Stratigraphy and The Pine Alley

One of the primary objectives for our current excavations at the Pine Alley is to determine whether or not the holes we are finding relate to (1) the original Madison-era tree planting, (2) the removal of a tree, or (3) a duPont period replacement tree planting. One area of excavation, the location of a tree closest to the temple and icehouse, provides some of the best evidence for explaining the series of events that took place at the site. One of the best ways to understand this information is by examining the soil stratigraphy.

What is Stratigraphy?

Stratigraphy is one of the basic concepts in archaeology. Essentially, stratigraphy is representation of the series of soil depositions that accumulate on top of each other through time and understanding these formation processes is critical to how archaeologists interpret the archaeological record. There are two primary laws that archaeologists follow when they are examining stratigraphy. First, that the oldest deposition is located on the bottom, and the most recent on the top. Consider a lasagne: the first noodle you put down when making lasagne is the oldest, and the delicious melted cheese on the top is the most recent. The second law is that a deposition will either come to a feathered edge, or conform to the shape of the basin or hole that is filling. Think about the difference between if you drop a pile of sand onto a flat surface, versus if you pour it into a glass: the former will create a pile that is taller in the middle and feathers out on the edges, while sand poured into a glass will take the shape of the glass. Archaeologists typically excavate sites “stratigraphically”—which means we excavate and record each depositional period, or strata, separately. This is because each strata represents its own event. We want to make sure that all the data (soil color and texture, depths, artifacts, etc.) from each event are recorded and collected separately, so we can learn as much about each distinct event as we can and understand how they relate to each other. Combining artifacts and historical information with stratigraphy is what allows us to tell time. We can often date artifacts, and when we find those artifacts in a strata we can figure out the earliest date the deposition could have occurred. The basic rule is this: if you find an object that didn’t exist before a certain date, then the strata can’t be older than that date. We call this the terminus post quem, or TPQ. For example, if you find a 1981 penny in your strata, there is no way that strata could have been formed prior to 1981. However, this isn’t a perfect measurement: a 1981 penny could have found its way into that strata in 2019. But it at least gives us a baseline. We can also date strata using event horizons. Event horizons occur if we have good historical documentation that describes specific events, and can associate those events with certain depositions. For example, if there is a documented fire that occurred in 1865, and we can see a strata that is all burned (e.g. lots of charred wood, burnt artifacts, etc.), we know all strata below this burnt strata occurred before 1865, and everything above it happened after.

The Tree Holes

One of the tree holes we are excavating this season presents an excellent example of  intact stratigraphy, and we can examine its stratigraphy to unpack the concepts of TPQ and event horizons.  One of the best ways to look at stratigraphy is by examining a cross section of a feature or a profile wall of an excavation unit. The photo above shows aerial photograph of the tree hole excavation area. The most notable component is the large circle, which is cutting into a number of strata. Three strata are particularly important, since they relate to event horizons that we know about from major building and landscape episodes on the property:

• H: This brick scatter likely relates to the construction of the Temple, which occurred in 1809.
• G: Below H is a bluish clay, which represents the deep subsoil that was dug out from the construction of the nearby Icehouse and Temple. This soil was spread out along the landscape around 1809.
• J: This strata is located below G, and includes a lot of slag and metal materials. It relates to the blacksmith shop that was in this area during the 18th century, prior to the construction of the Temple.

Because F cuts through all of these strata, we know that it post-dates 1809. You can see how this cut works in the lower right section of the image, where the fill of the tree hole is unexcavated: you can see the line of the circular hole cutting into G, the blue clay. The profile of this excavation unit can also help us understand our tree hole. Two holes appear to exist in this profile: the large hole (Layer F), and a smaller interior hole (Layers A-E). If we look closely, Layer F appears to be the mixed up fills of G, H, and J. This mixing of fills occurred when the tree hole was dug and the dirt was redeposited. Layers A-E, in turn, appear to relate to the planting of a tree: A, for example, is a series of root runs, and C appears to be a sandy ball, possibly part of the original root ball from when the tree was put into the ground. The line below E is the cut of the planting hole for this tree, and D and E represent the fill dirt put in around the newly planted tree. It appears that this tree did not survive long, however, since the roots are intact.   The sequence of excavation of the tree hole in 2013. Click to read the captions about each portion that was excavated. Though this stratigraphic relationship exists, what exactly can it tell us about the chronology of this feature? The stratigraphy suggests there have been multiple plantings and removals in this spot. Our current interpretation is that a tree was planted here in the 1810s. At some point, this tree was removed, quite likely before 1908, since there are no trees in this area on the 1908 property map. This may help explain the existence of the large hole, dug to remove the large stump and root system. In the 1920s, when a planting campaign was undertaken by the duPont family, a new tree was planted, represented by the second, smaller planting hole. This tree likely did not last long, was cut, and the stump left to rot in place. This would account for the tree roots identified in the excavations. However, since we still have to excavate the entire other half of this tree hole, these stratigraphic interpretations are preliminary. Hopefully, we will discover even more artifacts that will deepen our understanding of this feature by providing a TPQ for the associated strata. We expect to tackle the other half of this tree hole in July, following our field school—so if you are interested in being a part of this process of discovery, check out our July archaeology programs! Afterall, why read about stratigraphy when you could be digging it!