S1: okay everybody have a, copy of this, handout? okay, um, excuse me. i was going to um, once again make an attempt to um, uh place names to faces but uh, seeing that we're a couple minutes behind, i may have to wait and do that on Thursday. so again to (xx) assist me what i would like you to do is uh, uh keep more or less the same seats you had uh a couple of weeks ago or a week and a half ago last time we met, uh to assist me to uh, make a correlation with what i wrote down. okay what i would like to do today, uh is, continue the discussion about time, and in doing so, what i'd like to do is to make sure that you have, uh a good understanding of the difference between absolute time and relative time a topic that i think you discussed with uh, uh Professor Lohman last week. uh so to, lead off today's lecture we're going to be i'm going to be asking you questions about these terms and what they mean to you, and then very quickly review uh, the Geologic Time Scale. uh the handout i just gave you, has a summary of the Geologic Time Scale it looks like it's a very, involved, uh list of names and so on but there are a few basic concepts i'm going to point out, that you should remember, in order to facilitate future discussions you're not going to have to memorize all the names you see on there, i'm just going to point out the important, uh items in a minute. uh after we've done that, i am going to again ask you some questions about uh, uh sedimentary units, how they become so extensive as to cover an entire state or perhaps even, uh a large section of a continent, and, partly, to answer the questions we're going to ask about stratigraphic correlations, we will, go into what are known as transgressions and regressions. so number four, is really the answer to number three. and then if time allows at the very end i will introduce, uh radioactivity. okay? so let's begin, by revisiting uh this issue, of relative time versus, absolute time. anybody else, need a handout? you need one as well? okay <P :07> okay, uh, Professor Lohman, told me this morning that he talked about, relative dating, uh versus absolute dating, uh and <WRITING ON BOARD> very quickly i would like somebody to, remind us of what relative dating is and actually what it is that it accomplishes. what is relative dating? this is a course about the evolution of the earth right so that means that we're gonna have to put things or events in their proper sequence, and you can do this, either by actually putting a number on the event that this happened, five hundred million years ago. or we can do it, by using relative dating techniques. so what is relative dating?
S2: saying something is older or younger than something, (else of the same) 
S1: okay so this is simply to place, events in their proper sequence. <WRITING ON BOARD THROUGHOUT UTTERANCE> okay all we are doing, is putting events in their proper sequence, without necessarily, saying exactly when those events took place. and one example we could use here, uh is a cross sectional view, uh of let's say, a road cut. uh and you may identify in this road cut that there is some sort of an undulating line that we call an unconformity. <P :04> okay this is a line, along which there's been some erosion, uh take place. uh you may find, that, uh immediately below an unconformity like so, uh there are series of intrusions we call dikes. and let's have them represent several different uh, uh generations, okay so let's call this one dike A, and this one is going to be dike B, uh and to make it interesting let's put in, (a) third one. okay so this is cross sectional view, uh we're looking at a road cut, and we see sedimentary layers, that are horizontal like so <P :04> okay and these sedimentary units have been cut, uh by these intrusions that i'm calling, uh dikes. okay so sedimentary layers would look like this. this would be say unit one two, three and four. and in fact you may find that on top of that, there are other sediments deposited. let's call this unit six, seven and eight. okay, so relative dating, is the exercise, of placing geologic events in their proper sequence. and, if you came across a cross section like this in a road cut as you traveled to Arizona or New Mexico, or a place like that, would you be able to tell me what happened first and what happened second and third and so on? this is a cross section, that has sedimentary units one through eight, uh those sediments have been cut by, these features we call dikes. they are basically igneous material, that came up and solidified, in features that we call dike and it happens to be in this case, that we have three of them A B and C, we have an unconformity, and we have sedimentary layers on top of that. what we would like to do therefore, is put a history together, and in doing so, we are going to be carrying out the exercise of relative dating, placing geologic events, in their proper sequence, yes? 
S3: what happened to five or is the unconformity considered five? 
S1: well that's part of the story. [S3: oh ] i purposely omitted number five, uh because as we put the history, the geologic history of this cross section together, you'll have to tell me what happened to number five, [S3: alright ] okay? so what do you think happened first, in, yes Cliff? 
S4: uh i think like that layer one sediment was deposited [S1: okay ] one one through four was deposited.
