Villa & Courtin 1983 the Interpretation of Stratified Sites - A View From Underground

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Journal of Archaeological Scienc e 1983,10,267-28 1

The Interpretation of Stratified Sites:

A View from Underground

Paola Villa” and Jean Courtin*

This paper reports on an experiment designed to study the role of trampling in the

vertical dispersal of artifacts in the soil, and in the mixing of originally separate sets

of materials. The experiment is part of a study of the archaeological stratigraphy and

patterns of site use at a large stratified cave n southern France. The experiment was

designed to replicate conditions prevailing at the cave. The results strongly suggest

caution in the interpretation of living floors and stratified assemblages in sandy

deposits.

Keywords:

EXPERIMENTAL ARCHAEOLOGY, CULTURAL STRATIFICA-

TION, SITE FORMATION PROCESSES, TRAMPLING.

Intruduction

This article presents the results of a trampling experiment designed to study processes of

formation of the archaeological record at stratified sites. The hypothesis to be tested was

that trampling (a normal activity of prehistoric inhabitants of a site) can cause vertical

dispersal of artifacts in the soil and can create false stratigraphic associations. The first

part of this article provides the justification, background, and rationale of the experi-

ment. In the second part we present the experimental procedures and analysis of results.

The specific aim of the experiment was to provide information to aid in the interpre-

tation of dispersal patterns and relationships between superimposed sets of materials at

a particular site. We believe, however, that the results of this experiment are relevant to

general studies of the formation of archaeological deposits. This information can contri-

bute to a better assessment of the research potential of stratified sites and to the develop-

ment of more rigorous methods for drawing inferences from archaeological remains.

A case study

Background

The trampling experiment reported here is part of an ongoing study of the stratigraphy

and patterns of site use in Fontbregoua Cave, about 100 km NNE of Marseille, southern

France. Fontbregoua is a large cave (about 250 m2) under excavation since 1970 by

Jean Courtin. It has yielded a long cultural sequence with Upper Paleolithic, Mesolithic,

and Neolithic deposits totalling more than 9 m. The uppermost levels contain sparse

Chalcolithic, Bronze Age and historic materials.

aDepartment of Anthropology, University of Colorado, Boulder, Colorado

80309.

*Direction des Antiquit& Prehistoriquesde Provence,Alpes et CBte d’Azur,

21-23 Boulevard du Roi Rend, 13617Aixen-Provence, France.

267

0305-4403/83/030267+

15 03.00/O

0 1983 AcademicPress nc. (London) Limited


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P. V ILLA AND J . COUR TIN

The Neolithic layers (4 m) have yielded more than 24,000 objects including pottery,

grinding stones, sickle blades, remains of domestic and wild faunas, and carbonized

seeds of domestic wheat (mostly bread wheat), barley, legumes and acorns. The oldest

Neolithic occupation has a radiocarbon date of 4750f 100 bc. Below, and separated by

a layer of culturally sterile sand, are Mesolithic deposits with geometric microliths,

abundant bird bones but sparse mammal bones, remains of fresh water turtles and fish,

and wild legumes. The Upper Paleolithic occupation, with a radiocarbon date of 9250&

100 bc, is known only from a very limited test trench. Bedrock has not been reached

(Courtin 1975, 1976, 1978; Courtin & Erroux 1974; Cheylan & Courtin 1976).

Significance of the site

Being a deeply stratified site with a long history of occupation and repeated use, Font-

bregoua provides data for the study of change and continuity in material culture. The

site is especially important for a number of reasons. First, the cultural sequence spans

the transition from hunting and gathering to farming, a turning point in human history.

Second, the site has yielded a very large fauna1 sample and botanical remains for the study

of hunting and farming practices. In particular, Fontbregoua is one of only four Ear ly

