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Detecting Palaeolithic Activity Areas Through Electrical Resistivity Survey: An Assessment from Vale de Óbidos, Portugal
Journal of Archaeological Science (2002) 29, 563–570 doi:10.1006/jasc.2001.0691, available online at http://www.idealibrary.com on
Detecting Palaeolithic Activity Areas Through Electrical Resistivity Survey: An Assessment from Vale de O u bidos, Portugal Paul T. Thacker and Brooks B. Ellwood Department of Geology and Geophysics, E 235 Howe-Russell, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A.
Carlos M. C. Pereira Carta Arqueolo´gica de Rio Maior, Caˆmara Municipal de Rio Maior, 2040 Rio Maior, Portugal (Received 20 February 2000, revised manuscript accepted 3 April 2001) The open air Late Pleistocene campsite of Vale de O u bidos provided an opportunity to systematically assess the utility of electrical resistivity surveying for activity area detection at Palaeolithic sites. Detailed surface mapping facilitated the exploration of electrical resistivity effects caused by recent land use activities and post depositional processes. At Vale de O u bidos, tree throw events, ploughing, and earth moving have a more significant impact on soil resistivity than increases in soil moisture associated with pine tree stumps and roots. Resistivity results successfully discriminate activity areas containing hearth features and high densities of fire cracked rocks and artifacts. As an aid for understanding post-depositional processes at archaeological sites and to design excavation and recovery efforts, the technique has great potential for Palaeolithic archaeology and in other prehistoric contexts that lack major architectural remains. 2002 Elsevier Science Ltd. All rights reserved.
Keywords: PALAEOLITHIC, ELECTRICAL RESISTIVITY, GEOPHYSICAL SURVEY, SITE DETECTION, PORTUGAL.
nated areas of relative artifact density and a near surface large limestone block resulting from roof fall (Ellwood et al., 1993). In both Poland and Hungary, resistivity data have been used to trace the location of flint mine shafts that date to at least the Neolithic (with some shafts possibly begun in the terminal Pleistocene) (Herbich, 1993). At Konispol cave in Albania, the depth of cave deposits was determined through resistivity survey, and a large hearth feature was detected (Ellwood et al., 1993). In an open air context, Almeida used large area electrical remote sensing, including resistivity, to map the Late Pleistocene geology of open air landforms associated with rock art in the Coˆa valley of northeastern Portugal (1997). Resistivity results were reported from the large stratified Upper Palaeolithic site of Cabec¸o de Porto Marinho in central Portugal, but the degree to which electrical anomalies registered palimpsest effects and overlying later prehistoric levels has yet to be determined (Ellwood, Harrold & Marks, 1994). These preliminary reports indicated the promise of electrical resistivity surveys at appropriate Late Pleistocene sites. The Upper Palaeolithic open air site of Vale de O u bidos provided the opportunity to
Introduction ince the late 1950s, electrical resistivity remote sensing has been employed at archaeological sites containing relatively large architectural features such as masonry walls, paved foundations, or ditch construction (Weymouth, 1986). These archaeological situations are ideal for geophysical prospection methods since the targeted features are predictably regular in form, large in size, and usually contrast sharply with the background signal of the surrounding soil matrix (Carr, 1982). As a result, historic, protohistoric, and late prehistoric contexts account for most of the successful electrical resistivity surveys in archaeology. Electrical resistivity rarely is incorporated into research designs at early prehistoric sites, such as the Palaeolithic/Mesolithic periods in the Old World and Palaeoindian/Archaic periods in the Americas, as major architectural remains are scarce in the archaeological record of hunter–gatherers. When coupled with appropriate geological investigation, electrical resistivity surveys have been successful in a few Palaeolithic contexts. Resistivity survey results at the Pont d’Ambon II rockshelter in France discrimi-
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systematically assess the technique’s utility beyond that reported from any previous study. Results presented here demonstrate that geophysical site surveys can inform Palaeolithic site excavation and recovery techniques and also reflect critical post-depositional processes important for assessing the spatial integrity of such archaeological deposits.
The Palaeolithic Archaeological Record and Electrical Resistivity Methods Because of its great age, the archaeological record of the Pleistocene presents greater potential problems for geophysical prospection than more recent counterparts. For thousands of years, the majority of Palaeolithic sites have been subjected to numerous post-depositional geological processes. Both in caves and open air sites, burial depth, low contrast feature density relative to soil matrix, and underlying/ containing geological materials can limit the effectiveness of electrical resistivity remote sensing. In addition, geological processes such as pedogenesis and erosion badly alter or destroy sites and site contexts. Most Palaeolithic archaeological features are small in component size. For example, hearths are usually smaller than 2 m in size, and remains of constructed shelter walls are rarely wider than a metre (Gamble, 1997, 2000; Kolen, 1999). For electrical resistivity to detect features of this size, electrode spacing must be narrow, usually a metre or less, and roughly the size of the target diameter (Darwin, Ferring & Ellwood, 1990). Since measured resistance is averaged across the volume bracketed by the electrode array, a wider spacing usually results in a smoothing and integration of the feature signal with that of the adjacent soil (Weymouth, 1986). The depth of current penetration is determined by electrode spacing, such that in a Wenner array, measurement depth is roughly equal to the electrode spacing interval. Thus the small size of most Palaeolithic features necessarily constrains the application of electrical resistivity methods to sites buried within a few metres of the present surface. Archaeological application of electrical resistivity prospection relies on detecting the variation in resistance between a site’s sediment matrix and cultural features. Many locations favouring preservation of the Pleistocene archaeological record are inherently poor situations for electrical resistivity surveys. For example, many limestone cave sites contain levels that are heavily brecciated and cemented. In this context, generally smaller stone artifacts consolidated within a limestone breccia will rarely provide adequate resistance contrast with adjacent areas lacking artifacts. In similar fashion, apparent resistivity is significantly influenced by the matrix characteristics (or lithology) and relative position of underlying strata. In lithostratigraphic cases where a low density archaeological level is underlain by an undulating or irregular surface,
such as an eroded limestone bedrock, the resistance variation will most likely reflect the site’s geology rather than archaeology (Rapp & Hill, 1998). None of these phenomena negate the benefits of employing electrical resistivity surveys at Palaeolithic sites as long as the complicated nature of electrical resistivity data are understood. Resistivity measured during most archaeological survey applications is a composite of multiple effects created by site geology, surface disturbances, post-depositional processes, and subsurface archaeological variability.
