Society for American Archaeology

The Third Dimension in Site Structure: An Experiment in Trampling and Vertical DispersalAuthor(s): Diane P. Gifford-Gonzalez, David B. Damrosch, Debra R. Damrosch, John Pryor,Robert L. ThunenSource: American Antiquity, Vol. 50, No. 4 (Oct., 1985), pp. 803-818Published by: Society for American ArchaeologyStable URL: http://www.jstor.org/stable/280169 .Accessed: 20/09/2011 11:21

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THE THIRD DIMENSION IN SITE STRUCTURE: AN EXPERIMENT IN TRAMPLING AND VERTICAL DISPERSAL

Diane P. Gifford-Gonzalez, David B. Damrosch, Debra R. Damrosch, John Pryor, and Robert L. Thunen

Two measured and weighed assemblages of lithic debitage were subjected to human treadage, one set on a compact sandy silt ("loam") substrate, the other on unconsolidated sand. The assemblages were excavated, plotted in three dimensions, and documentedfor damage. Downward migration of pieces at the loam site was minimal: fracture of small pieces was the dominant damage pattern. Most sand site pieces migrated to 3-8 cm depth; vertical distribution of pieces approximated a normal curve, and edge-damage to larger pieces was the dominant damage pattern. Vertical distribution of artifacts at the sand site approximated a pattern observed in two other trampling experiments and a number of archaeological occurrences. Factors influencing these distributions are discussed.

Most archaeologists now recognize that assessing the influence of various site-formation processes is a prerequisite to inferences about human behavior from spatial or statistical patterning in ar- chaeological materials. Butzer (1982) has recently discussed cultural and sedimentological aspects of archaeological site formation in detail, and Schiffer (1983) has recently reviewed the growing body of literature on site formation processes in this journal.

Some site-formation research has focused on vertical artifact distributions within sites. Interest in closer specification of the processes that structure sites' vertical aspect arose more or less simul- taneously in analyses of archaeological sites (e.g., Barton and Bergman 1982; Cahen et al. 1979; Cahen and Moeyersons 1977; Moeyersons 1978; Stockton 1973; Van Noten 1978; Villa 1982), and in ethnoarchaeological studies (e.g., Gifford 1980; Gifford and Behrensmeyer 1977; Yellen 1977).

This article reports on the findings of an experiment assessing the potential of on-site human traffic (treadage and trampling) to cause subsurface, vertical dispersal of artifacts in a site's substrate. Our findings augment those of Courtin and Villa (1982; Villa and Courtin 1983) and Stockton (1973), and they are relevant to recent discussions of vertical dispersal of artifacts in archaeological sites (e.g., Barton and Bergman 1982; Cahen et al. 1979; Rowlett and Robbins 1982; Siirianen 1977).

Paola Villa's recent article (1982) in this journal summarized the literature on vertical dispersal; we will therefore discuss only those aspects of others' findings that provide a context for the results of our own experimental work. Conjoinable pieces of stone, bone, or pottery have been retrieved from two or more sedimentologically distinct strata, separated by up to 1 m vertically, with no traces of disturbance to the deposits perceptible to the excavating archaeologists (Courtin and Villa 1982; M. J. Mehlman, personal communication 1981; Villa 1982). In sites without strata but likewise lacking clear evidence for disturbance of sediments, artifacts from vastly different archaeological phases have been encountered in the same level, as was the case with Middle Stone Age and Iron Age materials at Gombe Point site in Zaire (Cahen 1976; Cahen and Moeyersons 1977). Conversely, conjoinable pieces have been recovered as much as 40 cm apart vertically in sites thought to result from relatively short single episodes of occupation, as in the case of Meer (Van Noten et al. 1978) and FxJj50, Koobi Fora, Kenya (e.g., Bunn et al. 1980). Materials in such sites have been recovered through 10 or more vertical cm of sediment. These findings clearly illustrate that vertical dispersal of materials in sites occurs in a variety of circumstances, and they raise questions about the processes that cause such movements of archaeological materials in their sedimentary matrices.

Diane P. Gifford-Gonzalez, David B. Damrosch, Debra R. Damrosch, John Pryor, and Robert L. Thunen, Board of Studies in Anthropology, University of California, Santa Cruz, CA 95064.