S1: okay, so the first thing that happened 
S4: one through five was deposited (one) 
S1: first thing that happened, was the deposition of sedimentary layers one through four. now, we can't be absolutely sure, that unit five was deposited. and that is an issue i'm going to come back to, after we've discussed what this unconformity is. so unit five, may have been deposited, uh more or less around the same time that one through four, were deposited. uh but there are some other, possibilities that we should consider. but before we do that, what would you say happened, next, uh yes.
S5: um, the dikes C B then A 
S1: are you Jerry?
S5: Drew.
S1: Drew, excuse me, Drew, dikes 
S5: C B then A.
S1: okay dikes were then emplaced in that order, C B and then A. <WRITING ON BOARD THROUGHOUT UTTERANCE> okay so we can say that it was the placement... [S4: ha- how do you know they're in that order? ] of dikes_ maybe Drew can, tell us, how he knows it's that order C, B, and then A in that order. okay how do you know they were emplaced in that order Drew? 
S5: because B overlaps C and then A overlaps B 
S1: okay it is because of the concept known as, crosscutting relations, okay so this is cross, cutting relations you can see, that, uh dike C must have been in place already before dike B was emplaced after all it's truncated by it or it's cut by it, and you can also see that dike B, is cut by dike A. so of the three of them C has to be the oldest one, it was here, when this one came in to cut it, and likewise B must have been in place, before A was emplaced because it's cut by A. so A, has to be the youngest one. okay? alright, then what happens... after these dikes have been emplaced in this sedimentary sequence, something very dramatic happens. what do you think it is, from the evidence you can see? Emmanuel. 
S6: sedimentary rock, um layer C, um, uh, is placed on top of the, 
S1: you mean most sediments were were deposited? 
S6: the sediment, after one, two four, six [S1: yes ] was placed, so 
S1: okay you're saying that number six is then placed on top of this? 
S6: yeah 
S1: well that will happen at some stage, but before we worry about the deposition of unit six what i would like us to worry about, is what happened, along this unconformity. an unconformity being, really, a surface of discontinuity in other words it's a surface along which there has been, erosion. but before erosion can take place, okay let's revisit the issue of deposition. units one through four, were deposited let's say in the ocean, this is a marine sequence if i didn't mention it already. uh, can we, erode, units that are still underwater? can sediments that happen to be, underwater, be eroded?
S3: yes 
S1: they can? 
S4: (a hurricane) 
S7: there's a movement of water, [S1: okay ] yeah they can be eroded 
S1: okay now, if we have, significant currents in the oceans, yes sediment can be, moved around. but when it is moved around, it goes from one place to the other, alright, and we still have deposition taking place so in other words if my ocean basin is like so, <WRITING ON BOARD> and you and i are looking at a cross section that developed, uh for sediments that sit right here, uh if we have strong currents there is sea level, uh the currents could indeed, move some of this sediment from right there, and deposit it someplace else, where, uh currents are not as, uh strong. but to have, a very profound erosion or surface develop, is our mountain or our pile of sediments, going to be underwater or exposed to the elements? 
S3: exposed. 
S1: it has to be exposed, right? in order to develop a significant erosion or surface, our sediments have to be exposed to the elements. in other words, we need rain, we need rivers, we need, uh wind, all of those, uh factors that are going to, uh help weather down or erode the surface, uh so that uh we develop what is known as an unconformity. so that brings us to the point of what happened to number five. it could very well be, that unit five had been deposited along the surface right on top of number four. but, because of the erosion that occurred, unit five was removed. but it is also possible, that unit five was never deposited in the first place. how would we know then that unit five, ever existed if it wasn't deposited right here...? yes.
S7: it wouldn't be if, if it was ever deposited it wouldn't completely, erode it away right?
S1: okay w- w- what we're, saying here really, is that uh we would be comparing, the cross section we are looking at right here, to a cross section we might be able to see in another cliff let's say a hundred kilometers away. so a hundred kilometers away, and by the way that's where we're going to start talking about stratigraphic correlations, we may be able to observe a hundred kilometers away, units one two three four five and then six. that's how we infer, that a unit five ever existed. cuz we see it some other place where it was not eroded away. okay? that's how we know, that unit five existed but in this particular location, uh it has been eroded away. okay, so having said, that, uh, you know profound amounts of erosion will take place only if these rocks are exposed to the elements, what would you say then happens, after the emplacement of dikes A B and C. what must happen Cliff? 
S4: like the sea comes back or wha- it, gets deposited under water again?
