Soil Stratigraphy in Geoarchaeological Contexts

The various kinds and states of soils, surface and buried, discussed in the previous chapter can be found in an almost infinite variety of combinations, and most can also be found in archaeological contexts. Furthermore, most soil stratigraphic relationships and conditions of soil burial can form a continuum through time or space or both, depending on local and regional variations in rates and depth of burial (i.e., rates and thickness of sedimentation). The most common and most extensive depositional environments with buried soils that illustrate these relationships are alluvial and eolian. These are the settings for much research on buried soils and soil stratigraphy. Alluvial settings likewise have been the loci of considerable archaeological and geoarchaeological research. Tephra—airfall deposits from volcanic eruptions—also commonly contain buried soils because of the episodic nature of eruptions. Though not as extensive as alluvial or other kinds of eolian deposits, tephra stratigraphy is locally important. Archaeological sites are also common in tephra layers from a variety of settings and regions. This chapter illustrates geoarchaeologically significant soil stratigraphic relationships in a variety of alluvial and eolian settings and at various spatial and temporal scales. Alluvial systems probably have been the site of more geoarchaeological research than any other type of depositional environment because they have always attracted occupants who left archaeological sites. A significant amount of archaeological research has also focused on riverine settings owing to “rescue” or “salvage” archaeology. In the United States, for example, this work included the federally funded River Basin Surveys of the 1940s, 1950s, and 1960s, followed by CRM studies beginning in the 1970s and continuing into the 21st century. The importance of alluvial stratigraphy in interpreting the archaeological record of alluvial settings has been recognized throughout most of this work (e.g., Mandel, 2000). Furthermore, the significance of soils in alluvial stratigraphic records has long been recognized; for example, soils were an important component of Haynes’s (1968) classic geoarchaeological model of an “alluvial chronology” for the central and western United States. Alluvial soil stratigraphy per se is more poorly known, however, being underrepresented in the traditional pedology or even traditional soil stratigraphic literature.

Conceptual Approaches to Pedogenesis

To fully appreciate and apply pedologic principals in archaeology, some of the theoretical underpinnings of pedology and especially soil geomorphology must be outlined. Pedologists and soil geomorphologists have attempted to describe, if not model, the processes of soil formation, the factors that drive the processes, and the evolution of soils as landscapes evolve (summarized by Smeck et al., 1983; Johnson and Watson-Stegner, 1987; and Gerrard, 1992, pp. 1–50, 217–220). The task is a difficult one, however, because of the complex and variable sets of processes responsible for soil development. Several of the resulting approaches have proven useful for conceptualizing pedogenesis and, more important, for interpreting soils. In addition to understanding soil-forming processes for interpreting soil profiles, understanding soil formation is important for understanding site formation. The conceptual approaches particularly useful in soil geomorphic and geoarchaeological research are summarized below. Soil-forming processes as components of site formation are discussed more fully in chapter 10. The following discussions of conceptual approaches to pedogenesis are roughly arranged in order of increasing complexity. The “multiple-process model” is essentially a categorization of soil-forming processes. It does not explain pedogenesis but is a useful way to sort and group the many soil-forming processes. The “state factor” approach and the “K-cycle” concept do not deal directly with soil formation, but instead focus on important external factors and processes that drive or affect pedogenesis such as climate and geomorphic evolution. The “soil evolution” model and the “new global view of soils” attempt to integrate pedogenic process with landscape evolution, climate, and other factors. This section closes with discussion of two important aspects of pedogenesis and pedogenic pathways that offer caveats in the use of soils for reconstructing the past. Soils are the result of biogeochemical processes determined and driven by the ecosystem (following Buol et al., 1997). This relationship is more simply described as “internal soil-forming processes” driven by “external soil-forming factors” (fig. 3.1; after Buol et al., 1984). A useful approach to categorizing the many and varied internal soil-forming processes responsible for pedogenesis is the multiple-process model of Simonson (1959, 1978).

