Soils and Time

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The influence of time on soil formation is a unique characteristic of pedogenesis among geomorphic processes that, like lateral variability, serves to distinguish soils and soil-forming processes from other geomorphic phenomena. Another unique aspect of time is that, among the five factors of soil formation, it does not contribute directly to soil formation. However, the passage of time allows the various pedogenic processes operating at a given location to alter the parent material and produce a soil. The physical, chemical, and biological processes of soil formation generally are much slower than many, if not most, processes of sedimentation and erosion. Moreover, most soil-forming processes are so slow that their effect on the soil is markedly time dependent (Birkeland, 1999, p. 144). Time as a factor of soil formation is a key concept in soil geomorphology and has driven much soil geomorphic research (Yaalon, 1975, 1983; Knuepfer and McFadden, 1990; Birkeland, 1999). Because time is also a key consideration of much archaeological research, the time-factor concept of soil genesis can likewise play a significant role in geoarchaeological research (Holliday, 1990a, 1992a). The concept that some time must elapse before a soil can form is arguably one of the most significant aspects of soil development in an archaeological context. This chapter is a discussion of some approaches to the issue of time in archaeology, using soils. The first section is a look at the archaeological implications of soils as indicators of stable landscapes and stratigraphic discontinuities. A number of case histories are presented. The validity of intersite and intrasite archaeological correlations using soils and interpretations of archaeological assemblages associated with soils are profoundly dependent on recognition of soils as depositional hiatuses. The subsequent section reviews the concept of the soil chronosequence and its use in archaeological dating. This is one of the most widely applied aspects of Jenny’s state factor approach to soil geomorphology, and it has considerable potential in archaeology. The last part of the chapter is a discussion of the radiometric dating of pedogenic features.