American Antiquity, 50(4), 1985, pp. 803-818. Copyright ?) 1985 by the Society for American Archaeology

804 AMERICAN ANTIQUITY [Vol. 50, No. 4, 1985]

Traditionally, archaeologists have conceived of the causes of vertical displacements to be post- depositional, "disturbance" phenomena, either biological or geological. A host of post-depositional disturbance processes have been documented, and their effects described (e.g., Wood and Johnson 1979). Cahen and Moeyersons (1977; Moeyersons 1978) demonstrated that cycles of wetting and drying in sandy deposits can effect substantial vertical movements of artifacts without creating any discernible traces of these movements. The same authors have proposed that reworking of loose sand matrices by termites and worms might, over long periods, cause substantial subsidence and creep of sediments, with resultant vertical displacement of archaeological materials, without leaving visible traces of such movements. These and other recent observations of the behavior of materials in uncemented sediments (e.g., Harris 1979; Limbrey 1975; Rolfsen 1980) all suggest that vertical movement of artifacts and other debris in their sedimentary matrices after deposition is more common than most archaeologists previously suspected. Particularly apt is Villa's (1982:287) state- ment that, without evidence to the contrary, "layers and soils should be considered as fluid, de- formable bodies ... through which archaeological items float, sink, or glide."

Treadage of debris into the substrate by the creators of a site during the time they live at that locale is another source of vertical dispersal that is neither post-depositional nor post-abandonment. The potential for on-site activity to cause downward migration of debris into loose substrates was recognized in the 1970s by a number of ethnoarchaeologists, including John Yellen (1977), James O'Connell and Peter White (personal communications 1977), and the senior author (Gifford 1980; Gifford and Behrensmeyer 1977). While excavating Shaw's Creek Shelter (1973), Eugene Stockton also recognized that treadage by humans could effect downward migration of debris. Stockton conducted the first experiment on treadage during his fieldwork; human traffic over glass fragments lying in a loose substrate caused subsurface migration of materials as much as 16 cm below their original locations. Stockton argued that treadage might also have been responsible for the size- dependent sorting of debris that he observed in the upper 20 cm of the Shaw's Creek archaeological site, where larger pieces were higher and smaller pieces lower in the section.

More recently, Jean Courtin and Paola Villa (1982; Villa and Courtin 1983) conducted another such experiment in connection with their excavation of La Baume Fontbregoua in southeastern France, and we have carried out the experiment described in this report.

The senior author became interested in treadage as a depositional process when excavating a small Dassanetch campsite at Lake Turkana, Kenya, in 1974. This site was created in 1973, over a period of three days, by eight men on a foraging trip. It was then abandoned, and buried in flood silts about six months later during the following rainy season of 1974 (Gifford and Behrensmeyer 1977). On the day it was abandoned, around 340 pieces of bone were observed and place-plotted on the surface of the site. It was, therefore, surprising when a smaller portion of the site, where only 200 bones had been noted on the surface, yielded an excavated sample of 1,955 pieces. Even allowing for the effects of equatorial sun on the efficiency of the first survey, this difference seemed excessive. Many of the bones recovered during excavation lay entirely within the site's original sandy substrate and were very small, less than 3 cm in maximum dimension.

It seemed possible that treadage by the camp's occupants might have sent many of these small pieces into the sandy substrate during their three-day stay. While this in itself might be a banal bit of archaeological lore, the small debris recovered were of the same size ranges specified by Schiffer (1977) and various ethnoarchaeologists (D. Anderson, L. Binford, J. O'Connell, personal commu- nications 1977) as highly likely to remain where generated as primary refuse. If microdebris were deposited well into a "permeable" substrate simply in the course of daily human activities, it would be even less liable to displacement away from the actual place of their production (see Schiffer 1983).

THE EXPERIMENT

We set up an experiment to assess how much subsurface migration of either microdebris or larger pieces can occur simply as a result of on-site human traffic. Since a number of post-depositional processes have been implicated in creating vertical dispersal of materials in sites, we also wanted

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to assess whether trampled assemblages exhibited any distinctive hallmarks-such as the size-sorting suggested by Stockton-that might distinguish them from those affected by other processes. Our experiment had three objectives: (1) ascertain the degree to which subsurface migration of artifacts occurred due to treadage; (2) ascertain which, if any, attributes of artifacts correlated with the distance pieces migrated subsurface; (3) assess the effects of differences in substrate on subsurface migration of pieces and damage to lithics subject to treadage.

