Tuesday, July 12, 2016

Shell-ter From The Storm

Oyster Midden Before It Was Excavated
Within the first few days of excavation at Penhallow, our crew was finding quite a few oyster shells...And when I say quite a few, I mean a lot! At the Western end of our trench, we uncovered an oyster shell midden that spanned the area of four 1x1 meter units, and when we started digging on the street-side of the house, we discovered that the midden also extended outward into those units. Although the midden takes up space in at least six units, we only excavated two and half of these units, and that still yielded over 60 bags of shells!

Midden After the Two Easternmost Units Were Excavated
The presence of all these shells at Penhallow leads us to many research questions, such as: do these oysters pre- or post-date the house? How old is the deposit? Where did the oysters grow? Are they local or imported? How old were the oysters when harvested? Was the aquatic environment where they grew healthy or polluted?

These are only a few of the questions that we can ask about the oyster midden, but in order to better answer all of these questions, we require a bit of background knowledge on oysters overall.

Although oysters come in many varieties, there are only 5 true species of oyster in the world. These species are the Pacific Oyster, Kumamoto Oyster, Atlantic Oyster, European Flat Oyster, and Olympia Oyster, and they are differentiated based upon where the oysters grow and their shell size and style. In general, these oysters all taste similar, but they do take on slightly different tastes based upon where they are grown. A lot of people liken it to wine, saying that each is distinctly different because of the regions and conditions in which they were grown and produced. The only oyster native to the US East coast is the Atlantic Oyster, which is defined by its teardrop shaped shell and its cream, brown, and dark green shell coloring.

A Water Tank Demonstrating How Oysters Filter Water
Oysters, as filter feeders are important to the health of an ecosystem. So important, in fact, that they are referred to as "Keystone Organisms," which means that without oysters, other organisms are unable to inhabit the area. This is because of several important roles oysters play in keeping the ecosystem healthy. Oysters filter organic and inorganic materials from the water, so they essentially clean the water by removing particles and pollutants. They are also helpful because they can remove solids from the water, purify them, and then package them as little bundles that are consumed by larger organisms. Oyster shells are also beneficial because they provide safe habitats for creatures on the bottom of the food chain. Oyster beds make a hard surface that creatures like barnacles, snails, crabs, and even baby oysters can live on. With oyster shells to attach to, these small organisms attract larger and larger organisms and help to create a complete and complex underwater ecosystem.

Growth Layers on an Oyster Shell
Because oysters are so important to the underwater environment, they can provide us with a lot of historic information. When examining a single oyster shell, there are several key things to look for that tell us about that oyster's individual life, and when we compile this data from all the oyster shells, we can learn about trends over time. One of the first things we can notice about the shell is its size. The larger the oyster, the longer it was able to live and grow, indicating a longer life in a healthy environment; smaller oysters mean just the opposite. For archaeologists, this can indicate levels of environmental health, but it can also indicate economic health. If oysters were able to grow to maturity before they were harvested, it is likely that people had other, more plentiful sources of food. If oysters are small it is likely because people harvested them before they were mature, indicating a higher demand for oysters, which could either mean a poorer economy or an increased human population. You can tell exactly how old an oyster was when it was harvested by looking at the layers on its shell. The layers on a shell grow out from the hinge area, like tree rings, so by counting these layers, we can tell the oyster's age. Similar to tree rings, these layers also indicate seasonality. Oysters grow more during warmer months, so the layers will be thicker in the summer and thinner in the winter. The layers closest to the hinge are the most recent, so you can look at these to determine in which season the oyster was harvested. Measuring the ratio between length and height also indicates whether the oysters grew in a bed, on sand, on a reef, or in a channel. If they grew in salty water, oyster shells will have holes in them that are caused by parasitic sponges feeding on the shells. Fresh water oysters, on the other hand, have no holes in their shells, and oysters that grew in brackish water have fewer, smaller holes. By noting how many oysters came from salt water versus fresh water, archaeologists can get a glimpse of changing harvesting patterns. People may change their harvesting patterns for a variety of reasons, such as if an oyster bed were completely depleted, if water was becoming too polluted, if access to the oyster beds became more difficult, or if they started intentionally growing and cultivating oysters. When people first harvest oyster beds, they are often pulling out oysters that have different sizes and styles of shell, because they are usually harvesting them for personal consumption. When oysters are being grown and harvested as a tradeable market good, however, their shells are often uniform across the oyster bed. This is because consumerism demands consistency in goods and products and it also demonstrates an knowledge of what makes a good eating oyster. Examining and studying all the excavated oyster shells and compiling data on their size, age, which season they were harvested, where they grew, and the similarity in shell shape and size can indicate quite a lot to an archaeologist in terms of environmental health, economic health, local markets and trade, what people ate, and changes to all of these over time.

So what, then, did people in the past do with all of these oysters? Besides eat them, of course. It turns out that there are quite a few uses for oyster shells, including, but not limited to:

  1. Ground up as feed for chickens to strengthen their egg shells
  2. Crushed up as pathways, much like gravel is used nowadays
  3. Restoring minerals to gardening soil by burying them in the ground
  4. Beds for future oysters to grow on
  5. Mixed into a building material called Tabby (similar to cement)
  6. Boiled into a broth, like how chicken bones are boiled in chicken stock
  7. Scattered across work yards to make muddy surfaces easier to walk upon
  8. Burned to create lime for plaster
  9. A remedy for the bite of a mad dog when burnt and crushed (According to the 1783 Virginia Gazette)
  10. Art

What do you think our oyster shells may have been used for?

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