Vale de O u bidos: An Early Upper Palaeolithic Open-Air Campsite Vale de O u bidos was discovered in 1994 following a forest fire on the ridge separating the Rio Maior and Jaleca drainages. A temporary access road (through an existing roadside construction borrow pit) was bulldozed for trucks conducting logging and clearing activities after the fire. A small section of the site was destroyed as sediments were pushed into the borrow pit to bank the access road. Displaced Palaeolithic artifacts on the surface of the access road led to the discovery of the site. In 1999, a cooperative archaeological project led by Thacker and Pereira began to investigate the in situ deposits and Upper Palaeolithic occupation of the hillslope. The relatively intact Upper Palaeolithic level at Vale de O u bidos is buried by about 75 cm of loose, medium sands. Unlike much of the landscape near Rio Maior, the hillslope has not been deep-ploughed for eucalyptus forestry. Intact and in situ pine stumps over much of the site area indicated the potential for preserved archaeological spatial patterning. A laser transit was used to map surface features and to establish the survey and excavation grid (Figure 1).
Field Methodology Electrical resistivity instruments introduce a current into the ground using two electrodes and measure the voltages developed between a second set of electrodes. Of many possible electrode array configurations, the evenly spaced Wenner array is commonly employed in archaeology especially when using instruments with a rotary switch (Aitken, 1974; Darwin, Ferring & Ellwood, 1990). The Wenner array and rotary switch allow the array to be reconfigured by moving only one electrode, which significantly increases data acquisition speed. An electrode spacing of one and a half metres was necessary at Vale de O u bidos to provide enough depth penetration to register features in the buried archaeological level. Due to its performance in high resistance situations, a Williams type resistivity device was used for field
Detecting Palaeolithic Activity Areas Through Electrical Resistivity Survey 565
North 99.4
Bulldozed access road Shallow plowing
Borrow Pit 0
10
Probe array Pine stump Tree throw 100.6
Excavated area Scale: 1 square metre
Figure 1. Surface map of Vale de O u bidos, Portugal.
measurement (Williams, 1984). Resistance is directly measured using this device, and the apparent resistivity calculated for each measurement using the function, a =(2)R(d)
(1)
where d is the distance between each electrode and a is in m. Apparent resistivity was predictably very high at Vale de O u bidos given the loose sand soil with low moisture content (Herz & Garrison, 1998). The centre point of each reading relative to mapped surface features and the excavation grid at Vale de O u bidos is shown in Figure 1. Variation in soil resistivity across space (Figure 2) is the cumulative result of numerous effects, only some of which are cultural in origin. In order to understand the cause of high and low anomalies, the extent of recent landuse activities and surface disturbances across the site was investigated. This mapping provided a control measure of recent, non-prehistoric effects that may obscure subsurface variation in apparent resistivity. Once understood, these non-archaeological signals can be filtered from the resistivity map, and this improves the detection of buried cultural features.
Excavation Research Design In order to systematically test the electrical survey results, preliminary excavation units at Vale de O u bidos were placed in several high and low apparent resistivity zones. By excavating a sample of units that cover the range of resistivity variability rather than focusing on the high contrast anomalies, the Vale de O u bidos project circumvents the analytical problem of confirmation common in many geoarchaeological resistivity surveys. Over 100 square metres were excavated revealing several well-defined Palaeolithic activity areas. Recovered artifacts include flaked stone tools, red ochre fragments, several stone-lined hearth features, and numerous large fire-cracked rock fragments (probably used for stone boiling). Organic preservation is predictably poor in the well drained sands, as the Early Upper Palaeolithic level yielded only a few very small fragments of bone and charcoal. The lithic artifact assemblage from Vale de O u bidos is technologically and typologically Gravettian (Marks et al., 1994; Zilha˜o, 1997). Distinctive thin blades and blade cores, as well as backed pieces, dihedral burins, microgravettes, and pointed backed bladelets cluster in
566
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65.00
Excavation Grid North
60.00
55.00
50.00
45.00
40.00
40.00
45.00 Excavation Grid East
50.00
Figure 2. Contour map of apparent resistivity (in m) at Vale de O u bidos. Excavated areas are indicated with white outlines. Dashed white lines indicate zones of high flaked stone artifact/fire cracked rock density. Identifiable anomalies include: (a) shallow ploughing of eucalyptus plot; (b) disturbance effects associated with bulldozing of access road; (c) tree throw disturbance; (d) effects associated with pine stumps and roots; (e) location of hearth feature pictured in Figure 3; (f) additional hearth features; (g) chert workshop activity area.
an area interpreted as a chert workshop where composite weaponry was manufactured. Thick flake scrapers and denticulates on quartz and quartzite dominate the tool assemblages in the activity areas
surrounding several stone-lined hearths. No radiocarbon dates are currently available for the occupation level as recovered charcoal fragments are under analysis for species identification prior to dating.
Detecting Palaeolithic Activity Areas Through Electrical Resistivity Survey 567
Geological studies into site burial processes indicate that the Early Upper Palaeolithic occupation level was buried by aeolian sands, and that no large pebbles or cobbles occur naturally in the vicinity of the site. No significant variation in sediment texture, compaction, or inclusions was found, indicating the ideal circumstance (for resistivity survey) of a homogenous subsurface matrix with the exception of cultural artifact and manuport deposits.