Neolithic sites in southern France known to have yielded plant materials (carbonized

cereal grains and legumes). Finally, an especially interesting feature of the site is con-

tinuing use of the cave across the Neolithic transition. The continuing use of caves-

together with open air sites-is not unusual in the Ear ly Neolithic of the Western Medi-

terranean and provides an interesting contrast to the situation in the Near East, where

caves, inhabited in Epipaleolithic times, were only sporadically used by Neolithic

farmers. In southern France farming first appears on the coast at around 5500 bc. Some

of the major domesticates-wheat, barley, perhaps sheep-are of exotic origin; their

wild ancestors are only found in the Near East. Thus, the evidence suggests that farming

was not a fully local, independent development. Its adoption must have been, at least to

some extent, the result of a process of transfer of new technologies, materials and ex-

pertise. The spread of the new economy eventually lead to the establishemnt of per-

manent, year-round villages and to the abandonment of caves as habitation sites. How-

ever, for the first two millennia after the appearance of farming, caves continued to be

used side by side with open air sites. Storage or trash pits and fireplaces, with thick ash

accumulations, are a conspicuous feature of the Neolithic deposits at Fontbregoua

suggesting prolonged periods of habitation. By contrast, during the Mesolithic, the cave

appears to have been used for short v isits only, The study of changing or continuing pat-

terns of site use through time is a major goal of the Fontbregoua research project.

Defining the Problem

Assemblages and layers

The first task facing the investgator of a deeply stratified site is to partition the deposits

into layers or levels representing successive time units (Wheeler, 1954, p. 43). It is then

possible to distinguish superimposed sets of material and group items into units of

association (assemblages) to be analysed separately.

Stratified sequences in prehistoric sites wil l often consist of a succession of contrasting

types of deposits, bounded by discontinuities reflecting changes in type of sedimentation,

temporary halts in deposition or erosional episodes. Within single geologic units, one

or more units of cultural stratification may be distinguished. In the latter case, their

boundaries are defined by the presence of steri le zones separating vert ical concentrations

of materials, or by distinct interruptions in the accumulation of individual man-made

deposits, such as ash or charcoal layers. Ideally each layer or level represents a well-


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STRATIFIED SITES

269

defined time unit bounded by clear discontinuity surfaces or by sterile zones. In fact,

gradational boundaries are common. When the cultural material forms unpatterned

vertical scatters within homogeneous deposits, arbitrary levels are used. Collections of

objects from single stratigraphic units form separate analytical units and are called

assemblages. In short, archaeologists subdivide and interpret a stratified sequence using

criteria of diverse kind, based on a combination of natural and cultural processes of

deposit formation(Wheeler, 1954, ch. 4; Michels, 1973, chs. 3 and 4; Harris, 1979, ch.

5; Straus, 1979; Laville ef al., 1980, pp. 9, 13, 152). These same criteria have been used

at Fontbregoua.

Clearly the definition of stratigraphic units and their boundaries is important for the

definition of assemblages. Boundaries between layers may be sharply delineated and

continuous; they may also be diffuse, discontinuous and irregular. The distinctness,

lateral continuity and temporal significance of these surfaces should be described and

documented (Harris, 1979, pp. 49-80). Methods used for defining layer boundaries and

for segregating superimposed sets of materials into assemblages should be crit ical ly

evaluated.

The assemblage concept plays an important role in Old World archaeology. In the

“assemblage approach” of Palaeolithic archaeology, pioneered by F. Bordes (Binford,

1981, p. 183; 1982) assemblage types are defined and compared on the basis of varying

frequencies of artifact types. Each artifact contributes to the description and diagnosis

of the assemblage. To be sure, assemblage typology and quantification are not emphasized

in Neolithic or proto-historical archaeology. In these disciplines cultural-historical

groupings and sequences are worked out on the basis of comparative studies of selected

classes of artifacts. More attention is given to diagnostic artifacts than to frequencies of

artifact types.

However, in behavioral studies the assemblage again is the basic unit of analysis.

Every item in the assemblage contributes to the diagnosis and description of the whole.

Implicitly or explicitly the material is referred to a single episode of occupation or, at

least, to a single mode of site use by a specific group of people. For instance, in studies of

seasonality, diet, and subsistence patterns collections of bones from the same layer or

level form the basis for estimating the minimum number of individuals represented in

fauna1 remains and for analysing hunting or herding practices. In spatial analysis we

try to distinguish areas in which various daily life activities were carried out. Again,

sets of associated materials are the basic unit of analysis.

Clearly, if all the elements of a set contribute to its diagnosis, then we should make

sure that they do, in fact, belong to the ensemble. For instance, Grayson (1979, pp. 205,

213) has suggested that the way a stratified sequence is subdivided into units of analysis

seriously affects sample size and, consequently, measures of relative abundance of faunas.

