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Archaeology and Technology

New Develoments in Geophysical Prospecting and Archaeological Research: An Example from the Navan Complex, County Armagh, Northern Ireland

Daniel O. Larson and Elizabeth L. Ambos


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Introduction

In this article, we review recent advancements in the application of geophysical prospecting techniques to archaeological field research. We specifically discuss the results of cesium magnetometer, ground penetrating radar, and aerial photographic enhancement studies of the Navan Archaeological Complex, County Armagh, Northern Ireland (Figure 1). The use of new geophysical instruments and recently developed computer software programs will, in the near future, revolutionize the way archaeologists structure their survey and excavation work. We argue that the application of this new equipment, however, must go beyond simply being "new" or "neat" techniques generating better "descriptions" for archaeologists. The ultimate contribution of these improved technologies will be in our ability to advance cultural evolutionary theory. Our objective here is to describe the research context that we have developed for geophysical exploration and how it can be applied to the study of culture change in archaeology.

Figure 1
Figure 1

The history of science contains numerous examples of innovations in methods that have led to great discoveries and the advancement of theory. Theory is of primary importance to archaeology because it functions to generate hypotheses, or models of the empirical world that can be falsified by comparing the models with descriptions of phenomena. Philosophers of science and critical archaeologists argue that the development of archaeological theory is repressed throughout much of the world because of inadequate empirical data against which specific hypotheses and theories can be evaluated. It is difficult to oppose this position because even within the American Southwest, where generations of archaeologists have applied their trade at hundreds of sites, we are still groping with the empirical quest and the linkage of data with theory (D. O. Larson, H. Neff, D. Graybill, J. Michaelsen, and E. Ambos,1996, Risk, Climatic Variability, and the Study of Southwestern Prehistory: An Evolutionary Perspective. American Antiquity 61:217-242).

Is theory repressed because of limitations of our data, or is it because of our inability to collect data in an efficient manner? We suggest here that more often than not, it is due to the latter. We believe that recent developments in remote sensing methods, aerial photographic image enhancements, and geophysical prospecting, coupled with strategically designed archaeological field surveys and test excavation programs, can dramatically advance the development of archaeology as a science.

The scientific equipment we operate in the field and the computer hardware and software used in our studies represent the latest advances in geophysical prospecting and photographic enhancement methods. The quality and quantity of data collected and processed using these tools is impressive, and the Navan Archaeological Complex is an excellent natural laboratory for demonstrating the utility of geophysical and archaeological research (D. O. Larson, E. L. Ambos, and M. Conway, 1996, A Strategy for Archaeological Remote Sensing Investigations. Emania, in press).

Principles of Geophysical and Photographic Image Research

The value and application of geophysical and aerial photographic methods in archaeology have been amply discussed by several eminent scholars (e.g., J. W. Weymouth and R. Huggins, 1985, Geophysical Surveying of Archaeological Sites, in Archaeological Geology, edited by G. Rapp and J. A. Gifford, pp. 191-235, Yale University Press: New Haven). European archaeologists and geophysicists have developed and pioneered many of the scientific applications discussed here. We neither endeavor to review this volume of literature nor present details about the history and use of the equipment and methods we employed. Rather, our purpose in this short presentation is to discuss our specific approach, project, and results.

The literature for geophysical applications in archaeology incorporates a number of differing opinions regarding proper applications of geophysical methods in archaeology. We argue that three principles should guide geophysical research in archaeology. First, it is prudent to apply multiple geophysical techniques in the field so that the particular strengths of each method are incorporated in the study. The use of magnetometers and ground penetrating radar are especially suitable in this regard. Second, the geophysical projects should be undertaken by archaeologists and geophysicists working in close coordination during the planning, implementation, and interpretive phase of the research program. Third, geophysical interpretations should be followed by field-testing procedures to "ground truth" the discoveries and any geophysical anomalies. Whether in research or cultural resource preservation work, geophysical interpretation should not be the sole basis for informed decision making; the element of proof must be based on field excavations in archaeology.

Geophysical and aerial photographic surveys are often conducted for the purpose of archaeological description. This may be appropriate for some preservation projects; however, it is our position that researchers should continuously strive to go beyond the presentation of graphics and site reports. Our research approach is driven by a set of specific research questions concerning culture change during the Bronze and Iron ages.