Correlating Features and Resistivity Survey Results The point biserial coefficient method is the most appropriate technique for testing correlations between apparent resistivity measurements, recent land use phenomena, and subsurface archaeological remains at Vale de O u bidos. Point biserial correlation examines the relationship between one variable measured on a continuous scale and another variable that can be characterized by a categorical dichotomy (Hammond & McCullagh, 1974). In the Vale de O u bidos case, either the resistance or the calculated apparent resistivity measurement at each grid point can be considered the continuous variable. Resistance values are more appropriate for point biserial statistics, however, as transformed apparent resistivity values increase the range of values by nearly a factor of 10, subsequently enlarging the standard deviation which can be problematic when assessing smaller sample sets (Shaw & Wheeler, 1994). For all of the statistical tests reported below, the point biserial coefficient was calculated using both resistance and apparent resistivity values. In each case, similar levels of significance and conclusions result. Two different dichotomies characterize the categorical axes reported below. In analysing surface feature contributions to resistance, locations either exhibit a feature potentially influencing soil resistance, such as ploughing or a tree throw, or the locations lack such features. Through the point biserial correlation coefficient, resistance values measured while each feature was within the electrode array are compared against the background variability of the hillslope in locations without such features. A second dichotomy, high and low artifact density, assesses resistance measurements as an indicator of subsurface archaeological deposits. Only excavated areas of the site, 70 measurement locations, were included in this latter statistical test. The point biserial coefficient requires the calculation of the means of each categorical subgroup and the standard deviation of the combined subgroups, and is given by the following equation:
where: N=total number of observations Np =number of observations in the first group of values
Nq =number of observations in the other group of values Mp =mean of the first group of values Mq =mean of the other group of values sy =the standard deviation of all the data The significance of the coefficient (which varies from +1 to 1) can be determined employing the following t-test:
The tailedness of the test will depend on the null hypothesis being tested. In the Vale de O u bidos case, surface or subsurface features were analysed in subgroups such that the anticipated effect should be consistent (that is increasing or decreasing resistance, but not both). Thus a single-tailed test statistic is warranted.
Soil Resistance and Post-Depositional Processes The surface map of the Vale de O u bidos hillslope that was generated with a laser transit prior to geological and archaeological investigations provides spatial control of recent disturbance events. Tree throw feature, pine stump (and related roots), ploughing, and earthmoving locations are all displayed in Figure 1. The remaining set of locations lacking such features (a total of 155 measurement points) is considered representative of subsurface resistance variation in the statistical comparison. Each subset of surface feature (single pine root, multiple pine stumps, tree throws, ploughing and earth-moving zones, and combination disturbances) was analysed independently against this background variability generated by the subsurface matrix. As Table 1 displays, some disturbance features do impact the resistance measures at Vale de O u bidos. While it is obvious in Figure 2 (features labelled ‘‘d’’) that pine stumps/roots can influence resistivity, Table 1 demonstrates that a significant reduction in electrical resistance does not always occur, even when multiple pine roots are situated within the electrode array. This lack of correlation may be due to the spatial positioning of the stump/roots relative to the electrode array, or it may reflect variability in the subsurface shape, extent, and relative moisture content of individual root masses. Physical disturbance of the sand sediments has the greatest impact on electrical resistance at the site. Areas that underwent earth moving such as the ploughed zone on the western edge of the site and the bulldozed access road display significantly lower resistance. Mechanical disturbance of the surface increases soil pore space and elevates moisture retention. The higher moisture content of these ploughed and disturbed areas results in lower electrical resistance.
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P. T. Thacker et al. Table 1. Surface disturbance features and electrical resistance
Surface feature
Number
Single pine stump/root Multiple pine stump/root Three throw Plowing and earth moving Combination/multiple disturbances Non-disturbed areas
28 4 8 11 16 155
Resistance Mean Stand. dev. 1871 2005 2648 1153 1654 1942
Tree throws produce a more complicated effect on soil resistance. The mechanical action of pine trees falling and uprooting creates a medium to large crater which, on the sandy hillslope of Vale de O u bidos, gradually infills over a period of years. In some cases (such as the feature labelled ‘‘c’’ in Figure 2), the pine tree was violently uprooted by heavy machinery conducting clearing activities. The subsequent rapid slumping of surrounding sands decreased compaction and increased soil moisture in the centre of the depression. In contrast, the peripheral edges of most tree throw depressions exhibited increased compaction and higher resistivity as infilling neared completion. Because of its burial context in sands, the identification of post-depositional processes is critical for archaeological interpretation of spatial patterns at Vale de O u bidos. The Gravettian occupation level, buried through aeolian deposition and low energy sheetwash, is vertically discrete across most of the site. In a few units, however, artifact locations are clearly disturbed with wide elevational ranges. In these areas, translocation and mixing of artifacts from later, overlying prehistoric occupations might have occurred. Identifying such artifact intrusion is especially important when palimpsesting is likely, as is often the case in Palaeolithic archaeology. The electrical resistivity survey at Vale de O u bidos is an important aid for assessing the geologic and archaeological integrity of excavation units, complementing particle size distribution profiles, diagnostic artifact data, and surface mapping.
