Posted by: signalhillarchaeology | July 31, 2010

It’s just one thing on top of another…

This is another post by a field school student, who writes about her experiences learning to understand and record the layers at an archaeological site.

Hi! I’m Erin Mundy, a 2nd year geology student at Memorial University. Taking part in the Archaeology field school at Signal Hill this summer has allowed me to notice striking correlations between archaeology and geology. Most people assume archaeologists just deal with human artifacts and structures. However, archaeologists also have to have an understanding about stratigraphic layers, and some of the concepts archaeologists use have been borrowed from geologists. For those of you who don’t have a background in Earth Science, here’s a little review. As layers of soil or rock accumulate, the oldest layers will accumulate at the bottom, with younger layers gradually building on top of them. This is called the Law of Superposition.

Also, layers have a tendency to accumulate in a horizontal fashion. Any rock layers that are not positioned in a horizontal unit have been affected by an event, causing the layers to fold or tilt. This is called the Law of Original Horizontality.

The understanding of these two laws is essential for archaeologists when dealing with a mapping technique called profiling. Here, archaeologists document the layers of each wall of their excavation trench by drawing them on a map. Wherever possible, archaeologists draw profiles of the North wall, East wall, South wall and West wall separately. To draw a profile map, archaeologists set up two stakes in line with their wall, placing them at opposite ends. Then, a string attaches the two stakes, using a line level to make sure this string is horizontally level. Then, a tape measure is taped onto the string.

(This picture shows the two stakes, string and tape measure of the East wall of our Signal Hill excavation.)

Now here comes the hard part. Archaeologists must look at the wall of their excavation trench and determine where the different layers of sediment are. Sometimes this can be easily seen, as two completely different stratigraphic layers (such a gravel layer and sandy layer) are easily distinguished from each other. Sometimes making this distinction is difficult. Stratigraphic layers can be folded, tilted and merge together, making it very hard for archaeologists to determine the lines between different deposits.

Here, three distinct rock layers can be seen. (Instructor notes: one of the reasons that this site is great for teaching students about archaeological excavation is that the stratigraphy is [for the most part] relatively clear. Trust me, it’s not always this clear…). Archaeologists then take horizontal and vertical measurements of the location of the different layers, which are then used to create a visual picture of the different layers.  Profiling is important for providing context for the artifacts found at the excavation site.

Measuring the location of stratigraphic layers...

By understanding the succession of layers, archaeologists can help build a timeline for the artifacts they find. Needless to say, rocks can’t be taken for granite! (haha – thought I’d throw a geology joke out there!) (Instructor notes: Erin, that is a *terrible* joke, heh). The links between geology and archaeology are based in our quest to uncover our past– our human past or the geological history of our planet. .. but even if you’re not an archaeologist or a geologist,  feel free to come out to the dig and explore our site!

–Erin Mundy, student.


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