The Assemblages

To accomplish these objectives, we controlled the size constitution of our artifact assemblages and the amount of treadage to which they were subjected, while varying the substrate. We created two assemblages of obsidian debitage, of 1,000 pieces each, each comprising the same number of pieces of three screen-segregated size fractions:

650 pieces 3.0 mm-6.5 mm (/8-l/4) 300 pieces 6.5 mm-13.0 mm (?/4-l/2')

50 pieces 13.0 mm

The relative proportions of these three fractions were determined by screen-sorting debitage from a flaking session in which we used several medium-sized obsidian nodules (15-30 cm maximum dimension). Thus, the relative frequencies of size categories in the assemblage were intended to reflect the products of core-reduction activities. Tables 1 and 2 show the actual size distributions (by length) of the recovered assemblages.

However, our choice to represent different size fractions proportionate to their occurrence in a core reduction episode introduced a weakness into our research design that, were we to reproduce the experiment, we would rectify. The largest size category contained so few pieces that, with loss of some pieces in the field, we had very few medium- to large-sized artifacts in our analyzed sample, with only eight pieces of greater than 6 cm maximum dimension in both samples. Therefore, our conclusions about the influence of weight or dimension on subsurface migration must be qualified by the recognition that larger pieces are underrepresented in our sample.

Each piece was measured for length, width, and thickness to the nearest mm, weighed to the nearest tenth of a gram, and numbered. To facilitate numbering and field recovery, pieces were spray-painted and the numbers scratched into the surface of the paint. As photographs in this report indicate, bones were also laid down with the lithic assemblages. However, the bones will not be discussed in this report.

Field Procedures

One assemblage (Site 1, the "loam site") was laid out and walked over on a level substrate of compact sandy silt (see Compton 1962), with a high proportion of rootlets and organic debris, in a clearing in second-growth forest on State Park property in a local river valley. The other assemblage (Site 2, the "sand site") was laid down on a level substrate of unconsolidated medium-fine sand (Compton 1962), in a vegetation-stabilized beach dune in a State Nature Reserve on the Pacific coast north of Santa Cruz. Field procedures were the same in both cases, with the exception of surface contour mapping strategies:

(1) A more or less flat 2 x 2 m area was demarcated for the experiment, staked, and strung. A fixed datum point was selected for each site's vertical elevation readings.

(2) At the first (loam) site, preliminary surface elevation readings were taken, at 10 cm intervals, over the entire 2 x 2 m area (400 stations). These surface readings were used to create a computer-generated surface elevation map for the 2 x 2 m unit (see below, Analytic Procedures). At the second (sand) site, surface readings were not taken before the experiment, because we hesitated to disturb the loose surface. The surface readings were instead taken after the experiment, a decision that hindsight indicates may have created more problems than it solved (see below, Results).

(3) Artifacts and bones were gently spread in a roughly circular pattern over the central zone of each 2 x 2 m unit. Time constraints did not permit us to plot the position of each of the 1,000 pieces as they lay in this original scatter. We instead simply demarcated the general area of the scatter within the 2 x 2 m

806 AMERICAN ANTIQUITY [Vol. 50, No. 4, 1985]

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REPORTS 807

Table 2. Occurrence of Fractures on Recovered Pieces.

Fractures No Fractures

N (%) N (%) Total

a. All Pieces Site 1 323 (50.4) 318 (49.6) 641 Site 2 104 (13.1) 688 (86.9) 792

b. Length <2 cm Site 1 88 (15.9) 465 (84.1) 553 Site 2 35 (5.4) 617 (94.6) 652

c. Length >2 cm Site 1 31 (35.2) 57 (64.8) 88 Site 2 60 (42.9) 80 (57.1) 140

grid. This meant that, unlike Courtin and Villa (1982; Villa and Courtin 1983), we were unable to trace the routes of lateral dispersal of individual pieces, but only their movement away from the area of the original scatter. Because we did note a substantial amount of horizontal dispersal at both sites, we would advise anyone doing similar experiments to record original coordinates of the pieces.

(4) Two persons wearing soft-soled moccasins or rubber sandals walked over the scatter and adjacent areas at a normal pace for two hours. The strings demarcating the 2 x 2 m squares' boundaries were removed to facilitate movement but were restrung on the unit pegs prior to excavation. Naturally, this foot traffic is much more intense than what one would expect for a comparable span of time during a real occupation of a campsite. We did not aim to simulate the conditions of such an episode but rather to subject materials and substrate to a substantial amount of treadage. The Villa and Courtin (1983) experiment more closely approximates normal rates of treadage over time than does our experiment.

