trampling the arh record

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Society for American Archaeology

Trampling the Archaeological Record: An Experimental StudyAuthor(s): Axel E. NielsenSource: American Antiquity, Vol. 56, No. 3 (Jul., 1991), pp. 483-503Published by: Society for American ArchaeologyStable URL: http://www.jstor.org/stable/280897

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TRAMPLING THE ARCHAEOLOGICAL RECORD:AN EXPERIMENTAL STUDYAxel E. Nielsen

This paper reports on several experiments carried out to explore the transformations of the archaeological recordaffected by trampling. These transformations include changes in artifact distributions andformal alterations thatshould be taken into account when carrying out studies of activity areas. The experiments were made on dry,hard-packed surfaces and in the same sediments after a rain. The materials used were bones, obsidian flakes,sherds, and fragments of brick and wood. The analysis focuses on vertical displacement, horizontal displacement,and damage (breakage, microflaking, and abrasion), paying special attention to the response of the troddensubstrate and its implications for the whole process. The interaction of trampling with otherformation processes(e.g., maintenance) also is considered. The main patterns observed in the trampled materials are vertical andhorizontal size sorting, and characteristic size distributions in sherds. These empirical generalizations are thenintegrated in a model that can help to identify trampled contexts and assess theirpotentialfor behavioral inference.El presente art?culo describe varios experimentos realizados con el prop6sito de explorar las transformacionesproducidas por pisoteo en el registro arqueologico. Tales transformaciones incluyen cambios en la distribucion de

artefactos y alteraciones formales que deben tenerse en cuenta al realizar estudios de areas de actividad. Losexperimentos fueron efectuados sobre superficies muy compactas, secas y luego de una lluvia. Se utilizaron huesos,lascas de obsidiana, tiestos y fragmentos de ladrillo y madera. Los aspectos que se analizan son desplazamientovertical, desplazamiento horizontal y danio (fractura, microlascado y abrasion), prestando especial atencion a larespuesta del substrato pisoteado y sus implicancias para el proceso en su conjunto. Tambien se considera lainteraccion delpisoteo con otros procesos deformacion (p.e., mantenimiento). Los principales patrones observadosen los materialespisoteados incluyen ordenamiento verticaly horizontalpor tamanioy distribuciones caracteristicasen la dimensi6n de los tiestos. Estas generalizaciones empiricas son luego integradas en un modelo que puedecontribuir a identificar contextos pisoteados, asi como a evaluar su potencial para establecer inferencias de cardcterconductual.

Since Stockton's (1973) pioneering study, trampling by humans and animals has been recognizedas a major process by which archaeological materials and deposits are transformed in their formaland spatial attributes (e.g., Schiffer 1983, 1987). Understanding the potential effects of this processis a prerequisite for many behavioral inferences in situations where treadage is likely to have takenplace.During the last two decades, for instance, many studies have attempted fine-grained reconstruc-tions of the spatial organization of living floors. Typically these analyses identify discrete areasdevoted to limited groups of activities like food processing and consumption, storage, trash disposal,tool manufacture and maintenance, resting, etc. In order to make these kinds of inferences it isnecessary to know minimally: (a) the activities in which the artifacts were used; (b) the circumstancesthat led to artifact deposition (whether they constitute primary, secondary, or de facto refuse); and(c) if there have been changes in their formal and spatial attributes after deposition. It is in thecontext of this last problem that trampling, along with other processes of disturbance have to be

taken into account. Intensive trampling modifies the horizontal distribution of artifacts, it obscurespatterns existing in their original deposition, and eventually introduces new trends in their spatialarrangement. By producing vertical migration of materials it also can move artifacts across strati-graphic units, and mix in the same deposits items originating in different occupations. When trodden,artifacts undergo several types of damage, like breakage, microchipping and abrasion. The resultingtraces sometimes mimic the damage produced by use or by other postdepositional processes, and

Axel E. Nielsen, Laboratory of Traditional Technology, Department of Anthropology, University of Arizona,Tucson, AZ 85721, and Cdtedra de Prehistoria y Arqueologia, Escuela de Historia, Universidad Nacional deCordoba, ArgentinaAmerican Antiquity, 56(3), 1991, pp. 483-503.Copyright? 1991 by the SocietyforAmericanArchaeology

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therefore can unwittingly lead to erroneous functional interpretations. Since trampling is a ubiq-uitous process on occupation surfaces, its effects cannot be overlooked when assessing the suitabilityof particular deposits for carrying out spatial studies at the microscale (Clarke 1977).Trampling also can be considered a broad category of human activity in itself, or a commonelement of various activities. Some models concerning the differential use of space can be charac-terized in terms of sharp differences in the amount of human traffic. By inferring the presence andrelative intensity of trampling in different spatial units rough functional distinctions can be made(e.g., storage rooms vs. habitation rooms; areas of domestic or restricted circulation vs. areas opento public traffic [cf. Whittlesey et al. 1982]).A number of authors have taken into account possible alterations of deposits resulting fromtrampling based on "reasonable assumptions" of what its effects are likely to be (e.g., Bradley andFulford 1980; Hughes and Lampert 1977; Rosen 1986, 1989). In addition, there have been severalattempts to explore trampling ethnoarchaeologically (De Boer and Lathrap 1979; Gifford 1978;Gifford and Behrensmeyer 1977; Wilk and Schiffer 1979) and experimentally under different degreesof control (Behrensmeyer et al. 1986; Courtin and Villa 1982; Flenniken and Haggerty 1979; Gifford-Gonzalez et al. 1985; Lindauer and Kisselburg 1981;Muckle 1985; Olsenndisselburg 19n and Shipman 1988; Pintar1987; Pryor 1988; Stockton 1973; Tringham et al. 1974; Villa and Courtin 1983). These studieshave focused primarily on two issues: (1) how human trampling disturbs stratigraphic sequencesby producing vertical migration of items, and (2) how treadage generates patterns of damage (mainlyin lithics and bone) in order to differentiate them from damage produced by use or butcheringactivities.

