The Interpretation of Stratified Sites: A View from Underground

Journal of Archaeological

Science 1983,10,267-28

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The Interpretation of Stratified Sites: A View from Underground Paola Villa” and Jean Courtin* This paper reports on an experiment designed to study the role of trampling in the vertical dispersal of artifacts in the soil, and in the mixing of originally separate sets of materials. The experiment is part of a study of the archaeological stratigraphy and patterns of site use at a large stratified cave in southern France. The experiment was designed to replicate conditions prevailing at the cave. The results strongly suggest caution in the interpretation of living floors and stratified assemblages in sandy deposits. Keywords: EXPERIMENTAL ARCHAEOLOGY, CULTURAL TION, SITE FORMATION PROCESSES, TRAMPLING.

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Intruduction This article presents the results of a trampling experiment designed to study processes of formation of the archaeological record at stratified sites. The hypothesis to be tested was that trampling (a normal activity of prehistoric inhabitants of a site) can cause vertical dispersal of artifacts in the soil and can create false stratigraphic associations. The first part of this article provides the justification, background, and rationale of the experiment. In the second part we present the experimental procedures and analysis of results. The specific aim of the experiment was to provide information to aid in the interpretation of dispersal patterns and relationships between superimposed sets of materials at a particular site. We believe, however, that the results of this experiment are relevant to general studies of the formation of archaeological deposits. This information can contribute to a better assessment of the research potential of stratified sites and to the development of more rigorous methods for drawing inferences from archaeological remains. Background

A case study The trampling experiment reported here is part of an ongoing study of the stratigraphy and patterns of site use in Fontbregoua Cave, about 100 km NNE of Marseille, southern France. Fontbregoua is a large cave (about 250 m2) under excavation since 1970 by Jean Courtin. It has yielded a long cultural sequence with Upper Paleolithic, Mesolithic, and Neolithic deposits totalling more than 9 m. The uppermost levels contain sparse Chalcolithic, Bronze Age and historic materials. aDepartment of Anthropology, University of Colorado, Boulder, Colorado 80309. *Direction des Antiquit& Prehistoriquesde Provence,Alpes et CBte d’Azur, 21-23 Boulevard du Roi Rend, 13617Aixen-Provence, France. 267 0305-4403/83/030267+

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The Neolithic layers (4 m) have yielded more than 24,000 objects including pottery, grinding stones, sickle blades, remains of domestic and wild faunas, and carbonized seeds of domestic wheat (mostly bread wheat), barley, legumes and acorns. The oldest Neolithic occupation has a radiocarbon date of 4750f 100 bc. Below, and separated by a layer of culturally sterile sand, are Mesolithic deposits with geometric microliths, abundant bird bones but sparse mammal bones, remains of fresh water turtles and fish, and wild legumes. The Upper Paleolithic occupation, with a radiocarbon date of 9250& 100 bc, is known only from a very limited test trench. Bedrock has not been reached (Courtin 1975, 1976, 1978; Courtin & Erroux 1974; Cheylan & Courtin 1976). Significance

of the site

Being a deeply stratified site with a long history of occupation and repeated use, Fontbregoua provides data for the study of change and continuity in material culture. The site is especially important for a number of reasons. First, the cultural sequence spans the transition from hunting and gathering to farming, a turning point in human history. Second, the site has yielded a very large fauna1 sample and botanical remains for the study of hunting and farming practices. In particular, Fontbregoua is one of only four Early Neolithic sites in southern France known to have yielded plant materials (carbonized cereal grains and legumes). Finally, an especially interesting feature of the site is continuing use of the cave across the Neolithic transition. The continuing use of cavestogether with open air sites-is not unusual in the Early Neolithic of the Western Mediterranean and provides an interesting contrast to the situation in the Near East, where caves, inhabited in Epipaleolithic times, were only sporadically used by Neolithic farmers. In southern France farming first appears on the coast at around 5500 bc. Some of the major domesticates-wheat, barley, perhaps sheep-are of exotic origin; their wild ancestors are only found in the Near East. Thus, the evidence suggests that farming was not a fully local, independent development. Its adoption must have been, at least to some extent, the result of a process of transfer of new technologies, materials and expertise. The spread of the new economy eventually lead to the establishemnt of permanent, year-round villages and to the abandonment of caves as habitation sites. However, for the first two millennia after the appearance of farming, caves continued to be used side by side with open air sites. Storage or trash pits and fireplaces, with thick ash accumulations, are a conspicuous feature of the Neolithic deposits at Fontbregoua suggesting prolonged periods of habitation. By contrast, during the Mesolithic, the cave appears to have been used for short visits only, The study of changing or continuing patterns of site use through time is a major goal of the Fontbregoua research project. Defining the Problem Assemblages and layers