Accuracy in Detection of Subsurface Archaeological Features The area excavation completed as of Winter 2000 encompasses a significant portion of seventy array fields and thus resistivity measurement points. The archaeological level at Vale de O u bidos mainly consists of flaked chert, quartz, and quartzite artifacts, firecracked rocks (fractured through thermal shock), and several stone lined hearths. While comprehensive size analysis and intrasite spatial patterning is under study, this paper incorporates preliminary data into a general density dichotomy for the occupation surface (high and low artifact density: see Figure 2). Every square metre unit excavated at the site contained at least a few
761 451 1368 491 968 787
Point biserial (r) 0·033 0·013 0·184 0·248 0·104
t significance
0·444 0·159 2·370 3·276 1·362
P>0·1 P>0·1 P<0·01 P<0·0005 0·05
artifacts. Based on field notes and initial lab records, high density areas contained an average of more than 35 artifacts per square metre, while low density ones contained an average of less than 20 artifacts per square metre. Table 2 confirms that the high apparent resistivity peaks at Vale de O u bidos correlate well with areas of high subsurface artifact density, as is graphically evident in Figure 2. The archaeological nature of these resistivity peaks is pictured in Figure 3, which shows a hearth feature and an associated discard zone containing numerous fire cracked rocks. Figure 2 indicates the location of these finds within the resistivity contour map (2(e)) along with two other hearth features (2(f)). A good correlation (P<0·0005; see Table 2) between these subsurface archaeological deposits and soil resistance is not surprising since no large pebble or cobble inclusions occur naturally in the sand matrix. The only area with high density artifact occurrence and relatively low electrical resistivity measurements was a specialized lithic workshop area on the north end of the site (Figure 2(g)). This activity area includes a very small hearth feature (less than 40 cm diameter) along with a very high density of chert debris resulting from the manufacture of numerous blade and bladelet tools, including burins and microgravette points. While there is a slight increase in local resistance at this location, the absolute value is much smaller than the other high artifact density areas of the site. Two possible explanations can account for this lowered absolute resistance value in the chert workshop area. Surface disturbance associated with the bulldozing of the access road only a few metres to the east of the workshop activity area may be masking an otherwise high subsurface resistance. The other possibility is that the size, composition, and treatment of artifacts within the sediment matrix are important Table 2. Subsurface artifact density and electrical resistance
Low density locations High density locations Combined (all excavated locations) r= 0·373; t= 3·319; P<0·0005.
Number
Mean
Stand. dev.
49 21 70
1745 2566 1991
815 1211 1015
Detecting Palaeolithic Activity Areas Through Electrical Resistivity Survey 569
resistivity surveys depends on similar knowledge of the complex geoarchaeology of Late Pleistocene sites. In most circumstances, electrical resistivity survey for Palaeolithic activity areas will be restricted to deposits buried within a few metres of the exposed surface due to the narrow electrode array spacing required to resolve small archaeological features. Detection also relies on a sufficient resistance contrast between archaeological deposits and the surrounding sediment matrix such as that provided by the large lithic artifacts, manuports, and especially features such as stone lined hearths at Vale de O u bidos. In many cases, these methodological parameters coupled with time and labour costs may limit the application of resistivity survey to determining the extent of activity areas at highly probable site locations or during the investigation of known sites. In addition, the systematic Vale de O u bidos study demonstrates the importance of various cumulative post-depositional processes contributing to soil resistance. The signal ‘‘noise’’ generated by recent land use activities is a useful and informative complement to field observations during the archaeological interpretation of occupation levels. While not all Palaeolithic site contexts are appropriate for electrical resistivity methods, these Portuguese results imply that the technique has great potential for Palaeolithic archaeology and is underutilized in prehistoric periods without major architectural remains. Figure 3. (a) Upper Palaeolithic hearth feature from Vale de O u bidos. The location of this feature is indicated in Figure 2. (b) General view of hearth feature and adjacent fire cracked rock discard zone detected through high apparent resistivity.
variables affecting soil resistance. Artifacts from the workshop area are mostly on chert and are noticeably smaller than the average quartz or quartzite pieces recovered in the hearth areas. Most artifacts and manuports from the hearth areas are burned, another contrast with the workshop assemblage. At present the former surface disturbance explanation appears most likely, but ongoing assemblage analysis may shed light on possible artifact assemblage composition effects. In any case, explaining such variability would only strengthen the already excellent statistical significance reported in Table 2.
The Utility of Electrical Resistivity Survey for Palaeolithic Archaeology: Conclusions The results of archaeological excavation at Vale de O u bidos indicate that electrical resistivity survey can be a good predictor of buried Palaeolithic activity areas given an understanding of local lithostratigraphy and geological context of archaeological levels. The method was valuable even in the extreme situation of a fairly deep archaeological level buried in very electrically-resistant loose sands. Success in future
Acknowledgements The archaeological and geophysical investigations reported in this paper were funded by the Carta Arqueolo´gica de Rio Maior, Caˆmara Municipal de Rio Maior, and an Organized Research Grant from Texas A&M University—Commerce. Work by Ellwood was supported by NSF grant BCS 9903172 to Ellwood and Frank Harrold. We appreciate the work and skill of Armando Manuel Santos Cruz who directed the surface mapping of the hillslope. The excavation supervision provided by Gabriela Mota Marques, Artur Ribeiro, Joa˜o Aguiar, Joa˜o Pinhal, and Nata´lia Fraza˜o was invaluable.
References Aitken, M. J. (1974). Physics and Archaeology. 2nd Edition. Oxford: Clarendon Press. Almeida, Fernando. (1997). Prospecc¸a˜o geofisica de depo´sitos qua´ternarios. In (J. Zilha˜o, Ed.) Arte Rupestre e Pre´-histo´ria do Vale do Coˆa. Lisboa: IPPAA, pp. 55–73. Carr, C. (1982). Handbook on Soil Resistivity Surveying. Evanston: Center for American Archaeology Press. Darwin, R. L., Ferring, C. R. & Ellwood, B. B. (1990). Geoelectrical stratigraphy and subsurface evaluation of Quaternary stream sediments at the Cooper Basin, NE Texas. Geoarchaeology 5, 53–79.
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Ellwood, B. B., Harrold, F. B., Petruso, K. M. & Korkuti, M. (1993). Electrical resistivity surveys as indicators of site potential: examples from a rockshelter in southwestern France and a cave in southern Albania. Geoarchaeology 8, 217–227. Ellwood, B. B., Harrold, F. B. & Marks, A. E. (1994). Site identification and correlation using geoarchaeological methods at the Cabeco do Porto Marinho (CPM) locality, Rio Maior, Portugal. Journal of Archaeological Science 21, 779–784. Gamble, C. (1997). Timewalkers: The Prehistory of Global Colonization. Cambridge: Harvard University Press. Gamble, C. (2000). The Palaeolithic Societies of Europe. Cambridge: Cambridge University Press. Hammond, R. & McCullagh, P. S. (1974). Quantitative Techniques in Geography. Oxford: Clarendon Press. Herbich, T. (1993). The variations of shaft fills as the basis of the estimation of flint mine extent: a Wierzbica case study. Archaeologia Polonia 31, 71–82. Herz, N. & Garrison, E. G. (1998). Geological Methods for Archaeology. Oxford: Oxford University Press.