Although several generalizations have begun to emerge as a result of this work, it is surprisingthat the results of different studies vary widely and are even contradictory in many respects (compare,for example, Tringham et al. [1974] and Flenniken and Haggarty [1979] on edge damage, or Gifford-Gonzalez et al. [1985] and Pintar [1987] on the relation between size/weight of artifacts and verticaldisplacement). This situation indicates that these kinds of experiments will have to be repeatedmany times for reliable generalizations to be drawn, and that considerable work is still neededbefore we are able to apply them to archaeological inference.The present paper reports on six experiments designed to examine some of the contradictoryresults achieved by previous studies and to explore aspects of trampling processes that have receivedlittle attention in the literature. These include: (a) patterns of ceramic breakage, (b) the influence ofthe different density of various materials on displacement, and (c) the interaction between tramplingand other formation processes.MATERIALS AND PROCEDURES

The six experiments are labeled TR-I through TR-VI and were carried out in backyards and ina park in the city of Tucson. A summary description of them is presented in Table 1. The nextthree sections offer details about the trampled substrate, the materials used, and the design followedin each case.The Substrate

Except for TR-III, all experiments were performed on dry, highly consolidated surfaces with novegetation cover. TR-III was carried out on the same sediments but five hours after a heavy rainin order to assess the effects of trampling on a wet, softer substrate.Two attributes of the substrate are considered to have the most influence on the way tramplingimpacts the archaeological record: texture and penetrability. A grain-size analysis of the sedimentsin the trampled areas showed that according to their texture they could be classified as "muddygravels" (Folk 1980): 79 percent gravel, mostly in the granule fraction; 10 percent sand; 11 percentmud.A pocket penetrometer (Bradford 1986) was used to measure the penetrability of the substrate,with limited success. This is a hand-operated, calibrated-spring tester that measures penetrabilityin kilograms per square centimeter necessary to stick its tip into the ground. This is the only technique

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Table 1. Summary of Features of Each Experiment.TR-I TR-II TR-III TR-IV

Number of items 318 173 78 88Materials used bones, lithics, sherds bones, lithics, sherds bones, lithics, sherds sherdsSize of original 1 x 1 m 1 x I m 1 x 1 m .5 x .5 m .

concentrationWet/dry dry dry wet drySoil penetrabilitya 2.49 >4.5 1.63 >4.5Number of cross- 1,500 800 800 100, 200, 300,ings 400, 800Variables consid- vertical-movement, vertical and horizon- vertical and horizon- fractureered damage tal movement tal movement

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Table 2. Size Distribution of Pieces Used in TR-I, TR-II, andTR-III.Size Categorya

1 2 3 4 5 6 7 TotalTR-I

Bones 11 25 23 19 6 1 15 100Sherds 25 30 26 19 6 1 0 107Lithics 28 41 27 7 5 2 1 111Total 64 96 76 45 17 4 16 318TR-II

Bones 4 5 9 3 7 2 4 34Sherds 2 12 1 3 11 14 9 12 73Lithics 17 20 15 10 2 1 1 66Total 23 37 37 24 23 12 17 173TR-IIIBones 1 5 10 6 3 0 5 30Sherds 0 7 7 1 3 2 2 22Lithics 7 8 5 3 1 2 0 26Total 8 20 22 10 7 4 7 78

a C,7,t =_, lr- " mm i - = '11m m. 4 = 4 l_50 mm; 5 = 51-60 mm; 6 = 61-70 mm; 7 = >70 mm.

for measuring the resistance to penetration of a surface, since others, like bulk density (Black 1965:381), measure the compaction of the top layer as a whole. Its results, however, are not preciseenough to be taken as an absolute measure of penetrability but rather as a relative estimation forbroad comparisons. Each locality was tested over an area of 5 m2 (10 points per m2) immediatelybefore and after the experiments in order to assess variations in penetrability. The means of thesemeasurements for each case are displayed in Table 1 and discussed in detail below.Materials

The materials used were obsidian flakes, coyote and sheep bones weathered 2-3 months (fragmentsof mandible, diaphysis and articular parts of long bones, and vertebrae), fragments of oak woodand brick, and sherds from the following five types of pottery: (a) High-tempered slabs made ofcommercial clay (Westwood EM-207) fired at 700?C for 30 minutes (thickness 7 mm); (b) smallMexican low-fired globular vessel (12 cm high, wall thickness 4.3-5.6 mm); (c) large Mexican low-firedglobular vessel (40 cm high, wall thickness 4.8-7.4 mm); (d) biglobular (gourd-shaped) Mexicanvessel (wall thickness 4.5-7.0 mm); (e) Italian high-fired flower pot (wall thickness 3.9-4.8 mm withan increase to 6.2 mm in a 30-mm band along the rim).These types are presented in order of increasing hardness, determined mainly by differences infiring temperature, and therefore the grades A through E can be considered a rough ordinal measureof the strength of the paste.The Experiments

During the experiments attention was paid to three different aspects of trampling. TR-I throughTR-III focused on horizontal and vertical movement and general damage in artifacts. TR-IV andTR-V were designed to examine patterns of ceramic breakage, and TR-VI focused on the influenceof material density and object "bulk"' on horizontal migration. Accordingly, three basic designswere followed.TR-I, TR-II and TR-III. Before each experiment all items were numbered and weighed, andtheir maximum length was recorded (Table 2). The flakes were spray painted to facilitate the

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identification of damage. All pieces were then placed in a 1 x- 1 m square, mapped, and exposed todifferent amounts of trampling. The areas were then excavated with paint brushes, recording thelocation of every object and noting the existence of abrasion, microflaking, or breakage. To facilitatethe mapping process in TR-II, instead of triangulating each item (as in TR-I and TR-III), proveniencewas recorded by 20-x-20-cm grid units.The amount of trampling was measured in terms of the number of crossings over the squarewhere the artifacts were placed. Each crossing represents two steps per square meter. Attention waspaid to enter the area evenly from all possible directions. All tramplers weighed 62-75 kg and woretennis shoes. The number of crossings was 1,500 for TR-I and 800 each for TR-II and TR-III.TR-IVand TR-V. In the first case, a gourd-shaped pot (type D) was dropped from 1.5 m on acement floor. The resulting fragments were measured (maximum length, width, curvature, andweight), and classified in 1 cm size categories by maximum length. Pieces smaller than 1 cm werenot considered, and those bigger than 71 mm were lumped in category "7." Sherds included in eachsize category were spray painted with different colors. The assemblage was then laid out randomlyin a 50-x-50-cm area, trampled 100 times (each crossing one step on the material), collected, andmeasured again. The size distributions for fragments belonging to each original size category (iden-tified by a distinct color) were recorded separately. The same procedure was repeated four times(100, 200, 300, 400 crossings) and again after 400 crossings, for a total of 800 crossings.TR-V followed the same general design using a flower pot (type E) and a very low-fired Mexicanvessel (type B). The size distributions of sherds were recorded after the initial fracture, and after50, 100, and 400 crossings.TR-VI. Fragments of oak wood and ceramic brick of three different sizes (73.4 cm3, 17.6 cm3,and 6.3 cm3) and 20 sherds were laid on a frequently used dirt path in a park in Tucson. Objectswere scattered along the line of most intensive traffic and mapped. Changes in their horizontaldistribution were recorded after three and six days.

