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How to date an archaeologist: Stratigraphy - Ep 33
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It’s time for the last in our series on dating methods for Tea-break Time Travel Training, and this time Matilda discusses the most common for of relative dating - stratigraphy. But what exactly is stratigraphy? Why are there so many rules and principles to something that seems extremely self-explanatory? And what does any of this have to do with dusty shelves? Listen it to find out!
Transcripts
- For a transcript of this episode go to https://www.archpodnet.com/teabreak/33
Links
- Science Direct stratigraphy topic search list
- Practices of Archaeological Stratigraphy
- Article on the history of Stratigraphy
- Principles of Geology ebook
- Harris Matrix activity sheet
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Transcript
Introduction to Tea Break Time Travel
00:00:00
Speaker
You're listening to the Archaeology Podcast Network. um You're listening to Tea Break Time Travel, where every month we look at a different archaeological object and take you on a journey into their past.
00:00:17
Speaker
Hello, and welcome to episode 33 of Tea Break Time Travel. I'm your host, Dr. Matilda Ziebrecht, and today I am s savoring a salted caramel, Roy Bosom black tea, which is absolutely divine, especially with some full fat milk. It's so creamy and so Sweet and really really really good. And
Archaeological Dating Overview
00:00:36
Speaker
there is no guest with me today because it's time for our next instalment of Tea Break time travel training, where we look at all of those theoretical and methodological backgrounds that every seasoned time traveller and archaeologist needs to know before they set off on their journey.
00:00:51
Speaker
So today we are continuing with probably the final installment of our theme of dating, as in archaeological dating, checking how old something is, and we're going to be talking about relative dating, specifically stratigraphy. So first of all, what is relative dating. So, so far, if you have been listening in to the last couple of episodes, you'll know that the dating methods that we've covered so far have been what's known as absolute dating methods. So you have absolute dating and relative dating. So absolute dating is when a date is given as an absolute date. So independent of other data,
00:01:28
Speaker
a particular period in time.
Absolute Dating Methods
00:01:30
Speaker
So not a date that's given in relation to other dates or sort of a period that's given in relation to something else. So let's have a quick look back at that. So we've looked at some radiometric dating. So this is based on the known and the constant rate of decay of different sorts of radioactive isotopes. So for example, we looked at radiocarbon dating.
00:01:51
Speaker
You also have something called potassium argon dating, which I'm not going to do a full episode on because the principles are pretty much the same as with radiocarbon dating, but instead of using the half-life of carbon-14, it uses the half-life of potassium-40.
00:02:06
Speaker
And you might remember that with radiocarbon dating, you can't really date things older than 60,000 years, but because the half-life of radiocarbon is relatively short, but actually the half-life of potassium-40 is 1.3 billion years. So it's very much possible to date much older sites than if you would have radiocarbon dating. The difference, of course, is that radiocarbon dating dates organic materials.
00:02:32
Speaker
ah Potassium argon dating looks at inorganic materials, specifically rocks, because that's where you'll find the potassium 40 isotopes. So that's radiometric dating, looking at the decay rate of radioactive isotopes. You then also have different kinds of luminescence dating, which we looked at last time. So this is based on the concept of luminescence, aka sort of light,
00:02:54
Speaker
building up over time. So when the energy is released in a certain way, it's released as just luminescence. So we looked at thermoluminescence dating, so check out that last episode, episode 31. If you're interested in finding out more about that, you also have optically stimulated luminescence, which again, I'm not going to do a whole episode on because it basically follows the same principle as thermo luminescence dating. But instead of looking at when something was last heated, it looks as when something was last exposed to the light. So similar concept in that respect. You have to be a lot more careful, of course, when you're taking samples for OSL dating, optically stimulated luminescence dating, because as soon as you expose it to the light, that is then the clock resets. So you need to be very careful when you're taking your samples.
00:03:40
Speaker
So we have radiometric dating, we have luminescence dating, we also then have dendrochronology. And now I know what you're thinking. You're thinking, wait a second, but she said that absolute dating methods is giving a specific point in time and it's not giving it in relation to other data points.