The first task facing the investgator of a deeply stratified site is to partition the deposits into layers or levels representing successive time units (Wheeler, 1954, p. 43). It is then possible to distinguish superimposed sets of material and group items into units of association (assemblages) to be analysed separately. Stratified sequences in prehistoric sites will often consist of a succession of contrasting types of deposits, bounded by discontinuities reflecting changes in type of sedimentation, temporary halts in deposition or erosional episodes. Within single geologic units, one or more units of cultural stratification may be distinguished. In the latter case, their boundaries are defined by the presence of sterile zones separating vertical concentrations of materials, or by distinct interruptions in the accumulation of individual man-made deposits, such as ash or charcoal layers. Ideally each layer or level represents a well-

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defined time unit bounded by clear discontinuity surfaces or by sterile zones. In fact, gradational boundaries are common. When the cultural material forms unpatterned vertical scatters within homogeneous deposits, arbitrary levels are used. Collections of objects from single stratigraphic units form separate analytical units and are called assemblages. In short, archaeologists subdivide and interpret a stratified sequence using criteria of diverse kind, based on a combination of natural and cultural processes of deposit formation(Wheeler, 1954, ch. 4; Michels, 1973, chs. 3 and 4; Harris, 1979, ch. 5; Straus, 1979; Laville ef al., 1980, pp. 9, 13, 152). These same criteria have been used at Fontbregoua. Clearly the definition of stratigraphic units and their boundaries is important for the definition of assemblages. Boundaries between layers may be sharply delineated and continuous; they may also be diffuse, discontinuous and irregular. The distinctness, lateral continuity and temporal significance of these surfaces should be described and documented (Harris, 1979, pp. 49-80). Methods used for defining layer boundaries and for segregating superimposed sets of materials into assemblages should be critically evaluated. The assemblage concept plays an important role in Old World archaeology. In the “assemblage approach” of Palaeolithic archaeology, pioneered by F. Bordes (Binford, 1981, p. 183; 1982) assemblage types are defined and compared on the basis of varying frequencies of artifact types. Each artifact contributes to the description and diagnosis of the assemblage. To be sure, assemblage typology and quantification are not emphasized in Neolithic or proto-historical archaeology. In these disciplines cultural-historical groupings and sequences are worked out on the basis of comparative studies of selected classes of artifacts. More attention is given to diagnostic artifacts than to frequencies of artifact types. However, in behavioral studies the assemblage again is the basic unit of analysis. Every item in the assemblage contributes to the diagnosis and description of the whole. Implicitly or explicitly the material is referred to a single episode of occupation or, at least, to a single mode of site use by a specific group of people. For instance, in studies of seasonality, diet, and subsistence patterns collections of bones from the same layer or level form the basis for estimating the minimum number of individuals represented in fauna1 remains and for analysing hunting or herding practices. In spatial analysis we try to distinguish areas in which various daily life activities were carried out. Again, sets of associated materials are the basic unit of analysis. Clearly, if all the elements of a set contribute to its diagnosis, then we should make sure that they do, in fact, belong to the ensemble. For instance, Grayson (1979, pp. 205, 213) has suggested that the way a stratified sequence is subdivided into units of analysis seriously affects sample size and, consequently, measures of relative abundance of faunas. Sampling error may distort our understanding of past patterns of human behaviour; its effects should not be underestimated. In short, we need to ask: - Do strata boundaries, observed during the excavation, represent meaningful breaks in a sequence of occupations ? Are our samples discrete units or arbitrary slices of a temporal continuum? - Do assemblages from individual layers represent the discrete residues of distinct human groups or are they the aggregate of different episodes? Can we be sure that our stratigraphic units relate to a single mode of site use, if not to a single episode of occupation ? It is clear that the answers to these questions have an important bearing on studies of site use patterns. If we can define single phases of occupation, then we can study activities and patterns of site use on a fine time scale. If our information is inadequate for

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such definition, then we should look only at gross patterns of change and continuity, focus our attention on the general picture, and develop detailed behavioural interpretations only in the case of features and site structures-such as storage pits-that clearly associate contemporaneous materials. Assemblages and time.

