volcano. They may stop on the way. And so, if they stop on the way, if they're headed from here to somewhere and they end up stopping for a while, then we call it sediment.

So, if they stop somewhere, the stream puts them down, the glacier puts them down, the wind puts them down, whatever, if they stop, we call the pile sediment. We call it sediment. And so, you can imagine that this is going to turn into sedimentary rock at some point.

And to turn into sedimentary rock, you have to make it hard. So, if you take sediment and harden it, hardening sediment makes sedimentary rock. Pardon me, that's not a technical term. We'll give you some technical terms here in just a second. But hardening sediment gives sedimentary rock.

We change the name, nature doesn't. There are all possible gradations between sediment and sedimentary rock. And at some point, we feel compelled to say, that's too hard. I'm going to change the name, OK?

There are several processes that go into this highly gradual transition. And it certainly is a gradual transition, a gradual change that takes one to the others. Up here above where I am, if you take your rock hammer and you hit a sedimentary rock, it goes, boom! It's really hard.

I have a friend that was studying sedimentary rocks that had been scraped off of the slab in Olympic National Park. And what they called rock-- they wanted to study erosion. So, they go out and hammer nails into it and then watch how fast the nails were washed out. And so, theirs are very, very much softer than ours, but they both were called sedimentary rocks.

And so, we have this gradual change of one to the other. And it is achieved primarily in three ways. The easiest one for doing this is how you harden it, so the hardening of sediment. The easiest one to think of is probably hard water deposits.

Most people these days have soft water. And if you have soft water, they've taken most of the rock out of the water. And you don't notice that your pipes get clogged.

But you know something? In an old house, you call the plumber and you say, I need help with my faucets. The plumber doesn't come in with a little screwdriver. The plumber comes in with a big wrench.

And the plumber's got a big wrench because all the pieces are stuck together. And all the pieces are stuck together because, whenever you put water in contact with stuff, it dissolves something here and it puts something down here. And there's usually bugs living in it, microbes that are changing the chemistry.

Anybody that's ever tried to take apart plumbing knows that it's stuck. And it's stuck by hard water deposits. Whenever you've got water going, whenever there's life, there's chemistry. And whenever there's chemistry, you're taking stuff from somewhere and you're putting stuff somewhere else. And eventually, you're going to end up gluing things together or else dissolving things and getting rid of them.

And so, we always see this in human activity. This stuff get stuck together. Nature is the same way.

If you leave a pile of sand long enough and you don't stir it and you let water go through it, eventually, you'll find out that minerals get deposited in the spaces. And it's stuck together. And once it's stuck together, we say, oh, that isn't a sediment any more. Now it's a sedimentary rock.

It's easier if you squeeze it at the same time because that keeps stuff from moving around. And so, you may also see squeezing going on or compaction. If you pick up a handful of sand and you squeeze it, you won't make it stick very well. You're not strong enough to do that.

If you're at the bottom of the Mississippi Delta, it's seven miles of mud on your head. And it's pretty hot down there at the bottom. And you put seven miles of mud on your head, you get squeezed. And so, that will help solidify things.

Way down at the bottom where it's hot and things are getting squeezed, you'll start to see new crystals grow. Things will dissolve. Other things will deposit. You'll get, what we call, recrystallization, or the growing of new minerals.

And you'll be sitting there and say, OK, he's talking about growing new minerals. And he's got a fancy word for that, which is recrystallization. What's the difference between this and metamorphism? And the answer is, nothing.

We humans have to draw lines. And if it looks muddy, we call it recrystallization in a sediment. And if it looks pretty, then we call it a metamorphic rock. And we change the name.

But this grades into metamorphosis. You heat things. You squeeze things. And they start to change.

The biggie for making sedimentary rocks is really, probably, the hard water deposits. We actually have a fancy name for that, as you might imagine. We call it cementation. You're putting cement between the grains. And so, cementation is usually what glues sediments together and makes something different.

OK, now, let us, then, go look at some more sedimentary rocks. So, we'll switch over to the pretty pictures again and see if we can go there. It's so hard to pick a favorite park. They're all so wonderful.

But if you want a beautiful fairyland, wonderful, just unbelievable place, go sit at Bryce for a day. Sit on the rim in the morning. Get up early and watch the sun come up on Bryce. Sit there in the evening and let the sun sink behind you and the shadows lengthen. It is just an amazing place. It's hard to believe that anything can be so pretty.