Kolen, J. (1999). Hominids without homes: on the nature of Middle Paleolithic settlement in Europe. In (W. Roebroeks & C. Gamble, Eds) The Middle Paleolithic Occupation of Europe. Leiden: Leiden University, pp. 139–175. Marks, A. E., Bicho, N., Zilha˜o, J. & Ferring, C. R. (1994). Upper Pleistocene prehistory in Portuguese Estremadura. Results of preliminary research. Journal of Field Archaeology 21, 53–68. Rapp, G. R. & Hill, C. L. (1998). Geoarchaeology: The Earth-Science Approach to Archaeological Interpretation. New Haven: Yale University Press. Shaw, G. & Wheeler, D. (1994). Statistical Techniques in Geographical Analysis. 2nd Edition. New York: Halsted Press. Weymouth, J. W. (1986). Geophysical methods of archaeological site surveying. Advances in Archaeological Method and Theory 9, 311–391. Williams, J. M. (1984). A new resistivity device. Journal of Field Archaeology 11, 110–114. Zilha˜o, J. (1997). O Paleolı´tico Superior da Estremadura Portuguesa. Lisboa: Edicoes Colibri.
Detecting Palaeolithic Activity Areas Through Electrical Resistivity Survey: An Assessment from Vale de O u bidos, Portugal Paul T. Thacker and Brooks B. Ellwood Department of Geology and Geophysics, E 235 Howe-Russell, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A.
Carlos M. C. Pereira Carta Arqueolo´gica de Rio Maior, Caˆmara Municipal de Rio Maior, 2040 Rio Maior, Portugal (Received 20 February 2000, revised manuscript accepted 3 April 2001) The open air Late Pleistocene campsite of Vale de O u bidos provided an opportunity to systematically assess the utility of electrical resistivity surveying for activity area detection at Palaeolithic sites. Detailed surface mapping facilitated the exploration of electrical resistivity effects caused by recent land use activities and post depositional processes. At Vale de O u bidos, tree throw events, ploughing, and earth moving have a more significant impact on soil resistivity than increases in soil moisture associated with pine tree stumps and roots. Resistivity results successfully discriminate activity areas containing hearth features and high densities of fire cracked rocks and artifacts. As an aid for understanding post-depositional processes at archaeological sites and to design excavation and recovery efforts, the technique has great potential for Palaeolithic archaeology and in other prehistoric contexts that lack major architectural remains. 2002 Elsevier Science Ltd. All rights reserved.
Keywords: PALAEOLITHIC, ELECTRICAL RESISTIVITY, GEOPHYSICAL SURVEY, SITE DETECTION, PORTUGAL.
nated areas of relative artifact density and a near surface large limestone block resulting from roof fall (Ellwood et al., 1993). In both Poland and Hungary, resistivity data have been used to trace the location of flint mine shafts that date to at least the Neolithic (with some shafts possibly begun in the terminal Pleistocene) (Herbich, 1993). At Konispol cave in Albania, the depth of cave deposits was determined through resistivity survey, and a large hearth feature was detected (Ellwood et al., 1993). In an open air context, Almeida used large area electrical remote sensing, including resistivity, to map the Late Pleistocene geology of open air landforms associated with rock art in the Coˆa valley of northeastern Portugal (1997). Resistivity results were reported from the large stratified Upper Palaeolithic site of Cabec¸o de Porto Marinho in central Portugal, but the degree to which electrical anomalies registered palimpsest effects and overlying later prehistoric levels has yet to be determined (Ellwood, Harrold & Marks, 1994). These preliminary reports indicated the promise of electrical resistivity surveys at appropriate Late Pleistocene sites. The Upper Palaeolithic open air site of Vale de O u bidos provided the opportunity to
Introduction ince the late 1950s, electrical resistivity remote sensing has been employed at archaeological sites containing relatively large architectural features such as masonry walls, paved foundations, or ditch construction (Weymouth, 1986). These archaeological situations are ideal for geophysical prospection methods since the targeted features are predictably regular in form, large in size, and usually contrast sharply with the background signal of the surrounding soil matrix (Carr, 1982). As a result, historic, protohistoric, and late prehistoric contexts account for most of the successful electrical resistivity surveys in archaeology. Electrical resistivity rarely is incorporated into research designs at early prehistoric sites, such as the Palaeolithic/Mesolithic periods in the Old World and Palaeoindian/Archaic periods in the Americas, as major architectural remains are scarce in the archaeological record of hunter–gatherers. When coupled with appropriate geological investigation, electrical resistivity surveys have been successful in a few Palaeolithic contexts. Resistivity survey results at the Pont d’Ambon II rockshelter in France discrimi-
S
563 0305–4403/02/$-see front matter
2002 Elsevier Science Ltd. All rights reserved.
564
P. T. Thacker et al.
systematically assess the technique’s utility beyond that reported from any previous study. Results presented here demonstrate that geophysical site surveys can inform Palaeolithic site excavation and recovery techniques and also reflect critical post-depositional processes important for assessing the spatial integrity of such archaeological deposits.
