Law of superposition essentially says the older rock is normally going to be found on

the bottom. And so right here we're looking at a section from the Grand Canyon. And you

can see that all the rock above this line looks very similar. It's actually called Navajo

sandstone. But if I go down to the next layer of rock, it's going to look the same. And

the next layer of rock is going to look the same. And the next layer of rock is going

to look similar. So horizontally rock looks the same, but the law of superposition says

the farther down you go, the older that rock is going to be. Another thing that you should

understand is that rock is going to be laid down horizontally. In other words rock will

be laid down flat because gravity is always pulling down. Now an example of where you

might see this in your daily life might be in your locker. And so I grabbed a sample

from a locker right here. So if I grab some of the stuff out, it looks just like that.

We got a book and a notebook and some papers. The stuff that's laid down most recently,

like this biology book is going to be on the top. Or like, this looks like a geologic key.

We might then have a notebook that was used fairly recently. But the farther I dig down,

I find crossword puzzles and doodling and way at the bottom I might find like a blank,

a sheet of black paper. Some headphones. Those are going to be found at the bottom. And so

that means that those were laid down more distantly or in the past. And so the law of

superposition is fairly simple. But the problems you can solve with it are fairly complex.

And so this would be an example of a geology problem that you have to solve. And so we

can see all these different rock layers. So the idea is to figure out well, which one

of these occurred most distantly and most recently. So could you put it in order from

the thing that happened in the distant past and the thing that just happened a little

bit ago? And so you might say some of these things might be easy like probably this rock

here E was laid down a long time ago. And this rock layer, F was laid down fairly recently.

But how do you do that? Well you have to use the law of superposition and the law of horizontally

laid down rock and you can do pretty good at it. When I was thinking about how I could

kind of explain this to you, the thing that made most sense for me was to use a drawing

program. And so imagine I were to ask you this problem right here. I were to ask you,

could you figure out all these rock layers, which one was laid down the oldest or which

is the oldest? And which is the youngest? And so some of this stuff would make sense.

Like this is probably older here. And younger up here. But it's always apparent until you

actually see it take place. So let's take a look at what happened here. So this is me

drawing it. I kind of sped up the drawing. Now let me pause it after a little bit. Okay.

So let me pause it right here. So what happens? Well we had this first layer of rock laid

down. And then a second and a third and a fourth layer of rock was laid down. So we

know that the oldest rock is going to be the rock at the bottom. But let's play what happens

next. Oh wow. Okay. So what happened there, well we had a fault. In other words those

rock layers, we'll say that look like this, were broken and lifted up. And when you're

looking at problems like this you try to match up the sides and so you know that the rock

was laid down before the fault occurred. And so we could say that this occurred before

the actual break because you couldn't have a break in this until it was actually laid

down. Let's keep going again. So what happened next? Well that whole rock layer was tilted.

That's kind of hard to figure out how that might work. Oh, what's this? That's me using

an eraser. Okay. So what happened next? Well it looks like we had erosion taking place.

And so there was rain and wind and it washed a lot of that rock away. Can we tell that

that occurred after the tilting? I think that would be kind of a hard one. So let's keep

going. What happens next then? Oh. It's a lava. So we had a lava flow and then that

lava eventually was eroded. And so this is horizontal now at this point. So we know that

this occurred before or after we had all of this erosion and we had the tilting. Looks

like some more rock layers are going to be added as well. So we're adding more rock layers.

What happens next? Looks like we have a little bit more erosion taking place. And is that

it? No. We have this. What is that? Okay. That is an intrusion. In other words that's

where molten rock is actually moving up. We call this a dyke and then that's causing this

bulge or like an intrusion, igneous intrusion or rock that's actually intruding into other

rock. And then it's hardening inside it. And so when did that occur? Well we can look on

here and we know that it actually, since it's cutting across all of these and cutting across

over here, it actually had to form after these rock layers formed. And so if you know a little

bit about the law of superposition and the law of horizontally laid down rock or horizontaility

I think is what it's called, you should be able to put together some of the geology or

how things laid down. Now you normally don't see it like this unless you're actually taking

a test. You usually see it something like this. And so what do you know using law of

superposition? Let's take a look at this? This is a road cut. Usually you can't see

geology because there's plants and stuff growing over it. But if you cut out a road or you

cut out a trail or something like that you can actually see the rock. And so here we

have several layers of rock that were at one time laid down horizontally. And so we would

say the oldest rock was down here and the youngest rock is up here. The whole thing

was then tilted. And then it was eroded. And so we have a unconformity here which is going

to be an area where it was eroded. And then we have new levels, layers of rock that are

added on top of that. Since these are horizontal, we'd say this is the newest and this would

be the oldest. So I hope that's helpful.