The flowers are there. The mountain lions are there. All sorts of wonderful things to see up on the rim. They've got prairie dogs and deer and what have you.

You go in the winter and, if you can get in-- the last time I was there in the winter, they were loaning out snowshoes at the visitor's center. And you could put on your snowshoes, and you could go snowshoeing along the canyon. It's just an amazing place.

The scale of it is immense. If you look in the right-hand picture here, you'll see at the bottom a whole bunch of people. I'm circling one of the CAUSE students right here. And I can't even see which one because you'll notice that this is a big place. And yet it looks so delicate. It looks so beautiful.

Now you will notice in all of these pictures that you see layers. The rocks are layered, and that is common in sedimentary rocks. You wash something in, it's put down. You wash some more in, it's put down. And so, you end up getting layers in the sedimentary rocks.

These happen to be from a lake. Today, there's a little lake left in Utah, the Great Salt Lake. During the Ice Age, when there was less evaporation, there were a lot more big lakes that extended across many places in Utah. And as we look back in time, we see the deposits of many lakes. And so, the Bryce Lake-- there's various names given to this-- was this big lake that sat down in Utah.

If you look carefully in many of these rocks, especially some just a little farther north of this, you will find fish fossils. You will find flamingo tracks. You'll find little snails and all sorts of things that say, OK, this was a lake. It was water.

Just glorious, you know? When nature starts beating up on these rocks, it just carves such beautiful things that it's hard to imagine. Here are a couple of our CAUSE students down here, Sam and Sameer, looking at a couple of really big trees that are sitting at the bottom of the canyon. It's hard to imagine how they live there, but they're doing just fine, thank you.

I don't think they do this any more. But you remember we've talked about sediment being carried in rivers and along beaches and so on. Here's a case that a stream is flowing towards you. So, the stream is coming towards you.

Now, there isn't much water in this stream right now. It only flows when it's raining. And the trail was getting buried in sediment, so the Park Service put these big logs across the stream.

And you'll notice what's happened is that sediment has piled up above the logs, it's filled it in, and that there's actually been erosion down here below. It's cut down a little bit. So, you can see the dynamics of sediment. What you've learned in earlier units keeps carrying through.

If this were to sit here a long time, you would get hard water deposits. And it would start to become a rock. It isn't there yet. It's still fairly loose.

This is a very interesting slide. Dave Janesko of the CAUSE class and I had the good fortune to go up and look at this one. And this one feeds into a story that we're going to tell about reading history, about inferring what happened.

We are just outside of Bryce. We're down in Red Canyon and the place they used to shoot a lot of Westerns. And we're up on the same rocks that Bryce is made out of.

Now, Bryce is mostly lake sediments. And you find all these fish fossils and stuff in similar rocks around. And occasionally you'll find such things in Bryce. But if you have a lake, there's probably a river feeding into it. And so, in fact, as you go west out of Bryce, you find the deposits of the rivers.

We know what a river gravel looks like. If you've ever been down to a river that has a little gravel bar in it and looked at the rocks, you know that they sort of look like this picture you see on the left. The rocks tend to be rounded. Rivers break off the hard points. And so, after a while, river rocks tend to be rounded.

The big rocks, cobbles, we call them, or pebbles, will have sand sitting in the spaces between them. This thing just looks like a river deposit. It's shaped like one. It's shooting down from a mountain range to the west, headed down into the lake that was Bryce Canyon.

And so, this is the sort of deposit that the geologist, who has looked at a lot of rocks, will be able to say, rivers make this. And it doesn't look like anything else I've seen. The size, the shape, the arrangement, the rounding, the sorting, all the things you go into this are pointing to this being a river.

What's very interesting, though, is that one of the pebbles in that river, which is this orange one in the middle here, one of the pebbles in that river is itself a river deposit. You can see that this orange one that I'm drawing on over here is made up of little, rounded sedimentary rocks itself. There's a big story in this. And it's a story that you should start thinking in your mind, how will am I going to tell this story?

What is it that has to happen that I can have a piece of an old mountain range and a piece of mud that's been squeezed and a piece of an old beach that have been put together in a river, bounced along, the corners knocked off, had sand deposited around them? Then they've been glued together by hard water deposits enough to make them hard.