(5) Readings were taken at 400 10-cm-interval stations over the post-experiment surface of the sand site. (6) Each site was excavated with brushes and all pieces recovered in place were plotted in three dimensions.

Artifact coordinates were measured to the middle of each piece. (7) Backdirt was screened for pieces missed in excavation, using '/8-inch mesh screens.

Analytic Procedures

Three dimensional coordinates of all recovered artifacts were added to computer files containing basic statistics on the original artifacts. A "depth-below-surface" statistic was generated for each retrieved piece by subtracting its depth below datum where it was retrieved from the depth below datum of a computer-interpolated surface at the same x-y coordinates. To generate the surface elevation statistic for a given x-y coordinate, the four nearest of the 400 readings taken for each unit were averaged. Regression analyses of each recovered artifact's depth-below-surface statistic against various artifact attributes were then carried out. Variables used were weight, length, width, thickness, length x width (as a rough index of the most extensive surface of the piece), and length x width x thickness (as a rough measure of volume).

RESULTS

Of the original 1,000 artifacts laid out at the loam site, 733 (641 numbered pieces plus 92 unnumbered fragments) were recovered. At the sand site 891 pieces (792 numbered artifacts and 99 unnumbered fragments) were recovered. The following discussion focuses on the numbered pieces only.

The effects of treadage on these materials are discussed in three sections, covering respectively: (1) the degree of horizontal and vertical dispersal of materials, (2) the role of their attributes in determining this dispersal, and (3) the types and degree of damage incurred by the lithics. Since the lithic assemblages were of the same raw material and roughly the same morphology, and since the treading activity was equivalent at the two sites, we have assumed that differences in observed effects were due to differences in the substrate types of the two localities.

808 AMERICAN ANTIQUITY [Vol. 50, No. 4,1985]

Cm East 0 50 100 150 200 250 300 350 400 450 500

0 - I I I I I I I I

50 -

100'

150-

= 200- 0

E 250

300

350-

400

450 Figure 1. Site 1: horizontal distribution of artifacts after treadage. Circle represents location and approximate

extent of the original scatter.

Horizontal and Vertical Dispersal

Artifacts at the loam site tended more to horizontal dispersal than vertical (Figure 1). The absence of artifacts on the north side of the unit is a result of our recovery methods. A dense brush cover lay on this side of the square, which we could not remove because of State Park restrictions on vegetation clearance. This probably obscured some pieces kicked out of the unit in that direction during the experiment. Widely dispersed pieces to the west of the unit may also have been hidden by underbrush growing further away from the square than the northern stand of bushes.

Only 10 pieces penetrated more than 2 cm below the surface (Figure 2a-d, Table 1), and 94% lay on the surface or in the upper 1 cm of matrix. Treadage actually increased the amount and depth of a loose layer that overlay the compact substrate, from an initial thickness of 0.1 cm to about 2.5 cm. This matrix caught and held small- to medium-sized flakes. Subsequent rains would probably have had the effect of consolidating this layer, incorporating the small pieces contained therein into the soil. Thus, although the loam site's substrate was not initially conducive to easy burial of small artifacts, the very process of human circulation on the site created a shallow, loose layer that promoted entrapment of these pieces.

By contrast, the loose sand of the second site was a highly efficient artifact trap. In the first few minutes of the experiment, most artifacts, including large pieces, disappeared below the surface

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Cm East

0 50 l00 150 2000 50 100 150 200

0gL- *-* a | z,_b|

I-I

6-

3-

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Figure 2. Site 1: vertical distribution of artifacts, by 50 cm combined sections, after treadage.

(Figures 3-5). By the end of two hours' treadage, very few pieces remained visible. Artifacts definitely circulated vertically during the experiment; experimenters saw distinctive pieces repeatedly surfacing and submerging over the two-hour span.

Excavation of the sand unit revealed a number of interesting features. First, the entire scatter had moved southward as a unit during treadage, while some horizontal dispersal of pieces occurred simultaneously (Figure 6). This southward shift requires an explanation. Unfortunately, ours must

71W~~~~~~7

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Figure 3. Site 2 (sand) scatter before treadage

3 rr fis a t~~~~~~~~~~~~~~~~W

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Fiur 3 Ste2 sad) cate bfoe redae

810 AMERICAN ANTIQUITY [Vol. 50, No. 4, 1985]

X s - ~ ~ ~~ ~~~ ~~~ ~~~ ~~~~~~ t '0.Z;I.V': .i-..r. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.. ... .