Understanding Relative Dating
00:03:54
Speaker
But dendrochronology as we remember, hopefully, from the last episode I did on that, is based on kind of a timeline that you build up, which is made up of overlapping tree rings. So you could argue that it's looking at a date in relation to other data. However, it is still classified as an absolute dating method because it can provide that independent absolute date. You're not looking at it even though the method itself arrives at it, one could argue in terms of looking at relative dates.
00:04:23
Speaker
the final data point that you have, the final date is absolute. So those are some of the sort of most common examples of absolute dating methods that we have covered so far on this podcast. But today we're having a look at relative dating. So relative dating is instead, to put it simply, is based on looking at whether something is older or younger than something else. So one of the most common examples is known as seriation.
00:04:47
Speaker
This is one of the earliest examples of archaeological dating, and it's based on the principle that objects develop over time into different typologies. And by typology, I mean the official definition in terms of the classification of things according to their physical characteristics.
00:05:05
Speaker
So to put simply, the assumption is that one style will slowly take over from another style over time. So if you identify changes in object typology, AKA and what they look like in the style of things, over time, that can be then related back to the age of those different areas of a site where those objects were found. If we think of it in terms of a modern example, think of mobile phones. If you have a Nokia 500 versus an iPhone 14,
00:05:34
Speaker
buried in different layers of a site, you'll be well aware of, oh, okay, well, this site must have been in the, whenever it was late 90s, early 2000s, whereas the iPhone 14 must be then the 2020s. So and we can see that even in the modern day and the same sort of things were happening in the past. However, there was a lot of the kind of issues with this idea and seriation and basing it solely on that.
00:05:59
Speaker
partly because that is very much based on interpretation of like how a style develops over time. But it is still one of the fundamental principles of archaeology. And if you look back in any sort of older analysis of archaeological sites, you will see that as the kind of principle way that different cultures different cultural development and different time periods were determined. Of course, it's important to remember that you know just because you see this development in the objects, that doesn't necessarily mean that the culture was changing as well. There's all sorts of papers that are written about this, and I don't want to go too much into this in this particular podcast.
00:06:36
Speaker
So there's been a sort of long standing debate within archaeology made famous by several different papers which stated that, you know, pots are not people. And there's, yeah, I'm not going to get too much into it now because it was a whole, whole thing. There was a massive debate about this idea that pots are not people. So you can't use objects and the sort of seriation of objects in that way. So looking at how the typology develops over time to then identify when cultures developed because pots are not people.
00:07:05
Speaker
a whole series of various papers flurrying around talking about that. But so if you're interested in learning more about the theoretical development of that argument, then definitely go and check out some of those papers. If you just type in pots are not people to Google Scholar, you'll already get a ah great little selection of papers in there.
Exploring Stratigraphy as a Dating Method
00:07:23
Speaker
But The fact remains that this this idea of kind of looking at things and how they develop over time and looking at the time layers and the different places that objects are deposited within a site still tells us a lot today about the different ages of those layers within a site. What do I mean by layers, by the way? So imagine we have so far, we've looked at various absolute dating methods, which look at things like you can
00:07:49
Speaker
to date organic materials. You can date pots that have been fired or flints that have been heated. You can date various wooden objects. If you do things like potassium argon dating or optically stimulated luminescence, you can also look at, for example, rocks and inorganic materials and objects. But What if you can't do those particular scientific dating methods? What if you want to be able to look at a site and see how it's developed over time and see the different layers within that site? and By layers, I mean the different ways in which the geology of a site has changed over time. This is strongly linked to the concept which is known as stratigraphy.
00:08:29
Speaker
Stratigraphy is the documentation of different layers within a site that relate to different time periods. and And this is probably going to be a little bit shorter this time, but we're going to have a very quick break and then we're going to come back and we're going to talk a little bit more about stratigraphy.
00:08:48
Speaker
Welcome back, welcome back. So I mentioned a little bit before the break that we are talking today about stratigraphy, which is kind of the ultimate relative dating method, really. So the official definition of stratigraphy, according to Waters 1992, I'm taking this from, is the study of the spatial and temporal relationships between the sediments and the soil.
