A publication of the Archaeological Institute of America
Before digging a site, archaeologists always create a site map. This map will be used to plan digging and surveying and to record the location of finds.
Prior to surveying and excavation, archaeologists abitrarily establish a reference point that all depths will be measured in relation to. This is called the "datum point." Another reference point is selected to help locate features on a horizontal plane. On a north/south oriented map where N, S, E, and W stand for north, south, east, and west, that point is labeled N0E0. So when you see something labeled N40 E35, you know that you are 40 meters (since we dig in meters and not feet) north of N0E0 and 35 meters east of N0E0. The excavation squares are labeled by their southwestern corner. All this does is allow archaeologists to better place them on the site map.
An excavation square is called by several names. The two most common names for this planed hole are "trench" and "unit." Most archaeologists trained to work outside of America call the excavation hole a "trench" while archaeologists who work in America tend to call it a "unit." Of course, to complicate matters, we at the Lott house call our holes trenches. Either way, it refers to the regular area that one is working in. Archaeologists choose whatever size unit best fits each specific site. At the Lott House site our trenches are 2 meters by 2 meters square.
The layers within an excavation hole are also labeled differently by archaeologists around the world. We call these different strata of dirt within the trench "units." However, those who call their trenches "units" call the interior strata "layers." It gets a bit confusing, so from this point on, just remember that for our purposes, the excavation holes are called "trenches" and named by their distance from the datum point, and our interior strata are called "units."
Finally, the term "scarp" is used to describe the walls of the trench, which are really a vertical cross section of many different horizontal layers. When scarps are cleaned--scraped and made level--the soil and artifacts collected must be analyzed separately as they no doubt come not from one discreet horizontal layer, but from many different layers representing different periods of time.