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Stratigraphy Part 3 - Layers and strata

MIKE SMITH: We have looked at sediments and now I want to look at layers and strata. Sediments are packaged as a layer, and a layer is a depositional unit that may contain a mixture of different sorts of sediments - and probably almost always does contain a mixture of different sorts of sentiments. A layer is a unit, but it is not necessarily an event. A layer can be something that builds up over several thousand years, because the inputs have stayed stable. The processes that have created that layer have continued to operate over a long period of time. Or a layer can simply be an event where a dyke has broken and a plug of sediment has been laid down very rapidly.

How do we go about looking at a layer? First of all, let’s think about it in terms of its components, in terms of the different sorts of sediments that it contains. Let’s think of a layer in terms of its structure, its horizontal and vertical structure. Let’s think of a layer in terms of its rate of accumulation. Let’s take each of those in turn.

First of all, the components. I made the point that it is not usually just a single type of sediment, but a mixture of different sorts of sentiments. You need to identify the different elements in terms of the overall sediment flux. You might have material that is input from the bedrock. You might have externally derived sediments which might be fluvial or aeolian. You will have anthropogenic material from people living on the site and bringing stuff in and lighting fires. Characteristically, a lot of Australian rock shelters, particularly arid zone rock shelters, reflect a shifting balance between external sediments, usually aeolian sand and weathering of the bedrock. Some layers which are dominated by weathering of bedrock are going to have a lot of relatively clean sand, rocks and rubble. Layers that are dominated by aeolian inputs are going to have red sand and maybe any clay that comes in bound to the sand and so forth.

When I analyse the particle size of sediments at my central Australian sites, I can often see some distinctive size modes in terms of peaks. We might get one for gravel, which represents material from the rock shelter wall. If you look at that gravel, you can see it is patinated. You can see that it has been on the wall and it has pinged off.

We might get a peak at the fine sand, which is typically 100 to 150 micron sand, which is absolutely characteristic of windblown sand. That is what all of the dunes are made of in the Simpson Desert. You might also find a smaller peak in the clay which, in the case of Puritjarra, is the stuff that came in around the sand.

Somewhere along the line you have to try to disentangle what the different inputs are. A very extreme case in terms of a riverbank and swamp deposit is one of the megafauna sites. I think it might be Reddestone Creek or it might be one of the other ones that Tim Flannery showed when he went back to reinvestigate the megafauna site, all the megafauna bone was in these big clods of earth that had been eroded out of a bone bed and moved - perhaps not moved very far - down the fluvial system. If you cut the section, you found you had clods of earth with bone surrounded by finer sediment. You’ve got to have a look at what’s in your sediment and how it’s packaged.

Structure. A layer has both a spatial and a vertical component. Sediments are configured in space. You might have variation across the site. Sediments against the wall might be quite different from those in the middle of the site, might be quite different from those in the dripline or outside the shelter.

It might also be all one layer, and it’s useful to introduce the concept of facies. Facies is the sort of local variability you get in an assemblage of material depending on its position. For instance, a classic case of facies is along an alluvial fan where you get coarse material here and finer material there.

In an archaeological site, you might find that external inputs, aeolian material blowing into the site, will strongly influence the deposits towards the front of the shelter, when you get towards the rear of the shelter, you’ve got more material derived from weathering of the rock shelter wall, but it might all be the one layer. So layers may vary in their characteristics across their length.

Layers may vary vertically as well. Classically, we’ve talked about how fluvial deposits grade. But small differences in the balance between the sedimentary input between external sediments and internal sediments or between whether you’re getting material blown in from the crest of a dune or the inter-dune corridors or whatever may gradually change the character of a layer vertically through its depth without there actually being any major layer boundary.

Of course, this raises the whole issue of how you correlate your stratigraphy between different trenches. You really need to have an eye to the concept of facies, otherwise you’re suddenly going to find that you’ve got a stratigraphic sequence at the rear of a rock shelter that really looks quite different from that at the front of the rock shelter or at the entrance to a rock shelter. It might all be just the one layer with variability across it.

You need to have an eye to how that layer has been draped over a site. Waterlain deposits are generally going to be horizontal, by definition. But other sorts of sediments, windblown sediments, may slope across a site. They may build up against obstacles. Sand may build up against a rock ledge.

They may horizontally bedded, as we’ve said, with fluvial deposits. They may be localised in depressions. You may have sand or organic material that accumulates in a depression in the floor, or you might have ponding where water flows in and forms a little deposit in a depression.

You might have a mantle or drape, you might have in situ decay of rocks. If you get a big patch of white sand in the middle of your section, what is it doing? Is it a burrow that’s filled in with sediment or is it just a big boulder that has decayed in situ?

You might get a mound or a bench of sediment. If you do, what’s going on? Has sediment been cut away?

Often in stratigraphic drawings people draw a line around an area of mottling or discoloration. They’re drawing it with a vertical face. What’s happening? Sediments are not laid down with vertical faces; they’re laid down in terms of lenses and beds. If you have a vertical face, it implies something has been cut away. You need to have an eye for that sort of structure.