Navan Complex Research

The excellent investigative efforts of the Navan Research Group have made it increasingly clear that the legendary prehistoric capital of Ulster at Navan, in County Armagh, Northern Ireland, evidences a sizable archaeological complex (C. J. Lynn, 1992, The Iron Age Mound in Navan Fort: A Physical Realization of Celtic Religious Beliefs? Emania 10:33-57). Indeed, previous research has identified more than 40 archaeological sites that suggest that the Navan Complex is similar to other major ritual complexes in Ireland, such as Tara and Rathcroghan. Our specific research interest is focused on the Navan Complex during the Bronze and Iron ages. We are particularly curious about the economic and demographic change that occurred during the transition between these two periods, 400 B.C. and A.D. 200.

Within the Navan complex, the two prominent sites are Navan Fort and Haughey's Fort, Late Bronze and Iron Age hillforts. Both sites are more than 300 m across and contain multiple features and subsurface structures (Figure 2). They are situated 1 km apart and are a rarity on the prehistoric landscape of Northern Ireland. Recent excavations in the Navan region have been concentrated in the interior of the Navan enclosure and ceremonial structure, at Haughey's Fort, and at the King's Stables, and it is from this research that most of our cultural and chronological inferences can be drawn.

Figure 2
Figure 2

It is at the end of the Middle Bronze Age that we begin to see the Navan complex taking real shape. Extant archaeological evidence for the Navan complex shows that during the Bronze and Iron ages there is increased intensification of agricultural production, greater emphasis on livestock herding, community aggregation, stratified social systems, complex exchange networks, and construction of large ceremonial structures (40 m across) and large public works. One such significant feature is the Black Pig's Dyke, which is a double ditch more than 5 m deep that winds its way across the landscape for more than 40 km. The labor required to construct this feature was extraordinary indeed. Both Navan Fort and Haughey's Fort produced evidence for intensive occupation, with gold, bronze, and iron artifacts. The existence of exceptionally large dogs and cattle and rare exotic trade items (an ape skull from Northern Africa) all imply a high status for the occupants. Interestingly, the Black Pig's Dyke and the Navan Fort ceremonial structure have both been dated to 95 B.C. by means of dendrochronology (R. B. Warner, 1994, The Navan Archaeological Complex: A Summary, in Ulidia: Proceedings of the First International Conference of the Ulster Cycle of Tales, edited by J. P. Mallory and G. Stockman, pp. 165-170).

Several researchers have emphasized the need to examine diachronic and synchronic variation in human behavior during the Bronze and Iron ages. Toward this end, our collaborative research specifically seeks to answer the following questions:

Research Framework

To answer these questions, we and our colleagues from Northern Ireland have begun the process of structuring a long-term research program that incorporates several major geophysical and archaeological study components. The relationship of these components is graphically illustrated in Figure 3. Archaeological theory is the cornerstone of any research design because the way archaeologists study the past is always based on implicit or explicit theoretical constructs. We pose problems, we choose units of archaeological observation, we sample the archaeological record, and we structure our data collection strategies in a deliberate manner. A number of archaeologists have discussed the value of selectionist theory in archaeology (see R. C. Dunnell, 1996, Foreword. In Evolutionary Archaeology: Theory and Application, edited by M. J. O'Brien, pp. vii-xv. Unversity of Utah Press, Salt Lake City.; M. B. Schiffer, 1996, Some Relationships between Behavioral and Evolutionary Archaeologies. American Antiquity 61:643-662). This theoretical focus requires that we collect data relevant to the study of variation in architectural and technological components of Bronze and Iron Age society. It is assumed that cultural evolution is a two-step process involving the generation of variation and its subsequent differential persistence. Clearly, geophysical prospecting and well-targeted excavations of units of empirical interest will greatly assist in the study of variation and evolution. The greatest advantage associated with geophysical prospecting is that the results can often be used to design archaeological data collection strategies in the field. Once they have been detected in geophysical surveys, sample data can be collected from specific features such as housefloors, hearths, metalwork areas, and ceremonial structures. Thus, the sample unit is the targeted features rather than some arbitrary notion of excavated space. As a result of this research process, the areas determined to be high-probability loci for archaeological material and environmental data are intensively investigated in the field using high-resolution geophysical methods and small incremental bore samples, column samples, and test excavations. All items collected can then be subjected to various material analyses that ultimately feed back into the research program design to explain the causal mechanisms of cultural evolutionary processes. In the following sections we discuss our specific approach, which uses aerial photographic images and geophysical prospecting methods, and our results.