Confronted by the problem of defining the temporal duration of their samples, archaeologists distinguish between homogeneous and mixed assemblages (some New World archaeologists use the term “component”). Homogeneous assemblages are associated sets of artifacts and bones, meaningful samples of a segment of a community. Mixed assemblages, on the other hand, are the telescoped residue of once separate and dissimilar events (Michels, 1973, pp. 22-25; Thomas, 1979, p. 231). This simple distinction hides a more complex situation. Homogeneous is an attribute that encompasses a graded series. The degree to which aggregates from individual layers can be treated as homogeneous assemblages depends upon the time scale we wish to adopt. An example from classical archaeology will serve to make this clear. Etruscan chamber tombs were often used for several generations. Pottery found in such a tomb may include, say, Middle Corinthian ware, bucchero ware and late blackfigure vases. These grave goods span a period of over a century. On a coarse time scale the material is associated because it is found in the same container (a tomb) and is homogeneous in that it is a typical and meaningful sample of Etruscan culture and burial customs. On a fine time scale the late black-figure vases (first decade of the 5th century BC) and the Corinthian ware (first half of the 6th century BC.) are not contemporaneous. They are in fact related to different events: the death and burial of a rich landowner and, much later, of his equally wealthy great grand-children. Seen from this point of view the tomb material forms a mixed assemblage. In prehistoric archaeology the strength of association of materials is traditionally evaluated in stratigraphic terms. Layers or levels are treated as containers of a sort (Spaulding, 1960, p. 211). But assemblages from individual layers represent temporal samples whose duration is very difficult to measure. Unlike historical materials, Stone Age materials are not clear time markers except on a very gross scale. Formal changes through time are too rare or unstable to allow time ranking with more than very few classes. Thus, neither stone artifacts nor Neolithic pottery provide direct criteria for separating the residues of different episodes of occupation that may have accumulated in a single layer. The time interval between the deposition of the first and last item in the set can rarely be specified. In most cases we do not know the scale on which an assemblage is homogeneous. Fine dissecting of strata does not provide the degree of time control required by behavioural studies, because geological cycles often cover spans of time that are too long for the study of human activities on a fine scale (Bordes et al., 1972, pp. 17-19). The Neolithic deposits at Fontbregoua have an average sedimentation rate of 1 cm per 17 years. Thus, it is likely that materials now aggregated in a single layer were, in fact, discarded during separate phases of occupation and possibly different modes of site use. Estimates of sedimentation rates in other Stone Age caves indicate equally low values. One cm of deposit may represent from 5 to 167 years, with an average of about 14 years (Speth & Johnson, 1976, pp. 47-48). Furthermore, layers appear to be rather leaky containers. Recent evidence provided by conjoined pieces in Old World sites has shown that vertical migration and dispersal of artifacts across different cultural levels is a fairly common phenomenon (Cahen & Moeyersons, 1977; Van Noten et al., 1978, 1980; Bunn et al., 1980; Villa, 1982; DelPorte, 1982, p. 161; see also Siriainen, 1977; Rowlett & Robbins, 1982). It is increasingly clear that in many stratified sites assemblages are not patterned sets of artifacts, tool-