00:09:11
Speaker
So what does this actually mean? So in order to understand the kind of basic principles of stratigraphy, you first need to know a little bit about the geological kind of formation processes. So let's dive into that a little bit. So you have a layer of soil.
00:09:26
Speaker
Right? On the ground now, you look out, there's a lovely layer of soil. Over time, another layer builds up on top of that. And then over time, another layer builds on top of that. And then another and another and another and another. Think of it like a shelf that doesn't get dusted, right? So even though you're not actively coming along and like dumping dust on this shelf, eventually over the years, different layers of dust will build up.
00:09:52
Speaker
And so the same thing happens with layers of soil on the ground, which you might not think of it. Because if you look outside now, you're like, yeah no, this is happening. If anything, things are eroding away. But actually, things are building up as well over time. And you'll know this a lot more if you are involved with with geology, because you can see it a lot in rocks. You can see it beautifully in quite a lot of times. And you might see this beautiful buildup of kind of lovely layers in in the different rock layers, which are very pretty.
00:10:20
Speaker
So depending on what happens environmentally, I mean you see the same sort of thing in the soil. So sometimes these soil layers are so distinct that if you dig a hole and you look at what we in archaeology call a profile or a cross section, so basically you've dug your hole and then you have a wall of the hole If you look at that, and you can see all the different layers of soil going down. And sometimes these layers are so distinct. They have different colors. They have different consistencies. They have different materials, et cetera. So those are the sort of stratigraphic layers that we look at when we are looking at stratigraphy. So this is what I'm referring to when I'm talking about the layers in the in the site or the layers of of a site.
00:11:02
Speaker
and This idea of stratigraphy was kind of concluded or collated, shall we say, from multiple scientists in a range of fields, mostly related to geology. The sort of idea is that an undeformed stratigraphic sequence, so this this little sequence of layers,
00:11:19
Speaker
you will have the oldest layer at the bottom. And this kind of general principle is called the law of superposition, which was first stated by Charles Lyell in his book, The Principles of Geology, which was a multiple volume book published between 1830 and 1833. Also, fun fact, the theories and the principles stated in this book were then part of the foundation for Charles Darwin's theory of evolution, because it showed that the deeper you go in these stratigraphic layers, the older things become.
00:11:49
Speaker
And the kind of guiding principle behind this is another lovely vocabulary. We're doing lots of vocabulary today called uniformitarianism, which is the kind of the main summary of that is the present is key to the past, which I would argue you is kind of it yes, it works.
00:12:07
Speaker
But in terms of interpretation, maybe it shouldn't always work. And in a lot of times also, the past is key to the present. But anyway, but the way that it works in terms of uniformitarianism is this idea that the way that things geologically or biologically happened in the present is the same way that things happened in the past. So if we see a particular geological process happening now, your shelf that's not getting dusted is building up with dust now, that also happened in the past. Shelves in the past also got lots of dust on them. I mean, but or, you know, geologically soil layers built up. And this seems very self-explanatory, right? You might be listening to this and thinking, yeah, duh.
00:12:45
Speaker
But it's it's important to consider that actually at this time when kind of science was really first getting solidified and sort of developing into something that could be replicated and could be theorized and could be interpreted, these principles were essential. Like it was really essential to have these principles, which might seem to us to be common sense, but actually at the time were groundbreaking. I mean, just think about things like Charles Darwin's theory of evolution, right? That was a ground literally a groundbreaking resource that came out, because and it shook the world because the way that people thought about how the world worked was so tied up in in kind of religious experience and that sort of thing as well. so This idea of the law of superposition, that things on the bottom, you know things deeper deeper layers are older, and uniformitarianism, that you know what happened now also happened back then,
00:13:36
Speaker
It might seem obvious to us now, but these were pretty revolutionary principles at the time in terms of guiding the way that we think about the world. Sorry, rant over. Right. so Archaeology started using this principle, so this law of superposition. Older things will be buried deeper, pretty much straight away.
