Geoarchaeology Research Paper

There are at least seven major concerns of geoarchaeology that must be integrated into research programs at the design, excavation and analytical stages and include: (a) site survey techniques that use geochemical, electromagnetic and other remote sensing procedures to locate sites or features within a known site; (b) documentation of site-formation processes and the regional context of a site; (c) using a wide array of approaches to differentiate cultural from natural (nonhuman) sediments and features, including disturbance processes by biological, pedological, and geological processes; (d) the development and correlation of intra and extrasite temporal contexts by relative and/or absolute dating; (e) integration of ancient landforms and biological information for the reconstruction of palaeolandscapes (after Gladfelter 1981, p. 347); (f ) reconstructing palaeoenvironments by the identification of pollen, wood charcoal, leaves, and seeds; and (g) the geochemical analysis of artifacts to document interaction between social groups. Examined here are a few techniques of geoarchaeology that are geared towards: discovering archaeological sites and documenting their internal structure; surveying site formation and disturbance processes; the analysis of soils and sediments; palaeoenvironmental reconstruction and the impact of humans on the landscape; and the physical analysis of archaeological materials.

1. Site Discovery And Definition

Traditionally, locating archaeological sites was accomplished by asking local residents in conjunction with a pedestrian survey; that is, simply walking over the landscape and looking for artifacts and food remains or larger features such as stone house foundations, terraces, or earthworks. In the 1960s with the advent of public archaeology programs—also known as ‘contract’ archaeology or heritage management studies formulated in response to development projects—archaeological surveys became more systematic, utilizing sophisticated sampling procedures, predictive modeling, and techniques of remote sensing, or archaeological prospecting as it is called in Europe.

1.1 Remote Sensing Or Archaeological Prospecting

Archaeological prospecting ‘is considered as an exercise in the deliberate planned measurement and evaluation of physical properties which lead to the detection and mapping of archaeological sites’ (Scollar et al. 1990, p. 27). Since all surface indications of many archaeological sites have been obliterated by modern urban encroachment or been buried under soils, excavation has been the technique of choice for locating subsurface cultural deposits. Remote sensing techniques are nondestructive and include aerial photography, and resistivity, magnetic, electromagnetic, and thermal prospecting (see Scollar et al. 1990 for a detailed explanation of these techniques).

1.2 Aerial Photography

The oldest form of remote sensing, aerial photography is used in archaeology primarily for discovering field monuments, whether earthen or stone constructions. A huge corpus of aerial photographs exist some of which were taken during various wars for reconnaissance purposes, others for urban planning, to record changing shorelines, and to document vegetation patterns. However, aerial photos made exclusively for archaeological site discovery are the most useful since they are taken at the appropriate scale, angle, and time of day for enhancing surface features. Prehistoric agricultural field systems in areas such as Mexico and the Pacific Islands, consist of low terraces and earthen mounds and embankments which are readily mapped from aerial photographs, saving countless hours of field survey and mapping.

1.3 Other Techniques Of Remote Sensing

Once archaeological sites are located, identifying internal structures (intrasite variability) can be accomplished with other techniques of remote sensing including resistivity prospecting, where subsurface anomalies (such as burial pits, compacted ancient floors, or buried house foundations) are mapped by identifying the variation in an electric charge as it passes through the ground (Scollar et al. 1990, pp. 307–421). Magnetic prospecting can locate the shape, size and depth of subsurface anomalies that can then be mapped. Thermal prospecting uses variations in surface temperatures as an indication of subsurface (often cultural) structures.

1.4 Coring Archaeological Sites

Identifying subsurface archaeological site boundaries, internal features, and differential distributions of artifacts and faunal material can be accomplished by coring (Stein 1986), especially in conjunction with the analysis of sediment chemistry. Small cores or bucket augers (about 10 cm in diameter) are used to collect sediment samples for the analysis of cultural content and to differentiate cultural from noncultural layers. Since organic matter, carbonates (e.g., marine shells), and phosphates (deposited in the form of urine, excrement and organic refuse) can increase relative to the intensity of human occupation, these residues are good indicators of past site use (Sjoberg 1976) and their distribution is used to identify subsurface cultural deposits with a minimum of site disturbance.

2. Site Formation Processes

The archaeological record is not static, but changes in response to a number of processes—both cultural and environmental (or noncultural). Once deposited in, or on, the surface of an archaeological site, cultural material (artifacts and food remains) and noncultural matter, such as pollen grains and terrestrial sediments, can shift their position or, in some cases, be removed completely. Since archaeological interpretations gain their strength from identifying and analyzing the vertical and horizontal distributions of material within and between sites, it is vital to understand how the positions of these materials may have changed since they were originally deposited—this is the challenge of site formation research defined as the processes, both natural and cultural, that build, disturb, redeposit, and destroy archaeological sites (Schiffer 1987).