Sampling error may distort our understanding of past patterns of human behaviour; its

effects should not be underestimated.

In short, we need to ask:

- Do strata boundaries, observed during the excavation, represent meaningful breaks in

a sequence of occupations ? Are our samples discrete units or arbitrary slices of a tem-

poral continuum?

-

Do assemblages from individual layers represent the discrete residues of distinct

human groups or are they the aggregate of different episodes? Can we be sure that our

stratigraphic units relate to a single mode of site use, if not to a single episode of

occupation ?

It is clear that the answers to these questions have an important bearing on studies of

site use patterns. If we can define single phases of occupation, then we can study activi-

ties and patterns of site use on a fine time scale. If our information is inadequate for


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P. V ILLA AND J . COU RTIN

such definition, then we should look only at gross patterns of change and continuity,

focus our attention on the general picture, and develop detailed behavioural interpreta-

tions only in the case of features and site structures-such as storage pits-that clearly

associate contemporaneous materials.

Assemblages and time.

Confronted by the problem of defining the temporal duration of their samples,

archaeologists distinguish between homogeneous and mixed assemblages (some New

World archaeologists use the term “component”). Homogeneous assemblages are associ-

ated sets of artifacts and bones, meaningful samples of a segment of a community. Mixed

assemblages, on the other hand, are the telescoped residue of once separate and

dissimilar events (Michels, 1973, pp. 22-25; Thomas, 1979, p. 231).

This simple distinction hides a more complex situation. Homogeneous is an attribute

that encompasses a graded series. The degree to which aggregates from individual layers

can be treated as homogeneous assemblages depends upon the time scale we wish to

adopt. An example from classical archaeology wil l serve to make this clear.

Etruscan chamber tombs were often used for several generations. Pottery found in

such a tomb may include, say, Middle Corinthian ware, bucchero ware and late black-

figure vases. These grave goods span a period of over a century. On a coarse time scale

the material is associated because it is found in the same container (a tomb) and is

homogeneous in that it is a typical and meaningful sample of Etruscan culture and

burial customs. On a fine time scale the late black-figure vases (first decade of the 5th

century BC) and the Corinthian ware (first half of the 6th century BC.) are not con-

temporaneous. They are in fact related to different events: the death and burial of a rich

landowner and, much later, of his equally wealthy great grand-children. Seen from this

point of view the tomb material forms a mixed assemblage.

In prehistoric archaeology the strength of association of materials is traditionally

evaluated in stratigraphic terms. Layers or levels are treated as containers of a sort

(Spaulding, 1960, p. 211). But assemblages from individual layers represent temporal

samples whose duration is very difficult to measure. Unlike historical materials, Stone

Age materials are not clear time markers except on a very gross scale. Formal changes

through time are too rare or unstable to allow time ranking with more than very few

classes. Thus, neither stone artifacts nor Neolithic pottery provide direct criteria for

separating the residues of different episodes of occupation that may have accumulated

in a single layer. The time interval between the deposition of the first and last item in the

set can rarely be specified. In most cases we do not know the scale on which an assem-

blage is homogeneous. Fine dissecting of strata does not provide the degree of time

control required by behavioural studies, because geological cycles often cover spans

of time that are too long for the study of human activit ies on a fine scale (Bordes

et al.,

1972, pp. 17-19). The Neolithic deposits at Fontbregoua have an average sedimentation

rate of 1 cm per 17 years. Thus, it is likely that materials now aggregated in a single

layer were, in fact, discarded during separate phases of occupation and possibly different

modes of site use. Estimates of sedimentation rates in other Stone Age caves indicate

equally low values. One cm of deposit may represent from 5 to 167 years, with an average

of about 14 years (Speth & Johnson, 1976, pp. 47-48).

Furthermore, layers appear to be rather leaky containers. Recent evidence provided

by conjoined pieces in Old World sites has shown that vertical migration and dispersal

of artifacts across different cultural levels is a fairly common phenomenon (Cahen &

Moeyersons, 1977; Van Noten

et al.,

1978, 1980; Bunn

et al.,

1980; Villa, 1982; Del-

Porte, 1982, p. 161; see also Siriainen, 1977; Rowlett & Robbins, 1982). It is increasingly

clear that in many stratified sites assemblages are not patterned sets of artifacts, tool-


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STRATIFIED SITES 271

kits that can be referred to the activities of a specific social group (Binford & Binford,

1969; Sackett, 1973; Whallon, 1973). Rather, they are aggregates of individual pieces

with individual histories and origins (Binford, 1982, pp. 17-18).

