December 2, 2024
Micromorphology is the study of the shape, orientation, and composition of individual grains that comprise layers of soil or sediment. The goal is to learn more about the cultural and natural processes that brought the grains together. In November, we began a collaboration with micromorphology consultant Howard Cyr to determine how the three distinct layers that fill a sub-floor pit at Site 30 were deposited. The results should advance our understanding of the domestic lives of Site 30's enslaved inhabitants and tobacco cultivation strategies at Monticello in the late eighteenth century.
Site 30, a late-eighteenth century domestic site that was home to enslaved agricultural laborers, is the current focus of our ongoing Monticello Household Archaeology Project. A key feature on the site is a subfloor pit: a small cellar that marks the location of a log house in which some of the site's enslaved residents lived in the 1770s and '80s. We have excavated half the pit and taken pollen samples at 0.1-foot increments down the profile through its three layers of fill.
Our analysis of the pollen, identified by our pollen consultant John Jones, suggests pulses of field clearance and abandonment on a decade-long time scale. Chenopodium and Amaranths (“Cheno-Ams”) and Asters are field weeds indicating active cultivation. Pines, on the other hand, are early successional species that indicate recent field abandonment. We plotted the proportion of pollen grains that belonged to the Aster and Cheno-Am family relative to grains belonging to the Pine family. High values of the proportion mean more Aster and Cheno-Am pollen and less Pine pollen. Plotting the proportions on the vertical axis against the depth down the profile at which we took the samples reveals what seems to be a cyclical pattern.
At the bottom of the pit fill (on the left of the plot), we see lots of field weeds. Moving up the sample column (toward the right on the plot), field weeds decrease and pine increases, then field weeds increase, then they decrease. The black line on the plot represents the trend in the data: a best estimate of the proportion of field weeds at a given depth, based on a regression model known to geeks as a “GAM”. The shaded area represents statistical uncertainty around the estimate.
Our current hypothesis is that fields were being actively cultivated close to the site at the beginning of the fill sequence. Then fields close to the site were abandoned long enough for pines to get started, then the same fields were cultivated again or new ones cleared nearby, then some were abandoned. The pattern matches what we expect based on descriptions of the "swidden" cultivation strategies employed to grow tobacco in the Chesapeake in the 17th and 18th centuries.
To test this hypothesis, we need independent information on how the pit was filled: gradually over the years or all at once. Enter micromorphology; The composition, shape, and orientation of sediment particles as they were originally deposited offer clues onto the agents responsible for deposition, where the grains came from, and what happened to the grains after they were initially deposited. In November, geoarchaeologist Howard Cyr took the necessary sediment samples from the profile through the pit fill at Site 30. The trick is to extract intact blocks of sediment from the profile without disturbing the original matrix. To do this, Howard pushes plastic conduit boxes into the profile and then removes them with the matrix trapped within the four sides of the box. The samples are then impregnated with epoxy, ground thin enough to be translucent, and examined under a petrographic microscope.
We hope the results will tell which of the three layers were deposited slowly, for example as sediment from the house's dirt floor sifted through the boards covering the pit while it was in use, or if each layer is the result of people filling in the pit with sediment from different sources at different times. We also want to know if the top of each layer represents a prolonged period of surface stability. Slow deposition is what we expect if our interpretation of the pollen is correct. We hope to be able to report on the results in the next couple of months.