The Palaeolithic Archaeological Record and Electrical Resistivity Methods Because of its great age, the archaeological record of the Pleistocene presents greater potential problems for geophysical prospection than more recent counterparts. For thousands of years, the majority of Palaeolithic sites have been subjected to numerous post-depositional geological processes. Both in caves and open air sites, burial depth, low contrast feature density relative to soil matrix, and underlying/ containing geological materials can limit the effectiveness of electrical resistivity remote sensing. In addition, geological processes such as pedogenesis and erosion badly alter or destroy sites and site contexts. Most Palaeolithic archaeological features are small in component size. For example, hearths are usually smaller than 2 m in size, and remains of constructed shelter walls are rarely wider than a metre (Gamble, 1997, 2000; Kolen, 1999). For electrical resistivity to detect features of this size, electrode spacing must be narrow, usually a metre or less, and roughly the size of the target diameter (Darwin, Ferring & Ellwood, 1990). Since measured resistance is averaged across the volume bracketed by the electrode array, a wider spacing usually results in a smoothing and integration of the feature signal with that of the adjacent soil (Weymouth, 1986). The depth of current penetration is determined by electrode spacing, such that in a Wenner array, measurement depth is roughly equal to the electrode spacing interval. Thus the small size of most Palaeolithic features necessarily constrains the application of electrical resistivity methods to sites buried within a few metres of the present surface. Archaeological application of electrical resistivity prospection relies on detecting the variation in resistance between a site’s sediment matrix and cultural features. Many locations favouring preservation of the Pleistocene archaeological record are inherently poor situations for electrical resistivity surveys. For example, many limestone cave sites contain levels that are heavily brecciated and cemented. In this context, generally smaller stone artifacts consolidated within a limestone breccia will rarely provide adequate resistance contrast with adjacent areas lacking artifacts. In similar fashion, apparent resistivity is significantly influenced by the matrix characteristics (or lithology) and relative position of underlying strata. In lithostratigraphic cases where a low density archaeological level is underlain by an undulating or irregular surface,
such as an eroded limestone bedrock, the resistance variation will most likely reflect the site’s geology rather than archaeology (Rapp & Hill, 1998). None of these phenomena negate the benefits of employing electrical resistivity surveys at Palaeolithic sites as long as the complicated nature of electrical resistivity data are understood. Resistivity measured during most archaeological survey applications is a composite of multiple effects created by site geology, surface disturbances, post-depositional processes, and subsurface archaeological variability.
Vale de O u bidos: An Early Upper Palaeolithic Open-Air Campsite Vale de O u bidos was discovered in 1994 following a forest fire on the ridge separating the Rio Maior and Jaleca drainages. A temporary access road (through an existing roadside construction borrow pit) was bulldozed for trucks conducting logging and clearing activities after the fire. A small section of the site was destroyed as sediments were pushed into the borrow pit to bank the access road. Displaced Palaeolithic artifacts on the surface of the access road led to the discovery of the site. In 1999, a cooperative archaeological project led by Thacker and Pereira began to investigate the in situ deposits and Upper Palaeolithic occupation of the hillslope. The relatively intact Upper Palaeolithic level at Vale de O u bidos is buried by about 75 cm of loose, medium sands. Unlike much of the landscape near Rio Maior, the hillslope has not been deep-ploughed for eucalyptus forestry. Intact and in situ pine stumps over much of the site area indicated the potential for preserved archaeological spatial patterning. A laser transit was used to map surface features and to establish the survey and excavation grid (Figure 1).
Field Methodology Electrical resistivity instruments introduce a current into the ground using two electrodes and measure the voltages developed between a second set of electrodes. Of many possible electrode array configurations, the evenly spaced Wenner array is commonly employed in archaeology especially when using instruments with a rotary switch (Aitken, 1974; Darwin, Ferring & Ellwood, 1990). The Wenner array and rotary switch allow the array to be reconfigured by moving only one electrode, which significantly increases data acquisition speed. An electrode spacing of one and a half metres was necessary at Vale de O u bidos to provide enough depth penetration to register features in the buried archaeological level. Due to its performance in high resistance situations, a Williams type resistivity device was used for field
Detecting Palaeolithic Activity Areas Through Electrical Resistivity Survey 565
North 99.4
Bulldozed access road Shallow plowing
Borrow Pit 0
10
Probe array Pine stump Tree throw 100.6
Excavated area Scale: 1 square metre
Figure 1. Surface map of Vale de O u bidos, Portugal.
measurement (Williams, 1984). Resistance is directly measured using this device, and the apparent resistivity calculated for each measurement using the function, a =(2)R(d)
(1)
where d is the distance between each electrode and a is in m. Apparent resistivity was predictably very high at Vale de O u bidos given the loose sand soil with low moisture content (Herz & Garrison, 1998). The centre point of each reading relative to mapped surface features and the excavation grid at Vale de O u bidos is shown in Figure 1. Variation in soil resistivity across space (Figure 2) is the cumulative result of numerous effects, only some of which are cultural in origin. In order to understand the cause of high and low anomalies, the extent of recent landuse activities and surface disturbances across the site was investigated. This mapping provided a control measure of recent, non-prehistoric effects that may obscure subsurface variation in apparent resistivity. Once understood, these non-archaeological signals can be filtered from the resistivity map, and this improves the detection of buried cultural features.
Excavation Research Design In order to systematically test the electrical survey results, preliminary excavation units at Vale de O u bidos were placed in several high and low apparent resistivity zones. By excavating a sample of units that cover the range of resistivity variability rather than focusing on the high contrast anomalies, the Vale de O u bidos project circumvents the analytical problem of confirmation common in many geoarchaeological resistivity surveys. Over 100 square metres were excavated revealing several well-defined Palaeolithic activity areas. Recovered artifacts include flaked stone tools, red ochre fragments, several stone-lined hearth features, and numerous large fire-cracked rock fragments (probably used for stone boiling). Organic preservation is predictably poor in the well drained sands, as the Early Upper Palaeolithic level yielded only a few very small fragments of bone and charcoal. The lithic artifact assemblage from Vale de O u bidos is technologically and typologically Gravettian (Marks et al., 1994; Zilha˜o, 1997). Distinctive thin blades and blade cores, as well as backed pieces, dihedral burins, microgravettes, and pointed backed bladelets cluster in
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65.00
Excavation Grid North
60.00
55.00
50.00
45.00
40.00
40.00
45.00 Excavation Grid East
50.00
Figure 2. Contour map of apparent resistivity (in m) at Vale de O u bidos. Excavated areas are indicated with white outlines. Dashed white lines indicate zones of high flaked stone artifact/fire cracked rock density. Identifiable anomalies include: (a) shallow ploughing of eucalyptus plot; (b) disturbance effects associated with bulldozing of access road; (c) tree throw disturbance; (d) effects associated with pine stumps and roots; (e) location of hearth feature pictured in Figure 3; (f) additional hearth features; (g) chert workshop activity area.