To understand patterns of site use, we must first investigate processes of artifact

accumulation and dispersal in the soil. Stratigraphic studies have traditionally focused on

the geologic context of cultural materials. Classic sediment analyses provide important

data for reconstructing paleoclimatic histories and for temporal ordering of assemblages

(e.g. Laville et al., 1980). However, the burial and stratification of cultural materials at

repeatedly occupied sites, such as caves, are due to a complex interaction of human

activit ies and geological processes. Comprehensive approaches to cave sediment forma-

tion stress human and biological influences (Butzer, 1978, 1981, 1982, ch. 7; Goldberg,

1979, 1980; Courty & Raynal, 1982).

Many techniques and procedures used in the study of site formation and modification

are borrowed from the earth sciences. Others are more strictly archaeological in origin,

in that the information is provided by the artifacts themselves, their interrelationships

and precise position in relation to strata boundaries. These latter methods of stratigraphic

analysis are based on the use of vertical projections and plots of conjoinable pieces (e.g.

Villa, 1982, pp. 280-281). Experimental and ethnographic studies of depositional en-

vironments and disturbance processes have been designed to answer specific archaeolo-

gical questions (Jewel1 & Dimbleby, 1966; Isaac, 1967; Stockton, 1973; Cahen & Moyer-

sons, 1977; Gifford & Behrensmeyer, 1977; Reynolds 1974, 1979; Schick in Bunn

ef al., 1980; see also Schiffer, 1975, p. 841; Lewarch & O’Brien, 1981, 307-311).

These different approaches must be combined and integrated to reach a better un-

derstanding of processes of cultural stratif ication and deposit disturbance. Our paper is

not intended as a review of all these methods. The potential of geoarchaeology and the

role of sediment analysis in such studies have been illustrated with concrete examples

by Butzer (1982, pp. 77-122). We present here an example of the experimental approach

that c losely reflects our professional interests. This approach should be seen as comple-

mentary to sediment analysis. As indicated below, the need for experimentation arose

as a consequence of studies of conjoinable pieces and of vert ical projections of the

Fontbregoua material.

Vertical displacement

of

archaeological remains at Fontbregoua

The study of conjoinable pieces found at Fontbregoua (mostly pottery sherds and bones)

is in progress. A preliminary analysis indicates that sherds belonging to the same pot

have a vertical separation of up to 25-30 cm. Vertical dispersal often occurs across what

during excavation had appeared to be distinct cultural horizons.

A variety of factors may be suggested to explain the vertical displacements observed

in the Fontbregoua deposits:

(1) Soil fauna. Animal burrows have been observed especially in the upper (Chalco-

lithic) layers; they are occasionally found in the lower units. However, these disturbed

areas are easily distinguished from the surrounding intact matrix and pieces from these

areas have been kept separate from the remaining materials. Earthworm castings are

uncommon; however, the occurrence of diffuse layer boundaries suggests earthworm

activit ies (Jacques E. Brochier, pers. comm.).

(2) Tree roots. The cave ceiling collapsed in ancient times; thus the central cave area

was exposed to sun and rain. When the excavation began, four live oaks and one cherry

tree were growing in the cave. Their roots have been found 4 m below the surface. Since

old roots rot away and may not leave visible traces, disturbance by tree roots may be

expected even in layers where they have not been observed.

(3) Alternate wetting and drying. According to Cahen dc Moeyersons (1977) alternate


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272


wetting and drying of sediments (by percolating rain water and sun) can lead to vert ical

movement of artifacts, especially if sediments are unconsolidated due to biogenic activi-

ties. It is interesting to note that some of the Fontbregoua layers and the sediments used

in Cahen & Moeyersons’ experiment have a very similar grain size distribution (analysis

of the Fontbregoua sediments is in progress). We have never observed clay cracks.