o~~~~~~~~. .. *:,,, i i i t_ 1!1! a g _ ,.., ,:; ,'E','.-.'::-s:,'@S,.'-:':'''

Figure 4. Site 2 (sand): after eight minutes of treadage.

remain speculative. Experimenters did not intentionally walk more in a southward direction; they entered and exited the 2 x 2 m unit from all directions. We believe that the entire block of sediment acted as a fluid, shifting down a slight north-south slope during the experiment. The post-experiment difference in north-south elevation averaged 12.1 cm over 200 cm at Site 2. (At Site 1, by contrast, the north-south elevation difference averaged only 4.3 cm over 200 cm.) Since both sites were chosen for their level surfaces, we think that some of the sand site's height differential developed during the actual treadage episode. A firmer layer of moist sand encountered 6-8 cm below the surface during excavation may have facilitated this process of lateral slippage by serving as a solid contour against which the loose sand and artifacts moved preferentially "downhill." We stress that this explanation is speculative, since we did not actually measure the surface elevations of the sand location before treadage.

Second, artifacts dispersed vertically to a remarkable degree in the sand site, as clearly shown in Figure 7a-d, with some pieces nearly 11 cm below the surface. The overall vertical distribution of artifacts from the surface to the greatest depth penetrated approximated a normal curve (Figure 8; Table 2). About 40% of the pieces were encountered between 3 and 8 cm below the surface, rather than at the surface, with frequencies falling off on either side of this mode (see below, Discussion).

Third, the moist zone of consolidated sand, present at 6-8 cm depth, apparently acted as a barrier to further downward migration of artifacts. A few flakes penetrated into this layer edge-on, but most pieces came to rest on it. The outlines of this layer cannot be seen in the 50 cm composite sections of Site 2 published here, but in some thinner composite slices (not published in this report) a line of demarcation between the dry and the moist strata is outlined by the artifact distributions.

Although the relative degree of consolidation between the substrates of the two sites differed, each experimental case shared two features: a layer of dry, relatively loose sediment, which served as an

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Figure 5. Site 2 (sand): after two hours of treadage (end of experiment).

artifact trap, and a deeper, moist consolidated layer, which impeded further downward migration of artifacts

Artifact Attributes and Downward Dispersal Site 2, the sand site, provides the most useful data for assessing the relationship between artifact

attributes and the distance pieces migrated below surface. Table 2 shows the frequencies of various length artifacts at centimeter intervals below surface. Tabulation of depth-below-surface keyed to artifact weight revealed a virtually identical distribution, differing only by fractions of a percentage point from the values keyed to length. None of the attributes indexing size or volume yielded a significant correlation with depth below surface. These findings are of interest because J. Moeyersons (1 978) has experimentally specified weight as a variable that does influence the depth to which artifacts migrate into sediments from initial locations on the surface when subjected to wetting and drying cycles.

Our results regarding the influence of weight could stem from either of two factors: either the size-skewed constitution of the artifact samples, in which large, weighty pieces were very rare, or the actual dynamics of the trampling process, in contrast to those of wet-and-dry cycles. As noted earlier, it would have been better to have included a greater proportion of heavy pieces in our experimental assemblage than are normally found in the by-products of core-reduction sequences. Paola Villa has noted the same problem with results of the experiment in trampling she and Jean Courtin conducted (Villa and Courtin 1983), since their experimental assemblage was likewise a "4natural" one. Given these considerations, we do not believe that either we or Villa and Courtin have decisively eliminated length, weight, or volume as attributes that might affect the distance an object migrates downward during a treadage episode.

812 AMERICAN ANTIQUITY [Vol. 50, No. 4, 1985]

Cm East 0 50 100 150 200 250 300 350 400 450 500

50-

100 QI 150-

200- 1

E 250.

300-

350-

400-

450- Figure 6. Site 2: horizontal distribution of artifacts after treadage. Circle represents location and approximate

extent of the original scatter.

On the other hand, this lack of correlation may be because the treadage was vigorous enough to keep artifacts from settling out according to any attributes that might have been influential in a less dynamic situation, up to the time when treading abruptly ceased. As noted earlier, experimenters observed distinctive specimens repeatedly submerging and surfacing during the experiment. The edge-damage on larger artifacts at the sand site (see next section) likewise reflects the vigor of subsurface circulation of artifacts during treadage. These circumstances may be analogous to those of a brief, violent flood episode in a stream, in which resulting deposits are poorly sorted. Clearly, further work is needed to ascertain which of these two explanations is the more likely.