You also need to have an eye towards the processes that lead to evolution of a rock shelter. There’s a classic process called rock shelter retreat. Rock shelters form by cavernous weathering, an area often at the base of an escarpment where a bit of moisture enhances chemical and mechanical weathering, and that cavity gradually retreats back and upwards into the rock. The overhang it creates may or may not be structurally stable. Characteristically, the lip of the overhang collapses over time, so the rock shelter retreats back.

This is Fromm’s Landing, one of the rock shelters on the lower Murray excavated by John Mulvaney. Twidale, who was the geomorphologist with the party, pointed out that there was an interesting relationship between how the sediment accumulated in a deposit and the erosion of the rock shelter wall. So that when you had a zone of occupation, you had a vertical wall, because you had rapidly accumulating deposit which protected the rock shelter wall. When you had a zone when you have very little occupation, you had the opportunity for the rock shelter wall to erode laterally, so you got a series of steps. There is an interaction here between the extent of cavernous weathering and the way occupation deposit protects the rear wall of a shelter from weathering.

If you are trying to decide where to dig to find the older deposits of your site, you need to understand what rock shelter retreat is doing. If you look at any of those pictures of excavations of the classic French Palaeolithic Dordogne sites, they are not actually digging in rock shelters, they are digging on the slope outside the rock shelters, which used to be a part of the rock shelter but is now part of the talus slope. They understand they are digging within what was once a rock shelter but the edge has collapsed periodically.

An example of what you can do here is given at Puritjarra. We explicitly dug a pit under the leading edge of the overhang. We wanted to see if it had had any rock shelter retreat over time. What we found was a column of rocks in exactly the same position right through our stratigraphic sequence. What this was showing was little rocks had been falling off the edge of the overhang for 10,000 years and landing more or less in the same position. There had been no real retreat of that overhang. That rock shelter was phenomenally stable over time.

When you are looking at the weathering of a rock shelter, also have a look at what is happening inside. You may have a domed roof or you may have ledges across the back wall that are collapsing. A domed roof might be an area where you get a lot of spalling and rock fall beneath. You might have a cone of rock fall beneath a domed roof.

Have a look where the structural controls are, where the joint lines are. There is something called ‘unloading’. When you take the pressure off a rock formation, things start to ping off. The classic case is Uluru, which is arkose sandstone, but the layers are pinging off like layers of onion skin as the pressure has been taken off. You can get things like this happening inside a rock shelter as well.

Rock shelters can change their form over time. The whole configuration of a rock shelter can change. You need to understand what you are looking at and which part of a site you are looking at.

This is a global diagram from Puritjarra rock shelter. It shows the changing proportion of coarse rocks and gravel, and the fine sediments in terms of sand, silt and clay, and also the proportion of grass in the phytolith column. What it shows on a global diagram like this is you can see a whole series of related changes between layers one and two at about 8000 years ago. The size of the rocks change. You get a lot more fine gravel. You get a lot less large rockfall. You get a change to a much more sandy deposit and a great increase in the proportion of grass. You have a whole range of changes here that relate to the changing local environment of the rock shelter. It is the ability to compare across these different materials that makes these diagrams useful.

You are also looking at the lateral relations of a layer, or series of a layer, across a site. You are looking at the bedding within a layer. Rockfall, I think, represents an opportunity. One of the few things you can see in a stratigraphic section, if you can’t see anything else, is you can see the rockfall and the rocks. Every rock that has fallen has fallen on a surface. It may be just one of these very temporary ephemeral surfaces, but it is a moment in time.

Every surface then gives you some clue as to the bedding and slope of that deposit. So have a look at how your rocks lie. They are the grain of the deposit. They are an opportunity to get some sense of its structure and an indication of a surface. Of course, when you lift a rock, you are lifting it off a surface that has a fair amount of integrity. What is on that surface? Is there a charcoal underneath the rock that you can use.

And then move toward the front of the rock shelter. Rock shelters may be at ground level or they may be raised. If they are raised above ground level, above the surrounding countryside, they will have a talus slope. Have a look at that. Is that just a collapsed lip of the rock shelter or is there more to it? How do the sediments within a rock shelter relate to those outside the rock shelter?

In many cases, we find that even the occupation extends outside the rock shelter. In fact, many rock shelter sites are just one end of a wider occupation site. It is essentially an open site. Have a look at how those deposits enter the rock shelter, how they feather in or blend into the rock shelter. You need to pay some sort of attention to the relationship between interior and exterior sediments and deposits.

We have looked at the components; we have looked at the structure; now let’s look at the rate of deposition. One of the key things that is important is: how fast has that layer built up? It makes a difference to how you interpret your archaeological remains, which we will talk about in a minute.

One of the first questions to ask is: what is initiating sedimentation? What might be triggering the buildup of sediment? A classic thing is where you get rockfall across the mouth of a rock shelter. A big boulder falls, and all of a sudden you have got something that will retain sediment, so sediment can start to build up. You might get some other trigger, which means that sediment builds up very rapidly. Or it might be something happening in the wider landscape whereby you have got a period of aridity, vegetation is dying away and all of a sudden you have got exposed land surfaces that are eroding, and that soil material is flowing or blowing into your site in some way. Have a look. What are the possibilities for factors that are triggering sedimentation?