Then they've been broken out. They've been bounced around in another river until they get rounded on the outside. They get deposited with pieces of all sorts of other things, old mountain ranges and old beaches and other things. They get glued together by hard water deposits. And they're setting up on a cliff far above any stream on the outskirts of Bryce.

There is a big story in this. And it's a story that, like I say, you want to start thinking about how you would tell that story and what it would mean. So, let's go back over to drawing things that might possibly prove useful. And what we're going to do now is we're going to take a look at sedimentary rocks, such as the ones you were just looking at.

OK, so we saw that sediment is either pieces, it's clasts, or else it's dissolved stuff. It's salts that dissolved. As you might imagine then, the sedimentary rocks that are produced from sediment come in two basic types. They either are pieces or they are the dissolved stuff that is put down. And so, we see chemical precipitates.

If you take a handful of ocean water and you let it evaporate, you will get a handful of salt, a little bit. If you were to wash ocean water into a little, shallow basin next to the ocean and then close it off and let it evaporate, you'll get a layer of salt. If that happens a few times, you'll end up with a lot of salt.

We saw, way back at the beginning, Death Valley early mining was for salts. It was actually a particular salt that had borax in it that had washed out of volcanic rocks. The water is carrying the dissolved stuff.

It goes down in Death Valley. The water evaporates. It leaves the salt. And then people went in and mined that.

And so, the chemical precipitates include things such as salt. It includes things such as the borax that led to the 20-mule teams in the early mining in Death Valley. And so, these things happen. You usually find them in fairly dry places because, when it's wet, the salts tend to wash away.

Other kind of sedimentary rocks, we can do them in a different color to have more fun. The other kind of sedimentary rocks are the ones that are made of clasts or pieces. And so, we call them clastic. And they are classified in various ways, usually by the size of the pieces.

And so, if you go in and you have a bunch of sand and you glue it together, you say, oh, it's a sandstone. This is really highly technical, let me tell you. So, sand gives rise to sandstone, when it gets hard enough that we decide we want to change the name.

A little bit smaller than sand, we have something called silt. And silt gives rise to siltstone. And there are actually very technical definitions for these things that you don't need to know. Silt is 1/16 of 1/256 of a millimeter in diameter, sand is 1/16, yeah, so on.

If you have clay, it gives rise to claystone. That is less than 1/256 of a millimeter in diameter. This is little stuff, although usually, people change the name. They use an old name for claystone. They call it shale. And so, you may possibly have come across that as one that you're familiar with.

If you get things that are bigger than sand, there's granules. People don't usually talk about granule stone. They get pebbles and they call it pebblestone. And they get cobbles and they call it cobblestone.

But usually, if you've got bigger pieces, they say, oh, let's just call that a conglomerate. And we'll not worry about that. And so, you'll end up with these being the main types of sedimentary rocks.

Now, what they are, how big they are, whether they're evaporated or not, whether the corners have been knocked off, a whole bunch of things of these rocks tell you a lot about the environment in which they were deposited. And so, what we can now do is look at it a little bit. And we can say that sediments and the sedimentary rocks that they turn into reveal the environment in which they were deposited.

And now, you can start to see that, if we can read the environment, if we can tell what the conditions were in which it was deposited, then we can read time, which came first and which came later. We can tell history. We can tell what the earth was doing and how it was changing.

For example, suppose that you go down and you find a rock. And you crack the rock open. And there's a fish fossil inside. It's not terribly likely that you're looking at a desert sand dune. You're probably looking at something that had water.

And so, what you can do is you start looking at things such as fossils. And fossils are pretty good indicators of what's there. If you see lizard tracks, you're probably not at the bottom of the sea. And you might be out on a sand dune somewhere. If you see ferns pressed into coal, you're probably not out in the desert where it's too dry for things to grow.

And so, fossils are going to give you a lot of clues as to the environment. And there's a big difference between a fish and a fern and a lizard and so on and so forth. And so, you can recognize these things. And you can start reading them.

There's other stuff. If you see a pile of boulders, the wind did not blow them there. We never see wind quite strong enough that they got boulders and put them there. Now, glaciers can bring piles of boulders. And so, you start saying we can look at what it's made of.

A beach is all sand. It's sorted. All that going in and going out and washing away, the little pieces are washed away fast. The big pieces are left behind until they get broken up. And the beach makes the sand-sized ones, the in between ones.