J. Moeyersons (1978) contended that artifact shape, specifically the relation of artifact edges to a sediment column, can affect rates of vertical migration of buried pieces. He predicted that objects with an edge directed upward and flat surfaces downward in a reconsolidating, downward-settling matrix are more likely to shift vertically than are those with edges pointing downwards and flat surfaces upwards. Moeyersons based his generalizations on experiments with wedge-shaped objects, offering no suggestions about the movements of flakes and other lithics, which may have edges both upward- and downward-directed.

Our observations do not reveal any clear relationship between edge orientation and vertical slippage. At the sand site, 87 (10.9%) of the flakes were encountered lying with edges upward- and downward-directed (found with either their longitudinal or transverse axes oriented at angles between

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Cm East 0 50 100 150 200 0 50 100 150 500

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39

Fiue7 ie2 etia itiuino atfcts dy5 mcmie etos feraae

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Figure 7. Site 2: vertical distribution of artifacts, by 50 cm combined sections, after treadage.

45 and 90 degrees relative to an arbitrary level horizon). The mean depth-below-surface of these edge-on flakes was not significantly different from that of the rest of the excavated sample. At the loam site, 56 flakes (8.8%) were encountered in similar orientations. These were flakes that happened to turn edgewise to the ground and be stepped on literally sliced into the compact loam substrate.

Substrate and Artifact Damage

Because individual obsidian artifacts were spray-painted light colors, breakage and edge damage associated with the experiment were readily perceptible. We suffered a great setback to our detailed analysis of damaged pieces when the entire set of fractured and edge-damaged pieces from the two sites was accidentally discarded prior to completion of the project. John Pryor undertook a careful examination of the color slides of 14 of the total 61 edge-damaged specimens. While his comments are relevant only to that proportion of the total set documented by photographs, they are incorporated in this section as an aid to interested researchers.

The ratios of artifact breakage differed substantially between the two sites (Table 2a). This contrast is perhaps expectable, given the compact, resistant substrate against which artifacts were pressed at the loam substrate site, as opposed to the sand. Viewed in more detail, a clear size-related pattern of breakage emerges (Table 2b, c). Significantly more smaller pieces were broken at the loam site than at the sand site (Chi-square exact probability for these results is less than .001). Artifacts greater than 2 cm in length showed less difference in breakage frequencies (Chi-square exact p = .25).

Edge damage frequencies (Table 3a-c) more or less reversed the size-associated patterns seen with breakage. Pieces under 2 cm in length displayed similar edge damage frequencies at the two sites. However, a greater proportion of pieces over 2 cm length from the sand site were edge-damaged (Chi-square exact p = .27).

We have considered the possibility that the two samples of pieces longer than 2 cm contained significantly different modal edge angles, with those of the sand site being more liable to detachment of flakes along their edges. Given the observed circulation of pieces during the experiment at the

814 AMERICAN ANTIQUITY [Vol. 50, No. 4, 1985]

0 5 10 15 20 25 30 35 40 45 50 55% 0 5 10 15 20 25 30 35 40 45%

cm Depth cm Depth 0-0.9

. 2- / STOCKTON 1.0-2.9 SITE 2(SAND) N=254 3.0-4.9 N=786

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Figure 8. Vertical distributions of objects trodden into sand. Site 2 compared with data from Stockton (1974) and Villa and Courtin (1983).

sand site, we think it more likely that artifacts came into vigorous contact with one another there at considerably higher rates than at the loam site.

The proportions of breakage and edge damage observed in our assemblages are much higher than one would expect with chert, flint, and other less brittle types of silicious stone. Villa and Courtin (1983) observed much lower rates of breakage and edge damage in their trampled flint assemblages.

The character of the edge damage cannot be specified closely because of the loss of our collection. However, close examination of the color slides of 14 edge-damaged pieces, 6 from Site 1 (1/3 of all edge-damaged pieces) and 8 from Site 2 (slightly less than 1/5 of all edge-damaged pieces) has yielded some interesting results. John Pryor concentrated on attributes specified by Tringham et al. (1974) as typical of edge damage resulting from trampling. Since these researchers' sample comprised 10

Table 3. Occurrence of Edge Damage on Recovered Pieces.