(4) The digging and leveling activities of the prehistoric inhabitants. Pits, depressions,

and large hearths in deep hollows are common in the Neolithic layers. Digging and

leveling activities, necessary to the construction of such features, disturb older deposits

and cause rearrangement and redistribution of archaeological material on younger sur-

faces. It is possible to imagine situations in which pits or dug-out depressions will not

leave visible traces in the soil. If the pit was in use for a short time, if it contained

perishable material, if the fil l is similar in colour and texture to the layer in which the

pit was dug, the feature will not be recognized and only the displacement of conjoinable

pieces will betray the presence of disturbances.

(5) Trampling. Recently several scholars have suggested that trampling is a kind of

occupational disturbance that is not easily recognized and may cause considerable ver-

tical displacement of artifacts (Stockton, 1973; Hughes & Lampert, 1977; Gifford &

Behrensmeyer, 1977 ; Gifford, 1978; Butzer, 1981, p. 155). At the Jean Cros rock shelter

in southeastern France sherds of the same pot have been found in three superimposed

levels; their vertical dispersal was attributed by the archaeologist to trampling by the

prehistoric inhabitants (Guilaine, 1979).

A trampling experiment has been carried out by Stockton in Australia (1973). Small

glass splinters (mean weight=4 g) were laid out on a level sandy surface, covered by

5 cm of sand and intensively trampled for one day. When excavated by levels, the glass

was found to be distributed over a depth of 16 cm. More than 50% of the sample had

remained on or near the original surface; 22% had moved 2-5 cm upward; the rest of

the sample had migrated downward to a maximum depth of about 10 cm below the

original surface.

Patterns of vertical dispersal of conjoined sherds in the central part of the Fontbregoua

cave suggest that trampling may have been a contributing factor. The density of material

indicates that the site had been intensively lived in; trampling on discarded material

lying on the surface or slightly buried must have been a common occurrence. Some of

the vertically displaced pieces are horizontally very close; thus it is unlikely that their

vertical distribution is due to the presence of irregular or sloping l iving surfaces. Groups

of conjoinable and vertically displaced sherds are scattered over large areas; this seems

to suggest a non-localized agency of displacement such as trampling. To test this hypo-

thesis, we decided to replicate Stockton’s experiment in such a way so as to approximate

more closely conditions prevailing in the cave.

Experiment Procedures

On the rocky slopes, near the cave entrance, there are several artificial terraces built with

dry stone walls to hold the screened backdirt of the excavation. During the summer,

when the excavations take place, these terraces are used by the excavating team (an

average of 12-15 people) for a variety of activities: screening, washing and sorting arti-

facts, eating and resting.

The material selected for the experiment was laid out on these terraces. We used

material similar in size, shape and kind to that found in the cave, viz.:

- flint flakes, blades and retouch flakes (down to l-2 cm in maximum dimensions);

- mammal bones (shafts and articular ends, teeth and skull fragments);

-

marine shells (Cardium, Patella, Columbella, Cerithium);


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STRATIF IED SITES

273

- pottery sherds (modern, unglazed);

-

a few flat limestone pebbles, to simulate the polished celts found in the cave.

Ninety-five y0 of all objects were 110 cm in maximum dimensions. Al l objects were

measured, weighed, numbered and catalogued; the stone pieces were coated with a thin

film of paint, in brilliant colours, to distinguish possible damage scars and to speed up

the conjoining of broken fragments.

To avoid downslope movement and image distortion in vertical projections, we flat-

tened the sandy surfaces by repeated trampling before laying out the objects. Footprints

no deeper than 1 cm were considered indication of an acceptable degree of compaction.

The precise location of each object was recorded with three spatial coordinates, the

relative elevation being measured with a transit. Some of the squares were covered with

2-4 cm of sand or sand and rubble from the cave before trampling; in others the material

was left uncovered. A sediment analysis of two squares done by Jacques E. Brochier

(Laboratoire de Sedimentologie, FacultC Des Sciences, Marseille) showed that the matrix

can be defined as a dry, loose, well-sorted sil ty sand (82-86% sand; 9-13% silt ) with very

litt le clay (5-8%); the median size was 96-108 pm. Small quantities of limestone rubble

(2-20 mm in diameter) were occasionally present in other squares; their proportions

varied from 0 to 14.5%.

In Stockton’s experiment intensive trampling was done for one day. We decided in

favour of natural, casual and prolonged trampling done by the excavators while walking

in and out of the cave to attend to their tasks. Al l the excavators wore only light sandals

or went barefoot.

