[MUSIC] If you think about the conditions under which sedimentary rocks are deposited, you can imagine that there are going to be certain kinds of features that develop on the surfaces of beds or sometimes inside of beds that reflect the conditions of deposition. For example, let's look at a sand dune that's being deposited in the desert. On this particular day when the photograph was taken, the wind was blowing very, very strongly. The wind was blowing from the left to the right across the photograph, bringing sand up the back side of this sand dune until it accumulated and then slipped down the face of the sand dune. Overall, beds tend to be horizontal. They can't be deposited on a slope, because if they were, gravity would eventually pull the sediment down to the floor of the basin, further offshore. So in this case, sand overall is being deposited in a horizontal layer. But within the sand, there are these successions of slopes developing. The overall layering is called the main bedding. The inclined layer, with inside the main bed, is called a cross bed. We're seeing them form here in these modern sand dunes. Here, we're seeing the relict of cross bed preserved in cliffs of sandstone that formed hundreds of millions of years ago in a place that's now exposed in Zion National Park in Utah. These are Jurassic sand dunes. You can see the main beds as the boundaries between the individual layers and the cross beds as the inclined slopes within the individual layers. At the time of deposition, keeping in mind the image we had in our last slide, the wind was blowing from left to right. Subsequent to deposition, these rocks have been tilted a little bit, so the layering is now no longer perfectly horizontal. Another feature, or another type of what we call sedimentary structure that forms during deposition, are ripple marks. If you go to a beach or you look at the floor of a stream, you'll see that the surface of the sediment typically is not perfectly planar, but is rippled into individual bumps and valleys, which have elongate crests. These features are called ripple marks. Here we're seeing an example formed along the coast of Cape Cod in Eastern United States exposed at low tide. Now, look at that image and compare that to this image. They look almost identical. But the rocks that we're seeing in this image, first of all, the beds are no longer horizontal, because mountain building processes tilted them to an angle of almost 45 degrees. But also, these rocks were deposited originally at the time the dinosaurs lived. In fact, just around the corner from this exposure, there are dinosaur footprints superimposed on the ripple marks. We've said that sedimentary rocks form in a variety of stages. First, there's production of the sediment. Second, there's the transportation of the sediment. Third, there's the deposition of the sediment. And fourth, there's burial and the process of turning the sediment into rock. Let's look at these final stages. There are certain places in the earth where the crust over time may start at, at one elevation. And if we look in cross-section here, over time, the crust gradually warps down as so. As I'm showing here. Now, this process by which the crest gradually sinks is called subsidence. The depression and its contents is called a sedimentary basin. In a sedimentary basin, the accumulation of sedimentary beds is much thicker than its surrounding regions. So you can imagine all these layers down here are composed of layers of sediment that have accumulated. Now, there are various tectonic reasons for why sedimentary basins form. Some of them are a consequence of loading of the crest by a weight, like a mountain belt that pushes an area down and creates a depression that then fills with sediment. Some are formed along the margins of continents, where the boundary between the continent and the ocean basin gradually sinks as it cools. There are various reasons, and we don't really have time to go into those now. But basically, this process of subsidence can take millions, tens of millions, even hundreds of millions of years, creating a low area that accumulates sediment over geologic time. Now, if you come down to some depth within the sedimentary basin and enlarge that area, here are the layers of sediments. You can imagine that there's a compression in the vertical, in the vertical direction, we're squeezing those sediments because of the weight of the overlying material. As that squeezing process takes place, water that was once in the grains, or once between the grains will escape. And the grains will pack closer together. Later on, we may have waters coming through that carry dissolved minerals. And out of these waters we precipitate water carrying dissolved minerals. That water can stick the grains together by cement. So basically, the process of lithification, the process of turning sediment into rock, involves two phenomena. First, the compaction of sediment, the squeezing it together, to remove pore space and to push the grains closer together. And secondly, the process of cementation, producing mineral cements that hold the grains together. Once that process is complete, we no longer call the material sediment. We call it a sedimentary rock. [MUSIC]