And so, we start looking at the size, the shape and the sorting, whether they're all the same size or whether some of them have been mixed with other sizes, so the sorting of the sediment. Size, shape and sorting of sediment clearly has clues to what you're looking at. Landslides may include all sizes. Glaciers can carry all sizes.

Wind does not. Wind is very selective. Little, tiny ones get blown far away. In between ones make sand dunes. Big ones are left behind. And so, when you see sand, it's usually wind or it's a beach, something like that. And so, these are pointing to indications of the environment.

Structures in the environment also can give you indications. If you see mud cracks, the mud dries and it cracks, you've got an idea that this was sitting in a place where it could dry so it could crack. And so, when you see mud cracks, you can start to tell a story.

If you see rain drop imprints, the blob that's made when the rain fell on the mud and then more mud washes in and buries it, moraines. If you see the pile left by a glacier and all the scratching and polishing that glaciers do, you're not in the middle of the tropics in a hot place. And so, if you see moraines, that is pointing to something else. And so, we can do environment by looking at the sediments and what is there and what it's doing.

OK, if you can do environment, add time, and you can do history. So, that's where we're headed. We want to tell the history of the planet, what was where.

Where were the continents? Where were the rivers? Where were the glaciers? Was the climate changing? Who was living here?

And so, we're going to try to put time together and we're going to see what we can do. Now, to do that, let me show you a couple more pictures. So, we will flop back over to pretty pictures. And I've got to get a little bit here.

Normally, you dump some mud, you dump some more mud, the younger stuff is on top. Nature doesn't raise the lid and shove mud underneath. It puts layer on top of layer, on top of layer, on top of layer.

If you're on the flood plain of the Mississippi, you build your house and the flood comes in and it puts mud on the grand piano. The mud is younger than the grand piano. The grand piano was there and then the mud is put on top.

So, normally, they pile up, and we can do a story. The young one's on top, and the old one's on the bottom, OK? There is, however, a bit of a tweak to that, which is, occasionally, nature causes trouble.

And you remember, in an obduction zone, you can cram things together. And if you cram things together too much, that they'll start to bend. And if they bend too much, they'll roll over. And if they roll over, now we've got to make sure that we know which way was up originally.

And there's a picture over here that we've got on the left. And if you follow one of these layers, start down at the green place down at the bottom, and then follow that layer around, and whoop, it bends. And now, it's upside down at the yellow place.

And so, geologists can't just say the young one's on top because, occasionally, nature tries to throw us a ringer. Fortunately, we can tell. There's all sorts of things that say this is the direction that was up when I was deposited.

And a geologist reads these. And then they can say, OK, am I down in the green place and I'm still right side up and the young ones are on top? Or am I up in the yellow place and I'm upside down and the young ones are on the bottom? And so, there are many of these indicators.

Something has to be, before you can cut it, you know? I could cut a piece of paper. That's fairly easy. But I can't cut it before it's made.

And so, what we have here is a picture taken in a cliff at Canyon de Chelly. And this is a fossil sand dune. And when there are sand dunes, the wind blows sand. And the sand piles up and makes little layers. And so, you'll see little layers here.

This picture is only about six inches across, but you'll see layers that are piled up. The red is actually hard water deposits. It's rust that's been put in here. And so, the wind was blowing into this picture from the right to the left. And it put down these various layers that you can see in the picture.

Then the wind picked up a little bit and it cut the top off. And so, all the way along the top there, you can see the layers go up, and then they're cut. They just end right there. And then there's more layers deposited on top of them.

And so, this one has to be right side up. The old ones have to be on the bottom because they've been cut. They've been whomped off on the top by the wind that put the next layer down.

Now, I can play games with this. The picture that you just saw is now in the upper left. And the blue arrow shows which way was up, where the sky was when this sediment was deposited.

Suppose that mountain building came in and it started turning things over. And so, you can look to the right of it there. And I've taken the picture and I've flipped it on its side.

And you can look farther down. And I've taken the picture and I've turned it upside down. All of these are possible. Nature can do this.

But you look at it and you say, I know because the cut is still there. And so, I know, if I come to the upper-right picture, I can see where the layers are cut. And so, I know that the layers had to be there before they could be cut. So, I know which way is up. I know what the pointing is.

This is a picture in the wall of the Grand Canyon. You're down at the South Rim. You're headed down to the bottom. And you're going to have a great time.

And you look at that cliff and, oh man, that's such a wonderful cliff. This picture is probably 100 feet high. This is an immense, immense cliff that's sitting there.

