Excavation Methods

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Excavation Methods

Excavation methods are the various techniques used within archaeology to dig, uncover, identify, process, and record archaeological remains. Archeological excavation involves the removal of soil, sediment, or rock that covers artifacts or other evidence of human activity. Early excavation techniques involved destructive random digging and removal of objects with little or no location data recorded. Modern excavations often involve slow, careful extraction of sediments in very thin layers, detailed sifting of sediment samples, and exacting measurement and recording of artifact location.

About the time of the American Revolution (during the last half of the eighteenth century), the then-future U.S. president Thomas Jefferson began excavating Indian burial mounds that had been constructed on his property in Virginia. His technique, which was to dig trenches and observe the successive strata, or layers of soil, anticipated the techniques of modern archaeology.

Between 1880 and 1890, English officer and archaeologist Lieutenant-General Augustus Henry Lane Fox Pitt-Rivers (1827–1900) initiated the practice of total site excavation, with emphasis on stratigraphy and the recording of the position of each object found. In 1904, English Egyptologist Sir William Matthew Flinders Petrie (1853–1942) established principles of surveying and excavation that emphasized the necessity of not damaging the monuments being excavated, of exercising meticulous care when excavating and collecting artifacts, of conducting detailed and accurate surveys, and of publishing the findings as soon as possible following the excavation. In the same year, archeologists R. Pumpelly and Hubert Schmidt, working in Turkestan, had already begun using sifting techniques to save small objects, and were recording the vertical and horizontal locations of even the smallest objects in each cultural layer.

Today, archeology is still equated with excavation in the popular mind. Most sites are no longer fully excavated unless they are in danger of destruction from building or erosion. Archaeologists leave a portion of some sites unexcavated to preserve artifacts and context for future research. Furthermore, there are now many alternatives to excavation for studying an archeological site, including surface archeology in which surface-exposed artifacts are detected and recorded; remote sensing; and the examination of soil and plant distributions. These techniques are nondestructive, and permit the archeologist to easily examine large areas. But even today, in almost all archeological projects, there comes a time when it is necessary to probe beneath the surface to obtain additional information about a site.

Before any excavation is begun, the site must be located. Techniques used to find a site may include remote sensing (for example, by aerial photography), soil surveys, and walk-through or surface surveys. The digging of shovel tests, augured core samples and, less commonly, trenches may also be used to locate archaeological sites. Soil samples may be collected from various sites and depths to determine whether any buried features are present.

When planning an archeological excavation, archeologists often use nondestructive techniques such as electrical resistivity meters and magnetometers to locate structures and artifacts on the site without digging. Soil testing may shed light on settlement patterns connected with the site. Aerial photography can also provide useful information for planning the excavation. In unobstructed fields, past human occupation of an area is evident through visible soil stains left by plowing, digging, and construction.

Before beginning the actual excavation, an archeologist prepares a topographical map of the site that includes such details as roads, buildings, bodies of water, and various survey points. This activity allows researchers to compare site location with natural land-forms or regional terrain to establish settlement patterns, a theory about where people used to live and why they chose to live there.

Prior to excavating the site, a series of physical gridlines are placed over it to serve as points of reference. In staking out the grid, the archeologist essentially turns the site into a large piece of graph paper that can be used to chart any finds. The site grid is then mapped onto a sheet of paper. As objects are discovered in the course of the excavation, their locations are recorded on the site map, photographed in place, and catalogued.

Excavation strategies

Archeology has undergone radical changes since the time when an excavation was simply a mining of artifacts. Today, the removal of artifacts requires that the spatial relationships and context in which they are found be fully documented.

When an archeologist documents a find, he/she considers both vertical and horizontal relationships. Vertical relationships may yield information about the cultural history of the site, and horizontal relationships, about the way the site was used. In vertical excavation, the archeologist may use test units to identify and/or remove strata. Many archaeologists excavate sites in arbitrary levels, small increments of excavation, paying close attention to any changes in soil color or texture to identify various strata. In horizontal excavation, the archeologist may plow strips along the surface of the site to expose any objects lying near the surface. The excavation of a site proceeds by these methods until, layer by layer, the foundations of the site are uncovered. Often, excavation ends when sterile levels, strata without artifacts, are repeatedly uncovered.