00:13:55
Speaker
So they considered that the objects or the features, by features I mean physical things like buildings, stuff that can't be moved, were buried deeper or discovered in deeper stratigraphic layers, were then older than those found in shallower layers.
00:14:11
Speaker
Again, might seem self-explanatory, but this created a fantastic way of then digging into a site and looking and seeing, okay, well, this pot was found, you know, below this pot, therefore it must be older. And by the way, when I say below this pot, I don't mean, you know, literally one millimeter below or something. Most of the time it's significantly deeper than that, but we're going to get into that in a little bit, because we're going to talk about how this actually works in practice. But the point is that you can look at these different layers, these stratigraphic layers, and you associate that with a particular time period. And therefore, any objects that are buried within that layer come from that time period. And if one layer is lower than another, that
00:14:54
Speaker
layer is older than the upper one, therefore that time period is older than the other one. And I should say that there's like many theories that were associated with this method, which are now considered quite outdated. So for example, in terms of cultural evolution occurs, or I mentioned earlier that pots are people and you can directly link an object with a culture that it represents, which is yeah, quite an outdated theory, I would argue.
00:15:18
Speaker
But it must still be said that the basic principles of kind of associating these different stratigraphic layers with different dates is still like the staple approach of archaeological excavation. So how does it actually work in practice? How does stratigraphy work as a dating method? So when you're digging at a site,
00:15:37
Speaker
You dig a hole, a test pit, or a trench, depending on where you are. They use different terminology and different kind of formats of the hole. But essentially, if you're an archaeologist, you'll dig a hole. And then you create what's known as a profile or a cross section. Again, the vocabulary is different depending on where you're digging. And what you do then is you dig your hole. You have a wall of the hole. Usually say you have like a round feature. So you see that the soil color on the surface is a bit different in different places. And this means that something was buried there at some point. Some changes occurred in this location.
00:16:11
Speaker
So you don't want to dig the whole thing because then you get rid of it, right? What you do is you dig half of it. You basically draw a circle around it from the surface, right? Draw a circle around it, cut it in half, and then you dig that half.
00:16:24
Speaker
So one wall of your your hole, your test bit, your trench, your cross section, whatever, will be the central kind of cross section or profile of this area. So you'll be able to see all of these lovely layers, all of this stratigraphy, this buildup of layers over time of what has happened in this particular area. So it's always, to be honest, it's always my least favorite part of an excavation is trying to make the profile look pretty because you want to when you're cleaning it off, you of course you don't want to be like smearing it right because you don't want to be smearing the layers together. So this is why a trowel is so important. And this is why we don't use brushes for this kind of thing. Because if you use brushes, you're basically just smearing stuff together. So you want to keep it
00:17:12
Speaker
as sort of clean as possible and is as clear as possible so that you can really clearly see different layers in the soil. And ideally, you have like lots of gorgeous layers in a very clear visual sequence, right? Like at the top, you'll have like the grass and the the newest soil. Below that you'll have like a sort of golden sandy layer. Below that you'll have like a thick deep dark brown clay. Below that you'll have like a red sort of brick thing and you know it goes down down down down down. But of course this hardly ever happened. You know people in the past they dug holes.
00:17:47
Speaker
They filled holes with things, which then filled more things. Stuff fell down. Stuff was disturbed by a rabbit a thousand years after it was buried by another layer. A landslide happened. You might have two different things happening at the same time, but in different parts of the site, at different parts of the stratigraphy, so they'll look different in the cross-section depending on where in the site they are. So you have to work out the puzzle.
00:18:10
Speaker
a little bit of which bits cut. It's called cut into little bits. So these are sort of very complicated drawings where you'll basically have like a lot of lines showing the layers, but then one line might go down and cut through four other lines because someone dug a hole at that point, right? And this requires a lot of kind of experience in knowing how exactly to interpret them.
Harris Matrix in Archaeology
00:18:31
Speaker
So you have a lot of different resources we can use for this. So you have something called a Munsell chart, which gives you the different definitions of different colors that you can use as a specific reference to be like, this is this kind of soil. But also, yeah, you have different soil types, which I'm not a field archaeologist, so I'm not as aware of all the different ones.