RESULTSIn the following sections the results of the experiments are described and discussed consideringseparately four kinds of transformations that trampling can produce in the archaeological record:

changes in the trampled substrate, in the vertical position of items, in their horizontal distribution,and various classes of damage.Trampling/Substrate Interaction

The measurements taken before the first experiment showed that soil penetrability is extremelyvariable, even within small areas (TR-I before: mean = 2.49 kg/cm2, s.d. = .99, range = .75-4.5).It was expected that trampling would increase the compaction of the soil, reducing its penetrability.However, the measurements taken after treadage demonstrated that the area was 14 percent morepenetrable (mean = 2.14 kg/cm2) and 17 percent more homogeneous (s.d. = .83). The erosionproduced by the feet, enhanced by the presence of the artifacts as abrasive media, resulted in theformation of a loose, more penetrable top layer. The excavation revealed that this top layer was 1-2 cm thick and rested on an extremely compact and almost impenetrable level. All artifacts thatmigrated downward were included in the loose layer; none of them penetrated the second one. Thesame phenomenon was observed in the other experiments performed on dry soils, though it couldnot be quantified because the surfaces initially were so hard that they exceeded the range of thepenetrometer scale (maximum 4.5 kg/cm2).To corroborate this point, observations were made on segments of intensively used paths ondifferent types of sediments (including dirt sidewalks and gardens with high humus content) on thecampus of the University of Arizona and across the city of Tucson. In all cases, a loose cover 4-15 mm thick overlaid an extremely compact layer. In three cases the loose material was brushedaway over 1 x- 1 m areas, exposing the hard surface. After one week of treadage the loose cover hadbegun to develop again. Later, a heavy rain compacted this material, but the same process ofloosening was again observed after three or four days.

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Table 3. Number of Items of EachKind of MaterialBuriedandon the Surface n TR-I and TR-II.Surface Subsurface Totala

TR-IBones 48 (51.6%) 45 (48.4%) 93Sherds 75 (40.8%) 109 (59.2%) 184Lithics 27 (21.9%) 96 (78%) 123150 (37.5%) 250 (62.5%) 400

TR-IIBones 22 (66.7%) 11 (33.3%) 33Sherds 179 (83.6%) 35 (16.4%) 214Lithics 36 (51.4%) 34 (48.6%) 70237 (74.8%) 80 (25.2%) 317

a The differencesbetween the numbersof items recoveredandthose in theoriginal assemblages as described in Table 2 are due to the combined effectsof loss and breakage.

The same phenomenon was recorded by Gifford-Gonzalez et al. (1985:808) in their experimenton loamy soil, and is familiar to ecologists (e.g., Liddle 1975; Weaver and Dale 1978) who conceivetrampling both as an erosive process that increases the depth of the paths and as a compactingprocess that increases the bulk density of the soil near the surface. For a given trampling agent thereexists a maximum stable compaction value that is a function of the microstructure of the soil. Theloose cover is a more dynamic element that is likely to vary in thickness depending not only on thesoil, but also on the intensity of treadage, slope, and patterns of rainfall.Trampling after rain (TR-III) had different effects. The muddy surfacewas initially very penetrable(mean = 1.63 kg/cm2; s.d. = .55), but doubled its compaction after treadage (mean = 3.25 kg/cm2;s.d. = .58). No loose layer developed, and very few artifacts were buried completely. Most of themwere stuck in the soft substrate during the first few crossings, and remained in the same position

throughout the experiment. At the end they still were visible from the surface.These various responses of the substrate are important for understanding many effects of trampling.They will be referred to while discussing particular aspects in the following sections.Vertical Displacement

This dimension of trampling processes has received the most attention because it has implicationsfor the interpretation of stratigraphic sequences and chronology. Most of the studies have beencarried out on loose sandy soils, where artifacts from the same original assemblage have beenrecovered in levels separated up to 16 cm (Stockton 1973). In a more compact soil (loam) Gifford-Gonzalez et al. (1985) recorded 3 cm as the maximum downward movement, with 94 percent ofthe items found within the first centimeter.Existing studies are contradictory regardingthe presence of a correlation between the size, weight,or density of the artifacts and their vertical migration. Villa and Courtin (1983:277) worked withdifferent kinds of material and found no correlation between this variable and vertical migration.

They only generalize that pieces lighter than 50 g may move, while heavier ones will tend to remainnear the level where they were placed. Gifford-Gonzalez et al. (1985:811) report that "none of theattributes indexing size or volume yielded a significant correlation with depth below surface."On the other hand, Pintar (1987) obtained a significant correlation value (Spearman's rank coef-ficient = -.8) for size/vertical displacement, suggesting that smaller items tend to be more displaceddownward. A similar correlation is apparent in Muckle's (1985:Table 16) trampling experimentswith shell on a loam substrate. In an ethnoarchaeological context, Gifford (1978:82) previously hadobserved a tendency of smaller objects to be trapped in loose sand surfaces.

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Table4. TR-II: T Tests for Length/Weight nd VerticalMigration.

Surface SubsurfaceMean s.d. Mean s.d. t value pAll items

Length 34.6 13.5 23.5 9.4 3.99 .000Weight 4.9 4.6 1.9 2.5 7.48 .000Bones

Length 54.7 21.5 26.3 9.6 5.24 .000Weight 3.9 3.6 .7 .5 4.02 .001Sherds

Length 30.9 12.3 23.5 9.2 4.09 .000Weight 5.2 4.4 3.0 3.2 3.50 .001LithicsLength 34.6 13.5 23.5 9.4 3.99 .000Weight 4.3 6.1 1.1 1.3 3.05 .004

Note: All lengths are in millimeters and all weights are in grams.

The maximum vertical migration recorded during the experiments here reported was 1.5 cm.IUnder dry conditions, this corresponds to the thickness of the loose top layer discussed in theprevious section. No artifact penetrated into the hard-packed bottom one. It follows from this that(1) the proportion of buried items will covary with the thickness of the loose top layer, and (2) sizesorting will occur, since only objects no thicker than the thickness of the top stratum can be buried.In TR-I, which was carried out on a more permeable substrate (2.49 kg/cm2) and trampled 1,500times, the top level averaged 1.5 cm and contained 62.5 percent of the artifacts. In TR-II (>4.5kg/cm2, 800 crossings), the loose layer did not exceed 1 cm and included only 25.2 percent of therecovered assemblage (Table 3).Size sorting is apparent when the proportions of buried and unburied pieces of each kind ofmaterial are considered. Lithics, which included smaller (Table 2) and flatter items than sherds andbones, were consistently buried in higher proportions. T tests comparing two indicators of size-length and weight-for surface and subsurface sets from TR-II show that these differences are verysignificant (Table 4).No such sorting occurred after treadage on wet ground. The proportions of surface/subsurfaceartifacts seem to vary randomly across material type (Table 5). Moreover, neither length nor weightrender statistically significant differences between objects recovered in various levels (Table 6). Noloose cover is developed in this situation. The materials, rather, are pushed down by the feet andstuck in the permeable substrate. If the surface is penetrable enough (ca. 2 kg/cm2 or less), no sortingoccurs. Trampling under these conditions will tend to fix in their initial horizontal location objectsof all sizes except the very large ones. Eventually, once the soil dries and hardens, erosion willdevelop the loose top layer releasing some of the artifacts, and sorting will start again.These contrasting observations call attention to the different mechanisms of vertical displacementof artifacts trampled on wet and dry ubstrates. Under dry conditions the artifacts tend to act aspassive elements (Pryor 1988) that are covered by the loose dirt scuffed onto them by treadage.Since this loose top layer is very thin, size becomes a critical factor for the materials to be covered.Since in hard-packed surfaces vertical displacement does not exceed 1.5-2 cm, no serious strati-graphic disturbance or archaeologically recognizable sorting by size will occur. These patterns of

vertical migration, however, limit the impact of other forms of disturbance on parts of the assem-blage, since burial will drastically reduce the horizontal movement of the small items and will protectthem from being removed during maintenance.

