the stratigraphic principles are the rules that we use to put the different geologic events in order so we'll start with the first one principle of superposition this one says that in a series of layered rocks the oldest layer is going to be on the bottom and the youngest layer is going to be on the top all right so here we have three sets of sedimentary rocks here the sandstone at the bottom is the oldest of those three and the limestone at the top is the youngest all right we can see this very nicely in the grand in the grand canyon sequence so all of the layers from the tipit sandstone up to the top at the kaibab formation those are all still horizontal so we know that the tipit sandstone is the oldest of that series of rocks the bright angel shale is is younger the moon limestone is younger than the bright angel shale etc alright so that one is pretty basic it's just that as rocks are deposited the oldest rocks will be deposited on the bottom of a stack of layered rocks and the youngest will be at the top next is the principle of original horizontality this one says that layered rocks are deposited in general horizontally like this over here okay so if we see something like this if we see layered rocks where we can clearly still see that they were layered but they are deformed like the folded in this example we know that the deformation event happened after the deposition of the layers all right so and again in our example here of vasquez rocks these are sandstones sandstone that would have been deposited horizontally and now since it is tilted at about a 45 degree angle we know that there was some tectonic event that occurred to tilt those rocks after the deposition of the rocks happened all right so we know that the the rocks are actually older than the deformation event next is the principle of original lateral continuity so this one says that layered rocks are deposited continuously in all directions throughout the extent of the depositional basin okay so for instance up here we have a sandstone layer covered up on the top by a limestone layer all right here we have eroded strata so if we were to come upon this example here okay we see that we've got sandstone here and limestone on top over here sandstone here limestone on top over here and then the same thing over in this this little valley as well so we can say that you know the erosion event occurred sometime after the deposition of the limestone right because if the limestone had been deposited on this shape of topography um the limestone would have filled in these gaps all right so you wouldn't have limestone just stopping here and just being deposited here and just being deposited here so when we see something like this we can basically kind of fill in the gaps and we know that there were layers there but now that material has been eroded away okay and so we can see that in this photograph as well you've got this kind of cliff forming layer right here and then if we go across the little valley there we see the same type of rock on the other side at the top of this little knob you've got this harder rock and if we go across here we see that we also have this kind of more hard cliff forming rock as well so these were continuous when they were deposited and now subsequent to that all of this material has been eroded away next is the principle of cross-cutting relationships and this one says that rocks or faults or any other feature are younger than the rocks that they cut across okay so let's look at this example first we've got four layers of sedimentary rocks here and we've got a dike so that's an igneous intrusion right so the dyke based on the principle of cross-cutting relationships has to be the youngest the dike couldn't intrude these layers if these layers didn't already exist okay so we could put this in order based on the principle of superposition we would know that this is the oldest rock next oldest next oldest next oldest and then the dike cuts across all of them so the dike has to be the youngest in this example we've got three different features we've got the gray rock which is labeled b we've got the quartz veins which are labeled a and we've got the fractures in this rock which are labeled c so if you i'll give you a second and try and think about what order these would be in from oldest to youngest well hopefully you notice that b is the oldest right so both a and c are cutting into b so b had to already be there in order to be cut across by these other features a is the next oldest right and we can tell that because c the fractures are cutting through those quartz veins all right so c is cutting through a and so c has to be the youngest all right so the order is b a c from oldest to youngest next is the principle of inclusions so this says that any material that is included in a layer is older than that layer all right so we've got two examples over here both with granite on the bottom and sandstone on the top let's start with this one here so the material that's included here are these chunks of granite alright so the surface of the granite was weathering and you had gravels forming all right so from the physical weathering process of granite of breaking granite down and then sometime later sand was deposited on top and then that underwent the lithification process to become a sandstone and that sandstone ended up with inclusions of granite inside of it so the material that is included has to be older than the thing it's included in so in this case the granite is older than the sandstone the granite had to be there and be weathering in order to make these particles that ended up in the sandstone now this one's a little trickier okay so here we've got the same orientation of the two rocks but now the sandstone is the inclusion inside of the granite so how could that happen well we need to think about how granite