The results we present now are based on the excavation and analysis of 292 pieces

from 11 l-m squares, 4 of which had two superimposed levels. Eight squares were dug

after 16 days; the others after 22, 32, and 36 days. All excavated artifacts were again

recorded with their new spatial coordinates with the exception of a few objects (2.9%)

which were recovered in the screens and could not be used in the analysis. The weather

remained warm and sunny throughout, with only a light rain one night. Moisture from

continuous water sieving operations considerably hardened the sand in one square.

Resul ts

Vertical displacement

Significant vertical dispersal can be achieved even with a limited amount of trampling.

The maximum range of vert ical separation we observed is 8 cm. After 16 days of casual

trampling, 20% of the objects in square S (Figure 1) had migrated downward 5 to 7 cm

below the surface.

After 16 days of trampling, the material of two superimposed levels in square R,

originally separated by 3 cm of sterile sand, was completely mixed and formed only one

level (Figure 2). To prevent the effects of sand compaction in the mixing of objects,

trampling was done in two stages. The lower level of artifacts was covered with 3 cm of

sand and trampled for 20 days. Other objects were then laid out on the compacted sur-

face and trampled for 16 more days.

Table 1 gives the frequency distributions of artifacts by depth in 5 squares trampled

for 16 days. The objects had not been covered by sand. Note that 21% of the objects

did not move while 28% was displaced 3 to 7 cm below the surface. The few millimeters

of upward displacement are the results of horizontal migration over a slightly irregular

surface.

Ten l-m squares covered by sand or sand and gravel were trampled for 16 to 36 days.

The thickness of the protective layer varied from 2 to 4 cm. Some squares happened to

be in areas outside the main traffic and were only minimally trampled. For the sake of


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274

50 -

cm

80-

+

+


50

lm

Figure 1. Square S , upper le vel, uncov ered. Vertical projections of artifac ts.

Top, before trampling, bottom, af ter 16 days of casual t rampling.

c c ‘V c-

I

0

m a 0

0 0 0 c o o o o o

0

0 0

c

+

lm

Figure 2. Vert ical projections of art ifacts in square R, lower and upper levels.

The lower level was covered with 3 cm o f sand and trampled for 20 days.

Objects were then laid out on the compa cted surface and trampled for 16

days. Top, the relative posit ion of objects in both levels before vert ical dis-

placement occurred. The f igure is a compo site image as the two levels were

deposited at dif ferent t imes. The elevat ion of each object was measured with

a transit in relation to a f ixed datum. Bottom , af ter 36 days of t rampling.

record a table of frequency distributions of artifacts in all covered squares is given below

(Table 2). However, the reader is warned that displacement distributions in individual

squares varied greatly.

In general, downward displacement is smaller in covered than in uncovered squares

even when trampling lasted longer. Upward displacement, of 1 cm or more above the

original position, is limited; its frequency varies from 0 to 17% in individual squares

with an average of 9-8o/o. The maximum observed value is +2-4 cm in a square covered

with 4 cm of sand.

We prefer to analyse distributions square by square and individually compare them


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STRATIFIED SITES

275

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

Displacement n cm o/0of artifacts

+0.9/-0.9

21.0

-l*O/-2.9 51.0

-3.0/-4-g 23.0

-5*O/-6.9 5.0

N= 100

Range: +0.3/--6.9

Table 2. Amount of displacement f rom the or iginal surface in covered squares

Displacement n cm 0/O f artifacts

+1*0/+2.9

+0.9/-0.9

-1.0/-2-g

-3*o/-4.9

--5-O/-6*9

N= 184

Range: +2.4/-5.2

to distributions in uncovered levels of the same squares. Table 3 shows vertical displace-

ments in two squares, R and S, each with two superimposed levels. As mentioned earlier,

in these squares sand was spread over laid out material and left to be trampled for 20

days. A second layer of objects was deposited and walked over for another 16 days.

The differences between the displacement distributions of the two lower levels appear

to be due to the differences in the thickness of the covering layer (3 and 2 cm), but could

also be partially attributed to more intensive trampling in square S which was close to

the lunch table and to the drinking water. It is also clear that, as the vertical distance

between the trampled surface and the artifacts increases, the effects of trampling de-

crease proportionally until downward displacement effectively ceases. Vertical displace-

ment is more limited in covered squares where objects were initially 2 to 4 cm below the

trampled surface.

