Where does the dirt between archaeological sequences come from on a site rich in layers?

Question

Guided by stratigraphic principles (e.g., superposition) largely borrowed from geology, archaeologists have obtained evidence of thousands of years of prehistory from layers of ceramics found on sites all over the world. How were these layers formed? Can't be in the same way as geological layers, where timeframes are much more stretched out!

Geology assumes that over the course of millions of years, the majority of the planet surface has interacted with water (e.g., in some drainage basin, or a rainfall-rich area) or wind. Water and wind make dirt from rocks and move it. It is easy to conceive erosion as the key mechanism creating geologic layers. But in archaeology? Rich ceramic layers only took a few millennia to form even in the most water- and wind-poor locations. So how did the dirt get between the pots? Did God deliberately blow Monte Alban over with dust at the end of each 'ceramic period' so the next one can start parking its garbage over it?

Answer 1

That is a good question.

Where does dust come from? If I don't wipe a counter for a week it has a layer of dust on it.

How about the works of worms, ants and beetles. They keep digging in the earth and burying the stuff on the surface.

It really is a good question.

I know that I do not have to wait a hundred years for a bottle that somebody tossed as litter to be basically buried.

In ten years it will be at least halfway under the earth and full of dirt.

Yet at the same time the farmer has rocks coming up to the surface in his fields.

Answer 2

Sediment can accumulate faster than you might think. My geoarchaeology instructor has a really great photo he took of of a single layer of sediment. Over 7 meters thick from top to bottom. Buried directly beneath it, as in, the soil was deposited right on top of it: a cache of modern beer cans, which could be dated by their labels and tabs to the 1950s.

Cultural materials generally accumulate on stable landscapes, sediment generally accumulates on unstable landscapes. Every region goes through periods of increased and decreased erosion or deposition, and this is where these inter-cultural sediment layers come from. Depending on conditions, they can be meters to centimeters thick. Thousands of years is more than enough time for a soil layer to form. These sediments don't have to come fresh off of eroded rocks, they can just be blown in from a nearby area (which is common). That's the short answer, anyway.

Answer 3

It depends on local conditions.

In some places flooding is common, and in others windblown sand is common, but it can also come from trash, rubble, etc.

Answer 4

One interesting source is mud bricks. In the Middle East where such building material is common, the bricks eventually crumble and become dirt. People then build on the new surface. Pottery being fired, lasts much longer and mixes in with the dirt.

Water, of course, is responsible form much of the new layers of earth. Just the dirt washed down a slope build up.

Animals are another source of dirt. Cows really tear up soil and their droppings add to the dirt. An archaeologist pointed at a field and remarked how there were 10 gophers in the acre. That they dug ten holes a year and after 1,000 years you had a lot of dirt moved.

Finally there's vegetation. The "duff" that you have to removed before you can open a pit is composed of new growth but also dead material and roots. It adds up over time.

Remember, with humans living in an area, the ground gets disturbed, more material is brought in then removed and trash accumulates. Stuff  gets buried.