Step 3: Excavation
The Grid | Stratigraphy | Tools
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A" Case Study: Excavations at the Stabler-Leadbeater Apothecary Shop
A Pictorial Guide to Site Selection and Backgound ResearchAfter the site is surveyed each excavation unit, or square, on the established grid is identified by a set of coordinates. The coordinates are designated by counting the number of units (meters or feet) north and east of the datum point. The datum point is located at the southwest corner of the site. Sometimes only a few test squares within the site grid will be dug, while on other sites the entire area is excavated.
Stratigraphy is the study of the strata or soil layers of an archaeological site. The effects of human and natural actions over a long period of time cause visible changes in the color and texture of the soil. The strata may have formed gradually by erosion, gardening activities and littering; or over a shorter period of time through activities such as construction, demolition, trash dumping, or landfilling.
Each stratum (an individual layer of soil and artifacts) is dug separately. Archaeologists start on the top layer and work their way down following the contour of each stratigraphic layer. The layers containing soil, structures and artifacts from different time periods are identified by careful excavation techniques. The archaeologists look for artifacts and any changes in the color and texture of the soil. These changes may indicate the beginning of a new layer.
As each layer is dug, it is measured and recorded on a special form. Artifacts are collected separately from each stratigraphic layer within each square so that their spatial relationship can be recorded. The artifacts are placed in a bag marked with the grid coordinates and the layer number. The exact location where the artifacts are found is called the provenience.
Each layer represents a segment of time, much like a timeline. Archaeologists also encounter features while they are digging. Features are areas which appear different from the surrounding soil because the ground was disturbed in the past. Features are usually the result of human activity. Some examples of features are building foundations, abandoned wells and privies, trash pits and burials. Usually the lowest layers are the earliest, although features such as wells, foundations and post-holes may have been dug through earlier layers.
Archaeological excavation techniques are tailored to the individual site. In some circumstances, such as in the excavation of some wells, stratigraphic layers cannot be identified by soil color and consistency. When strata are either indistinguishable or very deep, arbitrary levels are defined by the archaeologist. These arbitrary levels are all equal in depth (e.g., 10 cm.) and provide a way to record the relative placement of artifacts.
Archaeologists must be observant and aware of post-depositional factors, conditions that have occurred after the artifacts were deposited on the site. Seasonal weather changes, roots, worm and rodent activities can alter the placement or preservation of the artifacts. The very actions that can create a site, such as those noted above, may also affect the artifacts and features at a later time.
The basic tool for digging is the 5" pointing trowel; the same type of trowel bricklayers, or masons, use. The trowel is used to carefully and evenly scrape the soil off the excavation area. This not only allows archaeologists to uncover artifacts, but enables them to see changes in soil color and texture which may indicate a feature or the beginning of a new level. The Munsell Color Chart provides standard names for the soil colors. Smaller tools such as dental picks and paint brushes are used for very fine work. Once soil has been scraped, it is sifted through a screen to ensure that no artifacts remain in it.
A line level and plumb bob are used for mapping and drawing profiles of the stratigraphy seen on the side wall an excavation unit. Standardized record forms ensure that the same types of data are collected from every part of the site.
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