Earthworms and Archaeology:
The Unlikely Story of a Tiny Slimy Hero
By Gregory Vogel
This was originally published in Worm Digest in 2004, volume 35, pages 8-11. The article is non-technical, meant to be understandable to people who are not archaeologists or oligochaetologists (people who study earthworms - it's a new word to me, too). I hope it may teach you something about earthworms, and something about archaeology too.
Here I've adapted the article for the Internet and added a few new figures.
Imagine a worm with a brown fedora, a bullwhip, and stylish scar on its chin. It doesn't exactly present the popular view of an archaeological hero, does it? The truth is, though, that earthworms are greater heroes to archaeology than Indiana Jones - worms have helped preserve the archaeological record in many areas of the world by burying artifacts and keeping them safe from plowing and other modern disturbances. If it weren't for the burrowing action of earthworms, in fact, many archaeological sites would be completely destroyed. The burrowing action of earthworms can also help us understand how old some sites are compared to other sites, and even whether ancient artifacts have been moved from their original context by erosion.
Archaeologists have long understood that the natural environment affects archaeological sites, and that if we want to understand what people did in the past, we cannot take the artifacts they left behind at face value. We first need to understand how artifacts have been altered since they were last used - how they have been moved, buried, or changed in any way by geological or environmental processes. The study of how this happens is termedgeoarchaeology, and geoarchaeologists are coming to understand that earthworms do much more to archaeological sites than it may seem at first glance. What exactly do worms do for the archaeological record? First we have to understand something about a process termed bioturbation.
Bioturbation is the natural mixing of the soil through biological processes. Soil doesn't move very much from day to day, but over long periods of time it is mixed and turned and churned almost like a boiling pot of stew. To see how this works, imagine we have a window below the ground somewhere, so that we can see what happens inside the soil. Looking through this window we would probably see an upper layer of dark, organically stained soil and one or more lighter colored layers below that. If we looked closely we might notice that there were no big stones in the upper, dark layer of soil, but that there was a layer with lots of stones just below it.
Now imagine that we have a film of what happened through this window for several thousand years. Watching the film we might first notice a few ants and other insects scurrying industriously about through their tunnels, or a mole digging along just beneath the surface looking for tasty roots. We're interested in worms, though, so for now we will just pay attention to them. The worms would be munching their way through the soil, ingesting particles in front of them and leaving castings behind. From time to time they would eject castings onto the surface and grab a tasty tidbit of a leaf to eat, or make open tunnels down towards the bottom of the dark layer of soil. If we watched long enough we might see some of the worms burrow deeply, just below the upper, dark layer of soil, and encounter some of the rocks there. The rocks would be too big for the worms to ingest, so they could only burrow around or under them. Of course worms don't move very fast, so we might quickly get bored with watching the film at this speed.
We could speed up the film a bit so that we see a few weeks' action in one minute. At this speed, the worms would be moving so fast that we couldn't even see them. We could still see their burrows, though, as they were created and then collapsed or filled with soil from above, and we could see the castings building up at the soil surface. We would notice that even though castings were being ejected onto the surface regularly, the surface of the ground wouldn't actually get any higher, because the holes from which the castings came would collapse at the same rate the castings were brought to the surface. In this way, the worms move the soil about, but don't actually add much material to it or take material away. We would also notice that the worms only rarely ventured below the layer of rocks.
Let's speed up the film even more and see what happens when we watch a few years pass by in one minute. The summer soil would look like a storm of worms at this speed, with the burrows flashing across the screen like lightning and clouds of castings bubbling along the ground surface. The storm would slow down as the soil temperature cooled, and winter would be a very quiet time below ground. The worms would be nestled in burrows somewhere deep below, where the soil doesn't completely freeze. They might even burrow below the rocks in the lighter colored soil, and only come back up when the ground above thawed again. If they did this, we might see some of the rocks shift downward a little bit as the burrows under them later collapsed.
Now imagine that we see something happening above the ground surface so we slow the film down again - a small group of people has taken up residence in the area around our window. Maybe they are a group of Native Americans setting up camp for the summer. Exactly what they do there is the subject of archaeology, of course, but for this story we'll speed up the film again until they're gone. Now we can see some things they left behind - a few pieces of pottery from a jar that broke, a whole bowl, and the stone point of a spear (maybe the wooden shaft of the spear got left behind too, but this and all other organic material would likely degrade very quickly). The things that they left behind are artifacts, because they were made or modified by people.
If we look closely through our window just below the artifacts, we would see worms still going about their business and ejecting castings onto the surface. Because the worms wouldn't be able to move the artifacts or get to the surface immediately beneath them, the castings would build up around the edges of the artifacts and begin to bury them. As the burrows below the artifacts collapsed, the artifacts would begin so sink just a little. Eventually, the artifacts would have sunk far enough and the castings would have built up around them enough that the artifacts would be completely buried. If you were to walk around the area now, you wouldn't be able to see the artifacts at all.
These figures show the formation of a biomantle (and the burying of artifacts) through the burrowing of eathworms:
Now let's speed the film up more and watch the artifacts as they sink - we can see that the worms are really acting like a conveyor belt, moving the small soil particles (anything small enough for them to ingest) up through the soil, and the larger objects (artifacts and rocks too big for them to ingest) down through the soil. The artifacts don't move horizontally very much, because they are too big for worms to move all at once. They just fall deeper within the soil until they get to the bottom of the layer where the worms normally burrow, ending up in the same zone where the rest of the rocks are accumulated (this is called the "stone zone"). The upper, organic rich layer is again free from large objects. Together, these layers are called the "biomantle", and the processes responsible for creating them are termed bioturbation.