S1: okay we need sea level to do something don't we, right? in in in other words if these rocks are initially under water, but now we want, them to experience a period of erosion, that means that either sea level drops sea level drop is one possibility, what's the other possibility? 
S6: weathered away? 
S1: what's that? 
S6: it's weathered away?
S1: um... [S6: eroded? ] e- what's that? 
S6: it's eroded by
S1: by, currents is that what you're saying? no look either sea level can drop so that my rocks become exposed to the surface, so that wind and rain and so on are going to start working on those rocks and erode them away, or what else can happen, mountain building processes? 
S8: couldn't that part get pushed up? 
S1: okay we can have tectonic deformation exactly. the sediments themselves, can be uplifted. and that's what we can summarize as <WRITING ON BOARD> tectonic, deformation... okay so we either have sea level, change, sea level drop, or the land itself came up by tectonic deformation. either one, is going to accomplish the same thing. okay, um, then, it looks like, in order for, more marine sediments to be deposited so let's call these <WRITING ON BOARD> marine sediments... and these as well, are marine sediments, okay so what happens then in order for us to be able to deposit units six through eight? after this surface has been exposed to the elements and, weathering erosion and so on have taken place, then we have, what happening? 
S6: sediments from, other areas? 
S1: well, yeah sediments will eventually come into a basin but i'm telling you that these are marine sediments, <WRITING ON BOARD THROUGHOUT UTTERANCE> so we really just have to reverse, what we're talking about here, so we either have, uh sea level, rise, okay, or we are going to have, tectonic deformation in this case by the way created uplift, and in this case, tectonic deformation is going to create, uh subsidence, in other words, sinking. the ground sinks, and it goes below sea level yet again, and, therefore we can start depositing new sediments, uh on top of the original units one through four. so number five, is we have finally, uh the deposition, of units six, through eight. okay so the exercise we just went through, is one of placing, geologic events in their proper sequence but i haven't really told you exactly when, uh units one through four uh were, uh deposited, okay that, is something we can do, through what is known as absolute dating, okay so what is absolute dating again? from what Professor Lohman, told you last week? 
SU-F: putting in, (time on it)
S1: okay so putting in numerical value, to, a geologic event. how can we put a numerical value, to a geologic event that this must have happened fifty-three million years ago. how do we get that number? 
S9: looking at, radioactive decay? [S7: uh... radioactive decay? ] 
S1: okay radioactive decay alright so this absolute dating, as we shall see at the end of today, or beginning of next hour, is going to be based on, this is, a numerical value, and the numerical value's going to be based on, uh radioactivity. now well before radioactivity was discovered, anybody know when radioactivity was discovered? [S4: eighteen ] how long ago was that? 
S4: eighteen ninety-six 
S1: eighteen ninety-six, right eighteen ninety-six was when radioactivity was discovered, but by that time, uh geologic mapping had been going on, for probably two hundred three hundred years, okay two hundred or three hundred years. and the geologic mapping that was done early, used concepts of, relative dating, to come up with divisions and this is where i'm going to mention what i think you should remember. divisions of time, that we can cut into sections like there is a section right in here, known as the Cenozoic, <POINTS TO OVERHEAD> a section in here known as the Mesozoic, a section in here known as the Paleozoic, and the earliest time, for the earth, known as the Precambrian. where is the word Precambrian? right there, the Precambrian. now, it was much much later, after radioactivity had been discovered, that the values that you see here were added to the Geologic Time Scale. okay so this is the recent addition, to the time scale but prior to that, we already knew about the Precambrian, the Paleozoic, the Mesozoic and the Cenozoic. how were these divisions, put in place? okay that's the question, we should ask ourselves now. well, the Geologic Time Scale, okay the most important thing i can tell you about the Geologic Time Scale, is based on profound changes in life-forms. if you, walk your way from, the Colorado River, at this section of the Grand Canyon, you will discover that, close to the river, you're seeing very primitive life-forms. halfway up the section, as you come out of the canyon you will discover that you begin to see things that you recognize in today's life-forms. and then near the very top, just about everything you see, is something that still exists today. for that