so perfect example this is capitol reef in utah you can see the layers right here's the bottom middle another middle layer another one here and then the top if i were to ask you a question is it sequentially older from abc or cva right what would you answer is a older than b or is c older than b what would you say how would you answer this hopefully you are thinking that a is older than b and b is older than c because the bottom should be the oldest okay so that is how you would answer that question so stratigraphic succession this is one of those principles right so the chronological order or the chronological record of the geologic history in a region is as preserved as a vertical set of strata so layers of rock so what do we mean by that the stratigraphic record includes sedimentary rocks as well as this extrusive igneous rocks and in this case basalt so if you look at the columbia river basalts we're looking at here's all these different layers if you see the layers the layers are actually lava flow so this is one flow two flows three flows four flows five flows six flows there's a ton of flows within the columbia river basalts okay and this is up in the pacific northwest and you can actually go there i've been there it's amazing there's a whole giant area for you to be able to look at i mean you can follow this columbia river for very long ways so that's the stratigraphic record we're trying to figure out the ages relative ages of sedimentary rocks and also extrusivity igneous rocks okay so laws of stratigraphy this is all about relative dating and these laws you need to know okay so original horizontality this is just a principle that we're saying hey everything that gets deposited sediments wise are essentially laid out in horizontal beds so no matter what the bottom of the surface looks like when things are deposited things are going to be laid out in nice horizontal beds so see those nice horizontal beds that's what we're gonna assume okay so that's one assumption we're making a principle of original horizontality okay two superposition this is the principle of superposition that's saying that each layer of stratified rock in an undisturbed sequence remember undisturbed meaning not flipped or messed with tectonic-wise or with a fall is younger than the one beneath it and older than the one above it so just these layers are older while these layers are younger we just went through that so that's the principle of superposition i hope you're good with the fact that we're assuming sediments are laid down in a horizontal manner and that the ones below are older than the ones above super simple okay nice illustration of the superposition is in the grand canyon so we've got the supai group the hermit shale coconino sandstone and so on and so forth and you can see that as you go up everything's getting younger so literally if you walk down the grand canyon you are traveling back in time meaning if you start up here you're at the youngest of the sediment layers and as you walk down the grand canyon down to the center you're actually walking into older and older sediment which is pretty cool to think about next is the law of inclusions okay so this is the idea that when you include a previously existing rock within another rock you have to remember that the rock that contains the inclusion is younger but basically the inclusion is older the reason this is the way that i think about it is that the inclusion so the pebbles or the xenolist that are included within your sill or within a sandstone or within whatever so this is a nice example of one of those inclusions right there is that the only way for that magma that was hot at one point right to have an inclusion is for there to be pre-existing rock there that kind of breaks off and doesn't melt down all the way and therefore becomes an inclusion okay so it's the idea that the rock containing the inclusion is younger and the inclusion itself is older because it's pre-existing it was already there so this xenolith here inclusion is older than the sill that it sits in okay good cross-cutting relations this is something that you're going to see out in the field out here in san diego okay so these are disturbances of stratified rock layers so we're talking about faults and intrusions so dikes and sills remember dikes are vertical sills are horizontal and the big thing to remember here is that whatever is cutting is younger and whatever is cut or offset is older once again there's pre-existing rock there and a diker still comes through and kind of penetrates or forces its way through the older rock so whatever it's penetrating through is older and the sill or dike itself is younger so all this here right these horizontal beds right we're seeing the principle of original horizontality we're also seeing the principle of superposition meaning this is older younger younger younger younger younger right so all this was here already and then this dike comes through and breaks through intrudes through all of these previously existing rocks so therefore the dyke is younger than the rest of this picture okay here's an example of a dyke here and this kind of diagonal one i would still consider a dike it's not nice and horizontal okay so this whitish rock would be older and this darker colored intrusive dyke would be younger because this was existing before this okay now we also have baked contacts so thermal metamorphism occurs when country rock is invaded by plutonic igneous intrusion this is the same principle that we just had with those dykes and sills is that the baked rock had to have been there first so this rock here right this looks just like that picture from before these rocks were here first and then this pluton comes in and intrudes through and settles and cools and creates a nice baked contact around it that's that thermal metamorphism that we talked about previously and that thermal metamorphic baked contact would have been older than what was there previous than than this pluton i should say okay so baked contacts are older than the pluton um another principle is of lateral continuity this is idea that you know you deposit something um a long time ago say in a river or in a lake or in an ocean and then later on you're slowly eroding it away with a river right with water and you're actually breaking apart this area you're splitting this area into two so take this little square here and bring it over here now you've got these same nice layers that were there and they used to be connected but now a river has created a big gorge right big river gorge between the two what this is saying is that this layer is still the same as this layer but they might be miles apart um and that is something that we call the principle of lateral continuity that hey on this side they're going to match up with what's ever was on this side and you can see that in the grand canyon it's a perfect example okay principle of lateral continuity