Terminology and Methodology

The long history of soil science (e.g., well over 100 yr in North America) and its bureaucratic institutionalization as a component of agricultural research in many countries resulted in the evolution of a substantial vocabulary and methodology for the discipline. A wide array of methods for the field and laboratory investigation of soils also is available to geoarchaeologists. The first part of this chapter is a discussion of some basic terms and definitions used in pedology and soil geomorphology. Some specific terms (e.g., soil stratigraphic nomenclature) are discussed as necessary elsewhere in the text and in appendix 1. There is a sizable body of nomenclature in pedology and soil geomorphology for describing and classifying soils. Indeed, there is a tendency in soils research toward an over-abundance of nomenclature and jargon (e.g., Fastovsky, 1991). All scientific fields necessarily have a specialized nomenclature, however. Researchers in any field, and especially interdisciplinarians such as archaeologists working with soils and soil scientists working with archaeology, should be aware of the nomenclature, jargon, and lingua franca of the new fields they enter.A pedologist who becomes involved with North American archaeology would have to become familiar with terms and concepts such as “Paleoindian” or “Archaic” or “site.” Likewise, archaeologists and geoscientists interested in understanding soils for geoarchaeological purposes must learn some basic soil science terminology and the principles behind issues of proper use (or misuse) of some terms. This fosters communication and problem solving and avoids ambiguities. The rest of this chapter is a discussion of some of the more widely used approaches in the field and in the laboratory, especially in archaeological contexts. Key points to be made are that, first, investigators select the methods that best suit the field situation and the research questions being posed; second, if comparisons are made to other research, the comparable methods should be used; and third, all field and laboratory methods should be referenced in publications and deviations from standard practices or procedures should be described. Some terms introduced below and elsewhere are well defined and generally agreed on, whereas others are variously or vaguely defined.

Introduction

Soils are a potential source of much information in archaeological studies on site and feature-specific scales as well as on a regional scale. Soils are a part of the stage on which humans have evolved. As an integral component of most natural landscapes, soils also are an integral component of cultural landscapes. “Soils are active components of functioning ecosystems that reflect the spatial variability of ecological processes and at the same time have varying degrees of suitability for different kinds of human behavior” (Warren, 1982b, p. 47). Beyond physically supporting humans and their endeavors, however, soils are indicators of the nature and history of the physical and human landscape; they record the impact of human activity, they are a source of food and fuel, and they reflect the environment and record the passage of time. Soils also affect the nature of the cultural record left to archaeologists. They are a reservoir for artifacts and other traces of human activity, encasing archaeological materials and archaeological sites. Soil-forming processes also are an important component of site formation processes. Pedogenesis influences which artifacts, features, and environmental indicators (floral, faunal, and geological) are destroyed, which are preserved, and the degree of preservation. Those involved in field archaeology (as archaeologists, geoscientists, or bioscientists) routinely deal with soils—probably more so than most soil scientists or geologists (Birkeland, 1994, p. 143). However, what the soils or a soil scientist can tell archaeologists about the site and about the archaeological record is not always clear. In part, the integration of soil science in archaeology has been hampered by ambiguities in use of the term “soil” and confusion over what a soil is or is not. The bigger issue is that pedological research, particularly in the United States, has not traditionally been a component of geoarchaeology (the application of the earth science in archaeology) until recent years, in comparison with applications of other aspects of geoscience such as stratigraphy, sedimentology, or geomorphology. This situation evolved in large part because the academic study of soils typically is located in the agricultural sciences rather than the earth sciences.

Human Impacts on Soils

Literally since they first set foot on the Earth’s surface, humans and their hominid ancestors have affected soils. The degree of effect humans have on soils varies from the most subtle, which could include simply walking across the soil, to the most dramatic, such as wholesale removal, mixing, or burial associated with urbanization. Butzer (1982, pp. 123–156) and Davidson (1982) present very useful summaries of the history and nature of such influence in a geoarchaeological context. Geoarchaeologists can be confronted with soils subjected to a wide degree of anthropogenic alteration. The detection of these alterations and their differential distribution can, in theory, be used to determine site boundaries, define stratigraphic relationships, delimit intrasite activity areas and features, and aid in their functional interpretation (Woods, 1984, p. 67). The primary challenge is detecting the human-induced alteration and then, of course, interpreting it. In a very broad sense, the detectability of human impact is roughly proportional to the degree of impact; that is, very subtle alterations are difficult or impossible to detect, but more substantial changes are more obvious. As in most other aspects of archaeology, interpreting the meaning of anthropogenic effects on soils is much more problematic. The study of human impacts on soils is one of the oldest applications of soil studies in archaeology, particularly in regions with a long history of significant human modification of the environment. The topic is also an important part of soil science and agriculture. As a result, there is a very large literature on the topic, especially for the Old World. There well may be more writing on this aspect of soils research in archaeology than on all others combined. The topic is of such interest because human impacts on soils can be so obvious and pervasive in archaeological contexts; recognizing human impacts is critical in sorting out artificial versus natural pedogenic and other geogenic processes; it provides another avenue of research into understanding the relationship between humans and their environment, especially the landscape; and it offers another means of getting at human behavior, either directly, as in studies of mound construction or agriculture, or more indirectly, as in studies of humaninduced soil erosion. Regardless, the subject has long been of interest in archaeology and geoarchaeology.