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kits that can be referred to the activities of a specific social group (Binford & Binford, 1969; Sackett, 1973; Whallon, 1973). Rather, they are aggregates of individual pieces with individual histories and origins (Binford, 1982, pp. 17-18). To understand patterns of site use, we must first investigate processes of artifact accumulation and dispersal in the soil. Stratigraphic studies have traditionally focused on the geologic context of cultural materials. Classic sediment analyses provide important data for reconstructing paleoclimatic histories and for temporal ordering of assemblages (e.g. Laville et al., 1980). However, the burial and stratification of cultural materials at repeatedly occupied sites, such as caves, are due to a complex interaction of human activities and geological processes. Comprehensive approaches to cave sediment formation stress human and biological influences (Butzer, 1978, 1981, 1982, ch. 7; Goldberg, 1979, 1980; Courty & Raynal, 1982). Many techniques and procedures used in the study of site formation and modification are borrowed from the earth sciences. Others are more strictly archaeological in origin, in that the information is provided by the artifacts themselves, their interrelationships and precise position in relation to strata boundaries. These latter methods of stratigraphic analysis are based on the use of vertical projections and plots of conjoinable pieces (e.g. Villa, 1982, pp. 280-281). Experimental and ethnographic studies of depositional environments and disturbance processes have been designed to answer specific archaeological questions (Jewel1 & Dimbleby, 1966; Isaac, 1967; Stockton, 1973; Cahen & Moyersons, 1977; Gifford & Behrensmeyer, 1977; Reynolds 1974, 1979; Schick in Bunn ef al., 1980; see also Schiffer, 1975, p. 841; Lewarch & O’Brien, 1981, 307-311). These different approaches must be combined and integrated to reach a better understanding of processes of cultural stratification and deposit disturbance. Our paper is not intended as a review of all these methods. The potential of geoarchaeology and the role of sediment analysis in such studies have been illustrated with concrete examples by Butzer (1982, pp. 77-122). We present here an example of the experimental approach that closely reflects our professional interests. This approach should be seen as complementary to sediment analysis. As indicated below, the need for experimentation arose as a consequence of studies of conjoinable pieces and of vertical projections of the Fontbregoua material. Vertical displacement of archaeological remains at Fontbregoua The study of conjoinable pieces found at Fontbregoua (mostly pottery sherds and bones) is in progress. A preliminary analysis indicates that sherds belonging to the same pot have a vertical separation of up to 25-30 cm. Vertical dispersal often occurs across what during excavation had appeared to be distinct cultural horizons. A variety of factors may be suggested to explain the vertical displacements observed in the Fontbregoua deposits: (1) Soil fauna. Animal burrows have been observed especially in the upper (Chalcolithic) layers; they are occasionally found in the lower units. However, these disturbed areas are easily distinguished from the surrounding intact matrix and pieces from these areas have been kept separate from the remaining materials. Earthworm castings are uncommon; however, the occurrence of diffuse layer boundaries suggests earthworm activities (Jacques E. Brochier, pers. comm.). (2) Tree roots. The cave ceiling collapsed in ancient times; thus the central cave area was exposed to sun and rain. When the excavation began, four live oaks and one cherry tree were growing in the cave. Their roots have been found 4 m below the surface. Since old roots rot away and may not leave visible traces, disturbance by tree roots may be expected even in layers where they have not been observed. (3) Alternate wetting and drying. According to Cahen dc Moeyersons (1977) alternate

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wetting and drying of sediments (by percolating rain water and sun) can lead to vertical movement of artifacts, especially if sediments are unconsolidated due to biogenic activities. It is interesting to note that some of the Fontbregoua layers and the sediments used in Cahen & Moeyersons’ experiment have a very similar grain size distribution (analysis of the Fontbregoua sediments is in progress). We have never observed clay cracks. (4) The digging and leveling activities of the prehistoric inhabitants. Pits, depressions, and large hearths in deep hollows are common in the Neolithic layers. Digging and leveling activities, necessary to the construction of such features, disturb older deposits and cause rearrangement and redistribution of archaeological material on younger surfaces. It is possible to imagine situations in which pits or dug-out depressions will not leave visible traces in the soil. If the pit was in use for a short time, if it contained perishable material, if the fill is similar in colour and texture to the layer in which the pit was dug, the feature will not be recognized and only the displacement of conjoinable pieces will betray the presence of disturbances. (5) Trampling. Recently several scholars have suggested that trampling is a kind of occupational disturbance that is not easily recognized and may cause considerable vertical displacement of artifacts (Stockton, 1973; Hughes & Lampert, 1977; Gifford & Behrensmeyer, 1977 ; Gifford, 1978; Butzer, 1981, p. 155). At the Jean Cros rock shelter in southeastern France sherds of the same pot have been found in three superimposed levels; their vertical dispersal was attributed by the archaeologist to trampling by the prehistoric inhabitants (Guilaine, 1979). A trampling experiment has been carried out by Stockton in Australia (1973). Small glass splinters (mean weight=4 g) were laid out on a level sandy surface, covered by 5 cm of sand and intensively trampled for one day. When excavated by levels, the glass was found to be distributed over a depth of 16 cm. More than 50% of the sample had remained on or near the original surface; 22% had moved 2-5 cm upward; the rest of the sample had migrated downward to a maximum depth of about 10 cm below the original surface. Patterns of vertical dispersal of conjoined sherds in the central part of the Fontbregoua cave suggest that trampling may have been a contributing factor. The density of material indicates that the site had been intensively lived in; trampling on discarded material lying on the surface or slightly buried must have been a common occurrence. Some of the vertically displaced pieces are horizontally very close; thus it is unlikely that their vertical distribution is due to the presence of irregular or sloping living surfaces. Groups of conjoinable and vertically displaced sherds are scattered over large areas; this seems to suggest a non-localized agency of displacement such as trampling. To test this hypothesis, we decided to replicate Stockton’s experiment in such a way so as to approximate more closely conditions prevailing in the cave. Experiment Procedures

