 Okay, I'd like to talk a little bit about the distribution of time in sedimentary rocks. So in a previous video we talked about the principle of superposition where you have layers of rock and you have them, they're older on the bottom and younger on the top. So this seems like a very simple, obvious concept. You can't deposit sediments on top of something that doesn't exist. They don't stay suspended in the air to make rocks. But it's a very useful concept for understanding earth history. So I'm going to draw another set of layers over here. And one of the things that early geologists recognized is that if you use this idea of superposition, you can get the relative time. And one of the things that they noticed was that for phenerozoic rocks, the fossils present in the different layers varied with different fossils in the rocks on the bottom, which are older, versus the ones on the top. So I'm going to draw a little fragment of a trilobite. They often are sort of hooked here. And then that represents maybe the Cambrian time, and then maybe you have some sort of coral with a skeleton evolving at this time. Then I'll draw an ammonite, or that could even be a gastropod or snail. But the basic idea is that early geologists saw this change in the organisms through time using this idea of superposition. They also recognized that if you went to another place, and you saw similar fossils, that you could consider correlating these rocks and layers and basically say, okay, maybe these two groups of rocks are the same age. And by building up this framework, by studying rocks in a lot of different areas, we created the geologic time scale. And when we got better geochemical techniques, we could actually look at layers. So for example, maybe this is a volcanic ash here, and maybe there's a mineral zircon. We can actually look at the uranium lead distribution and lead isotopes to actually get an absolute age using the minerals that are present in these rocks. So this is one of the key importance of looking at older versus younger rocks through time. There are other interesting uses, and one of the key things that we'll be doing is using this framework for a lot of the quarter. And so I want to talk a little bit about how rocks don't record all of Earth history. So let's say we're going through time here, and so these are layers, so this will represent the thickness of the rocks here. So say we have layer one deposited on top of it is layer two. On top of that is layer three. And then at some point the environment changes or maybe there's some tectonic uplift, and parts of these rocks get eroded away. So let's have a time of erosion, and what's left of these layers, I'm going to just erase part of them so we have a representation that they still existed here. So what we now have is there used to be part of layer one, two, and three here, but they're now gone. And so maybe there's a sea level rise, and we start getting deposition on top of this unconformity. So let's say this is layer A and this is layer B here. Rock layers often thicken down into lows, like here. So if we were going to look at the sequence of layers here, and this time I'm going to plot it through time instead of thickness, we can mark on our timeline when each of one, two, and three were deposited. So let's say in this interval of time layer one was deposited, maybe there was no break and we got layer two and then three. But we had this time of erosion in here where some of the rock, where there might not have been any deposition. So maybe we have a gap and then we have A and B. So this interval in our time represents that time with erosion. So if we look at it right here, this zone right here, what we do, what we have is we have rock that represents time one, time two, time three, but then we have our gap and then at time A and B again we have rock. So there's this interval of time when time went on, but there's actually no rock represented and so there's sort of a gap in our geologic record. So if we go and look at an area over here, let's see, we'll choose this area here. We have layers one, part of layer two, then we have A and B. So in this particular case we only have a little bit of two down to one. Then we have our gap and then we have A and B. We're both recorded here. Let's make this the end of B. So even though A is a certain thickness here, it's thicker here, but in this view they represent the same amount of time, so they're the same thickness. But if we connect up the gap because we can trace this unconformity here, what we see is in this area over here our gap is actually thicker. And then if we do one more section all the way over here, I should say I left these lines in here which show that because I made up the scenario that you did actually have rock deposited here and this is in some sense our missing rock. The gap consists of two parts. One part is the missing rock and one part because there was actually no deposition. So if you go over here we just have a little teeny bit of one preserved. And then again we have A and B. Again even though they're thicker they represent the same amount of time. And here we have even more missing rock that's related to the erosion of these layers. So we're going to use this concept throughout the quarter and we'll keep coming back to it again and again. So thanks for watching.