00:18:50
Speaker
But a lot of my colleagues who are professional field archaeologists, I mean, they can identify exactly what kind of soil something is just by looking at it, because they've had so much experience in digging this down. And even bits of soil, which to me, I look at and I'm like, I mean, yeah, maybe there's a slight color change, but surely that's the same layer, they'll be like, no, that's clearly a different layer. And they can, you know, show you on your profile on your wall that you've cleaned, they'll be like, no look, this is a layer, this is a layer, this is a layer. And again, sometimes they're really clear, and they're really crisp. the The sort of best thing to happen is if, like some kind of disaster or whatever has happened in the past, and you know, like there's been a volcano eruption, and so a layer of ash forms a complete stratigraphic layer, and you're like, yes, Fantastic. that's That's that volcano eruption right there. And you can date that really precisely then because you know that that's when that happened or when there was a fire and the house burned down and then suddenly it's like a layer of sort of charcoal or like these sort of things that happen. And those are really nice because they're sort of very intentional events that happened. But most of the time, it's just changes over time. It's changes in the environment. It's changes in how a particular site was used, whether it was an outside area, an inside area, whether
00:20:00
Speaker
there was particular processes going on at that area. So you really need to have a lot of experience in order to be able to understand what it is you're looking at. And one of the other things, a tool that those archaeologists are very aware of, some people love it, some people hate it, is what's called the Harris Matrix.
00:20:17
Speaker
And this is a tool that is used to depict these kind of overlapping, complicated temporal successions, shall we say, so different time periods overlapping each other in the stratigraphic layers. It was first developed by Edward Harris in 1973.
00:20:32
Speaker
And it's based on five laws. Now bear with me. Don't worry. You don't have to remember all of these, but it's kind of important. Again, it's one of these things, these principles, which you might think, do but again, essential for basing ah sort of a solid and constructive theory and a toolkit. So you have the law of superposition, which we've already dealt with. That's deeper layers are older, right? You also have the law of original horizontality.
00:20:59
Speaker
which basically means that any layer which isn't intentionally built in a specific way will automatically adhere to the next horizontal surface. like If you have soil, it's going to lie on the ground. It's not going to like build itself up into a mount. right You then have the law of original continuity.
00:21:16
Speaker
And this is that a layer will sort of continue to spread out and won't necessarily, it will be ended by something, but it won't naturally end itself. It'll always sort of continue to to spread. So the point of this one is that if you think that, great, this stratigraphic layer has sort of ended now, it's kind of created a nice little bowl. It probably hasn't. Keep looking, keep extending that wall because you'll probably see it come back again later.
00:21:41
Speaker
you then have the law of stratigraphic succession, which is this idea that a unit of stratification falls between the layers directly above or below it, right? Pretty simple. This is sort of quite related to the law of superposition, I would argue, but like this idea that, you know, that that's the difference in the units, you have these layers.
00:22:01
Speaker
And then there's the law of original consolidation. And this basically makes a distinction between architectural stratigraphy, which is intentionally built by humans. So for example, someone laying down a floor, and they'll like really consolidate that floor. And that'll be a manmade floor, even if it's made out of soil, it's manmade, and it's sort of consolidated compared to other types of more natural photography, which are going on.
00:22:24
Speaker
So those are the five laws. And based on that, what you do is you look at all your different layers and you then understand them because you go, great, well, I see that this layer is spreading out here and that's going here and then this one's overlapping it there. Oh, and then this one's cut into there because someone's dug it down.
00:22:40
Speaker
And basically what you're trying to do is work out, okay, which, which of these layers came before these other layers? Because like I say, it's not always simply one on top of the other. The law of superposition can be complicated in this respect. So what you have when you have your Harris matrix is basically like a grid format with the different boxes are different stratigraphies. And then you have lines linking the different stratigraphic layers and showing the relationship between them. Because it might be that you have a layer, say, layer one in one test bit is below layer four in another test bit because it's been built down. But because you see that it's above in another one, it's actually done later, but it's dug down. So this is also why it's important to take kind of do test pits or do trenches of a lot of different areas of the site. Because if you just do it in one place, you'll have a bit of a warped view of what it looks like.