Frequency distributions from the other covered squares are intermediate between

those of the lower levels in squares R and S. In general, a comparison of frequency

distributions of artifacts by depth in all squares suggests that the degree of vertical dis-

placement depends on four major variables:

(1) the intensity of trampling;

(2) the degree of compaction of the sediments;

(3) the thickness of the deposits covering the pieces;

(4) the weight/size of the pieces (this point is discussed in the section “Sorting by

weight”).

During trampling, objects are constantly being displaced, sometimes lifted, sometimes

pushed down. When the trampling process begins most pieces not covered by sand are

pushed down and in a few minutes will disappear into the earth. But, after a while the

same pieces may reappear on the surface, having been brought up by a human foot.

Some pieces, however, appear to migrate gradually downward, below the zone of actual

disturbance and beyond the reach of feet. Intensive trampling will increase the propor-

tion of pieces pushed far below the surface.

The degree of compaction and resistance to object penetration varied in different

squares depending, among other factors, on the presence of rubble and moisture in the


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276 P. V ILLA AND J . COU RTIN

matrix. Unfortunately, we have not been able to provide an objective measure of packing

and we cannot quantify its effects on vert ical displacement.

Table 3. Amount of displacement f rom the or iginal surface in covered and un-

covered levels of the same square

Square R

Lower level

(36 days, covered

by 3 cm of sand)

Displacement in cm

%

+1.0/+2.9 15.8

+0*9/-0.9 84.2

-l.O/-2.9

-

-3*o/-4.9

-

-5.O/-6.9

-

N=19

Range +2*3/-0.8

Upper level

(16 days, not

covered)

29.4

52.9

17.6

-

N=17

Range -O-l/-4.5

Square S

Lower level Upper level

(36 days, covered (16 days, not

covered

Displacement in cm

by 2 cm of sand)

% %

+1.0/+2.9

+0.9/-0.9

- l.O/--2.9

-3-o/-4.9

-5.O/-6.9

-

41.2 5.0

29.4 30.0

235 45.0

,I : : 20.0

N=20

Range +O.S/-5.2 Range +O*l/-6.9

Sorting

by

According

decreasing

weight

to Stockton, trampling results in moderate size sorting, with mean weights

with depth. Our results are slightly different. Note that Stockton’s pieces were

lighter than ours, having a mean weight of 4 g; our pieces had a mean weight of 12 g.

Our correlation table (Table 4) suggests that pieces lighter than 50 g may move upward,

Table 4. Correlation between weight and amount of displacement

Amou nt of displacement f rom the or iginal surface (cm)

We ight +2.9/ +0.9/ -l.O/ -3-O/ -S.O/

@

-l-O -0.9 -2.9

-4.9

-6.9

1-9

10-19

20-29

30-39

40-49

50-59

60-69

70-79

80-89

90-99

100-109

100-119

120-129

10 90 24 4

4 27 4 I

3 11 6 1 -

4 1 1 -

1 4 4 - 1

3 1 - -

1 2 - -

- - -

-

1 1 - -

-

1 - - -


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S

STRATIFIED SITES

277

downward or remain in place. Pieces heavier than 50 g tend to stay on or near the sur-

face on which they were originally placed. However, the number of pieces heavier than

50 g is very small and this proposition, though intuitively justifiable, remains unproven.

We found no correlation between displacement and kind of material.

Horizontal displacement

The maximum observed horizontal displacement is 85 cm. In uncovered squares the

the horizontal displacement is greater than in covered squares and most pieces are

affected (Figure 3). The pieces had changed from a horizontal to an oblique or sub-

vertical position in 4.3% of the cases; 21-O”h had been overturned.

Figure 3. Horizontal displacements in squares R and S, near the cave en-

trance. 1, Squares R and S, lower levels, af ter 36 days of t rampling. The

objects w ere covered with 3 and 2 cm of sand respect ively. 2, Square R and

S, upper levels, af ter 16 days of trampling. The objects had not been covered

with sand. Arrows indicate direction and amount of hor izontal displacement.

Aster isks indicate pieces that were not displaced.