Many other organisms contribute to bioturbation, and their results are sometimes quite different from those of worms. Larger burrowing animals like moles and gophers form biomantles much the same way worms do, except that the size of particles they can move are much larger. The upper portion of biomantles created by these animals therefore includes larger particles. Burrowing mammals are capable of moving small and medium-sized artifacts like stone spear points, so these are constantly moved about in the "active" upper layer of the biomantle.
These figures show the formation of a biomantle (and the burying of artifacts) through the burrowing of small mammals. Compare results of this burrowing to the borrowing of worms illustrated above:
Other types of bioturbation don't contribute to the formation of a biomantle at all, but actually work against burrowing organisms to mix up the stone zone and the upper layer of finer particles. In forested areas, trees will sometimes tip over in high winds, or simply when they die. When they tip over, they often bring a mass of soil out of the ground with their root ball. If the roots that are brought to the surface were as deep as the stone zone, the biomantle will be completely mixed when it all washes back into the hole of the root ball.
There are other, non-biological processes that move particles about in the soil too. In cold, wet areas, water can accumulate underneath stones and artifacts in the soil. When it freezes, the water expands and pushes them upward slightly. This is called frost heave or cryoturbation (instead of bioturbation) and in some areas is responsible for bringing larger objects to the surface. Which soil mixing processes are dominant in any particular area depends on many factors, including climate, topography, soil type, environmental history, and the plants and animals present.
How fast and how deeply artifacts get buried depends on the number and species of worms (or other bioturbation agent) in the soil, and whether or not other factors such as frost heave are at work. In some areas of the American Midwest, biomantle formation has buried artifacts that are 5,000 years old more than 30 centimeters deep (about twelve inches), and this is deep enough to keep them below most plowing. In England, Charles Darwin found that Roman ruins 2,000 years old were completely buried by worm castings, and lime and charcoal put on some fields was buried by 6 centimeters (2 inches) or more within a few decades. He also started "worm gardens" with stones on the surface to see how quickly they would sink. Charles Darwin died before this experiment was complete, but his son Horace Darwin continued the study and found that the stones sank at a very quick rate of about 22 centimeters (9 inches) over a period of only 10 years. On the other hand, very old sites in some areas of the Ozarks are still not completely buried by biomantle formation, even after 10,000 years or more.
Two figures from Darwin's book The Formation of Vegetable Mould, showing how stones are buried throuh the action of earthworms. Click on each image for a larger version. (click here to read more about the book - did you know that Darwin yelled at worms? It's true!).
Where sites are buried by biomantle formation, they are not visible from the surface but are often safely protected below. This means that archaeologists can excavate the artifacts (carefully, now!), and see where each is in relation to the others. Are there patterns of artifacts that make sense? Maybe all of the artifacts associated with cooking and eating are in one area, and all artifacts associated with making pottery are somewhere else. This is how we begin to reconstruct what people did in the past.
We can also see which artifacts are deeper than others, and maybe understand which are older and which are younger. Recall our window into the ground, and the artifacts that are completely incorporated into the stone zone. Now imagine that another group of people come to live at the same location, and their artifacts are also incorporated into the soil. If we excavate the site before the newer artifacts are completely mixed into the stone zone (which may take a few thousand years), and we take very good notes about how deep each artifact is found, we will be able to separate the different occupations and see how the people lived differently. Without worms and bioturbation processes (such is the case in much of the American southwest deserts), artifacts from all time periods are piled up at or very near the ground surface, and telling one time period apart from another is a more difficult task.
All of these examples are from areas that are geologically stable; there is no fresh soil being added to the ground surface to build it up, and no erosion to take away the soil and artifacts that are there. Both of these processes occur, of course, at different times and places. Sometimes archaeological sites are buried by freshly deposited soil. Sometimes they are eroded away with the natural soil and deposited in an entirely different location. If this happens, the artifacts will be completely out of context, and if we don't know this we may be very confused about what the people who made the artifacts were doing.
If we look carefully at the artifacts within the soil, though, and compare them to the natural rocks that also occur in the soil, we can sometimes understand whether or not the artifacts have been moved very far. If the large artifacts are arranged vertically in the soil just like the large rocks are, and the small artifacts arranged vertically just like the small rocks are, we may be suspicious and question whether they were deposited at the same time (that is, whether both the rocks and artifacts were eroded, transported, and deposited by natural forces). In this case their vertical arrangement would be the same because bioturbation processes have had the same amount of time to work on both of them. If, on the other hand, there is a strongly developed stone zone in a soil but the artifacts are not completely incorporated into it, we can be more confident that the artifacts are close to their original context. In this case bioturbation processes have clearly worked on the natural stones for a much longer time than they have worked on the artifacts.
To really understand how some sites came to be buried or not, and to understand what has happened to the artifacts after the people left them, archaeologists must be able to understand an area's geological and environmental history. In order to do this, we need to understand quite a bit about geology, soils, and, of course, worms. Most detailed studies of how worms affect archaeological sites are fairly recent, but their importance was understood more than 100 years ago. Charles Darwin was probably the first to understand the connection. In 1882 he wrote, "Worms have played a more important part in the history of the world than most persons would at first suppose..." and, "Archaeologists ought to be grateful to worms, as they protect and preserve for an indefinitely long period every object, not liable to decay, which is dropped on the surface of the land, by burying it with their castings."
Archaeologists have therefore become very interested in the burrowing habits of different worms, studying how much soil they move, how large of particles they can ingest, where and when they eat and burrow, and what worm populations were like in the past. I think archaeologists are finally as grateful for worms as Charles Darwin hoped we would be. Thank you earthworms!