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Sediments under the Microscope

In this video geoarchaeologist Dr Mike Morley explains particular features of sediments from Liang Bua as we view them from a microscope.
So this is a profile of sediments from around 200,000 years ago up to the present day. And if you look on the profile, you can see that there are six blocks labelled MM4 at the top to MM9 at the bottom, three of which we’ll look at now. We’ve chosen these three because they’re representative of interesting features. So the kind of things that we’re looking at are features that would be either diagnostic of particular depositional environments or the way that sediments were deposited at the time– was it wet or dry, was it cold, was it warm– what’s happened to the sediments after they’d been deposited– have they been eroded, have they been heated, have they been chemically modified?
Those are the kind of things that we really want to try and get a handle on here. So we’re going to zoom in now onto that block MM7. We can see that there’s generally a very, very fine-grained sedimentation at this time. If we look closely, we can see that we can roughly divide it up into kind of three broad layers. We can see that kind of lower quadrant beneath the grey ash layer has only very small amounts of these kind of cracks and voids. And a lot of the time, this represents burrowing by either by small insects or by worms, or even by rootlets, or roots from vegetation growing on the surface.
So we think, because the lower part is quite dense, there’s not too many void spaces. And maybe in the upper 2/3 of that slide, we may have a period of time, for example, where the introduction of sediments to the site has been quite slow, and there’s been enough time for insects and other organisms to burrow into the sediments. The kind of most interesting feature, I suppose, of this thin section is this grey layer. And what this actually is a volcanic ash that’s been blown into the cave during a volcanic eruption, or immediately after a volcanic eruption. So what we see then here is we see this fine-grained sedimentation.
At some point, we have an eruption, and we get the volcanic ash being introduced to the site, banked up against the rear of the cave. At the front of the cave, it’s much thicker, up to about a metre in thickness. And then after that, we get the fine-grained sedimentation resumes. So now we’re going to hone in on sample MM6. And with our naked eyes, when we were down in the trench looking at this sequence of layers, we could see a very distinctive burnt layer. And we wondered if this might be evidence of human or hominin use of fire in the cave. And you can see that it does have a very diagnostic sequence of layers.
We can see, at the very base, we have these kind of fine-grained brown silts and clays. But if you look closely, you can actually see that there’s quite a few coarser particles. Very, very pale grey, almost white lumps. And this is, in fact, limestone gravel and degraded bits of speleothem that have broken off the walls in the roof of the cave. We can see that we have this kind of very distinctive orangey red layer. And then we have this dark distinctive middle layer. And then above that, we have kind of a pale grey layer that sits on top of that with a few other strange features, as well.
These three layers together are very, very indicative and characteristic of a fire, or a hearth. The lower layer, this kind of orangey red colour, essentially they’re no different from the sediments we see in the very base of the slide. But because of the heat of the fire, it’s kind of baked some of those clays and some of those silts. And then the dark layer in the middle, we see this kind of dark, orangey black grain in the middle, which again is a lump of kind of fire-cracked clay. But there are also lots of other black grains in there. Some of those are charcoal. Some of them very, very baked pieces of clay.
And then at the top there, I was talking about this kind of calcium carbonate layer, quite similar to the limestone at the base. But what this actually is is wood ash. We have, again, fine-grain sedimentation on top. We’re going to go right to close to the top of the sequence, up to MM4 now. We can see a lower area with kind of fine-grained sediments and some of these limestone lumps. And above that, we can see lots of this black material. And this black material is actually manganese oxide, which we see quite often in caves and rock shelters. So above this area of black manganese oxide, we see this kind of pale grey layer again.
This calcium carbonate is actually in the form of a flowstone. So following the deposition of this flowstone, bats roosting in the cave, the droppings from the bats have fallen on top of this flowstone. And as these bat droppings accumulate and as we get water coming into the site, as it percolates through the bat droppings, it actually becomes quite acid-rich. By the time the water’s percolated to the bottom of the bat droppings and comes in contact with the calcium carbonate flowstone underneath, it eats away at the calcium carbonate, and that gets replaced by phosphates coming out of the bat droppings. And during that process, the movement of manganese down from the flowstone, that oxidises, and we have manganese oxide.
So essentially the story here, at the base, we have this kind of fairly calm environment, where we get a little bit of fine-grained sediment coming from the rear of the cave. We get quite a lot of limestone breaking off the top. Then, we have water flowing over the surface. Because of the bat droppings there, it means that we actually get a very high acid-loaded environment. And that’s why we get all these strong chemical changes. And we call these– we call that change diagenesis, because the bat guano, or the bat droppings, are actually quite easily erodible. They’ve been eroded from the top of the flowstone, which has already been chemically modified. And then we have fine-grain sedimentation above that flowstone.
So there’s– essentially, we lose a period in time when the bat droppings were sitting on top of that surface, because they’d been removed.

In this video, geoarchaeologist Dr Mike Morley explains particular features of sediments from Liang Bua as we view them under a microscope.

Taking a close look at the slides helps to diagnose particular depositional environments and the ways in which sediments were deposited in the cave. We can then begin to answer the following questions:

  • Was it wet or dry?
  • Was it cold or warm?
  • What happened to the sediments after they were deposited?
  • Have they been eroded?
  • Have they been heated?
  • Have they been chemically modified?

Sediment Observations

The following summaries highlight key features of sediments from Liang Bua (as observed in the video above)


Cracks and voids indicate burrowing by insects or worms, or the penetration of roots by vegetation. The grey layer consists of volcanic ash blown into the cave during and/or after a volcanic eruption.


Distinctive burnt layer (consisting of fine-grained brown silts, coarse particles, limestone gravel, baked pieces of clay and charcoal) indicates evidence of fire use. The calcium carbonate layer consists of wood ash.


Presence of black material (manganese oxide) produced by water percolating through bat droppings (which are highly acidic) on to flowstones (calcium carbonate).

Conversation starter

Mike explains that bat droppings create a highly acid-loaded environment that causes chemical changes to the sediments (a process called diagenesis).

  • How might these changes affect the nature of information that can be gathered about the sediments, and the fossils and artefacts contained within them?
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