Archeological Sites

Archeologists are concerned with the activities of people and nature that create evidence of a cultural past. Such evidence, which may include any remnant of human habitation, is referred to as the archeological record. The processes that produce this evidence are called formation processes.

There are two types of formation processes: cultural and environmental. Cultural formation processes are those that follow the actual use of an artifact, e.g., reuse, discard, disturbance, and archeological recovery. Environmental formation processes are those agents that impact cultural materials at any stage of their existence.

Remote sensing and geophysical analysis

Although some archeological sites can be recognized above ground, the majority lie beneath the ground's surface. Remote sensing techniques allow archeologists to identify buried sites. In addition, by examining the ways human intervention have altered the surface near a site (e.g., through the construction of refuse pits or hearths), the archeologist may be able to identify patterns of previous usage.

Archeologists may employ techniques borrowed from the fields of geophysics and geochemistry to detect and map archeological sites and features. Many geophysical techniques, including electrical resistance measurements of the soil above a site and magnetic measurements of a pottery kiln, were first employed by archeologists in the 1940s and 1950s.

Satellite detectors have also been used to monitor the reflected solar radiation above a site. The characteristics of the reflected light allow the archeologist to identify differences in soil or vegetation covering a site, and at sufficiently high resolutions, to recognize archeological features. In this way, archeologists have been able to map out drainage canals once used by Mayan farmers, but now lying beneath the umbrella of the Yucatan rainforest.

Airborne thermal detectors, capable of monitoring the surface temperature of the soil and covering vegetation, take advantage of differences in the way materials retain heat to isolate archeological features from surrounding soil. With this technique, buried Egyptian villages appear to glow at night beneath thin layers of sand.

Electrical resistivity measurements of the soil are sensitive to the presence of water and dissolved salts in the water. Because constriction materials such as granite or limestone have a higher electrical resistance than the surrounding soil, electrical resistivity measurements may be of use in determining the locations of buried structures such as stone walls.

The magnetic properties of soil depend on the presence of iron particles, which when heated to sufficiently high temperatures tend to align themselves with the
earth's magnetic field. However, the earth's magnetic field and intensity change over time. Wherever human activity alters the iron compounds in the soil by subjecting them to high temperatures, for example by building fires in a hearth or firing pottery in a kiln, the heated soil upon cooling takes on magnetic properties that reflect the direction and intensity of the earth's magnetic field at the time of cooling. Since archeologically related changes in local magnetic fields may only amount to one part in 10,000, and because of daily fluctuations in the magnetic field due to electrical currents in the ionosphere, this technique usually requires monitoring of the earth's field at a reference point during the archeological investigation.

Other electromagnetic measurements used to probe a site may examine the soil for phosphates and heavy minerals often associated with past human habitation.

Three dimensional representations of a buried feature may be constructed using ground-sensing radar or resistivity profiling to obtain vertical geophysical cross-sections across a site. When placed beside each other, these sections create a three-dimensional image of buried objects.

Ground surveys

The techniques of ground surveying date to the 1930s and 1940s. Ground surveys require no special equipment, just an observant archeologist with some knowledge of what might be found at a site. Ground survey records may include notes about any visible cultural features and artifacts on the site, site measurements, preparing maps or sketches of the site, and sometimes gathering small collections of artifacts. Surface artifacts may be gathered either as random grabs or complete samplings in a given area.

Site assessments

Initially, the archeologist must determine the size, depth, and stratification of a site. Second, the age or ages of the site must be determined. Third, the types of artifacts and features present at the site must be identified. And finally, information about the environment and the way that it influenced human habitation at the site must be known.

Site assessment techniques fall into two categories: destructive and nondestructive. Surface collecting, testing with shovels, digging pits, and mechanical trenching all disturb the site, and are considered destructive. Nondestructive techniques include mapping and remote sensing.