On top are sand dune deposits. It's a giant, giant sand sea that came in here. On the bottom is a flood plain deposit. You might think of the Nile River flooding across its vast delta, its vast flood plain and then the desert of the Sahara blowing in over the top.

And so, the bottom rocks are flood plain deposits. You'll find leaf fossils in them. You'll find footprints in them. And then on top, it turns into these sand dune deposits.

Now, you'll notice right under the red arrow that the sand goes down a crack. If you've got mud and then it dries out, and this huge drought arrives, and the mud starts contracting, it'll crack. And when the sand blows in, the sand will fall down that crack.

And you can follow the sand way down this crack. This picture has to be right side up. Sand does not fall up cracks. It does not fall sideways in cracks. It falls down cracks.

And there is the sand. And there's the crack. And there it goes. This picture's right side up.

If we turned it on its side, you'd know that. You're now [? GEOSCI 10-ed. ?] You're geology of the national parks wise. You can figure this out.

These are other mud cracks. These are also right side up. This is from rocks that are just like Bryce, except just a little bit north in the Flagstaff limestone.

The cracks are going down. And so, it cracked. And then more mud will fall in. But we can see down in to these cracks. This one is right side up.

This one is a little tougher because it's a little hard to see. But you'll notice my finger in the picture in the lower left. And you'll notice the shadow of my finger.

And you'll notice the mud cracks up there in the middle. And you'll notice the shadow of the mud cracks right there and right there. So, these are sticking up at you.

This is the stuff that fell down into a crack that was below it. And then we turned the sucker upside down. And so, we're looking at the bottom of this one. And I'll draw this for you in just a minute, after we walk through this. And so, this one's upside down.

We're now out at Sunset Crater, big, beautiful volcano. And the lava came down. And it threw these little rocks.

And the little rocks that it threw actually made mulch for the native peoples that lived there, the Sinagua. And they had wonderful times for a while because their crops didn't dry out because those little rocks held the water.

But in the lava flows, when a lava flow comes down, it's usually bubbling and burping and belching and sorts of things. And the very top will freeze because it's in contact with the air and that freezes it. And the bubbles rise.

And so, if you look at this one, you'll see big bubbles at the top. And when you go down, there's a little bubbles. And then there's not many bubbles at all.

This one's right side up. The bubbles rose and they got trapped under the top. And it doesn't always work, but this one usually works too. And so, this says it's right side up.

If later mountain building turned it over, you would look at that and you'd say, I know what's going on.

OK, so let us draw a couple of those, just so we're on the same page and you're sure. This might prove useful some day. Suppose we start out and we have water. And it washes in some mud.

So, down here, we have mud. And up here, we have water. And you've got a fish swimming in the water, so you know which way is up.

OK, now what happens? The fish swims away, and the mud dries up, OK? So, now, we're going to dry this up. This fish is going off somewhere downstream.

And after the fish swims downstream to safety, then the water dries. And what happens after water dries is, usually, it will crack. And so, you'll get mud cracks.

And so, there we have a picture of some cracks. The cracks go down. They end. They don't go to the center of the earth. And so, the cracks go down and they end.

Now, what's going to happen? We're going to wash in some more mud of some other kind. Let's make blue mud. And so, more water comes.

And the mud comes in. And we get another layer in here. And what does it do? It squirts down in the crack.

OK, if you see this in a cliff, you say, oh, it's right side up. There's no problem. If you happened to come to a cliff and you happened to see something that looked like this-- let me see if I can get this-- suppose that you saw it this way. OK, and it was looking something like this.

You would look at this and you'd say, oh no, mud cracks didn't do that. The mud didn't start with a giant, overhanging cliff with cracks going up into space. This thing has been flipped over. So, this has been flipped, OK? So, this one is right-side up.

And the one down here has been flipped in a very funny way. It's not flipped quite all the way over, but nature doesn't do that. Sometimes it flips it all the way. Sometimes it flips it 3/4 of the way. There's all sorts of possibilities of what nature can do with these things, OK?

The same thing we'd do if you had footprints. If you want to think of as a footprint rather than a mud crack, it's the same sort of picture. And so, footprints do the same. Footprints go down.

Raindrop imprints, if the rain drop falls in mud and it makes a little hole, that goes down. Bubbles rise in lava flows. And they're trapped just below the top because that chills very early. So, they're trapped below the chilled upper layer, the chilled top.