Soil Genesis and Site-Formation Processes

Pedogenic processes that produce or alter the soils associated with a landscape (buried or unburied) also modify the archaeological sites and other traces of human activity associated with that landscape and buried landscapes. The wide range of processes that form soils can profoundly affect the archaeological record. Pedogenesis, therefore, is an important component of the processes of archaeological site formation. Archaeological “site-formation processes” are those processes that modify artifacts and archaeological sites from the moment they were formed until they are uncovered by archaeologists (Stein, 2001b, pp. 37–38). Understanding formation processes is crucial in archaeology because archaeologists use the patterns of artifacts in the ground to infer behaviors. Formation processes identify patterns that are created by ancient behaviors and separate those patterns from the ones created by later cultural and natural processes (Stein, 2001b, p. 37). In his influential volume Formation Processes of the Archaeological Record, Schiffer (1987, p. 7) notes that archaeologists try to infer past behavior based on the archaeological record, but the record “must be handled with great care by the investigator seeking to infer past behaviors, for the evidence that survives has been changed in many ways by a variety of processes.” These processes introduce variability and ambiguity into the archaeological record. Schiffer (1987, p. 7) further distinguishes between cultural processes, in which the agency of transformation is human behavior, and noncultural processes, which stem from processes of the natural environment. Natural formation processes are many and varied and include plants, animals, wind, water, ice, and gravity, among others. Soil formation is also identified as an important process of site formation. Schiffer (1987) provides a comprehensive discussion of natural site-formation processes, which are summarized by Stein (2001b). Nash and Petraglia (1987) and Goldberg et al. (1993) also provide a number of case histories of natural formation processes identified at archaeological sites. Because soil formation represents the alteration of rock and sediment (chapter 1), pedogenic processes are important natural processes in the formation of archaeological sites. Other weathering processes that are significant in site formation can be grouped as “diagenetic alterations.”

Soils and Landscape Evolution

Soils and archaeological sites are intimately related to the landscape. Investigating soils across past and present landscapes provides a means of reconstructing and understanding the regional environmental and geomorphic context of archaeological site settings and specific site locations, regional site formation processes, and aspects of the resources available to people in a region. Archaeological sites tend to occupy small segments of the landscape, but human activity may affect a much larger area, and in any case, people wander far and wide from sites, interacting with the environment—including the landscape. Thus, no matter whether a site is just a lithic scatter or bone bed or if it is a tell, understanding the regional landscape is an important part of understanding a site and human behavior, and soils are an important means of understanding a landscape. Soils are also important in reconstructing the evolution of landscapes and, consequently, the evolution of archaeological sites. That is, landscape evolution is an important external component of site-formation processes. Landscapes form the physical framework or underpinning for people and their activities and their resulting sites. As landscapes evolve, so do human activities and so do sites. Soils are key to recognizing and interpreting the evolutionary processes that shape the landscape and associated archaeological sites. Furthermore, the concept of landscape evolution also 1) is a logical continuation of the discussion of soil stratigraphy (chapters 5, 6) because it places soil stratigraphy in three or even four dimensions; 2) is a complement to the discussion of soils as environmental indicators (chapter 8), because landscape evolution can be linked to environmental change and because the evolution of the landscape itself, regardless of changes in other factors, represents a change in the environment from a human perspective; and 3) provides yet another means for predicting site locations. The discussion in this section, therefore, represents an integration of some of the principals outlined previously. Some of the studies presented in other chapters, such as the work on the Loess Plateau of China (chapters 6 and 8), and at Harappa and along the Ravi River (chapter 4), are good examples of landscape reconstructions for very large regions and are not repeated here.

