Objects are removed from a mound in a systematic process of mapping and retrieval known as excavation. To begin excavating a mound, an archaeologist may dig a trench around the periphery and proceed to dig in piewedged sections, exposing each successive layer of burial. The area is sketched and photographed, and the location of each individual object recorded. Skeletal remains are examined for position (for example, extended or flexed) and for the direction of the burial along the cardinal points (north, south, east, or west).
After removal from the mound, objects are assigned a date according to one of several procedures, the most common of which is carbon-14 dating. Carbon-14 (or radiocarbon) is a radioactive isotope that is present in the atmosphere and absorbed into the tissues and bones of all living things. After death, carbon-14 is no longer absorbed but begins to decay to nitrogen at a fixed rate, or half-life, of approximately 5,730 years. Because carbon-14 decays at this fixed rate, an estimate of the age of an object can be made based on the rate of decay of its radiocarbon.
New methods of excavation are being developed to avoid disturbing the underground contents of mounds. At the Cahokia site, researchers at the University of Southern Illinois at Edwardsville are using subsoil remote sensors. Linked to above-ground computers, the sensors relay electrical readings that determine the composition of the underground objects on the basis of their electrical properties.
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