Figure 3
Figure 3

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Aerial Photographic Inventory

The aerial photographic inventory is specifically designed to generate data on diachronic and synchronic changes in settlement location, organization, and variation in settlement type. Three photographs produced in 1972, 1982, and 1992 were provided by the Environmental Service in Northern Ireland and scanned using high-resolution equipment at California State University at Long Beach. In addition, a variety of computer software programs residing on powerful Macintosh and mainframe computers at the university were used to filter and enhance these images. We examined in detail each image on high-resolution computer color monitors (maximum resolution 1152 x 870).

In Northern Ireland, archaeological features show up as positive or negative markings in many agricultural fields. Linear/circular features, ring forts, and mounds are sometimes visible on the raw aerial photographs, but with new enhancement techniques, exceptional detail in archaeological features can be achieved. In other cases, archaeological features that are completely invisible on raw aerial photographs become apparent after the image has been filtered and enhanced. We incorporated several exploratory steps of filtering, smoothing, and re-expression of aerial photographic image data. This type of photographic enhancement coupled with statistical and visual evaluation often produces unanticipated results. We found that by filtering photographic data, a number of subtle archaeological features, not previously identified, were present within the Navan enclosure and along the slope of Haughey's Fort. Interestingly, we found that certain types of filters and image-enhancement methods were successful for different kinds of archaeological anomalies. We also noted that although image quality varied from excellent to poor, each of the three photographs offered evidence of archaeological significance not found in the other two. Multiple factors, including moisture content of the soil, crop rotation, season, film type, exposure level, and time of day for the photograph, all contributed to these differences.

We have found that there is no one correct method of aerial photographic enhancement; the analyst achieves the best results through long hours of visual exploration using a variety of techniques. The greatest advantages a researcher possesses are knowledge of enhancement procedures, patience, creativity, and sometimes luck. The power of modern computers and access to multiple software programs provide contemporary researchers with efficient means to perform mathematical and visual explorations that allow for the detection of previously unrecognized archaeological anomalies.

Results of Aerial Photograph Enhancements

Previous excavations at Navan Fort have revealed the existence of many subsurface archaeological features including Sites A and B (A. Selkirk and D. Waterman, 1970, Navan Fort, Current Archaeology, 22:304-308). Our careful reexamination of Navan Fort aerial photographs, including enhancements of three vertical images, documents the possible existence of the large ring feature between Sites A and B (Figure 4). The geophysical surveys (cesium magnetometer and ground penetrating radar) at this location, described in detail below, strongly support this conclusion. In addition, a linear feature detected west of Site B may be the prehistoric roadway into the Navan enclosure (Figure 4). It should be noted that Figure 4 is compromised by the reproduction limitations of this printed media.

Figure 4
Figure 4

At Haughey's Fort, two outer ditches have been previously identified by researchers using aerial photographs. Figure 5 is an enhanced image in which these features have been traced into portions of the field in which they had not previously been detected. Thus, the enhancement methods that we employed delineate the pattern of structures in a manner that goes well beyond simple visual examination of the photographs and offers archaeologists new and significant data on archaeological structures in the Navan region. Most importantly, the detection of these ditches provides a basis for the selection of specific areas for well-targeted cesium magnetometer surveys and later test excavations. The next objective of this project is to retrieve datable materials from several features so that their chronological context can be determined. This will be accomplished with relatively small-scale and well-targeted test excavations.

Figure 5
Figure 5

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Geophysical Survey Background Information

In 1994 and 1995 we conducted cesium vapor and proton precession magnetometer and ground penetrating radar (GPR) surveys at Navan Fort and Haughey's Fort. Geophysical data were collected along transects within 20-x-20-m grids. The cesium vapor magnetometer data were collected in 20-m-long parallel transects spaced .5 m apart. Ground penetrating radar and proton precession magnetometer data were collected in 20-m-long parallel transects spaced 1 m apart.