forms here okay granite is not a layered rock granite comes from magma which comes from the mantle so granite comes from lower down and intrudes upward into rocks okay so the way that this would have formed so based on the principle we know that the sandstone has to be older because that's what the principle says right and it's the one that is included in the granite so the way that this would have happened is that you would have had sandstone in the crust and then magma intruding into the sandstone and then some of the sandstone broke off in these chunks and then as the granite cooled and crystallized it ended up with the inclusions of sandstone inside of it okay so that's how that would work there so you've got to be careful with things like intrusive igneous rocks because those are not layered so we can't use the principle of superposition for this because granite comes from the mantle and sandstone is being deposited from the top this is an example from yosemite so what we're looking at here we've got a dark gray rock here and then we've got lighter colored rocks and there's some pieces of lighter colored rocks within the dark rock so which one is older so hopefully you said the white rock is older because the white rock is actually included inside of the gray rock so these are both igneous rocks but one is darker in color than the other and we can see that the lighter colored one is included within the darker colored one so it's older okay unconformities are essentially surfaces that are left usually due to erosion and or a period of non-deposition and they represent a gap in the geologic record so there's some amount of time missing there's some amount of rock missing which represents time but we don't know how much right so there are three different types of unconformities so when you look at these block diagrams and you see a squiggly line like this that indicates an unconformity the straight lines between the different rock types those are just contacts between two different rock types like in this case maybe between a sandstone and a shale so here we have a disconformity a disconformity is an unconformity where you have flat layers on the bottom and then flat layers on the top so that means that there was no deformation or faulting events that occurred there was just a period where this sandstone was being eroded down and then sometime later the limestone was deposited on top of that surface so that's a disconformity with with horizontal layers both below and above the unconformity an angular unconformity is where you have either tilted or folded rocks so non-parallel rocks underneath the unconformity and then layered rocks flat lying layered rocks over the top okay so that means that you know these layers were all deposited we know based on the principle of original horizontality that they would have been horizontal and so there was some tilting event that happened subsequent to their deposition and then there was a period where those tilted layers got eroded down forming the unconformity surface and then deposition started again forming these three layers and then finally we have a non-conformity a non-conformity is where we have crystalline rock either igneous or metamorphic rock below the unconformity and layered sedimentary rock on top okay so that is a non-conformity this place is called sicker point in scotland and this is a very famous place geologically this is where james hutton if you remember james hutton from the principle of uniformitarianism he is the person that kind of discovered unconformities or was able to actually describe what he thought was going on so you're going to watch a video after this about sicker point but i'll just quickly kind of show you what we're looking at here so we've got a b and c and that corresponds to a b and c in the cartoon down here so a is this sandstone that is tilted almost or i'm sorry almost vertically okay so you can see the layers that way but they are vertical now b is the unconformity so that's the surface between the two different rock types and then c is another sandstone but it is tilted at this angle here okay so it has a it has a different tilt than the than the a layer down here all right so b is the unconformity and this was where hutton realized that the age of the earth must be very very old in order for these processes to occur based on his observations and again this is this is kind of his idea of uniformitarianism so the the original sandstone the older sandstone here would have had to have been deposited horizontally all right and it's a quite thick set of layers here and then it would have had to undergo some kind of major tilting event to tilt it up like this then a period of erosion to form the unconformity surface then a period of deposition for the younger sandstone and then another tilting event to tilt that sandstone at the angle that we now see and so he realized that it must it must take a very very long time for these processes to occur and that the that's the earth must be very very old and this was his idea of deep time so you'll see more about that in a couple of the videos that you're going to watch okay so these different um principles are what we use to determine the order the chronological order of geologic events so you know if you were to see something like this image and number six here you would need to figure out what happened when right like what is the oldest thing and what is the youngest thing all right so that's this working out of a geologic history and this kind of goes through the whole process all right so if you had this like kind of how you would figure out how this worked um so we there's going to be a i have a video for you showing you kind of how this works and then we will be practicing this during class time as well but please do go ahead and read through this slide on your own