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Table 5. Number of Items of EachKind of MaterialBuriedandon the Surface in TR-III.Surface Semiburied3 Subsurface Total

Bones 7 (25%) 13 (46%) 8 (29%) 28Sherds 4 (17%) 17 (74%) 2 (8%) 23Lithics 8 (31%) 16 (61%) 2 (8%) 26Total 19 (25%) 46 (60%) 12 (15%) 77a Items that penetrated completely into the substrate but still were partiallyvisible from the surface.

From an archaeological point of view two situations can be expected when dealing with hard-packed sediments as those analyzed in the present study:1. If the surface was buried after a period of dry trampling (as can be assumed, for instance, inthe case of roofed areas), the less-disturbed evidence will be found in a thin (20 mm at most), looselevel overlaying a hard, compact, and probably sterile one (unless previous occupations exist in thesite). Holding constant other factors, the artifacts recovered in that upper layer should be very smalland could be considered primary refuse.2. If the last trampling period took place under wet conditions, items of all sizes will be foundembedded in a relatively hard layer.The previous discussion also illustrates the complexity of the formation of "living floors." Thewidely shared notion that intensively occupied surfaces are hard and highly compacted needs to betreated with caution. For instance, if a period of "dry trampling" preceded the burial of the surface,once the excavation reaches the hard "occupation floor," quite probably the most relevant behavioralevidence in the form of small remains and microartifacts already has been retrieved. Special tech-niques, like microarchaeological analysis (Hassan 1978; Rosen 1986; Stein and Teltser 1989), shouldbe employed to recover this information.Horizontal Displacement

Only two experimental studies have searched for patterns in the horizontal displacement oftrampled artifacts. Villa and Courtin (1983:277) observed that "the most displaced pieces are lightwhile the heavy pieces moved little" but "there is no obvious linear correlation between horizontaldisplacement and weight. . .thus weight is not a good predictor of displacement." Pintar (1987:16-18) arrived at a similar conclusion, obtaining an inverse but nonsignificant correlation betweenlength and horizontal migration of flakes.

Table 6. TR-IIl: T Tests for Length/Weight and VerticalMigration.Surface Subsurface

Mean s.d. Mean s.d. t value pBones

Length 37.4 8.8 35.9 19.0 .21 .84Weight 2.7 1.8 1.8 2.0 .98 .34Sherds

Length 54.7 29.1 26.0 7.1 1.87 .14Weight 21.6 21.7 1.2 1.2 .51 .15LithicsLength 29.( 14.9 27.5 16.3 .12 .92Weight 4.8 11.2 1.5 1.9 .78 .46

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Table 7. Mean Horizontal Migration of Materials in TR-II andTR-III.TR-II TR-III

Bone 78.1 cm (range 0-314) 52.7 cm (range 0-228)Ceramics 41.0 cm (range 0-336) 11.9 cm (range 0-83)Lithics 23.9 cm (range 0-126) 19.2 cm (range 0-122)

The results of TR-II and TR-III concur in general terms with those of Villa and Courtin andPintar. Although the correlations length/horizontal movement and weight/horizontal movementare positive in all the cases, they are not at all significant (range of r values = .0884-.5545). TR-IIIproduced similar results. The only observed difference is that materials moved less than in theexperiments performed on dry surfaces because they were "trapped" in the substrate from thebeginning of the process.

However, when the different materials are compared in terms of their mean horizontal migrationsome trends arise (Table 7). Bones moved more than lithics, whereas, at least in the dry-tramplingcase, ceramics had an intermediate response. Three factors could account for these results: density,size, and shape.Denser materials-like lithics-may have moved less because, holding size constant, they weighmore. It also could be argued that although length is not a predictor of horizontal migration, thereis still a weak positive correlation between both variables. Since bone assemblages included morelarge pieces than lithic and ceramic ones, differences in size still could be responsible for thedifferences in mean displacement.

Shape is a third variable that may be reflected in these figures. Three of the more displaced boneswere vertebrae which, by their very shape, are more likely to be kicked away than flat elements likesherds or flakes. In fact, size and shape are better conceived as a single attribute, that can be referredto as "bulk," which determines the probability of an object being kicked or scuffed by human traffic.TR-VI was designed to examine the relative influence of these variables on horizontal migration.An assemblage with equal numbers of prismatic fragments of oak wood (.59 g/cm3) and brick (1.84g/cm3) distributed in three size categories was used. The sizes were "large" (.57 x .46 x .29 cm =73.4 cm3), "medium" (.45 x .28 x .14 cm = 17.6 cm3), and "small" (.30 x .19 x .11 cm = 6.3 cm3).Each one included eight pieces of each material. Materials and sizes were chosen to maximizecontrasts in density and bulk. Sixteen small sherds (2.6-4.6 cm3, maximum length = 32 mm) wereincluded to facilitate comparisons with the assemblages used in previous cases. All pieces werescattered along a heavily used dirt path in a park in Tucson. The mean displacement recorded afterthree days is shown in Table 8.As noticed previously, denser materials tend to move less when size and shape are held constant.A t test run between wood and brick fragments of all sizes indicates that the differences in horizontalmovement between both materials are significant (t = 1.90; df = 32; one-tail p < .05).

Table 8. TR-VI: Mean Horizontal Displacement of Items afterThree Days of Trampling.Wood Brick

Large (N = 8) 259 69Medium (N = 16) 129 47Small (N= 16) 111 69All sizes 158 59s.d. 192 101

Note: All measurements in cm.

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12 1 6 days10- D 3 days

E 8-Q)0 6

E:32

4.6 cm3 or less 6.3 cm3 17.6 cm3 73.4 cm3Size

Figure 1. TR-VI: Number of pieces of various sizes cleaned up after three and six days.

Size,on the otherhand,showsno statistically ignificant orrelationwith the amountof displace-ment, whether wood and brick are considered together or separately. Sherds were not included inthe calculations because their mean movement was only 7 cm and therefore, they would have skewedthe results.