Several archaeological items and features were expected to be revealed by the magnetometer surveys, including metal artifacts, hearths, ceramic and metal production areas, burnt timber posts from residential features, ceremonial buildings, and hill fort palisades. We know that burnt materials have a particularly high magnetism because of the acquisition of thermoremanent magnetization (see W. M. Telford, J. P. Geldart, and R. E. Sheriff, 1990, Applied Geophysics, 2nd ed. Cambridge University Press: London). Positive magnetization may also occur in organic-rich materials, such as midden deposits, as these conditions may cause authigenic magnetic mineral growth (see I. D. G. Graham and I. Scollar, 1976, Limitations on Magnetic Prospection in Archaeology Imposed by Soil Properties, Archaeo-Physika. 6:1-125). Thus, we can expect ditch fill, agricultural settlements, and perhaps burial areas to be marked by small amplitude magnetic anomalies.

Although GPR and magnetometer data are complementary, they are by no means completely interchangeable. Because each method is based on measurement variation of different physical properties, appreciable differences in response often occur. For example, rock walls or tree roots in the subsurface may generate strong electromagnetic wave reflections that are clearly observable in the GPR record. Such subsurface features, however, might not register with the magnetometer. Conversely, a buried iron artifact 6 cm in length, buried .5 m below the ground surface, may result in a high-amplitude magnetic anomaly measurable over several meters in diameter. Unless the GPR instrument directly passes over the iron artifact, however, very little evidence of the artifact would be recorded by that instrument. The research results reported here demonstrate the complementary nature of these instruments and the value of using multiple geophysical techniques.

It should be stressed that the geophysical data produced by the GPR and magnetometer surveys can also inform the researcher about postdepositional processes. Geophysical survey data can identify areas that have been significantly disturbed by erosion, agricultural activities, and other natural and cultural factors. In the absence of the geophysical survey data, these processes may not be visible and therefore not considered as a potential bias in the archaeological record.

Magnetometer Surveys

We employed the EG+G Geometrics 856 proton precession magnetometer in the 1994 study and the EG+G Geometrics 858 cesium vapor magnetometer in the 1995 study. Both magnetometers measure the rates of change of certain atomic structures in the presence of a superimposed magnetic field (see H. R. Burger, 1992, Exploration Geophysics of the Shallow Subsurface. Prentice-Hall: New York, for information concerning the operating principles of these instruments). The primary difference between the proton precession and cesium vapor magnetometers is in rates of data acquisition and measurement precision. Using the single stable proton precession magnetometer, each measurement takes several minutes to acquire, whereas the cesium vapor magnetometer generates 10 measurements every second. Measurement speed and other factors associated with the use of the cesium magnetometer (equipment set up, data logging, and reduced number of crew), allow for both a significantly greater number of magnetic measurements (over 8,100 measurements per 20-x-20-m unit with transect intervals of .5 m) and greater areal coverage per field day. Under exceptional conditions, a 20-x-20-m survey unit can be completed in just one hour.

The cesium vapor magnetometer was also engineered to achieve a considerably more precise measurement than the proton precession magnetometer. The total magnetic field can generally be measured by a cesium vapor magnetometer to a precision of .05 nT (nanoteslas or gammas), while the proton precession magnetometer measurements attain a precision of less than .5-1 nT. The greater precision of the cesium vapor magnetometer is highly desirable in the geophysical studies of archaeological sites. In short, the anomalies in the earth's magnetic field that evidence archaeological features are often subtle, with amplitudes on the order of 5 to 10 nT, or approximately .01% of the earth's field. (J. W. Weymouth, 1986, Archaeological Site Surveying Program at the University of Nebraska, Geophysics. 51:538-552). The high density of magnetometer readings and the sensitivity of the instrument allow research to identify many archaeological features that would have been missed by proton magnetometer surveys.

The EG+G Geometrics 858 system includes an electronic console, carrying belt and shoulder straps, and a hand-held counterbalance staff with a mounted sensor (Figure 6). The console contains electronics to acquire magnetic field data and a LCD screen that displays the magnetic data, position, and information mapped during field operations. The console also emanates audible tones indicating magnetic field changes and survey cadence (pace), allowing the operator to survey in a "head up" mode. Data are stored in non-volatile RAM (250,000 compressed magnetic readings and associated positions and times) for playback and editing in the field. When the field survey is complete, data are quickly downloaded to a processing computer for statistical assessment, exploratory data analysis, filtering, and visual processing. Positional information can be derived from a connected GPS with better than 1-m accuracy or from regularly spaced fiducial marks (grid system) preset by the operator. Included with the system is a comprehensive software package to download, perform diurnal corrections, edit, interpolate and convert magnetic data into 2D and 3D contour reading format. The actual statistical analyses, exploratory data analysis, and visual processing are all conducted using third-party software.