People often look at shells on this. If you go down to the beach-- and this would be interesting for you to do-- this is not 100%, but it usually works. Suppose that you have a shell sitting on the beach. Beach, OK? And this is a shell. And here's a sea gull flying over the shell. OK, big shell.

Now, what happens when the wave comes in? When the wave comes in, it's going to get under this and it's going to rock it. And so, this what you would call your unstable, easy to rock position.

All right, well what's going to happen? It's sitting here and it's going to rock back and forth. And eventually, it's going flip over.

And so, it flips. And now, it's sitting here. And then it gets buried in the beach. And it will stay there.

And if you go down to a beach that has lots of curved shells and you look at them, you'll find that most of them, 80%, 90%, often are in this position. It's stable like this. And so, most shells end up this way.

They get buried. And you can tell which way was up. And so, there's all these things that are sitting there that are telling about it.

Let me try to draw the one I showed you in the cliff of Canyon de Chelly with the way that sand dunes work. And so, let's see if we draw a sand dune. We'll make it a red sand dune.

And so, the wind, we're going to bring in this way. And so, here comes the wind, and what happens? You know how sand dunes are. There'll be a pile here, but they start growing this way.

And so, over time, the sand is blown over the top. And it piles up in the lee of the dune. And so, the dune builds out that way. And so, it grows this way, grows sand there.

Now, what happens is there's usually changes in wind. Sometimes it's stronger. Sometimes it's weaker. And very often what you'll find is that the wind will come through and it will actually scour things off.

Oh, that wasn't what I wanted to do. OK, well let me draw it back in. We'll erase everything. And so, we'll go back and we'll say OK, we've got this sand dune, and it's sitting here. And it grows along like this.

And so, here comes our sand dune, and it's growing. Then the wind will come in and it will basically scour off the top. That gets cut away.

And after that's cut away, you may bury this in another sand dune that's coming through later. And so, now, let's put some more through.

The layers may be nearly horizontal. They may grow like this. But what you'll find is that, where the cut is, these things will always grade in to what's underneath them. They won't be cut, whereas, the ones on the bottom will be cut.

And so, the top ones are not cut. It's got to be there to be cut. And the bottom ones are cut. And so, whenever you see cut and not cut above it, you know which way is up. And so, there's an indicator there that you're dealing with up and not up, OK?

The other thing one notes is that most layers usually start sort of horizontal. These slant, but they don't make cliffs. You'll never go out and see a giant, 100-foot high cliff of sand. You'll see sandstone, but not sand.

And so, what you'll also find is that most layers start out nearly horizontal. And they start out nearly horizontal because of mass wasting. Otherwise, they'd schlump down.

So, that now gives us enough tools to tell stories. We can read environment. We can tell what the deposition was, whether it were a glacier, whether it were a lake, what have you. And we can read time. We can put events in order from oldest to youngest.

If you do that, a very, very remarkable result comes out. And this is something that was developed first by a canal engineer. This canal engineer, William Smith, was out trying to do canals. And he was trying to work in England.

And he found that there's a lot of sheep in England. There's a lot of grass in England. It's sort of hard to find the rocks. And he'd find the rocks in a few places. And he'd look around at the rocks.

And he started realizing that, when he put the rocks in order, that it put the fossils in order, that there were one kind of fossil here and another kind of fossil there. And it also turned out that some of the fossils, actually, he'd find in the dirt. And he could identify from a fossil which rock was underneath.

And so, there was this canal engineer. And his name was Smith, a canal builder named Smith in England. And this happened in the early 17th century. And what we found is, if you put the rocks in order, it puts the fossils in order.

What does that mean? It means that the fossils that are near each other in a pile of rocks are similar, if you do the same environment. Now, let's be very clear. You don't get exactly the same things if a lake turns into a sand dune. You have to compare a lake to a lake.

But if you compare a lake to a lake or an ocean to an ocean or something like that, what you find is the fossils that are near each other are similar. The fossils that are farther apart are more different. The fossils on top are more like the things that are alive today. And the fossils on the bottom are more different from the things that are alive today.

And so, what you find then is that the rocks of similar age have similar fossils. The rocks of very different age have very different fossils, so different age, different fossils. And the rocks on top, the youngest rocks, have fossils that look most like things still alive. And so, the youngest rocks, the youngest fossils, look most like things still alive.

