 Welcome to Open Geology. Open Geology is a project that we created to share geology with everybody. It is creative commons and has been created by a group of Salt Lake Community College professors. Some of these videos have been funded in part by Salt Lake Community College, but please note that the views and ideas expressed in this video aren't necessarily don't necessarily reflect the views of Salt Lake Community College. Today we're going to talk about sedimentary rocks, specifically sedimentary structures. So let's talk about them. Sedimentary structures are visible features in the rock that represent the processes that formed that rock. So anytime some kind of a process be it wind or water or even life is affecting sediment and that that effect then gets preserved in that sediment and turned into rock then you have a sedimentary structure. You can also have secondary sedimentary structures where the rock is altered a little bit after it's deposited or even after it's lithified and you can start to get things like lysogang bands and soft sediment deformation. But we won't talk about those for the sake of this introductory level material. Sedimentary structures are the embodiment of uniformitarianism. Uniformitarianism being the idea that the present is the key to the past. The processes that are going on now, we're likely the same processes that were going on in the past and we can carry forward our observation of today's processes to figure out what the rocks created in the past represent. Examples of sedimentary structures and in these examples we'll talk about today are crossbeds, ripple marks, mud cracks and fossils. Crossbeds are diagonal layering in sedimentary rock created by wind or water. Ripple marks are small waves on top of a bed that represent a beach or river environment. Mud cracks represent alternating wet and dry environment and fossils represent the remains of prehistoric life. Beds are probably the most important of the sedimentary structures and by beds we mean layers of sedimentary rock and these thicknesses can range from millimeters to you know many many many meters. Here is an example of an outcrop from Capitol Reef National Park here in Utah and you can see the different beds in the sedimentary rock and sedimentary beds, the strata is another term, they are distinguishing features of sedimentary rocks. If you see bedding it's a good indicator that you're looking at a cemetery rock. Please note though that there are other ways you can get layering in an outcrop like igneous dykes or things like that so you want to be careful but bedding is a good way to help you identify that what you're looking at is sedimentary rock. Notice that you have this dark brown bed in this photo and then some shale overlying that and then a nice thick bed of sandstone. The planes, the flat parts that separate these layers they're known as bedding planes. Cross beds are diagonal beds created from wind or river blown grains that are traveling over dunes or bumps in sand and as the sand travels over the top of a bump it falls down and kind of piles up into a layer and it does so again and again over time until you get these beautiful diagonal looking beds and this little GIF here is kind of supporting that and this particular bed, this guy standing on, you're either looking at it from an interesting cross-section view or it has changed direction a few different times. You get these really beautiful diagonal features. These can be on scales from many meters tall especially in the case of dune type deposits. If you can imagine a sand dune those can be very very tall or it can be fairly small especially if you're talking about a river type cross bedding where it's only less than a few centimeters in some cases. Here are some famous cross beds from Zion National Park here in Utah and you can see these alternating directions, these diagonal beds in the sandstone. This beautiful outcrop represents an ancient dune environment where there's a very large desert. Fossils are another type of sedimentary structure and they are kind of an easy and quick way of determining the depositional environment. Here's a shell hash where you have this fossil of first limestone just chocked full of shells and it's a good indicator that this formed in some kind of shallow marine type environment. Here's a piece of petrified wood and one of the famous Green River fish. So we'll talk more about these later but just know that fossils are a type of sedimentary structure. They're preserved traces of what used to be living. Mud cracks and raindrop imprints both kind of represent a nice muddy flat environment. I'm sure we've all seen mud cracks before in the modern world. We have these areas of alternating wetting and drying and it creates these cool kind of polygonal lines and features from that drying and these are all over especially near river banks and tidal flats and things like that. Ripples. Here's a picture I took of some ripples on the south end of the Great Salt Lake and notice the lake has formed the sand on its beaches or it could be the wind into these nice symmetrical ripple marks all along the edge of the shore whereas you have the same kind of features forming that formed to create this sandstone here. So this sandstone could have represented the environment very similar to this. You can use clues of other rocks nearby to help piece together that environment. These ripple marks can be formed both by wind and by water and sometimes their symmetry can indicate the direction of flow. So that's it for sedimentary structures. I hope you learned something. I hope this was informative and please do follow me if you enjoy this and want to hear some more.