What is a Unit?

by Charles SimpsonCART Archaeological Technician

As an archaeologist who has been working in the field for 3 years, I have had the pleasure to do a variety of projects across the East coast. When an archaeological survey is needed, different kinds of testing are required. In initial testing, archaeologists often dig circular or square holes called Shovel Test Pits (STP) down to subsoil and record artifacts found. When artifacts are found and the site being surveyed is suspected of being of significance, it typically requires avoidance or further excavation. A more in-depth survey is then recommended. This more in-depth survey is where archaeologists begin to dig test units. Which brings us to “What is a Test Unit?” in archaeology. A test unit is typically a 1 x 1 meter square area that will be carefully excavated and recorded in great detail.

The blackboard above shows details the coordinates of the SW corner and location of survey area. The North arrow shows the direction grid North and with scale below.

Test Units are often 1 x 1 meter squares placed in proximity to something that may have cultural significance. Archaeologists determine the placement of each unit on a number of factors. Sometimes they are placed in proximity to STPs with a high density of artifacts. Others are placed in areas where historical documentation suggests a high density of artifacts or a possible feature (see blog post on features) such as an old stone pathway, post holes, or a building foundation. Before getting started on excavation, archaeologists find a suitable location for the first nail, sometimes by using a transit to pinpoint an exact location. At CART we then use the transit again to place the other nails, either at one meter or at the hypotenuse. At other sites, archaeologists will use the compass bearing that aligns with the direction of the site grid and place the second nail one meter from the first nail. Using the Pythagorean Theorem to determine the hypotenuse of a right triangle, an archaeologist will then take two separate measurements using two measuring tapes from both nails (Point A and Point B in the figure below); one nail being measured to a meter while the other is measured to the hypotenuse (which equals to 1.414 m on a 1 x 1 meter square). This trilateration ensures a 90 degree angle at the corner. Using both measuring tapes, each tape is aligned until both points overlap one another.







Once both measurements meet up, a nail is placed in the ground (Point C in figure above). The process is repeated once more using different nails to find the last point creating, ideally, close to a perfect square.

After each nail is measured and strung up, the archaeologist takes a photo and writes down the information associated with the unit on a standardized form. Each level and/or layer dug receives its own sheet. Some of the information written includes: coordinates (which for CART always takes from the South West corner of the unit), a Munsell Soil Color sample, and perhaps a brief explanation on why the unit was placed where it was. Once the basic information is taken, measurements of the depth from each corner and center are taken. For some excavations, the archaeologist may be told to dig by strata, stopping their excavation when a change in soil is encountered.

Other times, excavators are told to dig up to an arbitrary depth and stop excavation at regular intervals such as every five centimeters to record their progress before continuing. Usually even with arbitrary levels, archaeologists will stop at a stratum change. Archaeologists usually shorten the word stratum to “strat”.

After the photos are taken, measurements are recorded and a brief description is written, it is time to dig. Using either a trowel or a shovel, the archaeologist begins removing soil. In many cases, one archaeologist will dig while another screens the buckets of dirt, collecting all artifacts found. It is the responsibility of the person digging to be on the lookout for changes in soil texture, color, or compaction that can signal a strat change, plow scars, or features. When the excavator is done, measurements are made and features (if any) are drawn to scale on a sheet of graph paper. A Munsell color of the new soil is determined and recorded on the form along with a list of all the artifacts found. When done excavating a layer or level, a detailed summary of what was found is written to help give an idea of what occurred. This will help when the project manager writes a report about the project at a later date.

Once all cultural layers are dug, archaeologists dig 10 centimeters into sterile subsoil. Subsoil differs by site, but in many sites, archaeologists end excavation once digging 10 centimeters into sub. This buffer minimizes the possibility that artifacts or features left behind by humans will be missed. Unlike previous cultural layers, sub (depending on the local geology and parent material) is composed mostly of clay mixed with traces of silt. Depth can also differ based on the location of the Test Unit. Excavations like the one going on in Colchester hit sub at about 35 centimeters below surface. Other regions with deep coastal plains may not see sub well past a meter below ground surface.

From there, the archaeologists clean up, take photos of the walls and floor of the unit, and draw a to-scale map of the different stratigraphy including any anomalies found within the unit walls. After the photos are taken and maps are drawn, it is time to write a summary on a separate form detailing everything from why the unit was placed where it was, the soils and disturbances that occurred, the artifacts and features found as well as overall impressions. Once all that is done, it is time to take all the dirt that was screened and shovel it back into the unit just dug. This is called backfilling.

Please note that methodology and terminology can differ in other regions or even, somewhat, between different organizations.


About cartarchaeology

We are the County Archaeological Research Team, part of the Archaeology and Collections Branch, Resource Management Division, Fairfax County Park Authority. We are tasked with understanding and managing the cultural resources on Park land throughout Fairfax County.
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