There is another way, however, of approaching the relation between bulk and horizontal migration.Wilk and Schiffer (1979:533), for instance, based on vacant-lot data, have observed that "[l]argeobjects (ca. 50 cm3) do not stay for long on paths. They are kicked or moved aside."The results of TR-VI support this statement. After three days only one large piece was found inthe path, whereas nine medium and 11 small ones were still in the line of most-intense traffic. Itfollows that the relation between size and displacement is better represented by a model consistingof a series of size thresholds that determine qualitative changes in the response of objects, ratherthan by a linear model.In other words, when trampled on unpenetrable surfaces (>2 kg/m2), objects follow at least threedifferent patterns of horizontal migration according to their size. Very small items (50 cm3) are kicked and scuffed rather than trodden and therefore willmove faster and more systematically to stable positions in the "marginal zone." Less dense elementswill tend to move farther, but again, a horizontal sorting by density is not likely to occur.TR-VI also serves to illustrate how maintenance, by acting selectively upon size, can modify

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Table 9. Breakage ndex(bx)for Nine CeramicAssemblagesAfter DifferentAmountsofTrampling.Number of Crossings Cumulative

50 100 200 300 400 800 1,500 IndexType A TR-I 1.84 1.84Type A TR-II 3.46 3.46Type B TR-V 4.00 1.22 1.23 6.00Type C TR-I 1.36 1.36Type C TR-II 2.05 2.05Type D TR-II 2.89 2.89Type D TR-III 1.45 1.45Type D TR-IV 2.01 1.17 1.08 1.24 1.11 3.18Type E TR-V 1.71 1.10 1.34 2.53

trampling patterns. After three days, seven of the large pieces (both wood and brick) had beenremoved.hremoved.hree days later fragments of all sizes had been cleaned up, but still the larger ones werethe most affected. On the other hand, none of the sherds (all of them smaller than 4.6 cm3) weremissing (Figure 1). In accordance with the McKellar Principle (Schiffer 1987:62), these observationsshow that smaller items are left behind in regularly maintained areas. From the point of view ofspatial analysis, they imply that maintenance eliminates part of the "noise" that trampling introducesin depositional patterns, since bigger, probably more displaced objects, are more likely to be removed.Damage

Different sorts of damage affect each material according to its physical properties. The presentsection focuses on ceramics and lithics. Bones were only abraded and will not be considered here.For a discussion of trampling marks on bone see Behrensmeyer et al. (1986) and Olsen and Shipman(1988).Ceramics. Sherds showed various abrasion traces (Schiffer and Skibo 1989), such as a slightrounding of edges, and in few cases microchipping and delamination, especially along the edges ofpolished surfaces. But breakage is certainly the most obvious kind of damage.To facilitate comparisons among assemblages, a breakage index (bx = number of fragments afterselected trampling episode divided by number of sherds before that episode) was calculated for eachtype of pottery in each experiment. The results are displayed in Table 9.The differences in fragmentation for the same type in different experiments indicates the criticalrole of surface hardness in the process of fracture. The three assemblages trampled on relativelypenetrable soils (A and C TR-I, D TR-III) had a breakage index lower than two, even though thesherds of TR-I were trampled twice as much; the rest of the assemblages (that were trodden onsurfaces harder than 4.5 kg/cm2) exceeded this value.Another trend reflected in these figures is the decreased fracture rate after the first few crossings,showing how the reduction in size increases the strength of the sherds (cf. Kirkby and Kirkby 1976:237).2 Eventually a stable size where no further breakage occurs would be reached. This value wouldbe a function of the microstructure of the paste, sherd thickness and curvature, and the nature(weight and contact surface) of the trampling agent.It follows that after a few crossings sherd size should be unimodal, distributed around a valuethat, when reached, would effect a significant increase in breakage resistance. Untrampled assem-blages, on the other hand, would present a random distribution of sizes produced by the originalfracture of the vessels. Further trampling would result in a slow reduction of the modal value andan increasing positive skewness of the whole distribution. When the modal value reaches the smallestsize category, the whole curve would approximate a Poisson distribution. If this proposition iscorrect, it could be a useful device for recognizing archaeologically trampled assemblages and perhapseven for determining the relative amount of treadage that occurred.

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50 * TYPEC TR-IN=2840 - TYPEA TR-IN=80

30- // - - -,\ TYPE TR-ii... / ....... \N=

..?' " "*< - - - -A-Y^ /v^^^< ...**** TYPE A TR-i20- // / / ^ ^\ \ / *%..:.===..= N=35TYPED TR-11

I0 |L-.---......X-.-..- 'X | N=221 2 3 4 5 6 7

,,~ , ' ",. /-..... - - --

size categoriesFigure2. Size distribution f six ceramicassemblagesbeforetrampling.Size categoriesare: 1 = 11-20 mm;2 = 21-30 mm; 3 = 31-40 mm; 4 = 41-50 mm; 5 = 51-60 mm; 6 = 61-70 mm; 7 = 71 mm or more.In order to test this hypothesis, the size frequency distributions for six ceramic assemblages beforeand after trampling were calculated and put in graphic form. Figure 2 shows the random distributionof size expected for untrampled assemblages, with frequent bimodality and large sherds consistently

represented in most cases. It should be noted that none of these curves reflects the distribution ofa "naturally broken" pot, the sherds of these six assemblages were chosen arbitrarily.

60 TYPEC TR-IN=38

50 - TYPEA TR-1

10 ^ \N=41Zo /t~~~~~~/ "~\~ ~TYPE D TR-II

1 - ..\........... 2... 12

< \ = L < .a,^--.^ l*-sr: TYPED TR-lii0 ' i l \i "^***^^.f--7?'"r> \^ ^^- N=331 2 3 4 5 6 7size categoriesize caotegories

Figure 3. Size distribution of six ceramic assemblages after trampling. Size categories are: 1 = 11-20 mm;2 = 21-30 mm; 3 = 31-40 mm; 4 = 41-50 mm; 5 = 51-60 mm; 6 = 61-70 mm; 7 = 71 mm or more.

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50 0 cross

100 cross40 ---_- .......-, *,; . 200 cross3..0 "-"^"\ 300 cross

400 cross20- -

800 cross

1 2 3 4 5 6 7size categories

Figure 4. TR-IV: Progressive reduction in size of one ceramic assemblage (type D) as a result of trampling.Size categories are: 1 = 11-20 mm; 2 = 21-30 mm; 3 = 31-40 mm; 4 = 41-50 mm; 5 = 51-60 mm; 6 = 61-70mm; 7 = 71 mm or more.After trampling (Figure 3) the least fractured assemblages (Type C TR-II bx = 2.050; Type D

TR-II bx = 2.889; Type D TR-III bx = 1.454) show an unimodal curve with the mode in category3, while the most reduced ones approximate a Poisson distribution. The "abnormally skewed"curves of types A TR-I and C TR-I, considering their relatively small breakage index (1.837 and1.357), are explained readily by their originally skewed distribution which, if types B, E (TR-V) andD (TR-IV) are assumed to be representative cases, are very unlikely to occur in "naturally" brokenpots. Furthermore, if the areas under analyss contain secondary refuse, their original size distributionwill tend to be skewed negatively, provided that smaller objects are more likely to escape maintenanceactivities and be left behind in original activity areas. This would provide an even stronger contrastbetween trampled and untrampled assemblages in secondary refuse.