Conclusion and Future Topics
00:23:31
Speaker
So that's the kind of most classic example I would say of a relative dating method is st stratigraphy. And this idea that if you look at things, you'll be able to see how old they are based on where they fall in the stratigraphic layers. And again, you're not then necessarily absolute dating it unless, for example, you find some beautiful well-preserved bones that you can send off for radiocarbon dating in one layer, you get that absolute date back for that layer, you've dated that layer, and then you can know what it is. But if you're looking at the stratigraphy, what you're doing is saying, well, this stuff is older than this stuff, and this stuff is older than that stuff. And this stuff is older, but it was dug into that stuff.
00:24:07
Speaker
So, yeah, it's something that field archaeologists are very, very familiar with, like I say, but it's just really interesting and fascinating to me to think how far back this dating method goes and kind of how established it is in the field of archaeology and in the field, like in the archaeological fields that we're digging.
00:24:23
Speaker
But also a very quick note on vocabulary. So I know that I've been a bit all over the shop today with my different stratigraphy, stratigraphy, stratigraphic layers. So I wanted to make sure I had noted at least somewhere on the podcast, what the different terms associated with stratigraphy actually are. So this is what I could gather from my research. So you have stratification or stratigraphic layer. Those are the different layers in the soil. You then have stratigraphy. That's the recording of those different layers in the soil. And then you have stratigraphic analysis. That's the interpretation of those layers.
00:24:58
Speaker
So of course, there's always subtle differences depending on your location. So if you're starting fieldwork and you're getting involved with this, do make sure to check the local or like the company glossary, but yeah, something to keep in mind. So yeah, that's about it really. I thought it would be a simple one to do this month, but actually it ended up being surprisingly complicated to try and explain in ah in a clear way. So hopefully it's clear enough.
00:25:21
Speaker
I think this will be yeah the last one of our dating methods. And there's going to be one more tea break time travel training episode next month, where I'll be making a little bit of an announcement. So look out for that one. Keep an eye out as well for this month's guest episode, and which comes out in two weeks where I chat to Dr. Alice Blackwell, all about the Lewis Chessman, which I'm very, very excited to talk to her about that. So keep an eye out for that on the fourth Tuesday of October.
00:25:48
Speaker
I hope that you enjoyed our little training session today. If you want to help support this show, as always, please do like, subscribe, follow wherever you get your podcasts. Also, if you want to support all of the other amazing series that form the Archaeology Podcast Network or Why Not Become a Member, you'll be helping us to create even more amazing content and will also have exclusive access to ad-free episodes and bonus content. Speaking of fantastic bonus content, on the 24th of November,
00:26:15
Speaker
So in hopefully two weeks or one and a half weeks from the time this episode is released, we will be celebrating 10 years of the Archaeology Podcast Network. We'll be running a whole day of live events from all the different shows. I'll also be doing something. I'll be having a Tea Break time travel 20 questions with some wonderful guest speakers.
00:26:34
Speaker
All of the information can be found on our Discord server or on our social media. So 24th of November, mark it in your diaries and keep an eye on our social media in order to see when exactly all of the different shows are going. It's going to be a really great day. And if you can't make it live and are an APN member, you'll be able to watch and listen to all of those recordings after the event as well through our members area of our website.
00:27:01
Speaker
So yes, for more information on that, do follow the socials, and for more information on becoming a member and other ways to support the APN, check out the homepage at archaeologypodcastnetwork dot.com. I hope that you enjoyed our journey today. If you did, make sure to like, follow, subscribe wherever you get your podcasts, and I'll see you next month for another episode of Tea Break Time Travel.
00:27:22
Speaker
This episode was produced by Chris Webster from his ah RV traveling the United States, Tristan Boyle in Scotland, DigTech LLC, Cultural Media, and the Archaeology Podcast Network, and was edited by Rachel Rodin. This has been a presentation of the Archaeology Podcast Network. Visit us on the web for show notes and other podcasts at www.archapodnet.com. Contact us at chrisatarchaeologypodcastnetwork.com.