The Archaeological Site and Its Environment

In geological and archaeological studies, landscapes and environments of the past can be re-created through studies of geomorphology and sedimentology. Since a supply of water is essential for survival, as well as for many human activities, habitation is commonly near water: along floodplains or near rivers, around lakes, and coastal estuaries. In such environments, landscape changes can be extreme because of floods, drought, sediment deposition, erosion, tectonics, or changes in sea level. Landscape is subject to dynamic changes, so today's is not identical to landscapes of the past. Geomorphic and sedimentologic studies offer the tools to determine the appearance of ancient landscapes.

A favored site for urban civilizations, and indeed the birthplace of urban civilization, is along and in the floodplains of rivers. The great river systems that witnessed the development of the first advanced civilizations—the Nile, Tigris-Euphrates, and Indus—all show dynamic changes in their courses marked by channel migration, floodplain development, and changing deltas as they flow seaward and landlock coastal sites (figure 1.1).

Figure 1.1 Radiocarbon dating of sediment accumulation from the Danube River delta (modified from Panin et al., 1983)

The changing course and delta of the Tigris-Euphrates resulted in political boundary changes and were directly responsible for the recent war between Iraq and Iran. Ur, which was an important deltaic port in the 3rd millennium BC, is now more than 200 km from the Persian Gulf. The present Nile has two main branches in its delta, but Herodotus reported five in the 5th century BC, and Ptolemy noted eight in the 2nd century AD. Sediment from the Danube delta carried south ward by currents cut off the Graeco-Roman ports of Histria and Argamum from access to the Black Sea.

Shorelines have always been prime real estate property for both play and settlement. However, the littoral is a dynamic environment, with changes in landforms brought about by erosion and deposition by the sea itself or more radical changes caused by storms and changing sea level.

Methodologies that have been successfully used to reconstruct ancient littoral settings include:
1. geomorphic surveying to detail the evolution of landforms and land-sea relations;
2. determining the original coastline relationship of coastal archaeological structures that are now submerged or elevated;
3. core drilling in coastal areas and radiocarbon dating of the sedimentological sequence to establish the changes in geological environments.

Detailed studies of sediments in fluvial and deltaic settings have recorded the first environmental misuse of the land by humans. In the eastern Mediterranean, beginning about the time of the Early Bronze Age (ca. 5000 BP), rapid sediment infilling along river valleys and in deltaic regions took place. This was a direct result of deforestation, intensive agriculture, and grazing of newly domesticated sheep or goats—the first clear evidence that humans had arrived, settled down in the area, and changed their lifestyle from hunting and gathering to husbandry and agriculture.

Dating Techniques. Both relative and absolute dates are important for artifacts and remains and to establish the chronological history of a site. Relative dates can be obtained by (a) geological, (b) archaeological, and (c) chemical techniques. Absolute ages are obtained largely by radiometric and biochemical techniques but also, in some places, by archaeomagnetic measurements.

Relative Age Dating. Relative age can be determined by stratigraphic position, style, and absorption of transient elements.

Objects found in the same undisturbed stratigraphic horizon should have the same age, those in a higher horizon should be younger, and those lower should be older.

Style, including method of fabrication and ornamentation, can be used to correlate objects from a known stratigraphic column to other objects found in neighboring sites; that is, similar styles suggest similar dates of manufacture. A more precarious application of this technique occurs when archaeologists attempt to date far-removed objects, such as pottery, on the basis of ornamental or technological style.

More positive identification is possible by knowing that objects buried at the same time in a site should absorb equivalent amounts of transient elements, such as fluorine, uranium, calcium, nitrogen, and water. Differences in the fluorine content of bones allegedly found in the Piltdown site were decisive in exposing the hoax. All should have had about the same amount of fluorine if they were buried at the same time. Some bones, however, had a high fluorine content and were presumed to be more ancient than others which had a low fluorine content and thus were clearly modern and completely unrelated to the ancient bones.

Absolute Age Dating. Absolute age dating by radioactivity is based on two general principles: damage by radiation and radioactive decay.

Radiation damage is produced in crystalline or other materials by (a) the emission of radioactive particles from disintegrating atomic nuclei and (b) the flight of heavier nuclei from fissioning atoms. The flight of these particles, especially of the heavy nuclei, will produce measurable damage in the material. By measuring the extent of the radiation damage in an artifact, an indirect measure of the age of the material or the time of manufacture can be obtained. The older the artifact, such as a ceramic or glass, and the higher the original radioactive element content, the greater will be the measurable radiation damage. Methods based on radiation damage include thermoluminescence, fission tracks, and electron spin resonance spectroscopy.

Radioactive elements and their daughter products produced through radioactive decay have a measurable half-life. If the elements produced in the decay series of the radioactive element and their half-lives are known, then a direct determination of the age of the material is obtainable. For example, radioactive potassium, ⁴⁰K, decays at a known rate to argon, ⁴⁰Ar. By measuring the amount of the radioactive parent, potassium, that is present in an artifact and the amount of its daughter product, argon, that resulted from the decayed potassium, the age of the artifact can be calculated.

Radiocarbon (carbon 14, ¹⁴C) dating is the most widely used direct radioactive age determination method in archaeology. Traditional laboratory techniques of radiocarbon dating have been successfully used on materials younger than about 40,000 years, and now tandem accelerator mass spectrometer methods can be used for up to an additional 10,000-20,000 years. Uranium-series dating of bone and cave deposits for materials older than about 50,000 years and K-Ar dating of volcanic ash older than 100,000 years have been widely used.

Biochemical Dating. Amino acids in organic materials undergo a chemical change known as racemization that is time and environment dependent. Unfortunately, precise knowledge of the temperature history and other environmental conditions that prevailed during burial are required for a reliable age determination. Greater precision can be obtained only if the dates determined for some artifacts can be calibrated against other methods, such as radiocarbon. Then a calibration curve can be set up for use on artifacts and remains uncovered at that particular site.

Archaeomagnetic Dating. Archaeomagnetic dating of objects up to a few thousand years old has been carried out with great success. The method is based on the fact that the earth's magnetic field is constantly changing, both in direction (declination and inclination) and in intensity. In order to use this method, a paleomagnetic databank must first be established for the area in question. The paleomagnetic history is worked out by measuring the magnetic properties of accurately dated and spatially fixed samples such as pottery kilns. Then objects can be dated by comparing their magnetic properties to those of the dated samples.