reason, the Paleozoic is known as early life, okay the word Paleozoic means early life, and among the creatures of the Paleozoic can anybody think of, one what was the dominant life form of the Paleozoic? this would have been five hundred and thirty to, two hundred and forty-four forty-five million years ago. what was the dominant life back then? there were a few fish in the oceans but the dominant life-form, was actually amphibians. okay this is before, the emergence of reptiles, so this time, segment in here is dominated by amphibians. Mesozoic everybody has seen Jurassic Park, okay so from that you would be able to tell me, that, this time horizon in here is dominated by reptiles, and Cenozoic which means recent life is dominated by marmal uh mammals, like you and me okay? now, i would like you to remember those three terms, Paleozoic Mesozoic and Cenozoic the reason being that uh later on in our discussions we will need to know what they are i'll simply tell you life-forms of the Mesozoic and you should know what Mesozoic means. if you, if you want to you can remember, uh exactly when each one of these ends but it's not absolutely necessary, uh the Paleozoic begins five hundred and thirty million years ago ends two hundred and forty-five million years ago, and everybody has probably seen these, uh, uh doomsday, movies about asteroids bombarding the earth and so on. well we think that one may have happened sixty-six million years ago, when the dinosaurs go extinct. in other words reptiles, uh many reptiles go extinct at this particular time, and mammals become the dominant life-form, uh subsequently. the only other thing i'd like you to remember about the Geologic Time Scale, is that, ninety percent of geologic time, is actually squeezed into, just the bottom right here. okay so ninety percent of geologic time, keep in mind that the earth is four-point-six, billion years old, four-point-six billion years old, but ninety percent of the existence of the earth, or time for the existence of the earth, is something that we don't know very much about. and so we put it more or less, under the rug, way down here, and most of what we know, about evolution and so on, is since five hundred and thirty million years ago. now has anybody worried about, how we know the age of the earth? how do we know that the earth is actually four-point-six billion years old, yes.
S2: don't they, um they assume that all, the whole universe is, or, all the other, space were, made at the same time and then they, find the age of the rocks then 
S1: okay so this is an assumption that uh, she's pointing out that we'll come back to a little bit later. we have to make an assumption about, the age of all the objects in the solar system. our earth has been churned around, so that the rocks that, existed on its surface initially are no longer there. okay those rocks have disappeared, they've gone inside the interior of the earth, so that we have to make the assumption, that all the other bodies in the solar system that haven't undergone this churning of the surface, formed at the same time, as the earth did. and we'll be evaluating that assumption when we come to that uh, uh discussion in another uh few lectures from now. okay so, this age for the earth four-point-six billion years, is really based on dating meteorites. okay these objects that, come flying through space and fall right here on on earth, are the ones we use to date the earth or dating the solar system in general, and we'll be re- evaluating that, a little bit later on. so, most of what we know, about geologic time, comprises only about ten percent, of the entire history of the earth. that's, very very important. any questions about this, yes.
S6: i read that, uh the rock found in, on the moon? [S1: yes ] it's used to uh, help date the uh the age of the
S1: right right [S6: i mean ] (no no) this churning i'm talking about, where the surface of the earth renews itself uh, probably every two hundred million years or thereabouts, every rock on the surface have pretty much, uh disappeared. in the ocean basins that is. on the moon on the other hand, uh there is absolutely no movement, of any crustal blocks, so that the earliest rocks to form on the moon, are still on its surface. and when you date rocks on the moon, the oldest ones are about four-point-four, billion years old. so they're not exactly four-point-six which is what we are claiming the age of the earth to be, but they are awfully close, to the age of the earth. so the oldest, uh rocks on the moon are about four point four billion years. and in fact if you look at our syllabus, you'll discover that uh, we're going to talk about the origin of the moon, and one of the issues we're going to be addressing at that time is why the moon is slightly younger, than what we think the age of the solar system to be. okay? another comment, yes? 
S6: how do we know that the moon hasn't been (changing since,) its formation?