2.1 Nonhuman Disturbance Processes

Since the soils and sediments that make up an archaeological site are part of a dynamic open system, archaeological sites are best viewed in a regional context. It is at this level that soil development is best understood because it is influenced by such large-scale processes as precipitation and temperature and erosional agents—including wind and surface water (e.g., rivers, marine shore waves, and sheet wash). Wood and Johnson (1978) have outlined a number of disturbance processes that affect archaeological deposits. Some of the more important processes include faunalturbation, or the mixing of deposits by animals, especially the burrowing kind (rodents, crabs, insects, earthworms, and birds). Digging burrows not only brings buried material to the surface, but open holes are traps for surface material to accumulate beneath the ground. The mixing of deposits by plants that occurs during root growth and decay is known as floralturbation. Uprooted trees can also bring material to the surface, often from a meter or more below the ground.

Discrete layers within archaeological sites can be homogenized by alternating freezing and thawing episodes which sort sediments by size. This is a problem with sites in the higher latitudes that consist of clay soils that swell and contract in response to precipitation and temperature. Gravity can cause mixing and movement of soil and rock debris downslope, as well as settling or subsidence of relatively large areas. Aeroturbation occurs when wind winnows fine sediments from deposits leaving a ‘pavement’ of artifacts. This is a significant problem in areas with active sand dune systems. Despite the range of disturbance processes that operate on a regional and site-specific basis, most cultural deposits maintain some order of integrity and, after the site formation processes are understood, valid interpretations are possible.

2.2 Human Disturbance Processes

In addition to natural or environmental site formation processes, people intentionally contribute to the formation of archaeological deposits by bringing to a site raw materials such as fine-grained rock to fashion into tools, finished artifacts of bone, shell, wood and stone, cooking fuel, building materials, and food. Many of these items are discarded on site as waste. Even sand grains adhering to feet, after countless thousands of trips to a site, can add significantly to the cultural matrix. Digging pits for earth ovens and trash disposal, small holes excavated for wall or fence posts, or trenches dug for building foundations can displace subsurface cultural deposits during later periods of occupation. Accurate interpretations of the archaeological record can only be made after understanding both noncultural and cultural site formation processes.

3. Definition Of Soils And Sediments

Soils and sediments figure prominently in discussions of site formation processes, so it is essential to know how these terms differ. Soils develop in situ at or near the ground surface through the various processes of weathering in conjunction with plant growth and decay, other biological elements, time, climate (temperature and moisture), and local topography. Sediments are particulate matter that has been transported from one location to another by one or more processes such as wind, water, or people.

3.1 Principles Of Sedimentation

All particles, including artifacts and food remains, found in archaeological deposits can be viewed as sediments (Stein 1987, p. 339) and, because of this, the four principles of sedimentation (Shackley 1975) apply equally to natural and cultural sediments. First, the source of sediments found at an archaeological site can be unconsolidated particles of bedrock as well as humanly-transported stone tools, shells, bones or building stones, to name but a few examples. Second, the transport history of sediments is usually determined by textural and compositional analysis in that biological agents (people, plants, and animals), wind, and water move sediments according to certain physical constraints. For example, wind normally transports evenly-sorted sand-sized sediments, while water—in the form of rivers, sea waves, and glaciers— can move anything from clay particles suspended in its volume to huge boulders that are pushed under enormous power. Third, the environment of deposition, in this example, is the archaeological site, but offsite examples can include sedimentary basins, aggrading shorelines, sand dunes, and till and eradict boulders deposited from melting glaciers. And last, post depositional alterations, discussed above, are caused by people, other animals, plants, and geologic erosional processes.

3.2 Sampling Soils And Sediments

Soils and sediment samples are usually taken from a profile in an erosional exposure or from the sidewall of an excavation unit or trench. Beginning from the bottom up, samples are taken from separate stratigraphic layers or in a continuous column. The sample volume varies according to the kinds of analyses undertaken and the size of sediments. For example, small samples of about 20 grams are needed to analyze the texture of clays and silts, while several kilograms are necessary for analyzing cultural content or, if sediments are gravel, to cobble size. The key is to collect a representative sample of the sediment.

3.3 Analytical Techniques

Among other things, particle size analysis permits the detection of the agent of deposition (e.g., wind, river, sea) and the environment of deposition such as beach, flood plain, and dune (Shackley 1975, p. 87). As mentioned above, well-sorted sand is often wind deposited and certain agents of deposition have particular sediment signatures. Mineral identification of individual grains can suggest a likely source of the sediments, especially if the regional geology is well known. Grain shape (roundness and flatness) can indicate the transport agent, depositional environment, and history. Grains can be, for example, platy or flat; angular, or granular. Roundness refers to the general grain surface curvature and is more a function of mineral composition (certain minerals are harder or softer than others and respond to friction differently), depositional history, and final depositional environment (Shackley 1975, p. 46). Grain surface textures, examined under high-power magnification, can be: unworn and angular, suggesting a fresh or minimally transported history; rounded and glossy, the result of transport by running water; or matt-surfaced grains, the result of wind transport. A good review of the analytical methods of sediment analysis can be found in Shackley (1975).