an area interpreted as a chert workshop where composite weaponry was manufactured. Thick flake scrapers and denticulates on quartz and quartzite dominate the tool assemblages in the activity areas
surrounding several stone-lined hearths. No radiocarbon dates are currently available for the occupation level as recovered charcoal fragments are under analysis for species identification prior to dating.
Detecting Palaeolithic Activity Areas Through Electrical Resistivity Survey 567
Geological studies into site burial processes indicate that the Early Upper Palaeolithic occupation level was buried by aeolian sands, and that no large pebbles or cobbles occur naturally in the vicinity of the site. No significant variation in sediment texture, compaction, or inclusions was found, indicating the ideal circumstance (for resistivity survey) of a homogenous subsurface matrix with the exception of cultural artifact and manuport deposits.
Correlating Features and Resistivity Survey Results The point biserial coefficient method is the most appropriate technique for testing correlations between apparent resistivity measurements, recent land use phenomena, and subsurface archaeological remains at Vale de O u bidos. Point biserial correlation examines the relationship between one variable measured on a continuous scale and another variable that can be characterized by a categorical dichotomy (Hammond & McCullagh, 1974). In the Vale de O u bidos case, either the resistance or the calculated apparent resistivity measurement at each grid point can be considered the continuous variable. Resistance values are more appropriate for point biserial statistics, however, as transformed apparent resistivity values increase the range of values by nearly a factor of 10, subsequently enlarging the standard deviation which can be problematic when assessing smaller sample sets (Shaw & Wheeler, 1994). For all of the statistical tests reported below, the point biserial coefficient was calculated using both resistance and apparent resistivity values. In each case, similar levels of significance and conclusions result. Two different dichotomies characterize the categorical axes reported below. In analysing surface feature contributions to resistance, locations either exhibit a feature potentially influencing soil resistance, such as ploughing or a tree throw, or the locations lack such features. Through the point biserial correlation coefficient, resistance values measured while each feature was within the electrode array are compared against the background variability of the hillslope in locations without such features. A second dichotomy, high and low artifact density, assesses resistance measurements as an indicator of subsurface archaeological deposits. Only excavated areas of the site, 70 measurement locations, were included in this latter statistical test. The point biserial coefficient requires the calculation of the means of each categorical subgroup and the standard deviation of the combined subgroups, and is given by the following equation:
where: N=total number of observations Np =number of observations in the first group of values
Nq =number of observations in the other group of values Mp =mean of the first group of values Mq =mean of the other group of values sy =the standard deviation of all the data The significance of the coefficient (which varies from +1 to 1) can be determined employing the following t-test:
The tailedness of the test will depend on the null hypothesis being tested. In the Vale de O u bidos case, surface or subsurface features were analysed in subgroups such that the anticipated effect should be consistent (that is increasing or decreasing resistance, but not both). Thus a single-tailed test statistic is warranted.
Soil Resistance and Post-Depositional Processes The surface map of the Vale de O u bidos hillslope that was generated with a laser transit prior to geological and archaeological investigations provides spatial control of recent disturbance events. Tree throw feature, pine stump (and related roots), ploughing, and earthmoving locations are all displayed in Figure 1. The remaining set of locations lacking such features (a total of 155 measurement points) is considered representative of subsurface resistance variation in the statistical comparison. Each subset of surface feature (single pine root, multiple pine stumps, tree throws, ploughing and earth-moving zones, and combination disturbances) was analysed independently against this background variability generated by the subsurface matrix. As Table 1 displays, some disturbance features do impact the resistance measures at Vale de O u bidos. While it is obvious in Figure 2 (features labelled ‘‘d’’) that pine stumps/roots can influence resistivity, Table 1 demonstrates that a significant reduction in electrical resistance does not always occur, even when multiple pine roots are situated within the electrode array. This lack of correlation may be due to the spatial positioning of the stump/roots relative to the electrode array, or it may reflect variability in the subsurface shape, extent, and relative moisture content of individual root masses. Physical disturbance of the sand sediments has the greatest impact on electrical resistance at the site. Areas that underwent earth moving such as the ploughed zone on the western edge of the site and the bulldozed access road display significantly lower resistance. Mechanical disturbance of the surface increases soil pore space and elevates moisture retention. The higher moisture content of these ploughed and disturbed areas results in lower electrical resistance.
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Surface feature
Number
Single pine stump/root Multiple pine stump/root Three throw Plowing and earth moving Combination/multiple disturbances Non-disturbed areas
28 4 8 11 16 155
Resistance Mean Stand. dev. 1871 2005 2648 1153 1654 1942
Tree throws produce a more complicated effect on soil resistance. The mechanical action of pine trees falling and uprooting creates a medium to large crater which, on the sandy hillslope of Vale de O u bidos, gradually infills over a period of years. In some cases (such as the feature labelled ‘‘c’’ in Figure 2), the pine tree was violently uprooted by heavy machinery conducting clearing activities. The subsequent rapid slumping of surrounding sands decreased compaction and increased soil moisture in the centre of the depression. In contrast, the peripheral edges of most tree throw depressions exhibited increased compaction and higher resistivity as infilling neared completion. Because of its burial context in sands, the identification of post-depositional processes is critical for archaeological interpretation of spatial patterns at Vale de O u bidos. The Gravettian occupation level, buried through aeolian deposition and low energy sheetwash, is vertically discrete across most of the site. In a few units, however, artifact locations are clearly disturbed with wide elevational ranges. In these areas, translocation and mixing of artifacts from later, overlying prehistoric occupations might have occurred. Identifying such artifact intrusion is especially important when palimpsesting is likely, as is often the case in Palaeolithic archaeology. The electrical resistivity survey at Vale de O u bidos is an important aid for assessing the geologic and archaeological integrity of excavation units, complementing particle size distribution profiles, diagnostic artifact data, and surface mapping.
