 So, sedimentology and stratigraphy are about as old as mineralogy as a field of study. Leonardo da Vinci provided one of the first environmental interpretations from sedimentary rocks. He interpreted fossils at the top of the Italian Apennines as evidence for an ancient ocean. The logic he used behind this is now called the Principle of Uniformitarianism. The idea is similar organisms produce similar shells now and in the past. The logic is, if you see shells on the tops of mountains that look like those from organisms in the oceans today, the shells on the mountain tops were probably once in the oceans too. He didn't know at the time whether the mountain tops went up or the oceans went down, and that's not a question that we could answer until we understood plate tectonics and understood that the Italian Apennines were faulted up during a collision between Europe and Africa. But the idea is still the same. We can look at ancient rocks and we can use our understanding of modern processes to interpret how those formed. That's one of the key concepts we're going to be using for going from the modern processes to the ancient rocks. If we stated it as a key concept, we can say the characteristics of sedimentary rocks can be used to determine the environmental conditions under which they were deposited and the environmental conditions allow you to predict the characteristic of sediments that are likely to be deposited in that environment. This is the principle of uniformitarianism and it was formulated officially by James Hutton in the mid-1700s. And we are going to use that throughout the class. We can use the principle of uniformitarianism to interpret things like the sand that we see on the beach. Here you can see there's an interesting topography on that sand and we have some footprints here for scale. We can watch in a natural environment to see when this various topography forms or we can actually do experiments. These particular structures are ripples due to currents and there's this nice example from YouTube with ripples forming in a flume. So in this case the flow is going from left to right and you can watch the sand move in a cross-sectional view. So we're looking at it sideways. And what you see is that the back of the ripple has a fairly narrow slope somewhat like you see in this upper part here and then the crest of the ripple is much deeper like you can see on the upper part and in general those ripples are migrating downstream. So we can actually link what we see in terms of the experiment with what we see in the modern environment. We can also link that to the ancient environment. So here's a photograph from some Neoprotorzoic sandstones from Namibia and they show the characteristics of what the ripples look like in cross-section. So if I trace a line and I'm going to make my line a little smaller here. So we have a hand lens for scale up here. It's a little over a centimeter across. So what we actually see here are these lines. I can draw them well. They show a more gentle slope on one side and a steeper slope on the other a lot like we see in the modern ripples and a lot like we see in the experiments. So that suggests that we can interpret these ripples that are hundreds of millions of years old, these structures as ripples from a current. If we interpret the current as going one direction that tells us something about the environment and the flow. We can similarly do that with other structures. So there's a difference if you have a current versus a wave and so this video shows we're looking down in a flume where the water is moving back and forth to the right or to the left to the right to the left to the right and you can see that the sand is getting transported back and forth and these ripples have a more uniform slope on the two different sides. So this is a downward looking view and we can actually look at that in modern beaches. So this is from Broom Beach in Western Australia here and it's in an area where we would normally have waves and we have centimeter scales, this whole thing's about 10 centimeters long and you have ripples that have about the same slope on either side and again we can compare those to ancient ripples. For example here and these are our key in again with a hand lens for scale and what you see in terms of the lamina is that you see the lamina dipping in both directions about evenly. Not everywhere in the bed but really, really commonly and so that looks fundamentally different than what we see for the current ripples where most of the lamina you have a series of lamina that are coming down this way and surface is going across the top like this. So the principle of uniformitarianism allows us to match what we're seeing in the rocks with what we see in the sediments, with what we see in experiments and so it's that connection between the process and the result that is really what makes uniformitarianism different. So the principle of uniformitarianism has been a little bit controversial. For example, sometimes it's interpreted as processes having to be continuous and uniform. That's not how I view it. So in my interpretation it can have rare events. The key is connecting the process with the deposit. For example a meteorite impact is something that happens very abruptly. It's an event and the way I look at it is meteorite impacts no matter when they happen cause similar deposit. So for example the meteorite impact, the evidence for meteorite impact at the Cretaceous tertiary boundary is the presence of a large crater, the presence of tsunami waves and impact spherules where a rock was vaporized and put up into the atmosphere where it condensed into these little spherules that then fell back to earth. So the idea is that you can have events and things that don't happen continuously through time and still apply the principle of uniformitarianism. Again, if the similar processes produce similar events it's one of the key aspects of uniformitarianism. So I want to add that right now that I'm using the principle of uniformitarianism as a scientist working on the NASA Mars Science Laboratory rover. We are looking at sedimentary rocks on Gale Crater on Mars and what we're doing is we're looking at the grains, their size, their rounding, their orientation, we're looking for things like current ripples. We found some of those, we have not yet found wave ripples. But when we make those observations we're using this concept of the principle of uniformitarianism. We are assuming that we can interpret those rocks as representing specific processes as we know them on earth and from experiments on earth. In some cases, because Mars is a smaller planet, there might be some differences in the details that we can evaluate and model those. But we still think that to get current, if you have a flow going in one direction, you're at the right speed, you're likely to get current ripples. Whereas if you have waves going back and forth, you're likely to get the more rounded wave ripples. Thus, the principle of uniformitarianism is useful even when looking at sediments, for example, on another planet. The physics is very similar no matter where you go. Thanks for watching.