Edge Damage No Edge Damage

N (%) N (%) Total

a. All Pieces Site 1 14 (2.2) 627 (97.8) 641 Site 2 43 (5.4) 749 (94.6) 792

b. Length <2 cm Site 1 4 (0.7) 549 (99.3) 553 Site 2 3 (0.4) 649 (99.6) 652

c. Length > 2 cm Site 1 14 (15.9) 74 (84.1) 88 Site 2 40 (28.6) 100 (71.4) 140

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SiV ~ ~ ~ ~~~~~~~~S

. : ' is . '.: S:yB': ~~~~~~~~~~~~~~~~~~~~W-1.2

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Figure 9. Edge-damaged flake from Site 2 (sand), with 16 flake scars.

pieces, our own observations may contribute to a clearer understanding of the variability involved in damage generated by human treadage.

As predicted by Tringham et al. (1974), the placement of flake scars along the edges of artifacts from both sites was essentially random. Rlakes tended to be sparse along any given edge, with only 2 of 16 cases displaying more than three scars. One of these two artifacts, however, showed 16 closely placed scars along its edge (Figure 9). However, contrary to predictions by Tringham et al. (1 974) regarding the orientation of trampling scars, flake scar orientations on pieces from both our sites were not random, with 48 of 67 being perpendicular, 17 right-oriented, and 3 left-oriented.

Tringham et al. (1 974:192) further predicted that flake scars produced by treadage would be more "6elongate" (by implication, longer than wide) than would those produced by use. Our sample, however, exhibited significant differences in this trait according to the substrate on which the artifacts were trampled. The sand-site artifacts showed a slight tendency toward a greater number of flake scars with lengths greater than widths than did the loam site (sand: 13 of 31 = 42%; loam: 3 of 16 = 18.7%; significant at the .10 level). This might result from differences in the dynamics of detachment events at the two sites, with artifacts at the sand site subjected to a higher rate of free impacts and the loam-site assemblage subject to higher rates of compression events. The assemblage produced by Tringham et al. (1 974) was apparently trampled near the Peabody Museum at Harvard University, where the substrate is likely to have more closely resembled Site 1 than Site 2.

DISCUSSION

Among the most interesting results of our experiments was the pattern of vertical distribution of artifacts in the sand site. This normal curve pattern has also been reported in the two other trampling experiments cited in this report (Figure 8). Stockton's (1 974:116) tabulations reveal a similar overall pattern, although of a shallower general vertical distribution. Some of Villa's and Courtin's (1 9 83)

816 AMERICAN ANTIQUITY [Vol. 50, No. 4, 1985]

multiple experiments also approximate a normal distribution. These researchers conducted several separate trials on and in the same substrate with small sets of artifacts and bones totalling around 20 pieces each. Some assemblages were laid out on the surface and directly trodden, and others were covered with 2-3 cm of sediment before treadage. Only those assemblages trampled directly from the surface exhibited the "normal curve" vertical distribution pattern. The covered assemblages tended to contain most pieces either in the precise level of deposition or a few cm lower, more closely resembling a Poisson distribution in vertical configuration.

The precise mechanisms involved in creating these differences remain to be specified, but we suggest, as have Villa and Courtin (1983), the following scenario. When objects are trod into a loose substrate directly from the surface, some pieces swiftly migrate down below the effective "reach" of subsequent footfalls, while others above them are caught up in a zone of constant circulation. The movement of most pieces from the surface into a subsurface zone where they remain may reflect the tendency of objects to move downward in moving sediments despite repeated upward churning. The falloff below the modal frequency zone must reflect something of the tendency of some objects to penetrate more deeply into a loose matrix, according to properties or dynamics we do not at present understand.

Recently, N. Barton and C. Bergman (1982) have documented a different type of vertical dis- tribution of materials at Hengistbury Head, an Upper Palaeolithic site in eolian sands on the southern coast of England. At Hengistbury, the majority of artifacts tend to lie in the uppermost zone in which they are encountered, with a downward falloff in frequency below. Barton and Bergman contrast their findings with those reported by Van Noten (1978) for artifact distributions in the sand matrix of Meer, in the Netherlands. In various sections at Meer, artifacts are distributed either more or less evenly from top to bottom, or in a somewhat curvilinear frequency distribution over some 48 cm vertically (see also Van Noten et al. 1980:51). Barton and Bergman further note a positive correlation between depth and artifact weight at Hengistbury (no comparable data are available for Meer).