Figure 6
Figure 6

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Results of the Cesium Vapor Magnetometer Survey

In this section, we limit our presentation to two geophysical survey areas: a 40-x-40-m section between Sites A and B at Navan Fort and the upper ditch identified in the aerial photographic survey at Haughey's Fort (Figures 4 and 5). A significant result of the cesium vapor magnetometer surveys was the production of a high-resolution image of a double-ring structure, approximately 30 m in diameter, centered between Sites A and B (Figure 7). The new double-ring structure was not previously identified from past excavation work, but it is visible in the aerial photograph enhancement reported above. The magnetic anomaly that cuts diagonally through the double-ring feature is a historic field boundary, and it is the only feature visible on the ground surface when one walks over the site ( K. Kvamme, 1996, A Proton Magnetometry Survey at Navan Fort, Emania, in press). The magnetic dipole features (paired positive and negative anomalies), or dark areas in Figure 7, may well reflect the position of structural timber. That is, the dipole structures appear as "bull's eyes" within the double ring with dimensions of less than 1 m (Figure 8). We suspect that this magnetic anomaly may evidence a burnt posthole (similar features were noted for the Waterman excavations). Targets like this are extremely important for obtaining dendrochronological and radiocarbon samples. We also detected a possible superimposed structure in the northwest corner of the double-ring structure in our enhanced computer screen image (Figure 8). Lastly, we found what appears to be several interior, circular features of some sort (Figures 7 and 8).

Figure 7
Figure 7


Figure 8a Figure 8b
Figure 8

At Haughey's Fort, the precise location of the outer ditch was discovered. The location of the cesium magnetometer survey was determined on the basis of the enhanced aerial photographs, as discussed previously. A 20-x-20-m grid was surveyed with the geophysical equipment, and as a result, a deep linear structure approximately 15 m wide was detected (Figure 9). Irish scholars are particularly interested in the study of material derived from these features because of the extraordinary preservation of archaeological materials and environmental data in deep ditches (i.e., pollen, plants, insects, and dendrochronological specimens).

Figure 9
Figure 9

It should be emphasized that although the fieldwork for the Navan magnetic surveys was rapid (we completed thirty 20-x-20-m units in 12 days), the computer processing of data including exploratory data analysis and visual data analysis is time consuming (we estimated 5 days per 20-x-20-m unit). Accumulated data are assessed using a number of statistical and visual display programs. Our purpose then is to detect the patterns and outliers in the raw magnetic data and in the reexpressed magnetic data. In this process, various filters (e.g., gaussian, Fourier transform, low pass) are applied to data, and multiple images are closely examined on high-resolution computer screens for subtle anomalies that may shed light on interesting targets of past Bronze and Iron Age behavior.

Without question, traditional methods of archaeological fieldwork would have required decades to detect the complete ring feature and ditch described above. A significant advantage of cesium magnetometer work is that it allows the archaeologist to place test excavation units directly over the archaeological features that interest them. Indeed, test excavations are presently being conducted by our Irish colleagues to "ground truth" these discoveries at Haughey's Fort. We can also retrieve artifacts from areas of known concentration such as along slots, ditches, or within structures and specific features. Cesium magnetometer studies may be particularly helpful in the retrieval of metal artifacts that were thrown into wet environments such as bogs, perhaps as symbolic sacrifice during the Bronze and Iron ages.

Ground Penetrating Radar (GPR) Method

The ground penetrating radar (GPR) method was used at the Navan Fort site in the summer of 1994. The GPR data corroborate the magnetometer evidence for the existence of the new ring between Sites A and B. Although the GPR data will be discussed in detail in an upcoming Bulletin article, we present here a brief overview of the instrument design, field operation, data analysis, and reduction procedures.

GPR involves the transmission of electromagnetic wave energy into the ground from a broad-band antenna source. The source is put in contact with the ground surface and then the source antenna is pulled along the 20-m profile. A single two-dimensional profiler record is obtained for each 20-m-long line. As the GPR energy penetrates subsurface materials, some of it will be absorbed and some will reflect back to the surface. Reflected energy can be recorded by a receiving antenna and their signals stored on tape media for later examination and processing. Reflections occur where abrupt changes in subsurface materials take place. Transitions from dry to wet soil, soil to bedrock, and soil to buried walls or floors often generate marked reflections events. In the Navan Fort region, prominent reflections may be expected between soil and limestone bedrock and between loose and packed soils that characterize the floors of ring structures.

