Soils and Archaeology
Dr. John E. Foss, Yul Roh, and Debra Phillips
The disciplines of soil science and archaeology make a natural combination to study landscapes and environmental history of sites from previous civilizations. Archaeologists have been excavating sites for centuries, but scientific excavations have only taken place for perhaps the last 70 to 80 years. Soil science has been involved in archaeological excavations for 50 to 60 years in the United States, but in the early years only soil chemistry was used to evaluate the impact of humans on the soil system. In the last few decades, more sub-disciplines of soil science have become interested in the archaeology, especially pedology (study of soil formation and classification). In many archaeological projects, pedology is one member of a team of scientists studying a site; team members may include scientists from geology, botany, zoology, palynology, surveying, and other disciplines.
Perhaps a definition of soil is in order to establish the boundaries and general nature of this natural resource. Soils are defined as natural bodies on the earth’s surface with unique morphological characteristics resulting from past or present combinations of physical, chemical, and biological weathering processes. The upper boundary is clearly defined but the lower boundary is quite variable; generally soils include the upper 2 meters of the earth’s surface. Soils, however, will vary from just a few centimeters in thickness to 20 to 30 meters in some areas of the tropics. This upper surface of the earth’s crust is where most of the archaeological activity takes place and thus the interest in soils.
The applications of soil science to archaeology used in this article come from the study of al-Mudaybi’, a Moabite fort, and from the survey of sites on the Karak Plateau in Jordan. The soil investigation was initiated in 1997 and continued in field seasons in 1999 and 2001. These field seasons provided many examples of using soil interpretations for clarifying the archaeological settings and specific stratigraphic characteristics of sites. In this article we will provide some general information on soils and landscapes and their application to archaeological investigations. A general study of the soils on the Karak Plateau is available in an earlier VKRP article (Foss, 2002).
The study of soils at archaeological sites includes two major components; these are the (1) field study of landscapes and description of individual soil profiles and (2) laboratory characterization of soils. The field study comprises a vital aspect for all future interpretations of stratigraphy and ultimately the environmental history of the site. Field soil morphology includes detailed evaluation of soil horizons that provide descriptions of soil texture, structure, color, consistence, horizon boundaries and classification. Figure 1 shows a soil profile (cross section of soil) in Field A at al-Mudaybi’; note the alternating dark and light-colored bands in the lower portion of the profile. The dark layers are periods of stability when organic matter was accumulating in the soil from plant decomposition, and light-colored zones are areas of rapid deposition of wind-blown sediment. Many archaeological sites are in a dynamic position on the landscape, such as along rivers and streams, base of colluvial slopes, or in areas of eolian (wind) deposition. Loess deposits (Figure 2) were the dominant parent material for soils on the Karak Plateau, although a wide range of loess thickness occurs on the various landscapes.
Laboratory characterization of soils for archaeological interpretations is used to verify and supplement the field morphology of soil profiles. In addition, laboratory analysis can provide information on soil system modification resulting from human occupation. One of the best examples is the amount of Pb and other heavy metals introduced into soil systems during Roman times in the Mediterranean region.
Some of the major pedologic contributions to the interpretation of archaeological sites include the following (modified from Foss et al., 1993):
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Determining site delimitation
Soil and landscape analysis provides stratigraphic data and the size and extent of archaeological sites
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Pedological stratigraphy
Soil morphology is the key to understanding the stratigraphy of a site. Soil horizons are related to site history, especially periods of stability and rapid deposition and this relates to habitation of a site
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Soil-landscape relationships
Soils are three-dimensional features on the landscape and, thus, soils provide information on drainage, erosion, deposition, and overall age of site deposits
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Identification of geologic parent materials
The geology of a site has many implications to archaeological sites including parent materials for soils, natural source for building stone, clay and overall structure of the site
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Correlating soil morphology and archaeological levels
The identification of certain soil horizons as artifact-bearing can aid archaeologists in providing physical evidence for further testing of other similar horizons.
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Identifying lithologic (changes in parent material) and pedologic (soil
weathering) discontinuities
The breaks in sedimentation or soil formation are key locations to observe for artifacts. These breaks in sediment history or soil formation intervals, of of sufficient length, may have provided time for habitation.
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Approximating soil age
By combining field morphology with laboratory characterization, soils provide a general time estimate that a surface has been exposed to weathering. These observations are especially useful in developing an excavation plan early in the study.
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Identifying paleosols (fossil soils)
Buried paleosols are useful for evaluating the environmental history of a site. The paleosol itself indicates the length of the weathering period for the its development but also gives information on the climate, vegetation, and landscapes during this period.
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Contributing to the overall interpretation of the archaeological sites
Pedology is just one of the numerous disciplines that provide the comprehensive environmental data that are needed to develop complete site history.
The soil-archaeology project the past three field seasons on the Karak Plateau has essentially been involved in all of the interpretations above. The on-going laboratory analysis of the major soils will also provide additional interpretation of the relationship of soil and landscape to archaeological levels.