S1: that it hasn't been churning its surface? well partly, because we find very old, rocks on its surface, but also because it's so small, okay because it is so small, it is not able to sustain, um, convection in its interior, to be able to destroy, its original crust. so its original crust is, is still there, still preserved. and that is a topic that uh, we're going to address in detail, when we talk about the origin of the moon, uh in another few lectures. now, another topic i would like to bring up, uh which as i said a moment ago was very influential, in putting the Geologic Time Scale together, okay remember that in identifying recent life, middle life and early life, correlations were being made over very large distances. and, on the handout i gave you, the example i would like to use, is of sedimentary layers very much like the ones we drew in our cross sections over there, but in this case we are looking at northern Colorado. uh i found this convenient to show you because, an oil company went and drilled five holes, right here in northern Colorado, uh over a distance of about a hundred and fifty kilometers, they put in five different holes, and discovered the following. there's hole number one, they went all the way down to whatever that is um, a hundred meters or so, perhaps deeper than that, and they found, a sandstone layer there, they found a shale right there, and they found this uh, formation that i colored in yellow right there. then they moved about what uh thirty forty kilometers away, put another hole in the ground and discovered exactly the same relationships they had seen over here, and repeated that five times, and, you can see this uh, orange unit appears in all five holes, and the yellow unit likewise, appears in all five holes. does that bother anybody? that you can actually find a unit like a sandstone, shown in orange, over a distance of, a hundred and fifty, two hundred, perhaps even a thousand kilometers? the reason i ask you, wha- whether it bothers you or not is the following... okay this is something i've given you, as well. if you ever stood, at the waterfront at the beach, where sediment comes into a standing body of water like the ocean, you would be the first one to tell me, that right next to the beach here, um it is not a terribly wide zone over which we're depositing very coarse grains of sand and, pebbles and cobbles and so on. okay so, if we take a picture, that is an instant in time, what we'd discover, is that in areas where sediments are deposited, we have a very narrow zone over which coarse sediment is going to be deposited, and if we go in slightly deeper waters it's a very narrow zone over which sand is being deposited, and in deeper waters, a very narrow zone over which mud or mudstone eventually, is being deposited. so the question i'm asking you, is how do we go from, very local deposition, of sediments, in a zone that's no more than, a kilometer or so in width, to having such a unit actually cover an entire state. everybody see what the problem is here? okay the, oil wells that we just drilled in northern Colorado show us that a sandstone unit can actually exist all the way from the Utah border, to the Nebraska border. this is, a very very wide area, and yet when we look at environments in which sediments are being deposited today, we see that, any given unit like a sandstone or a conglomerate, is being deposited over just a very narrow belt and my question to you is, how do we go from, having sediments deposited in a very narrow area, to having that particular sediment cover, an entire state. how does that happen? [S3: con- ] yes? 
S3: conditions were different? 
S1: uh, in what sense, it's gonna have to do with conditions yes you're right,
S3: as far as like, well, um
S1: conditions are changing, but in what way? Drew.
S5: um maybe it was all covered by ocean before. 
S1: okay what did the ocean do? [S5: uh ] so, i mean, what you're saying is if uh, if we have an ocean that's rather extensive, okay it will still have a beach somewhere. and what i'm pointing out is, well let's take the example of uh, the eastern seaboard of the United States. okay there's Florida, New York City is over here somewhere, Long Island, what i'm pointing out here, is that we are depositing sand, over a very narrow strip, if we assume that all of this is beach, from New York to Florida. so it's a very very narrow belt, how do we go from having sediments deposited over such a narrow belt, to actually covering, <WRITING ON BOARD> several states, like so, as i showed you in the example of Colorado? yes.
S7: glaciers? 
S1: what did the glaciers do? 
S7: they carried sediments? 