Accuracy in Detection of Subsurface Archaeological Features The area excavation completed as of Winter 2000 encompasses a significant portion of seventy array fields and thus resistivity measurement points. The archaeological level at Vale de O u bidos mainly consists of flaked chert, quartz, and quartzite artifacts, firecracked rocks (fractured through thermal shock), and several stone lined hearths. While comprehensive size analysis and intrasite spatial patterning is under study, this paper incorporates preliminary data into a general density dichotomy for the occupation surface (high and low artifact density: see Figure 2). Every square metre unit excavated at the site contained at least a few
761 451 1368 491 968 787
Point biserial (r) 0·033 0·013 0·184 0·248 0·104
t significance
0·444 0·159 2·370 3·276 1·362
P>0·1 P>0·1 P<0·01 P<0·0005 0·05
artifacts. Based on field notes and initial lab records, high density areas contained an average of more than 35 artifacts per square metre, while low density ones contained an average of less than 20 artifacts per square metre. Table 2 confirms that the high apparent resistivity peaks at Vale de O u bidos correlate well with areas of high subsurface artifact density, as is graphically evident in Figure 2. The archaeological nature of these resistivity peaks is pictured in Figure 3, which shows a hearth feature and an associated discard zone containing numerous fire cracked rocks. Figure 2 indicates the location of these finds within the resistivity contour map (2(e)) along with two other hearth features (2(f)). A good correlation (P<0·0005; see Table 2) between these subsurface archaeological deposits and soil resistance is not surprising since no large pebble or cobble inclusions occur naturally in the sand matrix. The only area with high density artifact occurrence and relatively low electrical resistivity measurements was a specialized lithic workshop area on the north end of the site (Figure 2(g)). This activity area includes a very small hearth feature (less than 40 cm diameter) along with a very high density of chert debris resulting from the manufacture of numerous blade and bladelet tools, including burins and microgravette points. While there is a slight increase in local resistance at this location, the absolute value is much smaller than the other high artifact density areas of the site. Two possible explanations can account for this lowered absolute resistance value in the chert workshop area. Surface disturbance associated with the bulldozing of the access road only a few metres to the east of the workshop activity area may be masking an otherwise high subsurface resistance. The other possibility is that the size, composition, and treatment of artifacts within the sediment matrix are important Table 2. Subsurface artifact density and electrical resistance
Low density locations High density locations Combined (all excavated locations) r= 0·373; t= 3·319; P<0·0005.
Number
Mean
Stand. dev.
49 21 70
1745 2566 1991
815 1211 1015
Detecting Palaeolithic Activity Areas Through Electrical Resistivity Survey 569
resistivity surveys depends on similar knowledge of the complex geoarchaeology of Late Pleistocene sites. In most circumstances, electrical resistivity survey for Palaeolithic activity areas will be restricted to deposits buried within a few metres of the exposed surface due to the narrow electrode array spacing required to resolve small archaeological features. Detection also relies on a sufficient resistance contrast between archaeological deposits and the surrounding sediment matrix such as that provided by the large lithic artifacts, manuports, and especially features such as stone lined hearths at Vale de O u bidos. In many cases, these methodological parameters coupled with time and labour costs may limit the application of resistivity survey to determining the extent of activity areas at highly probable site locations or during the investigation of known sites. In addition, the systematic Vale de O u bidos study demonstrates the importance of various cumulative post-depositional processes contributing to soil resistance. The signal ‘‘noise’’ generated by recent land use activities is a useful and informative complement to field observations during the archaeological interpretation of occupation levels. While not all Palaeolithic site contexts are appropriate for electrical resistivity methods, these Portuguese results imply that the technique has great potential for Palaeolithic archaeology and is underutilized in prehistoric periods without major architectural remains. Figure 3. (a) Upper Palaeolithic hearth feature from Vale de O u bidos. The location of this feature is indicated in Figure 2. (b) General view of hearth feature and adjacent fire cracked rock discard zone detected through high apparent resistivity.
variables affecting soil resistance. Artifacts from the workshop area are mostly on chert and are noticeably smaller than the average quartz or quartzite pieces recovered in the hearth areas. Most artifacts and manuports from the hearth areas are burned, another contrast with the workshop assemblage. At present the former surface disturbance explanation appears most likely, but ongoing assemblage analysis may shed light on possible artifact assemblage composition effects. In any case, explaining such variability would only strengthen the already excellent statistical significance reported in Table 2.
The Utility of Electrical Resistivity Survey for Palaeolithic Archaeology: Conclusions The results of archaeological excavation at Vale de O u bidos indicate that electrical resistivity survey can be a good predictor of buried Palaeolithic activity areas given an understanding of local lithostratigraphy and geological context of archaeological levels. The method was valuable even in the extreme situation of a fairly deep archaeological level buried in very electrically-resistant loose sands. Success in future
Acknowledgements The archaeological and geophysical investigations reported in this paper were funded by the Carta Arqueolo´gica de Rio Maior, Caˆmara Municipal de Rio Maior, and an Organized Research Grant from Texas A&M University—Commerce. Work by Ellwood was supported by NSF grant BCS 9903172 to Ellwood and Frank Harrold. We appreciate the work and skill of Armando Manuel Santos Cruz who directed the surface mapping of the hillslope. The excavation supervision provided by Gabriela Mota Marques, Artur Ribeiro, Joa˜o Aguiar, Joa˜o Pinhal, and Nata´lia Fraza˜o was invaluable.
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