To determine how much trampling is necessary for the size distribution to adopt each shape,Figures 4-6 wre constructed. In these cases, the curves drawn as solid lines do reflect the size ofsherds produced by the initial breakage of whole pots. In TR-IV, after just 100 crossings only onesherd larger than 71 mm is left (which remains unbroken throughout the process), and the generalcurve already shows the characteristic configuration. Changes after this first stage are much moregradual. This process is even clearer for type B TR-V (Figure 5) in which no sherd exceeds 45 mmin length after 50 crossings.As noted previously, type E was a high-fired flower pot that had the hardest paste among thevessels used. Consequently, it experienced the slowest size reduction (Figure 6). Several fragmentscorresponding to the reinforced rim of the vessel remained relatively large. Unlike type B, whichwas exposed to the same trampling conditions, the mode of the distribution for type E did not reachcategory 1 by the end of the process.The response of this type suggests that particularly strong wares-beyond the range consideredin this study-probably will depart from the trends described thus far. Fragments of storage vesselswith extremely thick walls or ceramics fired at exceptionally high temperatures can be so strongthat the stress of human trampling may not be enough to effect a significant amount of fracture.

The preceding observations can be summarized as follows:1. Holding constant other factors, it can be inferred that a sherd sample has been trampled if itssize distribution is found to be unimodal, with the mode lower than 30 mm, and with no fragments

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0 cross

50 cross100 cross400 cross

- - - -


Figure 5. TR-V: Progressive reduction in size of one ceramic assemblage (type B) as a result of trampling.Size categories are: 1 = 11-20 mm; 2 = 21-30 mm; 3 = 31-40 mm; 4 = 41-50 mm; 5 = 51-60 mm; 6 = 61-70mm; 7 = 71 mm or more.larger than 50 mm in length or just very few corresponding to especially strong parts of the vessels(like the articulation of the body and the base).2. If the penetrability of the soil, the nature of the trampling agents, and the strength of the ceramicmaterial can be assumed approximately constant across samples (i.e., they show a similar range ofinternal variability), and no other cultural formation processes are acting upon this dimension of

0 cross50 cross100 cross400 cross

- - - -


Figure 6. TR-V: Progressive reduction in size of one ceramic assemblage (type E) as a result of trampling.Size categories are: 1 = 11-20 mm; 2 = 21-30 mm; 3 = 31-40 mm; 4 = 41-50 mm; 5 = 51-60 mm; 6 = 61-70mm; 7 = 71 mm or more.

496

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40

30

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10

0

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20

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40 0+100 cr.

- k 0+300 cr.~/' *\ 0+800 cr.

10 -

0 'I I I I1 2 3 4 5 6 7size categories

Figure 7. Size distribution of three hypothetical assemblages combining trampled and untrampled sherds.Size categories are: 1 = 11-20 mm; 2 = 21-30 mm; 3 = 31-40 mm; 4 = 41-50 mm; 5 = 51-60 mm; 6 = 61-70mm; 7 = 71 mm or more.the material, then the degree of positive skewness of the distribution is a relative indicator of theamount of trampling undergone by different assemblages.

Two objections could be raised against these generalizations. First, if "freshly broken" assemblagesare mixed with previously trampled ones, as may happen, for instance, if a path crosses a secondaryrefuse area, would the patterns described above still be recognizable? Would it be possible to detectthe presence of both kinds of artifacts in the mixed assemblage?

To answer these questions, the figures for some trampled and untrampled sets chosen at randomwere combined in three ways (type D TR-IV 0+100 crossings; type A TR-II 0 + type D TR-IV 300crossings; and type D TR-II 0+type D TR-IV 800 crossings) and the distributions represented ingraphic form for the resulting "mixed assemblages" (Figure 7). As can be seen, the "tramplingpatterns" still are perfectly visible. The slightly "high" proportion of large pieces shown in the curveas a solid line could serve as an indicator of the presence of the untrampled set, if these sherds donot have special attributes that would give them higher breakage resistance.

Another process that could produce a similarly skewed size distribution in the assemblages ismaintenance (Schiffer 1987:64), since the biggest artifacts would be retrieved and discarded insecondary refuse areas. Two alternatives exist to distinguish both processes. First, after trampling,a few large fragments corresponding to stronger parts of the vessels that should not be present inrdual primaryefusesidualrimarprobably will remain. Second, in trampled assemblages not subjected tomaintenance, one should find a consistent proportion of big pieces of other kinds of materials moreresistant to fracture (e.g., bone, lithics). It should be recalled, however, that these large items mayhave migrated to the margins of the areas of most intense traffic (see section on horizontal displace-ment above).

A second possible objection stems from the fact that, if the size distribution for untrampled sherdsis random, it is possible-though improbable-that such distributions fall within the specificationsfor trampled material. Type C (TR-I), represented by the solid line in Figure 2, would be a case inpoint. Although it does not represent any "natural breakage," if such a distribution was found itwould be interpreted erroneously as a trampled assemblage.To provide an additional control, it was postulated that there should be a correlation betweenthe mean size of the sherds and their size variability (as measured by the standard deviation of the

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30 beforetrampling25- A A25 A after

0 - trampling._ 20-> - 4)

10-

5-

10 20 30 40 50 60 70Mean Size (maximum length)

Figure 8. Size and size variability for eight ceramic assemblages before and after different amounts oftrampling. Before trampling, r2 = .2438; after trampling, r2 = .8576; a = -3.2830; and b = .4686.

set) through successive stages of reduction. Figure 8 displays a scatter plot for these two variablesin all the assemblages before (nine samples) and after different amounts of treadage (16 samples).As predicted, the points representing the sets before reduction are dispersed throughhe diagram(Pearson's r = .4938), while those corresponding to trampled assemblages are aligned in a regularpattern (r = .9271) indicated by the regression line in the graph. The trajectory of individualassemblages through increasing treadage show even higher correlations (r = .99 for types D TR-IVand B TR-V). Certainly, the general validity of this pattern should be tested and, eventually,readjusted using a larger sample. However, considering that the materials used were extremelyheterogenous, it is likely that the pattern will hold in any archaeological situation regardless of theinternal variability of the ceramic material.Therefore, a second procedure for differentiating trampled and untrampled samples can be pro-visionally postulated. If a given ceramic assemblage has been trampled, the mathematical expressionof the regression shown in Figure 8 (y = bx + a) should predict its standard deviation from itsmean size. In other words, the expression S = .4686 X - 3.283, where 5S= standard deviation oflength, and X = mean length, should hold with a margin of error of ? 1.19 (measurements takenin mm), corresponding to the standard error of the regression line. The range of values predicted

Table 10. PredictedandActual StandardDeviations of Lengthfor Nine Ceramic Assemblages Before Trampling.Assemblage Mean (mm) Predicted S Actual SA TR-I 29.59 9.39-11.77 12.28A TR-II 41.63 15.03-17.41 18.79B TR-V 42.29 15.34-17.72 25.60C TR-I 33.86 11.39-13.77 9.08C TR-II 54.00 20.83-23.21 13.76D TR-II 59.33 23.33-25.71 23.85D TR-III 41.36 14.91-17.29 18.58D TR-IV 40.86 14.67-17.05 21.16E TR-V 45.73 16.96-19.34 24.09

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25 3 ass.trampled

20 - 1 ass. trmp.? + 1 untrmp.C - A'> A o....- . 3 ass.qa 15C) o ,,-- untrampled

^ -U *e ---'"" o -~0

5-^ -6^ 00 i

15 20 25 30 35 40 45Mean Size (maximum length)

Figure 9. Size and size variability for three hypothetical ceramic assemblages combining trampled anduntrampled sherds. The line represents the pattern obtained from the eight assemblages with various amountsof trampling (small circles) (see Figure 8).

by this procedure and the actual ones for the standard deviation of the nine assemblages used inthis study before trampling are displayed in Table 10. Most values of S fall, as expected, out of thepredicted range for trampled assemblages, including C TR-I that could have been mistaken astrampled following the size-distribution procedure. The only exception is D TR-II, that, in anyevent, can never be mistaken as trodden if its size distribution-35 percent of the sherds larger than71 mm-is considered (Figure 2, long dashed line).