We chose to use a GSSI SIR-10 GPR instrument for our data collection and the GSSI Radan and Fortner Research Spyglass software for processing. After GPR reflection data have been recorded on tape media, they are ready for processing. The steps we employed included filtering and data compression and then interpolation of the individual two-dimensional data profiles into a three-dimensional data volume.

Summary Discussion and Future Work

According to Albert Spaulding, the "scientific method is concerned with the process to follow once the question has been asked." The research context that we have designed for Northern Ireland is fundamentally based on this premise. The application of geophysical methods is just one part of the scientific endeavor in archaeology--but we feel it can and should be an important part. In this report, we have discussed our approach to geophysical and archaeological research in the Navan Complex. Our research demonstrates that archaeologists can investigate intra- and intersite settlement patterns with a high degree of resolution, yet with a relatively small expenditure of time and labor when compared with traditional methods. Archaeologists and geophysicists, however, must be willing to invest a considerable effort to learn about each other's disciplines to successfully implement a collaborative program of this kind.

The most important point of our presentation is that this new technology offers scholars a unique opportunity to execute well-developed empirical studies designed to test and evaluate explicit theoretical frameworks. The level of efficiency for targeting archaeological features of particular interest is unprecedented in the history of archaeology (Figure 10). We believe that this coupling of research strategies (geophysical exploration and archaeological field excavations) will, in the not too distant future, significantly contribute to the advancement of archaeology as a science. This is a critical point because at present there are over 100 different general theories of cultural evolution advocated by archaeologists. Scholars argue that this condition exists because we are in the early developmental stages of our discipline. For archaeology to grow intellectually, we must begin the process of reducing the number of competing theories. We expect that part of this process will include the effective use of geophysical equipment in the design of excavation programs.

Figure 10
Figure 10

Training of future archaeologists in these advanced methods of geophysical exploration and aerial photographic enhancement techniques will be of tremendous value to the profession. Clearly, these technologies will continue to be developed by engineers at a tremendous pace, and those academic programs in archaeology that offer training in geophysical methods coupled with a strong commitment to develop and scientifically test and evaluate evolutionary theory will offer graduate students a competitive edge in an increasingly competitive field. Where possible, university-based archaeologists and geologists can develop joint proposals for purchasing geophysical equipment. In addition, courses can be cross-listed in each department, thereby increasing enrollment and linking courses for a focused curriculum. Our experience shows that university administrators are particularly receptive to this kind of approach.

Finally, our work with colleagues in Northern Ireland and Southern California shows that archaeologists and cultural resource managers can plan for the development of particular areas without extensive excavation, thereby preserving archaeological sites and cultural heritage values in perpetuity. This is an issue that is becoming increasingly important to cultural groups around the world who are interested in preserving their cultural heritage.

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Acknowledgments

This research would not have been possible without the help and support of the following institutions and individuals in Northern Ireland: Ann Hamlin and Chris Lynn of Environmental Services; James Mallory and Michael Baillie of the Department of Paleoecology and Archaeology at Queen's University, Belfast; David Weir of English Heritage; Richard Warner of the Ulster Museum; Danny Sutherland, Anne Hart, James McKee, Jim Finnegan, Stephen Gilmore, Tommy McCreesh and many other wonderful individuals at the Navan Research Centre. We are especially grateful to Jim Mallory and Chris Lynn who have encouraged us and advised us at all points during this research. The British Council supported our first visit to Northern Ireland and we acknowledge the interest of Peter Lyner, director, and the unfailing support of Carmel McGill, cultural exchanges officer. At the California State University at Long Beach, we received support and strong encouragement from President Robert Maxson, Karl Anatol, Keith Polakoff, James Jensen, Stanley Finney, Toni Beron, and Gloria Carver, and were assisted by various archaeology students the fall of 1995 in processing the aerial photos. At the Jet Propulsion Laboratory, we received guidance in imaging data volumes and translating data formats from Michael Martin and his colleagues, and unwavering support from Fred Shair. Finally, we thank the students of California State University, Long Beach, for their support through the Instructionally Related Activities Program and Mark Aldenderfer and John Kantner for their help with this article.

Daniel O. Larson is in the Department of Anthropology and Elizabeth L. Ambos is the Department of Geological Sciences at California State University, Long Beach.


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