S1: uh, they're going to do it, locally okay they are going to do it locally. when we talk about, the sediments you find, in let's say the Grand Canyon, <WRITING ON BOARD> you know if this is a, map of the United States, the four corners area, uh New Mexico Arizona Colorado and Utah, are all covered, by, more or less the same sediments. okay it's a very very large area. and glaciation had nothing to do with it. so, you're kind of beating around the bush you say conditions will change, the oceans you're all mentioning the right things, but in what way specifically, did the ocean change? if you want to cheat a little bit you can look on the next page that i handed you... okay, the, answer to this question, or the answer to this riddle, has to do with the fact, that sea level... is not invariant. it actually changes through time. okay so the image i just showed you a moment ago, of having a beach along which we're depositing sand this image right here, is just a snapshot if you came back a thousand years from now, sea level would not be, where you see it today. that is illustrated, by these two diagrams right here. where in the first diagram, we can see that the shoreline, is right there. and if we fast forward in time, we can see that the shoreline has moved, to over here. okay so, in the first image, the new shoreline would be somewhere over here, and that is because sea level, has risen. or you could also argue that the basin in which these sediments are being deposited, has gone down. so, the idea, of shorelines moving inland, from where we find them to be today, is known as, transgression. transgression is the, movement of a shoreline, inland, from where they are at any given time, and regression is the opposite. okay that would be shoreline, moving away, uh from the continents. okay, so if shorelines, can transgress in other words they can move inland, what is that going to do, to where we're depositing sediments? and what does that have to do, with sediments being able to cover a very very large area, such as several different states? in other words i'm asking you to compare, picture A to picture B. okay what happens when the shoreline transgresses, moving inland. <P :04> let's see, i have a pointer right here. uh let's consider, a particular point such as right here, this is a depositional environment that early on, you can see, is ideal for depositing fairly large pieces of rocks, so that the rock that's going to form is known as a conglomerate. in that particular area. but now shoreline change- the sh- shoreline changes in other words sea level has gone up, and the area that we were monitoring before which is right there, corresponding to right there, is now in deeper waters, and you can see that, where before we were depositing fairly large sediment, we are now depositing, fairly, fine sediment. okay you appreciate that? so in other words we've changed the depth of the water, at that particular locality, and we asked previously was ideal for conglomerate to accumulate, now after the shoreline has moved inland, that particular point is under deeper water, than previously, so we are depositing, sand... well, imagine, the shoreline being able to move even further inland, in other words that basin is either sinking, or sea level is rising very very rapidly, and therefore this particular unit, being able to form a very very large area, that keeps going on and on and on, for several, hundreds of kilometers, uh inland. so typically therefore, <WRITING ON BOARD> what we would find, is what is known as a transgressive, uh sequence of sediments. in a transgressive sequence of sediments, what you're likely to find, as a package, is that at the very bottom, you're going to have, conglomerate... okay immediately above that, you're going to have sand-sized, material so the Rockies are sandstone. and immediately above that you'll have fine-grained material the Rockies are shale, and then finally, you'll have the limestone. now if you're hiking into the Grand Canyon, and you look in these cliffs, and you see conglomerate, then you see sandstone, then you see shale, then you see limestone, you can actually begin to tell us what sea level was doing, when those sediments were being deposited. you can be reasonably sure, that when those sediments were being deposited, sea level was rising. okay because we've just demonstrated that right here, uh again to, continue with the example i was providing before, the first rock to be deposited for this particular locality, was what turned out to be conglomerate, sea level rose so that right above conglomerate we deposited sand, uh later to become sandstone, and you can imagine that uh, if i raise sea level even more, to this level right here, eventually, at that particular locality, water will be so deep, that i can't deposit sand there anymore, but instead this unit right here, is going to march inland, and it will be deposited on top of that. so i'll have the sequence, of conglomerate, sandstone and then on top of that i'll have shale, very much like we've illustrated here. okay does everybody get that? you can have transgressions that actually cover, uh hundreds and hundreds of square kilometers, so that eventually, you might have sediments that cover, almost half a continent. yes, Emmanuel.
S6: so, the different types of ro- uh rocks have been deposited [S1: yes ] due to due to their suitability to the environment? 
S1: exactly, right what we deposit in any one location, is going to be a function of, how far we are from the shoreline, in other words, how high sea level happens to be, uh in that particular locality, okay? so, eventually, over geologic time, keep in mind that we're talking about millions and millions of years, you will build a transgressive sequence like the sediments we find, in the Grand Canyon today, that are not just, uh randomly distributed, but there is indeed a systematic order, to what you see, and that represents, sea level rising. now, this invites the other question, okay, uh of, if sediments are deposited during sea level drop, what order should they, exist in? as we look at a stratigraphic column, very much like this one, but, those sediments happen to have been deposited when sea level was going down, what order should they exist in? again it's something you can figure out by looking at uh, uh, uh a column li- or rather by looking at a cross section like this, okay now let's say, that, well let's begin with this unit right here. it's a shale, okay, where it is right there. if sea level drops, so that now sea level is right here, what's going to be deposited on top of the shale? 
SS: sandstone 
S1: it should be sandstone, right? because the water's going to be now relatively shallow, so that the environment here will be ideal for sandstone, and, uh if we drop sea level even further, what should be deposited on top of the sandstone? [SU-F: conglomerate ] conglomerate. so, really, for regressive sequences, you should simply flip this upside-down, and you'll see conglomerate on top sandstone shale and then limestone is going to be, at the very bottom. but why is it then, that when we go into the Grand Canyon, it's very common to find transgressive sequences like this, but it is not common to find, regressive sequences. why would that be? i mean you should be able to figure this out. it's easy to find, transgressive sequences in any sedimentary pile you look at anywhere in the world, but it's very rare, to find a regressive sequence preserved, Cliff.