Finally, Figure 9 shows the ability of the size-variability procedure for detecting assemblagescontaining various types of untrampled material and for classifying correctly other hypothetical"mixed" samples. The line represents the pattern obtained on the basis of all trampled sets (smallcircles). As expected, the solid circle (three trampled sets added) falls close to the line, within therange of variability predicted for trodden materials. The combination of one trampled and oneuntrampled assemblages (solid triangle) is situated out of this range, and three untrampled setstogether fall even farther away from the line.Consequently, it is suggested that the application of both procedures, size distribution and sizevariability, by using two different dimensions of the data, can discriminate with a high degree ofconfidence between trampled and untrampled sherd samples, and even establish relative amountsof treadage (or activity) on the material if certain conditions are met.Lithics. The three kinds of damage considered here (breakage, microflaking, and abrasion) wereobserved on lithics. Table 11 shows the number of pieces representing the first two alterations in

Table 11. Number of Flakes Showing Breakage andMicroflaking.TR-I TR-II TR-III

Broken 29 (24.8%) 17 (19.8%) 5 (19.0%)Microflaked 31 (26.5%) 32 (37.2%) 6 (23.0%)Broken and microflaked 11 (9.4%) 25 (29.7%) 2 (8.0%)Undamaged 46 (39.3%) 12 (13.9%) 13 (50.0%)Total 117 86 26

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TR-I, TR-II, and TR-III. As noted by Gifford-Gonzalez et al. (1985:813), breakage is more frequenton harder surfaces (TR-II). Artifacts in TR-II were more damaged even though artifacts in TR-Iwere trampled twice as much. Lithics trampled on a wet substrate (the most permeable) were theleast damaged. Abrasion was especially severe on prominent parts of the flakes, such as percussionbulbs and dorsal ridges.There are considerable differences in the literature concerning the type of edge damage producedby trampling. Working with obsidian flakes, Tringham et al. (1974:113), who performed the firstexperiment on this subject, established three criteria for differentiating trampling from use damage.The scars are randomly distributed around the perimeter of the flakes; they occur only on the surfaceopposite to the trampler; and they lack fixed orientation or size, but are characterized by markedelongation.A later experiment carried out by Flenniken and Haggerty (1979) contradicts these criteria. Theyfound that out of 157 flakes (37 percent) that underwent edge modification during treadage, 56 (13percent) could be classified as "tools," and their edges were remarkably similar to used ones. Theypointed to the absence of polish as the only definitive indicator of trampling damage as opposed touse.On the other hand, Gifford-Gonzalez et al.'s (1985) and Pryor's (1988) studies agreewith Tringhamet al. concerning the sparseness of the scars along the edges, but these scars were not elongated andoriginated on both surfaces of the artifacts.The results of the experiments reported here are in general agreement with the conclusions of thelatter authors. Most pieces show one to three isolated scars randomly distributed along the edges,regardless of their angle. They originate on either surface and no distinctive shape or size could beidentified, except for a trend of larger scars to occur on steeper edges. However, six or seven piecesfrom the dry-trampled assemblages depart from this general trend. They show rows of continuousparallel scars along one or more edges that could be mistaken easily for intentional retouch.

CONCLUSIONSSeveral transformations that occur in spatial and formal attributes of materials exposed to tram-pling have been discussed separately in previous sections. These observations are now integratedto outline sets of traces in the archaeological record that can help to identify trampled contexts. Ofcourse, the applicability of these generalizations is restricted to situations where the relevant con-ditions are comparable to those considered in the present study. Minimally, these conditions are:similar materials in terms of size, density, and fracture properties, and trampling by humans onhard-packed substrates (ca. 2 kg/m2 penetrability or more when dry).A small amount of trampling will cause the migration of bulky items to the margins of thetrampled area where they will stay stationary unless affected by other factors. Small and mediumsize objects will move randomly within the trafficzone, blurring previous patterns in their horizontalarrangement that might have existed within therampled area. Only very small items will remainin their original location by being "absorbed" inheriginal substrate. In other words, even moderatelytrampled areas will be composed of a "marginal zone" characterized by a high proportion of bulkyartifacts, and a "trafficzone" with small- and medium-size items randomly scattered and very smallones buried close to their original spot of deposition.In this initial stage the sherds already will exhibit the typical relation between mean and standarddeviation of size and will adopt a size frequency distribution that resembles a bell-shaped curve ora Poisson distribution, depending on the strength of the paste, the thickness of the sherds, and theircurvature. Few lithics will break and present isolated and randomly distributed flake scars alongtheir edges. A few damaged edges may mimic retouching.If trampling continues and the area is not cleaned, the original pattern, preserved by the smallpieces initially trapped in the substrate, will be obscured increasingly by the "absorption" of newsmall pieces produced by the fracture of objects after having been displaced horizontally. On theother hand, medium-size items that are unlikely to be trapped in the substrate will reach graduallystable positions in the "marginal zone."

