How do Archaeologists Know What They Know?

Ever wonder how archaeologists know that something is older than something else? Or why we take so much care when we dig a hole in the ground? I thought I’d write about some of these questions while we’re catching up on the lab work and staying inside out of the ridiculous heat that has the entire country in its grip.

Elsie and Jean Marie excavating a foundation in Port Tobacco.

Context is Everything

Let’s start with some very important information about WHY we are so careful. Archaeology is a destructive process - once the soil is removed and once an artifact is removed it can never be put back the way it was. Good field methods retrieve data about the past. We’re concerned with an artifact’s context. Context consists of three basic things:

·      The soil matrix (or soil layer) where an artifact is found

Each of the different soil layers or strata
 in the photograph
 is from a different time period

·      The location or provenience of the artifact. This mean where it’s located in space (the horizontal location across a site) and time (the vertical location or depth below the ground).

·      The association between all the artifacts in the same vertical soil layer across the horizontal plan of the site. Are they all associated with food preparation? Are they household trash? Are they from a blacksmith’s shop or some other industrial site? And so forth. Also important is their temporal association, are they all from the same time period or multiple time periods?

These bottles from Port Tobacco
were excavated in the 1970s.
We have no context for them,
which means we have no information
about where they came from in the village
or how deeply buried they were.
They have no informational value.

We use this information to help us determine what type of activity took place on the site or the social or economic status of people living on the site. This information can also help us determine if a site was destroyed by a fire, a flood, or if it was just abandoned by the people who lived there. If an artifact is removed from its context without recording information about the soil matrix, the depth of the artifact, or its location across the site it loses its information potential and becomes just a pretty knickknack. The information potential is the most valuable thing about an artifact. Does this mean that archeologists are immune to the beauty of an object? Heck no, we’re only human! But the information is of the highest value because it allows us to reconstruct the past. 

How Old is That Artifact?

Archaeologists use dating methods such as Carbon 14 dating, Dendrochronology (tree ring dating) or Thermoluminescence dating to get absolute or more exact dates. A more general dating method we use is called relative dating, and here’s where the soil matrix and vertical position become key!

Copied from https://www.southalabama.edu/org/archaeology/news/stratigraphy.html

For relative dating we use a law – The Law of Superposition – to get a general idea of the time frame for a layer and for the artifacts in that layer. This is a really simple law – it states that the bottom soil layer (or stratum) in multiple layers (or strata) of soil is the oldest layer. The stratum above the bottom stratum is younger and the stratum above it is even younger! As one of my former mentors once said – “First thing in is the last thing out.” Take a look at the diagram above of a soil profile (or wall) in an archaeological unit (the squares we dig in). The newest soil stratum is at the top in this column of strata and the oldest is at the bottom. We call this relative dating because the artifacts are dated relative to their position to one another in the ground.

When we dig in agricultural fields the top layer is often a mixture of all sorts of different artifacts because the plow has mixed different strata together to make one stratum that archaeologists call the plow zone. A simple way to illustrate this is with a piece of Smith Island cake (which by the way was delicious). Each of the layers of cake and icing in the photograph on the right represent a different soil stratum. Now let’s pretend that the top six layers of the cake have been blended together, just like when a plow runs through the soil in a field. As you can see in the second photograph, the top six layers

of cake can no longer be distinguished from one another – this is precisely what happens when the plow runs through the soil layers. [Note - Mom if you’re reading this I WAS NOT playing with my food - Tim was. Does anyone hear a bus backing up?].

The Information Potential of Archaeological Deposits

Early 18th-century Rhenish Mug Fragments.
These sherds and other mug and wine glass fragments recovered during 
excavation suggest that this site was once used as a tavern.

Does this mean there’s no information potential for the artifacts recovered from the plow zone? There is less information potential than there would be in an intact soil stratum, but a mixed plow zone can tell us a few thing. First, it tells us yes there is a site here (because there are artifacts). Secondly, the types of artifacts in the plow zone might give us some information about what kind of site was located here in the past. Lots of horseshoes, nails, wagon and carriage parts? We’re probably looking at a blacksmith shop. Buttons, thimbles, cloth seals, and broken scissors? Perhaps a tailor shop. Broken bottle glass and pottery, roofing slate, tobacco pipes? Perhaps someone’s house. Tavern artifacts (photograph above), you guessed it - a tavern! The location of concentrations of these artifacts on the surface guide us in the selection of where to put our units.