Conventional excavation tools include, roughly in order of decreasing sensitivity, a magnifying glass, tape measure, pruning shears, bamboo pick, whiskbroom and dustpan, grapefruit knife, trowel, army shovel, hand pick, standard pick, shovel, and perhaps in some cases, even a bulldozer. Most of the excavation work is done with a shovel, but whenever fragile artifacts are encountered, the hand trowel becomes the tool of choice.

Mapping and recording

Archeologists record spatial information about a site with the aid of maps. Measuring tools range from simple tapes and plumb bobs to laser theodolites. The accuracy of a map is the degree to which a recorded measurement reflects the true value; the precision of the map reflects the consistency with which a measurement can be repeated.

In the course of an excavation, the archeologist carefully evaluates the sequential order that processes such as the collapse of buildings or the digging of pits contribute to the formation of a site. In addition, the archeologist typically notes such details as soil color and texture, and the presence and size of any stones.

The way the research proceeds at the site will depend on the goal of the excavation. If the purpose of the excavation is to document the placement of all retrieved artifacts and fragments for the purpose of piecing broken objects back together, the level of recording field data will be much finer than if the goal is simply to retrieve large objects. In cases where the goal of the site research is, for example, to recover flakes and chips of worked stone, digging at the site typically involves a trowel and whisk broom, and almost always, screening or sifting techniques. One-quarter-inch (6 mm) screens are usually fine enough for the recovery of most bones and artifacts, but finer meshes may be used in the recovery of seeds, small bones, and chipping debris. When a screen is used, shovels full of soil are thrown against or on the screen so that the dirt sifts through it, leaving any artifacts behind.

Another technique frequently utilized in the recovery of artifacts is water-screening. By using a water pump to hose down the material to be screened, the process of recovery is sped up and the loss of objects that might be missed or damaged in the course of being dry-screened can be avoided. A drawback in using this recovery technique is that it generates large quantities of mud that may cause environmental damage if dumped into a stream.

Elaborate flotation techniques may be used to recover artifacts, seeds, small bones, and the remains of charred plant material. In these techniques, debris from the site is placed in a container of pure or chemically treated water. The container is then shaken, causing small objects to float to the surface where they can be recovered. Archeologists have developed elaborate modifications of this technique, some involving multiple trays for the sorting of objects, to facilitate the recovery of artifacts. Many of these artifacts are analyzed under a microscope to look for clues about manufacture and use. The smallest artifacts, microartifacts such as pollen and seeds, require the use of a microscope for simple identification.

KEY TERMS

Electrical resistivity— A remote sensing technique that determines the character of subsurface sediments or the presence of objects based on variations in the resistance to an electrical current passing through the subsurface.

Magnetometer— An instrument designed to measure the strength of a magnetic field; magnetometers detect the presence of metallic objects that distort Earth’s magnetic field.

Stratigraphy— The study of layers of rock or soil, based on the assumption that the oldest material will usually be found at the bottom of a sequence.

Theodolite— An optical instrument consisting of a small telescope used to measure angles in surveying, meteorology, and navigation.

Topographic map— A map illustrating the elevation or depth of the land surface using lines of equal elevation; also known as a contour map.

Prior to being sent to the laboratory for processing, artifacts are placed in a bag that is labeled with a code indicating where and in which stratigraphic layer the artifacts were found. All relevant information about an artifact is recorded in the field notes for the site.

Publication of findings

Because excavation permanently destroys at least a portion of a site as a source of archeological data for future generations, it is essential that the results of an excavation be promptly published in a form that is readily accessible. Current practice is to publish only portions of the complete field report, which is based on analyses of physical, biological, stratigraphic, and chronological data. But many archeologists are of the opinion that the public, which is widely viewed as having collective ownership of all matters relating to the past, has a right to view even unpublished field records and reports about a site.