S4: it got eroded away? 
S1: okay why? 
S4: mm because uh, after the sea regressed it was exposed to the elements again and, [S1: right ] caused the unconformity? 
S1: exactly, okay notice that, during transgression, the sediments being deposited, are immediately covered up by others, by younger sediments, so that there is a much better chance, of that section being preserved, in the geologic record, than it is if sea level is dropping. if sea level is dropping, the moment you deposit something like, this sand right in here, okay and then sea level went to down here, well, those loose sediments are immediately exposed, and they're going to be eroded away. so, regressive sequences are not very common in nature, because very shortly after, deposition, of the sediments, they're eroded away, simply eroded away. okay anybody, have a question about that? transgressions and regressions? so basically, sediments exist, as a unit, over very long distances, because of sea level transgression. and in fact i think uh, i give you a couple of examples to finish off here. uh one example is a modern, modern example of sea level transgression, which uh the Netherlands, are experiencing today. okay so i have a couple of cross sections here. on top we are looking at the North Sea, and, here is the Netherlands or Holland, right here, and you can see that uh, four thousand B-C or rather four hundred B-C so that this is a mere twenty-four hundred years ago, the shoreline was actually right here. in the Netherlands. and at the present time, the shoreline has moved to way over here, this is why the Dutch have to deal with these dikes, in order to keep sea level, uh from, inundating their land. okay the shoreline used to be there, now it has marched, all the way uh to here, and you can see that, in a place where, previously, sand was being deposited, that's what these little dots are, the water now is so deep, that mud, is what's being deposited, so on top of, sandstone, we are now having, uh shale being deposited. and when you go north of there, to the Scandinavian countries, uh you see exactly the opposite. okay, so let's see where was, uh sea level, at uh, let's see where does this go? six thousand B-C uh the shoreline, was way over here, okay and at the present time, the shoreline is right there. so it has actually, retreated, so in other words, sea level happens to be dropping in Scandanavia. how do we explain that? let's see if we have uh, a map to show the region we're talking about here. okay there is uh uh Great Britain, so the Netherlands would be, somewhere in here, and here is Scandanavia. so the area we're talking about is actually not very large right? uh Holland over here, Scandanavia over here we're saying that sea level, has gone up over here, sea level has gone down over here. how could that be? how could it be that sea level is doing two different things, not very far... from each other, those two localities. how could that be? 
S6: different, rock formations.
S1: different rock formations? [S6: that ] can you think of anything that, has happened in the last say, fifteen twenty thousand years that might be influencing what we're looking at here...? the last fifteen thousand years or so? [S4: (it was glaciers) ] fifteen thousand years ago could we have had, the University of Michigan right here? 
S3: (we couldn't)
S1: why not? 
S3: (covered by) glaciers 
SU-F: glaciers 
S1: there were glaciers okay, there were s- about three kilometers of ice, right where you're sitting. okay very very thick ice, and certainly, there was a tremendous amount of glaciation in the Scandinavian countries, and believe it or not, when you have several kilometers of ice sitting on top of the crust, it depresses the rocks. very much like putting a big chunk of rock on a sponge, okay the sponge, uh is depressed downwards. rocks, that form the crust can likewise, be depressed, and after the glaciation has gone away, uh Scandinavia happens to be rebounding, very much like a sponge, would rebound, and guess what happens to s- to uh Holland over here? when this rebounds, it's almost like, pulling on a handkerchief or a shirt, you can see that a moat has to build around, this topographic high, and okay so Scandinavia's going up, and all the area around Scandanavia, has to be going down. this is known as isostatic rebound, re-equilibration, <WRITING ON BOARD> isostatic rebound. okay so, it is not that the North Sea is changing, uh uh its sea level, what is actually happening is that uh, over here, the land is going down, and over here in Scandanavia the land is going up, as a result of, uh glacial rebound. and uh okay so we've run out of time, the final page on the handouts is just an illustration, of a transgressive sequence, in the, Grand Canyon. okay, and of course you'll notice that we have, sands at the bottom, uh then shale, then limestone, very much like we've just described, uh over here. okay on Thursday, i'll begin by, describing or defining exactly what radioactivity is and we can continue on from there. see you Thursday. 
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