Soils and Paleoenvironmental Reconstructions

One of the earliest uses of soils in archaeological research, in addition to stratigraphic markers, was as paleoenvironmental indicators. Similar to soil stratigraphy, the use of soils as environmental indicators in archaeological research probably has its roots in Quaternary geology (e.g., Leighton, 1937; Bryan, 1941a, 1948; Bryan and Albritton, 1943; Movius, 1944; Ruhe, 1965; Haynes, 1968; Valentine and Dalrymple, 1976). Quaternary geologists and geomorphologists working with archaeologists were quick to use soils as clues to past environments (e.g., Leighton, 1936; Antevs, 1941; Bryan, 1941a; Hopkins and Giddings, 1953; Haynes, 1968). Likewise, the nature of prehistoric environments has long been a fundamental question in archaeology. Recognition of the relationship of soil development and morphology to environmental conditions goes back to the beginning of modern pedology, in the later 19th century in Russia and in the early 20th century in the United States (Thorp, 1941, 1949; Tandarich and Sprecher, 1994; Johnson and Hole, 1994). Climate and vegetation in particular were understood as important soil-forming factors long before Jenny produced his landmark volume on Factors of Soil Formation (1941). What Jenny (1941, 1980) brought to the discussion was a theoretical means, using the state factor approach, of assessing the effect of vegetation and climate on soils. By understanding these relationships via biosequences or climosequences, we are theoretically able to pick out the morphological and chemical characteristics of soils that are linked to climate or to vegetation. Climate most directly influences pedogenesis through precipitation and temperature and influences pedogenesis indirectly through vegetation. The most direct effects of biota probably come from the addition of a wide range of chemical compounds, from bioturbation, and from rooting. This chapter is a discussion of those characteristics of soils that have some utility for environmental reconstructions, including climate and vegetation estimates. The chapter also includes some discussion of the potential pitfalls in using soils as paleoenvironmental indicators. Longer and more in-depth discussions of soil–environment relationships in the context of soil geomorphology or environmental reconstruction are presented by Birkeland (1999, pp. 268–306) and chapters in Wilding et al. (1983b) and Martini and Chesworth (1992, pp. 155–306).

Soils and Time

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.

Soil Stratigraphy

Soils have been employed in archaeological stratigraphy since at least the 1930s, including topical discussions of the significance of soils in stratified deposits (e.g., Leighton, 1936, 1937; Bryan and Albritton, 1943). This apparently was for several reasons. The unique physical and chemical properties that distinguish soils from sediments make soils quite useful for stratigraphic subdivision and correlation. In particular, pedologic features, most notably soil horizons, are often the most visually prominent features in stratified deposits. Furthermore, much of the early archaeological pedology was done by individuals trained in Quaternary geology (e.g., Leighton, 1937; Bryan, 1941a; Bryan and Albritton, 1943; Movius, 1944, pp. 49–62), in which soils have been recognized as stratigraphically important since the late 19th century (Bowen, 1978, pp. 10–56; Finkl, 1980; Tandarich, 1998a). The recognition of soils and the differentiation of soils from sediments in archaeological contexts is one of the most fundamentally significant aspects of geoarchaeological stratigraphy. This initial step in stratigraphic interpretation is crucial to most of the applications of pedology and soil geomorphology discussed in subsequent chapters. Because soils indicate periods of stability or hiatuses in deposition, the identification of soils or the lack thereof in a stratigraphic sequence provides information on the number of depositional episodes and intervals of stability. The identification of specific soil horizons also provides clues to the degree and duration of soil development, the nature of the soil-forming environment, and the kinds of soil-forming processes that may affect the archaeological record. Further, tracing of soils from exposure to exposure is a key aspect of correlating strata and interpreting the evolution of archaeological landscapes. This chapter presents a discussion of some principals of soil stratigraphy, and the following chapter focuses on the archaeological significance of soils as stratigraphic units. This chapter begins with a discussion of basic stratigraphy, which is one of the fundamental components of field-based geoscience. That section is followed by a closer look at soil stratigraphy, including a summary of both formal and informal soil stratigraphic nomenclature as well as a discussion of the unique characteristics of soils when used as stratigraphic markers and their archaeological implications.