|My pit partner Kaylee and I breaking ground into|
the alluvium of our 1 x 2 meter unit
Tuesday, July 18, 2017
The Stratigraphy of Fort St. Joseph
Hi! My name is Bryan Schutte and I am a student participating in this season's field excavations! Yesterday was the first day of the second full week of field school and the start of our third week in total. Yesterday also marked the start of the middle school summer camps, where the students helped us excavate around Fort St. Joseph. Currently the excavations at the site are well underway and most of the excavation units will be breaking into the plow zone in the next few days. One group has already reached this layer and are very excited for what they could soon be finding. For this post, I will be talking about the stratigraphy of the site. Stratigraphy refers to the different levels of soil within an excavation unit or site. It is also used to give an estimation of the age of artifacts based on the context relative to the context of other artifacts found. At our site, we have three layers of soil, otherwise known as stratum.
The first layer present throughout the soil of the floodplain site, represented by the letter A in the diagram, is called the Alluvium. If you are walking through the floodplain, you are walking on top of the alluvium. This layer is comprised of the sediments that have been deposited by the river over time. This layer has wet and thick soil, almost mud like. This is also where most of our roots are found. This layer is from 0 centimeters below datum (cmbd) to 20 cmbd. Datum refers to one of the corner points on every unit that is used to make consistent measurements throughout the unit. We do not see artifacts from the fort’s occupation because this is the newest soil. We find mostly modern artifacts/trash at this level such as McDonald’s straws, plastic bottle caps, styrofoam, etc.
The second zone, represented by the letter B, is what we call the plow zone. During the second half of the 19th century, to the early 20th century, the land that we call the flood plain was cultivated farm land that was frequently plowed over. Artifacts from the time the fort was occupied are found in this layer, but we can’t get a context from what time frame of the fort they came from because the plow that went through, stirred up the stratigraphy from this layer. Imagine that you are cooking a bag of popcorn in the microwave and the bag is ready to be opened, but first you shake the popcorn to get the butter to cover the kernels, this is what the plow did to this layer. So, artifacts in this layer are oriented in a way that we can tell they are disturbed, such as being found vertical within the soil. We cannot get a context of these artifacts within the soil because we can’t tell what came first or last because the soil is all mixed together. We can tell the difference between the first and second layers based on the color, texture, and moisture of the layers. This layer extends from a range of 20 cmbd to about 45 cmbd.
The final zone, represented by the letter C, is what we call the occupation zone. This is the oldest of the layers and the one we are the most interested in studying. Artifacts from this layer are indisputably from the time of the forts occupation (hence the name occupation zone). The artifacts from this zone differ from the plow zone in that they are undisturbed, or In situ, meaning that the artifacts are oriented in the way from when it was originally placed on the ground. This means they are typically oriented parallel to the surface of the earth. An example of this would be if you dropped a playing card on the ground, it wouldn’t land vertical to the ground, it would be face down. We can get a better context from the objects in this zone (we can tell which artifacts are older), because they are undisturbed and not in a jumbled mess like the plow zone. This makes it easier to study the occupation time of the fort, and that’s why we are more interested in this layer than the others. This layer is typically found below 45 cmbd.
Overall, excavations are going well. We have had good weather so far this week and have been getting acquainted with nature and how the site functions. Come see us at 12:30 p.m. on Fridays (weather permitting) for pit tours of each of the excavation units that we are working on and see what we have discovered so far. See you then!