Based on these observations, Barton and Bergman propose a "passive" mode of downward deposition at the Hengistbury site, in which inertia, weight, and perhaps shape (see Moeyersons 1978) have been responsible for most downward migration of artifacts from an assemblage buried by windblown sands after minimal treadage or other disturbance. Based on our own observations, we are inclined to agree. The distribution at Hengistbury does not resemble that of any trampled- from-the-surface assemblage we have examined. Although the Hengistbury distribution does re- semble in general form the patterns documented by Courtin and Villa in their trampled-under- sediment assemblages, the weight-dependent sorting at the English site was not observed in any of the latter researchers' experiments. Moeyersons's findings indicate a positive correlation between weight and depth migrated from the surface, but he notes that weight seems to have less effect on pieces buried in more than 30 cm of sediment (1978:122). Thus, it seems to us more likely that such weight-dependent sorting went on while the assemblages were lying close to the surface.

The Meer assemblage looks like a "good" trampled-from-the-surface one, although spread over a much greater vertical distance than any of the experimental assemblages we have seen. Ellen Kroll (personal communication 1980), who has worked on distributions from a number of African Lower Pleistocene sites, reports similar "normal curve" vertical distributions at several of these sites, although over vertical distances exceeding a meter.

Before rushing to diagnose trampling as the major agent in any such distribution, however, we must ask what other kinds of processes might produce a vertical falloff in artifact frequencies on either side of a mode. One may imagine, for example, bioturbation or wetting/drying cycles acting on a buried layer of objects in such a way as to raise some and lower others relative to the majority of pieces. A recent study by Rowlett and Robbins (1982) highlights this caution. The authors report on a similar vertical distribution of artifacts, in this case ceramic coin molds, in an Iron Age hillfort in Luxembourg. They argue that this distribution is the result of post-depositional upward and downward migration of pieces from a buried horizon, an inference at least partly supported by the internal relations of datable materials, site structure, and overall stratigraphy. However, the authors go on to argue that all such vertical distributions result from this process and seek to induce an

REPORTS 817

average rate of vertical dispersal for buried materials. The results of our own and others' work on treadage-produced distributions indicate that this conclusion is unwarranted.

CONCLUSIONS

This study, along with those of Stockton and Courtin and Villa, clearly establishes that treadage by humans can cause substantial downward migration of objects in loose, sandy substrates. Our study indicated no clear correlations between size attributes of pieces and depth below surface, although this finding needs further elucidation through experiments involving a greater proportion of large pieces. Damage to artifacts varied according to the compactness of the substrate, and there may be substrate-related differences in the shape of flakes along damaged edges.

A difference seems to exist between vertical distributions of assemblages trod upon from an initial position on the surface versus those trampled after sediments have covered the scatter. While this distinction could shed light on the relationship of on-site human activities to the configuration of excavated materials, further research is needed to specify other processes that might either create similar patterns or modify those initially created by trampling.

"Treadage" should not be considered a monolithic category, but a general descriptive term for a human or animal activity that has depositional and object-modifying potential. Variations in sub- strate, in the intensity of human activity, and in the resulting interactions of objects with both substrate and one another may produce disparate distributions and damage patterns. Research on these interactions will clarify the dynamics that generate vertical distributions in archaeological sites. The need for better controlled, replicative experiments of this sort is obvious, as is the need to further isolate other types of generative processes and their signatures. We hope this report will encourage others to undertake such work.

Acknowledgments. Field expenses for this project were partially funded by a University of California Pres- ident's Undergraduate Research Fellowship, awarded by Cowell College, U.C.S.C., to Debra Damrosch and John Pryor. The senior author wishes to note that all fieldwork was carried out by the junior authors while they were undergraduates at U.C. Santa Cruz. Because of the conditions of the Undergraduate Research Fellowship, they were entirely unrecompensed for long and difficult hours of work, either by salary or academic credit. Without their commitment, this study would never have been accomplished. This article was drafted by the senior author, and only she should be held responsible for the inferences and opinions offered. We are grateful to Bruce Morris and Knute Maddock for their help and advice in locating suitable sites, and to the personnel of Henry Cowell Redwoods State Park and Afio Nuevo State Reserve for their cooperation and help. Data processing expenses were supported by a Faculty Research Grant from the Committee on Research, U.C.S.C., to the senior author. We thank Brian Beach, who wrote the program that generated our surface contours and depth statistics, and Tim Bessie, who wrote programs that produced our plots of artifact distribution. Annette Whelan produced the graphics in their final form. We also thank Nick Barton, Chris Bergman, Jean Courtin, and Paola Villa for sharing their data with us prior to publication of their articles.

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