4. Palaeoenvironmental Reconstruction And Human Impact

The relationship of people to the landscape (and here I use ‘landscape’ to mean the physical and biotic environment) is of worldwide interest, not only to archaeologists, but to biologists and politicians alike, to name just a few. How humans have historically managed their resources—whether successfully or with tragic consequences—is of utmost importance to contemporary society—especially as the world becomes increasingly one ‘global village.’ Archaeology is better positioned than any other discipline to understand the historical details of human–landscape relationships over tens of thousands of years and across an enormously varied set of environmental conditions that posed opportunities and problems for our ancestors.

To document and understand the variation in human–landscape interactions, the analysis of soils and sediments is especially useful when used in conjunction with the differing distribution of floral remains (see Hastorf and Popper 1988 for a review of analytical methods). The analysis of faunal material is also of importance here and is addressed in Zooarchaeology. Whether of carbonized wood fragments, pollen, opal phytoliths (minute silica bodies formed in the cells of living plants that can be diagnostic of certain species), or leaves, plant remains are often indicative of vegetation on or near an archaeological site and, in the case of wind-dispersed pollen, can reveal regional floral patterns. Recovered from dated stratigraphic contexts, vegetation history can be charted over time—before, during, and after human settlement. In essence, archaeological sites hold an historical record of local and regional palaeoenvironmental change, often in response to human modifications to the landscape.

5. Physical Analysis Of Archaeological Materials

A growing area of interest in geoarchaeology is the elemental analysis of artifacts such as ceramics and stone tools so links can be drawn from the geological source or, in rare occasions, the site of production, to the place of deposition or archaeological site. The emphasis is on establishing lines of interaction or communication between or within social groups. The distribution of exotic artifacts across space and through time (that is, artifacts recovered from dated contexts or layers) is thought to reflect changing aspects of social organization—a topic of great interest to archaeologists.

5.1 Physical Analysis Of Ceramics And Stone Tools

Ceramics are manufactured from clay, water, and nonplastic inclusions (temper). Pots were often decorated with an exterior clay slip, incised or stamped with various intricate patterns. Of interest to interaction or ‘sourcing’ studies is the elemental composition of the clay and temper. If the latter consists of crushed rock or terrigenous (land-based) sands, then chemical analysis of the temper may isolate its source. Stone tools, manufactured from obsidian, basalt, chert, or other fine-grained rocks, also can be geochemically analyzed to determine the artifact’s geological source. Economically-important rock sources were generally well known in prehistory and had a limited distribution, thus facilitating sourcing studies.

There are many useful techniques for determining the elemental composition of ceramics and stone tools. (See Parkes 1986 for a description of various techniques.) These include, for example, x-ray fluorescence (XRF) and lead isotope analysis at the newer and more sophisticated end of the techniques (Weisler 1997), to thin-section petrography which has been used in geologic investigations for over a century.

5.2 X-Ray Fluorescence Analysis

Simply stated, XRF analysis consists of bombarding a specimen with primary x-rays. The sample emits fluorescent x-rays that are indicative of the kind and frequency of its elemental composition (oxides, such as iron or silica, are measured in weight percent, while elements are counted in parts-per-million or ppm). Dedicated computers are used for data analysis, while statistical clustering programs can aid in grouping artifacts with possible sources. Once sources are adequately characterized in terms of their geochemical variability, then unknowns or artifacts can be more confidently assigned to a specific source.

5.3 Thin-Section Petrography

Thin-section petrography is used extensively today in ceramic sourcing studies when the mineral identification of temper is desired. Samples measuring up to 25 mm by 40 mm are cut from ceramic sherds and ground thin enough to pass light through them. Placed under magnification, minerals (such as olivine and feldspar) are identified and texture measured. Within a geologically well-known region, temper composition can be assigned to a particular river drainage (in the example of placer sands), a marine beach, or a larger region such as a group of hills or a mountain range.

Geoarchaeology uses the techniques, methods, and concepts of the physical sciences to address archaeological questions. In this short article, I have only touched on a few of the techniques and methods applied to research domains that are of worldwide interest. This subdiscipline of archaeology is continually expanding the range of techniques and their applications to innovative questions. The successful melding of interdisciplinary research typical of geoarchaeological pursuits continues to have a bright future.

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