How do You Know Where to Dig?

How did you know to dig here?

This is the most common question we get when we are out excavating. Visitors are always curious: what is it about this spot that made you think you’d find something? There must be a way that archaeologists narrow it down. And they’re right: we employ a number of different techniques to figure out where sites are located, so that we can can maximize our resources to conduct our excavations. In this post, I’m going to talk a little bit about some of the methods we use at Montpelier, and invite you to join us on one of our new spring programs, the Site Location Program, where you will get a chance to participate in some of these survey methods.

The process of identifying archaeological sites is called Survey. The idea is to cover large areas of ground with minimally invasive and efficient means of systematic data recovery. There are a number of different types of archaeological survey, each best used for identifying different types of sites in different types of contexts. I’m going to cover a couple of them here because they are ones that we are using for our archaeological investigations of the Home Farm and Overseer’s house.

For our current research project at the Home Farm, archaeological survey is critical to how we are going to identify the location and function of the many different work and living spaces that were occupied by enslaved African Americans and white overseers. Because we have such a large area to cover, survey is integral to this process. And because we want to not only know where those sites are, but what they are and, hopefully, where buildings are within those sites, we are applying a number of different methods to narrow things down before we start excavating.

Metal Detector Survey

At Montpelier, we are one of the few places that relies on metal detector survey as our primary means of identifying historic period sites. We use this method as our broadest means of finding sites – and are in the process of surveying the entire property with this tool. The method is pretty simple: first, we cover areas on a 20 meter grid, counting the number of hits we record in each grid, and digging a small sample of hits in each grid. The more hits, the more likely a site is located in that area. One of the major areas we’ve done the survey is at the Home Farm, which has led us to identify almost 15 potential sites – the map below shows the red clusters of high density hits – these are all representative of potential sites.

Then, we come back to the areas with high densities and do a 10′ grid, and do it again. At the tighter interval of hits, we get a better idea as to the extent of the possible site. We have done this at the Overseer’s Site, and it has helped us narrow down the area that we should be conducting our excavations, and we will be replicating this process at all the high density areas identified on the map above. This scope of survey is critical to our entire understanding of the Home Farm, since it will help us to better understand the size of these sites.

Shovel Test Pit Survey

The most common type of survey is shovel test pit survey, because it can be used to identify the widest variety of sites. Commonly called “STP Survey”, the process is quite basic: holes anywhere from 16-30″ in diameter are excavated with shovels on a grid, and typically excavated to subsoil – no more than a couple feet at Montpelier. Artifacts are screened and collected. Depending on the context for the survey, the grid could range anywhere from 10 to 100 feet spacing. Once the STPs are dug, the artifacts are cataloged and their densities are mapped.

Often, STPs are excavated and have no data in them, called a negative STP. For projects that are digging on a large grid, when a positive STP is found, the archaeologist excavates radials – this means digging additional STPs around that positive hole at a pre-specified distance from that hole. This helps further delineate the extent of the potential site.

At Montpelier we use STPs a little differently because we rely on MDS to cover the broadest areas for survey. Once we’ve narrowed the site boundaries through 20 meter and 10′ metal detector survey, we use STPs on a 10′ grid to provide us with more information. One advantage of STPs is that they recover all kinds of artifacts, including glass, ceramics, and brick, which can help us better understand the location and function of buildings or trash deposits. We also use our STPs to gather soil chemistry samples, which give us a consistent data set of soil chemistry across the site. STPs are also an important part of our Home Farm project, as they will be used at each site we’ve identified through metal detector survey to better understand the function of these potential sites.

Geophysical Survey

One of the ways that people ask a lot about are whether or not we use geophysical survey techniques such as ground penetrating radar (GPR), electronic resistivity, or magnetometer survey at our sites. We do, with GPR being the one we have relied on the most. Again, these survey techniques are best used for certain applications, and I will admit that it is not an area I am an expert in. Geophysical survey requires skilled technicians to carry out, and specialized tools that we currently don’t have at Montpelier. So, we rely on partnerships and colleagues who have done this work for us.

The basic idea behind these geophysical tools is that they are ways for us to look into the earth without disturbing it. GPR sends signals into the ground, and detects reflected signals that come off of subsurfaces. So, brick or stone walls often result in clear signals, and in some cases so do changes in soil density or type, depending on the conditions. We have used GPR with varying levels of success – our Piedmont clay often makes things more difficult for these forms of survey.

Used in conjunction with other survey methods, however, GPR results can point us towards “anomalies” that we can target our excavations on. This spring, we will be focusing our excavation units on a number of anomalies identified in GPR survey, in hopes that some may turn out to be related to architecture. Similarly, we will be using GPR at a number of different sites found on the 20 meter and 10 foot metal detector surveys, hoping to identify architectural components of those structures as well.

Piecing it all Together

Archaeological survey is a critical part of how we know where to dig – and it is also an incredibly important part of our research project over the next three years. In order for us to really understand how the agricultural complex at the home farm is designed, and how people who live there are using the space, we need to employ as many different types of survey as we can to narrow down our sites. And of course, we want you all to be a part of that investigation! This is why we’ve designed the very first Site Location Program – a 3-day program where participants will be helping us with Shovel Test Pit Survey, doing a little metal detecting, and getting a demonstration on GPR. This year, we will be doing survey at the Overseer’s Site, and in subsequent seasons we will be working at other sites across the property.

So, if you’ve every found yourself wondering just how archaeologists do find archaeological sites, this is your opportunity to not only get that question answered, but try your hand at finding some sites yourself!