On the rocky slopes, near the cave entrance, there are several artificial terraces built with dry stone walls to hold the screened backdirt of the excavation. During the summer, when the excavations take place, these terraces are used by the excavating team (an average of 12-15 people) for a variety of activities: screening, washing and sorting artifacts, eating and resting. The material selected for the experiment was laid out on these terraces. We used material similar in size, shape and kind to that found in the cave, viz.: - flint flakes, blades and retouch flakes (down to l-2 cm in maximum dimensions); - mammal bones (shafts and articular ends, teeth and skull fragments); - marine shells (Cardium, Patella, Columbella, Cerithium);

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- pottery sherds (modern, unglazed); - a few flat limestone pebbles, to simulate the polished celts found in the cave. Ninety-five y0 of all objects were 110 cm in maximum dimensions. All objects were measured, weighed, numbered and catalogued; the stone pieces were coated with a thin film of paint, in brilliant colours, to distinguish possible damage scars and to speed up the conjoining of broken fragments. To avoid downslope movement and image distortion in vertical projections, we flattened the sandy surfaces by repeated trampling before laying out the objects. Footprints no deeper than 1 cm were considered indication of an acceptable degree of compaction. The precise location of each object was recorded with three spatial coordinates, the relative elevation being measured with a transit. Some of the squares were covered with 2-4 cm of sand or sand and rubble from the cave before trampling; in others the material was left uncovered. A sediment analysis of two squares done by Jacques E. Brochier (Laboratoire de Sedimentologie, FacultC Des Sciences, Marseille) showed that the matrix can be defined as a dry, loose, well-sorted silty sand (82-86% sand; 9-13% silt) with very little clay (5-8%); the median size was 96-108 pm. Small quantities of limestone rubble (2-20 mm in diameter) were occasionally present in other squares; their proportions varied from 0 to 14.5%. In Stockton’s experiment intensive trampling was done for one day. We decided in favour of natural, casual and prolonged trampling done by the excavators while walking in and out of the cave to attend to their tasks. All the excavators wore only light sandals or went barefoot. The results we present now are based on the excavation and analysis of 292 pieces from 11 l-m squares, 4 of which had two superimposed levels. Eight squares were dug after 16 days; the others after 22, 32, and 36 days. All excavated artifacts were again recorded with their new spatial coordinates with the exception of a few objects (2.9%) which were recovered in the screens and could not be used in the analysis. The weather remained warm and sunny throughout, with only a light rain one night. Moisture from continuous water sieving operations considerably hardened the sand in one square. Results

Vertical displacement

Significant vertical dispersal can be achieved even with a limited amount of trampling. The maximum range of vertical separation we observed is 8 cm. After 16 days of casual trampling, 20% of the objects in square S (Figure 1) had migrated downward 5 to 7 cm below the surface. After 16 days of trampling, the material of two superimposed levels in square R, originally separated by 3 cm of sterile sand, was completely mixed and formed only one level (Figure 2). To prevent the effects of sand compaction in the mixing of objects, trampling was done in two stages. The lower level of artifacts was covered with 3 cm of sand and trampled for 20 days. Other objects were then laid out on the compacted surface and trampled for 16 more days. Table 1 gives the frequency distributions of artifacts by depth in 5 squares trampled for 16 days. The objects had not been covered by sand. Note that 21% of the objects did not move while 28% was displaced 3 to 7 cm below the surface. The few millimeters of upward displacement are the results of horizontal migration over a slightly irregular surface. Ten l-m squares covered by sand or sand and gravel were trampled for 16 to 36 days. The thickness of the protective layer varied from 2 to 4 cm. Some squares happened to be in areas outside the main traffic and were only minimally trampled. For the sake of

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Figure 2. Vertical projections of artifacts in square R, lower and upper levels. The lower level was covered with 3 cm of sand and trampled for 20 days. Objects were then laid out on the compacted surface and trampled for 16 days. Top, the relative position of objects in both levels before vertical displacement occurred. The figure is a composite image as the two levels were deposited at different times. The elevation of each object was measured with a transit in relation to a fixed datum. Bottom, after 36 days of trampling.

