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The Law of Superposition

Superposition is a relative dating method that looks at the layers of soil beneath the Earth’s surface. Layers of soil that are younger are found on top of layers of soil that are older.  The Law of Superposition isn’t only used by archaeologists though, it is also very important to other scientists like paleontologists and geologists.

At first superposition might seem pretty simple, older things on the bottom, newer things on the top—but what happens if something moves the soil around, like a farmer’s plow or an animal burrowing into the earth?  That is when things can get a little tricky.

A good way to think about superposition is to imagine a messy desk, full of four weeks of mail!  Say one day you need to find a letter from three weeks ago in those layers of envelopes, how will you know about where to look in the pile when there are two weeks of newer mail? To get to the letter from three weeks ago you will have to dig and sift through the other two weeks before you can find the one you are looking for.  When you’ve found it there will still be about a week of even older mail left in the desk.

For archaeologists this means that the artifacts that are found in the upper layers of soil are younger than those found below them.  By looking at the layer of soil that an artifact is found in you can learn how old it is relative to another artifact.  Newer artifacts on top of older artifacts. Archaeologists can’t tell exactly how old artifacts are using the layers of soil or how many years older or younger the artifacts are than each other. A different, absolute dating method would need to be used to do that.

You may be familiar with the word stratigraphy. Stratigraphy is the study of how layers of earth are deposited and overlap, leaving patterns in the soil.  Take a look at the photo below. Superposition just means that the upper layers of stratigraphy are newer than the lower layers. New layers are formed later, or more recently, than the older ones so they sit on top.

stratigraphy
Stratigraphy, or layers of soil, at an archaeological site in the Islands of Orkney, Scotland. The upper layers are younger than the lower layers. Any artifacts found in the dark band in the middle of the picture would date from roughly the same time. Photo source: https://www.flickr.com/photos/25889317@N06/2431080454/in/photostream/, Creative Commons license.

Superposition can be really useful when trying to figure out the history of a village site, like Lawson.  When archaeologists find three longhouses, and one crosses over two of them, you know they couldn’t have been used at the same time.  Some of them are newer, and some of them are older.  Take a look at the image of the below. Which longhouse, or longhouses are older, and which are newer?

Which longhouse do you think is the youngest? Which one is oldest? Why?

Which longhouse do you think is the youngest? Which one is oldest? Why?

In this example the red longhouse is newer than the blue and black longhouses.  Archaeologists learn this by noticing that the posts and pits from the red longhouse cut into, and overlap those of the blue and black longhouses.  The blue and black longhouses don’t superpose, or overlap, each other so archaeologists can’t determine which one is older and which one is newer.  Chances are they may have been neighbours, used at the same time.

What archaeologists can’t learn about these longhouses, and what we can’t tell from the image, is how long after the blue and black longhouses  exsisted was the red longhouse built? Was it two weeks later, or 200 years later?

 

Want to learn more about dating methods? Be sure to check out the series introduction or follow the links below!

Relative Dating Methods in Ontario Absolute Dating Methods in Ontario
Superposition Radiocarbon
Seriation Dendrochronology
Index Types

 

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