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Thus, while original "fine-grained" horizontal configurations within the traffic zone will be nolonger recognizable, the contrast between traffic and marginal zones will be stronger. The formerwill be characterized by a high frequency of small artifacts and microartifacts randomly scattered,low proportion of medium-size items, and virtually no bulky ones (cf. O'Connell 1987:95). Thelatter will have high frequencies of artifacts in large size categories and very few in the small ones.All of them will be displaced far from their original locations.The substrate of the traffic zone will consist of a top loose layer (5-20 mm thick) containing manysmall artifacts and microartifacts and an extremely compacted, sterile bottom layer. The latterusually is identified during excavation and can orient the application of microarchaeological tech-niques to recover the former. Differences in penetrability between traffic and marginal zones alsocan be recognized in the archaeological record (see Koike 1987).In addition, the whole assemblage should show severe damage--randomly scarred edges andabraded ridges in most flakes, and rounded, microchipped, and delaminated edges in sherds. Thesize frequency distribution of sherds should be extremely skewed as well.When applying these generalizations to archaeological cases it should be kept in mind that everydeposit is the result of multiple formation processes, including human activities and the action ofenvironmental factors. Sometimes the material effects of these processes overlap, this is, differentprocesses can produce similar traces. For instance, ethnoarchaeological studies have demonstratedthat horizontal size sorting also can result from the spatial organization of activities themselves andcorresponding disposal modes (Binford 1978; O'Connell 1987), or cleaning (DeBoer and Lathrap1979; Simms 1988). When inferring trampling in archaeological cases, therefore, one should consideras many traces as possible, as well as relevant independent data, in order to differentiate it fromother processes that may have acted upon the materials and generated similar patterns.On the other hand, the interaction with other formation processes can modify the effects oftrampling itself. For example, as has repeatedly been demonstrated, maintenance operates selectivelyon larger items. If an intensively trampled area is frequently cleaned, clear depositional patterns arelikely to be preserved in the distribution of small artifacts in the traffic zone. Bigger objects, thereforemore displaced, will be cleaned up systematically and will not remain long enough to be randomlydisplaced and fractured in different locations, contributing additional small artifacts that wouldobscure existing patterns reflected in this size fraction. On the other hand, in rarely maintainedareas the contrast between marginal and traffic zones will be stronger, and damage will be moresevere because artifacts will be exposed longer to treadage.Differences in environmental conditions, or in exposure to such conditions, can effect variationsin these patterns as well. Rain, for instance, generates a muddy and very penetrable surface thatwill "trap" all artifacts regardless of their size. This will prevent the objects from moving horizontallyand will reduce damage. However, once the surface dries and treadage erosion generates the loosecover again, larger items will be released, start moving horizontally, and be exposed to retrievalduring cleaning activities, while smaller ones will remain embedded in the top layer. If rains arevery frequent and evenly distributed throughout the year, very little horizontal movement (andpostdepositional patterning) will take place in unroofed trampled areas. Even in spaces regularlycleaned, compact occupation floors will be found with artifacts of various sizes embedded anddamage will be less severe. On the other hand, if the amount of rainfall is small and unevenlydistributed during the year (as is often the case in semiarid environments), trampling patterns willbe much clearer, more so if the surfaces were buried or abandoned after a long period of "drytreadage."Future studies can define variations in the patterns discussed above when treadage occurs underdifferent conditions. This "basic" understanding gained through experimentation, together withrelevant ethnoarchaeological observations, can provide criteria applicable to the archaeologicalidentification of trampling, and hence contribute to assess the potential of particular contexts forvarious kinds of behavioral inference.

Acknowledgments. The researchreportedon here has been supportedby the Laboratoryof TraditionalTechnology, Department of Anthropology, University of Arizona, Tucson. I want to express my gratitude to

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several persons that help me during the work. Michael Schiffer gave me access to the lab and made insightfulcomments on drafts of the paper. William Walker, James Spicer, and Nieves Zedeniio ent their feet and sharedgenerously their time and ideas. James Skibo, Chuck Bollong, William Walker, and the editorial staff ofAmericanAntiquity helped to make several points clearer in the text. I also benefited from the comments of Steven Simms,Duncan Metcalfe, Harold Hietala, and an anonymous reviewer. Finally, I am indebted to the Fulbright Com-mission for their support.

REFERENCES CITEDBehrensmeyer, A., K. Gordon, and G. Yanagi1986 Trampling as a Cause of Bone Surface Damage and Pseudo-cutmarks. Nature 319:768-771.Binford, L. R.1978 Dimensional Analysis of Behavior and Site Structure: Learning from an Eskimo Hunting Stand. Amer-ican Antiquity 43:330-361.Black, C. A. (editor)1965 Methods of Soil Analysis. Part 1:.Physical and Mineralogical Properties, Including Statistics of Mea-surement and Sampling. American Society of Agronomy, Madison.Bradford, J.1986 Penetrability. In Methods of Soil Analysis. Part 1.:Physical and Mineralogical Methods. 2nd ed., editedby A. Klute, pp. 463-477. American Society of Agronomy, Madison.Bradley, R., and M. Fulford1980 Sherd Size in the Analysis of Occupation Debris. Institute ofArchaeology, Universityof London, Bulletin17:85-94.Clarke, D.1977 Spatial Archaeology. Academic Press, New York.Courtin, J., and P. Villa1982 Une experience de pietinement. Bulletin de la Societe Prehistorique Francaise 79:117-123.De Boer, W., and D. Lathrap1979 The Making and Breaking of Shipibo-Conibo Ceramics. In Ethnoarchaeology: Implications of Ethnog-raphyfor Archaeology, edited by C. Kramer, pp. 103-138. Columbia University Press, New York.Flenniken, J., and J. Haggerty1979 Trampling as an Agency in the Formation of Edge Damage: An Experiment in Lithic Technology.Northwest Anthropological Research Notes 13:208-214.Folk, R.1980 Petrology of Sedimentary Rocks. Hemphill, Austin.Gifford, D.1978 Ethnoarchaeological Observations of Natural Processes Affecting Cultural Materials. In Explorationsin Ethnoarchaeology, edited by R. Gould, pp. 77-101. University of New Mexico Press, Albuquerque.Gifford, D., and A. Behrensmeyer1977 Observed Formation and Burial of a Recent Human Occupation Site in Kenya. Quaternary Research8:245-266.Gifford-Gonzalez, D., D. Damrosch, J. Pryor, and R. Thunen1985 The Third Dimension in Site Structure:An Experiment in Trampling and Vertical Dispersal. AmericanAntiquity 50:803-818.Hassan, F.1978 Sediments in Archaeology: Methods and Implications for Paleoenvironmental and Cultural Analysis.Journal of Field Archaeology 5:197-213.Hughes, P., and R. Lampert1977 Occupational Disturbance and Types of Archaeological Deposits. Journal of Archaeological Science 4:135-140.Kirkby, A., and M. Kirkby1976 Geomorphic Processes and the Surface Survey of Archaeological Sites in Semi-arid Areas. In-Geoarchaeology, edited by D. Davison and M. Shackley, pp. 229-253. Duckworth, London.Koike, H.1987 Measurement of Soil Hardness of Floor Surface for a Reconstruction of Activity Patterns in a Prehis-toric Dwelling. Asian Perspectives XXVII:5-13.Liddle, M.1975 A Selective Review of the Ecological Effects of Human Trampling on Natural Ecosystems. BiologicalConservation 7:17-36.Lindauer, O., and J. Kisselburg

1981 Primary and Secondary Breakage. Paper presented at the 46th Annual Meeting of the Society forAmerican Archaeology, San Diego.

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NOTES1It was not considered appropriate to assess relative amounts of vertical displacement within such a smallrange. As a consequence, when analyzing its relation with artifact attributes in the following discussion, verticaldisplacement is treated as a categorical variable (surface, semiburied, subsurface).2 The evolution of type D in TR-IV illustrates the influence of the development of the loose top layer previouslydescribed on fragmentation rates. From the second round on, the breakage index declines regularly throughoutthe process except in the third round (200-300 crossings) when the value falls below the trend (bx = 1.082, ascompared to 1.175 in the second round and 1.124 in the fourth). During this experiment each trampling roundwas carried out on a different spot to eliminate variations in breakage resulting from alterations in the penetrabilityof the soil. The only exception was the third round, in which the artifacts were placed on the same spot as inthe second one. The more penetrable character of the previously disturbed surface clearly was reflected in thedrop of the breakage index.

Received July 17, 1990; accepted March 5, 1991

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