record a table of frequency distributions of artifacts in all covered squares is given below (Table 2). However, the reader is warned that displacement distributions in individual squares varied greatly. In general, downward displacement is smaller in covered than in uncovered squares even when trampling lasted longer. Upward displacement, of 1 cm or more above the original position, is limited; its frequency varies from 0 to 17% in individual squares with an average of 9-8o/o. The maximum observed value is +2-4 cm in a square covered with 4 cm of sand. We prefer to analyse distributions square by square and individually compare them

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Table 1. Amount of disprclcement from the or@kal surface in uncovered squares

Displacement in cm +0.9/-0.9 -l*O/-2.9 -3.0/-4-g -5*O/-6.9 N= 100 Range: +0.3/--6.9

o/0of artifacts 21.0 51.0 23.0 5.0

Table 2. Amount of displacement from the original surface in covered squares

Displacement in cm

0/Oof artifacts

+1*0/+2.9 +0.9/-0.9 -1.0/-2-g -3*o/-4.9 --5-O/-6*9 N= 184

Range: +2.4/-5.2 to distributions in uncovered levels of the same squares. Table 3 shows vertical displacements in two squares, R and S, each with two superimposed levels. As mentioned earlier, in these squares sand was spread over laid out material and left to be trampled for 20 days. A second layer of objects was deposited and walked over for another 16 days. The differences between the displacement distributions of the two lower levels appear to be due to the differences in the thickness of the covering layer (3 and 2 cm), but could also be partially attributed to more intensive trampling in square S which was close to the lunch table and to the drinking water. It is also clear that, as the vertical distance between the trampled surface and the artifacts increases, the effects of trampling decrease proportionally until downward displacement effectively ceases. Vertical displacement is more limited in covered squares where objects were initially 2 to 4 cm below the trampled surface. Frequency distributions from the other covered squares are intermediate between those of the lower levels in squares R and S. In general, a comparison of frequency distributions of artifacts by depth in all squares suggests that the degree of vertical displacement depends on four major variables: (1) (2) (3) (4)

the intensity of trampling; the degree of compaction of the sediments; the thickness of the deposits covering the pieces; the weight/size of the pieces (this point is discussed in the section “Sorting weight”).

by

During trampling, objects are constantly being displaced, sometimes lifted, sometimes pushed down. When the trampling process begins most pieces not covered by sand are pushed down and in a few minutes will disappear into the earth. But, after a while the same pieces may reappear on the surface, having been brought up by a human foot. Some pieces, however, appear to migrate gradually downward, below the zone of actual disturbance and beyond the reach of feet. Intensive trampling will increase the proportion of pieces pushed far below the surface. The degree of compaction and resistance to object penetration varied in different squares depending, among other factors, on the presence of rubble and moisture in the

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matrix. Unfortunately, we have not been able to provide an objective measure of packing and we cannot quantify its effects on vertical displacement. Table 3. Amount of displacement from the original surface in covered and uncovered levels of the same square Square R Lower level (36 days, covered by 3 cm of sand) % 15.8 84.2 N=19 Range +2*3/-0.8

Displacement in cm +1.0/+2.9 +0*9/-0.9 -l.O/-2.9 -3*o/-4.9 -5.O/-6.9

Upper level (16 days, not covered)

29.4 52.9 17.6 N=17 Range -O-l/-4.5

Square S

Displacement

in cm

+1.0/+2.9 +0.9/-0.9 - l.O/--2.9 -3-o/-4.9 -5.O/-6.9

Lower level (36 days, covered by 2 cm of sand) % 41.2 29.4 235 ,I:: Range +O.S/-5.2

Upper level (16 days, not covered % 5.0 30.0 45.0 20.0 N=20 Range +O*l/-6.9

Sorting by weight

According to Stockton, trampling results in moderate size sorting, with mean weights decreasing with depth. Our results are slightly different. Note that Stockton’s pieces were lighter than ours, having a mean weight of 4 g; our pieces had a mean weight of 12 g. Our correlation table (Table 4) suggests that pieces lighter than 50 g may move upward, Table 4. Correlation

between weight and amount of displacement

Amount of displacement from the original surface (cm) Weight +2.9/ +0.9/ -l.O/ -3-O/ -S.O/ $-l-O -0.9 -2.9 -6.9 -4.9 @ 10 90 24 4 1-9 10-19 4 27 4 I 3 11 6 1 20-29 30-39 4 1 1 1 4 4 1 40-49 50-59 3 1 1 2 60-69 70-79 80-89 90-99 100-109 1 1 100-119 1 120-129