Even within a shelter, if you are starting to get a lot of material that derives from the rock shelter walls, what is likely to be causing it - changes in moisture, temperature, salts? Is it because animals and people are changing the microenvironment within the shelter? Is it because external climate is changing and that is changing the moisture regime? Is it a gradual process of weathering or is it some structural collapse?

You may have a rockfall layer. That rock fall layer might be a gradual accumulation of rockfall over several thousand years, or it might be a single event in one catastrophic fall. Have a look. Is there any internal structure within that rockfall that indicates that it is a series of smaller rockfalls?

I want to move, as we look at the rate of deposition, to a really key concept: net sediment budget. We know that fine sediments are removed from sites. Most archaeologists really don’t quite grasp the idea that sediments are routinely removed from the sites. We would not have rock shelters if sediments were not removed.

You go to a rock shelter that might have a roof or ceiling height of two metres, it might be 20 metres across and 10 metres deep. There is quite a few tonnes of sediment that have been removed to create that void. If it had not been removed, we would not have a rock shelter; we would just have a pile of sand. There are processes that evacuate the sediments, and there are processes that introduce sediments. So we have always got a balance between removal and accumulation.

The deposits build up if the inputs exceed removals, and vice versa. Ingrid Ward developed a really nice model for sediment accumulation in northern Australian rock shelters. You can imagine it as a bit of a clock. When the hand is horizontal, sediment removal equals sediment accumulation. You have zero net sediment budget so you’ve actually got a surface, you’ve got a palaeosurface. That means that all your activities, all your occupation, is happening on the surface, and you are going to get a palimpsest of material over time, however long that surface survives - thousands of years, hundreds of years or whether it is a more ephemeral surface.

When the hand is pointing vertical, you have a very high positive sediment budget so you are getting rapid accumulation of sediment. You will find that discrete occupation events might be quite nicely separated stratigraphically. So you can get a band of occupation, then a band of sterile sediment and a band of occupation. It is really an ideal situation, because you can get high-level resolution in your archaeological record. If the hand is starting to point closer to horizontal, sediment accumulation is not so rapid and you do not get as good a separation of your occupations; you tend to get overlays.

If the hand is pointing straight down on this clock where we’ve got extremely negative sediment budget, your sediment is being removed from the site. In fact, sediment that might have accumulated over a period of time is just gradually wiped away. What it will leave behind, depending on what processes are removing the sediment, might be a lag of artefacts. For instance, we see this at Cuddie Springs where there is a stone layer, which is almost entirely made up of stone artefacts, that represents all of the coarse material that has been left behind as wind erosion has removed everything else. At some stage at Cuddie Springs sediment has built right up and then were cut right back, just leaving the stone band.

Similarly, in dune sequences, you often find where you’ve got a blowout and you have got lots of stone artefacts. Those stone artefacts have been let down as the fines have been removed. They may then be reburied and recycled and let down again. You may have coarse material of stone artifacts or it might be tooth enamel, megafauna bone, that is moving through an aeolian sequence.

Somewhere between this extreme case and a zero sediment budget you might have a site where sediment removal is always exceeding sediment accumulation, but not in an extreme way. That might be gradually creating conflated deposits where you are gradually getting a condensation of your archaeological remains.

So you need to have an eye to a sediment budget. At least you need to understand the concept that sediments can be removed and not just accumulate.

ROBERT PATON: Like you, I have excavated in Britain, the thing that struck me about those excavations was the planning of the trenches, the shape of the trenches and the placement of additional trenches was almost entirely based on the variation in stratigraphy across the site and really thinking carefully about planning the dig based on what we knew about the stratigraphy and what we were discovering about the stratigraphy. If you could perhaps comment on the Australian case where it has been my experience, and it may have been yours, that digs do not really take into account the stratigraphy. People just go in and dig. There is already a plan to dig a one by one or a two by two. I think the planning of a dig for the most part is not really based on what the stratigraphy of the site is.

MIKE SMITH:I think excavation has always got to be a dialogue with the site. You may have limited capacity in terms of the area that you can excavate so you may ultimately have to say, ‘I am going to put a one-metre square in the middle of the site.’ You need to be asking what is happening in other parts of the site. If you do have the capacity to do a large dig, you need to be probing those areas: I really want to know what is happening at the front of the site, what is happening in the middle of the site, what is happening at the rear of the site. They will differ in terms of the sediment characteristics but also differ in terms of how the site is used.

Characteristically, you might find the discard areas against the rear wall or against rockfall embedded in the floor of the deposit. If you want to find grind stones or your larger stone artefacts, you have to look for these secondary discard areas. The sleeping areas of the site with all small sleeping parts will be at the rear. The flaking areas might be towards the dripline. Where you place your trench makes a lot of difference as to what you’re going to find.

If you do have the luxury of digging more than one square metre, it’s an iterative process of asking questions about the site and dissecting it in a certain way. I don’t think enough people are alert to the possibility that, if you dig quite a small excavation pit, you could actually come down inside an entirely intrusive feature. You may end up with something that bears no relationship to the rest of the site at all. And that has been done actually.