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).
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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 worms!