Now, this was done for purely practical reasons. This is an engineer looking for a shortcut. It's to help him identify the rocks that are down underneath all these sheep and the goats and the heather and the what have you. This clearly turned into something much bigger, or it helped contribute to something much bigger, that we're going to talk about. We will come to that in just a little bit.

Right now, we just need to add a couple of things. One is that this gained a name. It was called The Law of Faunal Succession.

Now, this is not a law that was passed in Congress. This is just a rule that is found to work over and over and over again. And after you check it 10 times, you check it 100 times, you check it 1,000 times and it just keeps working, and eventually you say, well this works.

And so, we should give it a fancy name so that people know that we've checked it a whole lot and the thing keeps working. So, they call it a law, but there's no law here. It's just a summary of observations. It is The Law of Faunal Succession.

Another thing that came out of this-- and this will be the last thing we need to get through for today-- is that, when people had put all these fossils in order, they got really tired of talking about, well, you know, the time that I had those big, funky trilobites with the big, giant eyes and then the little horn curls? But not after you showed up with the big, [? iridescent ?] clams.

They needed names. They needed some way to talk about this aside from talking about the big funky eyes of the trilobites. So, they came in and they named everything. And you know that geologists, and scientists in general, have a habit of trying to name a lot of things.

And so, they gave their names-- and if we go from young rocks on the top to old rocks on the bottom, the big names that they gave it, they said, OK, today we're going to call this Cenozoic, all right? Now, you'll recognize in the word Cenozoic, "zo" for life down there. "Ceno" is actually something like new. And so this is the time of new life.

There are lots of things alive today. There are insects and all sorts of things. But the big land critters are more mammals than anything else, so you'll often hear this is called, The Age of Mammals. Although, if you've ever been out in a really mosquitoey place, you begin to wonder about that.

Now, below that, they said, well there's different things. We need another name for that. We don't see mammals dominating all the way back. So, below that, they called the Mesozoic.

And "meso" is middle. So, this is something like the middle life. This is biased for us, big land critter, so this is middle life. And the big land critters were dinosaurs then, so this is what you often hear as, The Age of Dinosaurs.

Now, below that, they had-- you might imagine, if we have new life and middle life, well let's have old life. So, let's call it the Paleozoic. And so this is something like the old life. There weren't a whole lot of land critters until the latter part of this. So, this is old life.

And if you want a catchy name, you could call this The Age of Shellfish because there's lots of marine life that's making shells. And we have good records of that.

Below that, there are a bunch more "zoic" words. If you were taking a course in historical geology, we'd pound them into your head. We're going to use an old term that's a little easier and lumps a whole bunch of them together. We're going to call it the Precambrian.

It turns out that the Paleozoic and the Mesozoic and the Cenozoic have subdivisions. And one of the subdivisions of the Paleozoic is named for Wales, or Cambria, in the United Kingdom. And the rocks that are exposed there have these cute goggle-eyed trilobites.

Since they called that the Cambrian, and then this is older than that, so it's the Precambrian. And if you want, that's just the rocks that are before the Cambrian, so, don't worry about that. And there isn't a lot there, a lot of algae, blue-green algae, cyanobacteria, various other things. So, let's just call this the Age of Algae. That is a little bit loose, but I think it'll get us there.

And so, we have come a fair distance here in a very short period of time. You've seen some beautiful things. I hope that you enjoy seeing these. I certainly enjoyed going there with the CAUSE students to take the pictures.

We have seen that sediment, which is washed down from the hills, which is land-slided down, which is brought by glaciers or what have you, can be glued together by hard water deposits to make sedimentary rock. We can look at those and say, are you a sand dune? Are you from a glacier? Are you from a landslide? Are you from a lake?

We can put them in order. We can tell if nature has tried to fool us by flipping them upside down. And so, we really can say this one's younger and this one's older. We can put the events in order. And once we do that, we are going to start telling histories.

One of those histories is going to involve who lived where, when. Some others are correlating changes in where the continents were. There are going to be changes in all sorts of things in the climate. But one of those stories is going to be who lived when. And we've got a few words now down here that sort of help us summarize who lived when.

Notice that we don't have any numbers on this young and old. And so, we have to go look at putting numbers on the young and old before we can go further. So, that will be our next task is to try to put years on this. And then we're going to tell history.

Credit: Dr. Richard Alley

Want another look?

Check out the Unit 9 Presentation used in the online lecture.