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downward or remain in place. Pieces heavier than 50 g tend to stay on or near the surface on which they were originally placed. However, the number of pieces heavier than 50 g is very small and this proposition, though intuitively justifiable, remains unproven. We found no correlation between displacement and kind of material. Horizontal displacement

The maximum observed horizontal displacement is 85 cm. In uncovered squares the the horizontal displacement is greater than in covered squares and most pieces are affected (Figure 3). The pieces had changed from a horizontal to an oblique or subvertical position in 4.3% of the cases; 21-O”h had been overturned.

S

Figure 3. Horizontal displacements in squares R and S, near the cave entrance. 1, Squares R and S, lower levels, after 36 days of trampling. The objects were covered with 3 and 2 cm of sand respectively. 2, Square R and S, upper levels, after 16 days of trampling. The objects had not been covered with sand. Arrows indicate direction and amount of horizontal displacement. Asterisks indicate pieces that were not displaced.

There is no obvious linear correlation between horizontal displacement and weight. Figure 4 shows that the most displaced pieces are light while the heavy pieces moved little. However, many light pieces were not displaced or were displaced only a short distance. Thus weight is not a good predictor of displacement. The missing third variable is the haphazard occurrence of effective scuffage. The observation that larger objects on paths get greater horizontal displacement (Wilk

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& Schiffer, 1979, p. 533) does not seem to apply to people with no or light footgear. It should be noted, however, that our trampled areas probably received less traffic than Wilk & Schiffer’s urban paths and that only 5% of our items were larger than 10 cm in any one dimension (see also Figure 4 and Table 4). Breakage

Damage to pieces by breakage was limited since the substratum was nowhere very compact and artifact density was low (mean=20 artifacts per square metre). The fragmenta(a 1

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tion index (no. of fragments x lOO/total no. of pieces) is higher in covered squares: 26 versus 13 in uncovered squares. A few flints had very limited visible edge damage; predictably a few long blades were snapped in two. Bone and shells broke more easily than pottery or flint. However, the bone material was slightly weathered and the modern pottery we used is harder than the prehistoric pottery. Conclusions

The experiment demonstrates that trampling can cause mixing of materials belonging to two separate levels. Thus, horizontally associated materials in a layer may derive from the mixing of distinct episodes of site use. Size sorting may appear to be a clue to the occurrence of vertical displacement since pieces lighter than 50 g are more mobile (both upward and downward). In sites with superimposed levels, however, it would be very difficult to know for sure whether small objects in a given level are displaced or in situ, unless information from conjoined pieces is used. It should be noted that, in Stone Age assemblages, many pieces are lighter than 50 g. The degree of vertical dispersal suggests that in caves whose rates of sedimentation are low, some archaeological items may be considerably younger than the matrix in which they are found. The horizontal displacement is not negligible. When doing a spatial analysis of sites where trampling and scuffing might have been a significant factor, it is probably wiser not to trust statistics based on precise measurements of horizontal distances (such as used in nearest neighbour analysis). Otherwise, one may be applying precise methods to ambiguous, imprecise data. The vertical displacement we observed (7-8 cm) is smaller than the displacement observed by Stockton (16 cm). Crucial differences were perhaps the use of small glass splinters in his experiment and the intensive trampling. Finally, both experiments suggest that in sandy deposits the effects of trampling are limited to a zone 10-16 cm deep. More intensive trampling or trampling over long spans of time may well cause larger-scale vertical displacements. However, the evidence, as it stands now, suggests that other agencies should be considered for large-scale vertical separations of archaeological items. Of course, these conclusions apply only to sandy deposits and to archaeological materials of the kind we used. Clearly the degree of consolidation of living surfaces at the time of occupation is a factor that should be considered in stratigraphic studies. Note. While this paper was in review, we received a manuscript by Gifford et al. (in press) which dealt with two trampling experiments. These experiments support Stockton’s and our conclusions about the effects of trampling and the extent of vertical displacement sandy deposits. Acknowledgements

The work of Paola Villa has been supported by grants from the American Philosophical Society and the Leakey Foundation. We also wish to thank Karl W. Butzer, John W. Fisher, Jr., Patricia Phillips and one unknown reviewer for their careful and constructive criticism of this paper. References

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