There is no obvious linear correlation between horizontal displacement and weight.

Figure 4 shows that the most displaced pieces are light while the heavy pieces moved

little. However, many light pieces were not displaced or were displaced only a short

distance. Thus weight is not a good predictor of displacement. The missing third variable

is the haphazard occurrence of effective scuffage.

The observation that larger objects on paths get greater horizontal displacement (Wilk


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278 P. VILLA AND J. COU RTIN

& Schiffer, 1979, p. 533) does not seem to apply to people with no or light footgear. It

should be noted, however, that our trampled areas probably received less traffic than

Wilk & Schiffer’s urban paths and that only 5% of our items were larger than 10 cm

in any one dimension (see also Figure 4 and Table 4).

Breakage

Damage to pieces by breakage was limited since the substratum was nowhere very com-

pact and artifact density was low (mean=20 artifacts per square metre). The fragmenta-

50 -

l

(a 1

40.

5 30.

i

.o

520-g

l

l

L

10-r’ l l

.

.

l *

y. l +

.

04

=’ +a’. I.

. l + .

.

.+ . l l

.t

l

c

120

1

I

(b)

50-

.

,”

40-

i

.

.F N’

s

30-

l . .

20.

l

l .

.

IO- . . .

.

l

: ’ l .

. . . . .

0

IO 20 30 4b 50 60 70

80

Horizonta l displace ment (cm)

Figure 4. Sca tter diagram of the relationship betwe en weight and horizontal

displacement in (a) two uncovered squares trampled for 16 days and (b) two

covered squares trampled for 36 days.

,.


13/15

STRATIFIED SITES

279

tion index (no. of fragments x lOO/total no. of pieces) is higher in covered squares: 26

versus 13 in uncovered squares.

A few flints had very limited vis ible edge damage; predictably a few long blades were

snapped in two. Bone and shells broke more easi ly than pottery or flint. However, the

bone material was slightly weathered and the modern pottery we used is harder than the

prehistoric pottery.

Conclusions

The experiment demonstrates that trampling can cause mixing of materials belonging to

two separate levels. Thus, horizontally associated materials in a layer may derive from

the mixing of distinct episodes of site use. Size sorting may appear to be a clue to the

occurrence of vertical displacement since pieces lighter than 50 g are more mobile (both

upward and downward). In sites with superimposed levels, however, it would be very

difficult to know for sure whether small objects in a given level are displaced or in situ,

unless information from conjoined pieces is used. It should be noted that, in Stone Age

assemblages, many pieces are lighter than 50 g.

The degree of vertical dispersal suggests that in caves whose rates of sedimentation

are low, some archaeological items may be considerably younger than the matrix in

which they are found.

The horizontal displacement is not negligible. When doing a spatial analysis of sites

where trampling and scuffing might have been a significant factor, it is probably wiser

not to trust statistics based on precise measurements of horizontal distances (such as

used in nearest neighbour analysis). Otherwise, one may be applying precise methods to

ambiguous, imprecise data.

The vertical displacement we observed (7-8 cm) is smaller than the displacement ob-

served by Stockton (16 cm). Crucial differences were perhaps the use of small glass

splinters in his experiment and the intensive trampling.

Finally, both experiments suggest that in sandy deposits the effects of trampling are

limited to a zone 10-16 cm deep. More intensive trampling or trampling over long spans

of time may well cause larger-scale vertical displacements. However, the evidence, as it

stands now, suggests that other agencies should be considered for large-scale vertical

separations of archaeological items. Of course, these conclusions apply only to sandy

deposits and to archaeological materials of the kind we used. Clearly the degree of

consolidation of living surfaces at the time of occupation is a factor that should be con-

sidered in stratigraphic studies.

Note. While this paper was in review, we received a manuscript by Gifford et al. (in

press) which dealt with two trampling experiments. These experiments support Stockton’s

and our conclusions about the effects of trampling and the extent of vertical displace-

ment sandy deposits.

Acknowledgements

The work of Paola Villa has been supported by grants from the American Philosophical

Society and the Leakey Foundation. We also wish to thank Karl W. Butzer, John W.

Fisher, Jr., Patricia Phillips and one unknown reviewer for their careful and constructive

criticism of this paper.

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Villa & Courtin 1983 the Interpretation of Stratified Sites - A View From Underground

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