As funerary archaeologists, we need to consider the whole range of behavior surrounding death and burial. This includes the ritual surrounding preparation of the body for burial, modes of transportation from the site of death to preparation site to burial, any funerary behavior associated along the way, the inclusion of items with the deceased, the ritual surrounding the physical act of burial, and the actual placement of the individual within the grave. That’s it… or is it? In our modern western culture, this is often where the direct funerary behavior comes to an end. The mourners return to their cars as the coffin is lowered into the ground. But an important process is occurring here- the process of backfilling the grave. Sometimes the dirt from the six foot deep hole is enough to cover up the coffin and put the ground back to a normal level, sometimes more dirt is needed and it is taken from elsewhere. While this process may be something today that we don’t consider, in archaeological contexts it has important implications.
Overburden is the rock or soil that lies on top of an archaeological deposit. In burial sites, this includes both the soil placed over the body by mourners as well as the soil that has built up over time. A new article by McGowan and Prangnell (2014) examines how the weight of overlying soil within a burial can lead to changes in bone degradation that affects our ability to properly interpret the remains. Soil movement has a major effect on what appears in the archaeological record and how it appears to archaeologists. When we examine a burial, we need to consider the difference between how it looked when the individual was first buried and how it appears to us as archaeologists. As the body decays within a wooden coffin, the bones may move from their original position. As the coffin degrades, the soil pressure may cause it to warp and over time, the pressure can crush the skeletal. While we know this process happens, there has been little explicit discussion of what overburden can actually do to the remains and how this affects our ability to interpret them.
McGowan and Prangnell (2014) worked on the excavation of a cemetery during construction of a new sports stadium. Despite the relative newness of the cemetery (only 160 years old), there was very little left of the remains- of 397 burials, only one burial was complete. Of the remaining burials 54.2% of the human remains revealed only stains in the soil, 29.9% of the skeletons were compressed into dust, and 15.7% were broken and fragmented. The degradation was not due to water or highly acidic soils as is found elsewhere. The skeletons were destroyed due to the weight of the overburden.
In order to properly investigate this, McGowan and Prangnell (2014) used a mathematical equation from the construction industry that has been used to calculate the pressure exerted by fill over buried pipelines. It takes into account the pressure of backfill settling into a rigid space over time, and how this will effect what has been buried beneath it. They calculated vertical soil pressure of the overburden with different equations for six situations: 1) Adult burial at a traditional depth of 6 feet, 2) Child burial at a traditional depth of 6 feet, 3) Adult burial at a shallow depth of 3 feet, 4) Child burial at a shallow depth of 3 feet, 5) Adult burial at a depth of 12 feet, and 6) Adult burial at a depth of 29 feet. The deepest depth of 29 feet was what was found at the stadium cemetery excavated by McGowan and Prangnell (2014) in areas where the cemetery was covered by almost 7 meters of landfill refuse.
This method developed by McGowan and Prangnell (2014) provides a means of quantifying the magnitude of soil pressure and overburden acting upon a given burial. This allows archaeologists to weigh the effects of the overburden against other taphonomic processes such as animal disturbance or environmental changes like waterlogging and acid soil. Human factors such as placement of landfills or more soil over a burial can be factored into how the burials appear when excavated.
What is the most fascinating about this study of overburden is that it applies a civil engineering equation to a historic cemetery site in order to calculate vertical soil pressure. Archaeology benefits from multidisciplinary imagination. The more disciplines we learn about, the more creative and improve our interpretations of the past.
McGowan, G., & Prangnell, J. (2014). A method for calculating soil pressure overlying human burials Journal of Archaeological Science DOI: 10.1016/j.jas.2014.09.016