Swann House Ceramics
Those on the left date to the period circa 1760-1800.
The four on the right date from the 1830s through the early 20th century.

The last thing we can learn from a plow zone assemblage (or collection of artifacts) is the temporal dimension. Last week (July 10th blog) I wrote about the 30 year gap in artifacts at the Swan Site. I based this observation on the artifacts we recovered from the plow zone. There are artifacts from the period 1760 through about 1800 and from about 1830 to the early twentieth century, but there are no artifacts from the period circa 1800 through 1830. This suggests that the site was abandoned for about 30 years.

Valerie Recording information about the level before continuing excavation
If context is everything then it must be recorded!

The place where we find the most information about a site is in areas where no plow has gone through and there is no other type of distrubance (such as ground hog burrows). These intact soil stratum can tell us a great deal about what happened on a site. For example, a few years ago we excavated the corner of a building in Port Tobacco that was used as a print shop. We recovered printer’s type from in situ stratum, or soil layers that haven’t been disturbed by plowing or or other activities. This gave us information about the type of activity that took place at the site. The sequence of strata from the bottom up (or their position relative to one another) enabled us to determine that the building had been erected after the Revolutionary War because the base stratum only had ceramics and other items that post-dated the war (labeled 1 on the photograph below). This was under a stratum with mid-nineteenth century ceramics, bottle glass, and printer’s type, which gave us a date for the print shop (labeled 2 on the photograph below). Above the shop layer was a destruction stratum from the late nineteenth century when the town of Port Tobacco was all but abandoned (labeled 3 on the photograph below). And, on top of these three strata was a plow zone of mixed artifacts (labeled 4 on the photograph below). The context of these artifacts allowed us to date the area and begin to form an interpretation about activities in the town before the county seat and the center of business and economic activity moved to La Plata in the 1890s.

Left is a strata profile from the print shop excavation
Right piece of printers type recovered during the excavation

Other types of deposits that give us information about a site are features. These can be foundations for buildings, post holes for building supports or fences, trash pits, wells, hearths, garden features (planting holes, paths, or buried terraces), sepulcher, roads, and other items that can’t be picked up and taken into the lab. All of these features give us information about how people conceptualized and used the landscape.

Building Foundation with a post hole by the corner. Port Tobacco.
Photograph from a drone, courtesy of Maj. David Kelly

For example, what’s the proper way to set up a farmstead in the seventeenth century? Is the house next to the stable or barn? Is the layout similar to contemporary farms in England? Are there specialized buildings for the dairy, the kitchen, the smokehouse – or is that something that only happens later during the eighteenth and nineteenth centuries? How is the layout of this farmstead like/unlike other farmsteads? And if the latter, why? The features also give us information about the types of material culture people owned and used. Are they using new fashionable tea wares? Or are there hardly any ceramics in the trash pits, which for an eighteenth century site might suggest they were using pewter dishes. By the way pewter hardly ever gets thrown out in the trash so we rarely recover any. It doesn’t end up in the ground because it was used for decades or it might have been melted down and used to produce other items.

Archaeology at George Mason's Gunston Hall in Virginia
located the remains of the quarters for the enslaved house servants
(brown rectangle on the left) and the location of
the kitchen, dairy, and laundry (which have been rebuilt on the right).
All of these were adjacent to the the main house.
The archaeology informed the reconstruction of the landscape.

These are a few examples of what archaeological deposits can tell us about our past, about the everyday people, free and enslaved, who worked as farmers, printers, tailors, tavern keepers, housewives, and so forth. The people who built the foundations of the farms, towns, and cities that form our own landscape today. So the next time you’re tempted to just dig up a site, think about all the information, the history, that you’ll destroy. Call an archaeologist instead, I’m sure they’ll be interested in your site and they might just give you some help in finding out more about it.

John Lewis Krimmel, Bar Room Dance, 1820