trampling the archaeological record an experimental study

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Society for American Archaeology

Trampling the Archaeological Record: An Experimental StudyAuthor(s): Axel E. NielsenReviewed work(s):Source: 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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2/22

TRAMPLING

THE

ARCHAEOLOGICAL

RECORD:

AN

EXPERIMENTAL

STUDY

Axel E. Nielsen

This

paper reports

on several

experiments

carried

out to

explore

the

transformations

of

the

archaeological

record

affected

by

trampling.

These

transformations

include

changes

in

artifact

distributions

and

formal

alterations

that

should

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

trodden

substrate

and its

implications

for

the whole

process.

The interaction

of

trampling

with other

formation

processes

(e.g.,

maintenance)

also is considered. The main

patterns

observed in the

trampled

materials

are vertical and

horizontal size

sorting,

and

characteristic size distributions in sherds. These

empirical

generalizations

are then

integrated

in a

model that can

help

to

identify trampled

contexts and assess their

potentialfor

behavioral

inference.

El

presente

art?culo describe varios

experimentos

realizados con el

prop6sito

de

explorar

las

transformaciones

producidas 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. Los

experimentos

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

desplazamiento

vertical,

desplazamiento

horizontal

y

danio

(fractura,

microlascado

y

abrasion),

prestando especial

atencion

a la

respuesta

del substrato

pisoteado

y

sus

implicancias para

el

proceso

en su

conjunto.

Tambien se considera la

interaccion

delpisoteo

con otros

procesos deformacion (p.e.,

mantenimiento).

Los

principales patrones

observados

en los

materialespisoteados incluyen

ordenamiento

verticaly horizontalpor

tamanio

y

distribuciones caracteristicas

en la dimensi6n

de los tiestos. Estas

generalizaciones empiricas

son

luego integradas

en un modelo

que

puede

contribuir a

identificar

contextos

pisoteados,

asi como a

evaluar su

potencial para

establecer

inferencias

de cardcter

conductual.

Since

Stockton's

(1973) pioneering study,

trampling by

humans

and

animals has

been

recognized

as a

major

process

by

which

archaeological

materials and

deposits

are transformed

in

their formal

and

spatial

attributes

(e.g.,

Schiffer

1983,

1987).

Understanding

the

potential

effects of this

process

is a

prerequisite

for

many

behavioral

inferences

in

situations

where

treadage

is

likely

to have taken

place.

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 areas

devoted 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 is

necessary

to know

minimally: (a)

the activities

in

which the artifacts

were

used;

(b)

the

circumstances

that

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

the

context 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 obscures

patterns

existing

in

their

original

deposition,

and

eventually

introduces new trends in

their

spatial

arrangement.

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

resulting

traces

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 de

Cordoba,

Argentina

American

Antiquity, 56(3), 1991, pp.

483-503.

Copyright

? 1991

by

the

Society

forAmerican

Archaeology

483


3/22

AMERICANANTIQUITY

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

suitability

of

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

common

element

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

and

relative

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

open

to

public

traffic

[cf.

Whittlesey

et

al.

1982]).

A

number of authors have taken

into

account

possible

alterations

of

deposits

resulting

from

trampling

based

on

reasonable

assumptions

of what its

effects are

likely

to be

(e.g., Bradley

and

Fulford

1980; Hughes

and

Lampert 1977;

Rosen

1986,

1989).

In

addition,

there have been several

attempts

to

explore

trampling

ethnoarchaeologically (De

Boer

and

Lathrap 1979;

Gifford

1978;

Gifford and

Behrensmeyer 1977;

Wilk

and

Schiffer

1979)

and

experimentally

under different

degrees

of 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; Pintar

1987;

Pryor

1988;

Stockton

1973;

Tringham

et al.

1974;

Villa and Courtin

1983).

These studies

have

focused

primarily

on

two issues:

(1)

how

human

trampling

disturbs

stratigraphic sequences

by

producing

vertical

migration

of

items,

and

(2)

how

treadage generates patterns

of

damage (mainly

in

lithics and

bone)

in

order to

differentiate them from

damage

produced

by

use

or

butchering

activities.

Although

several

generalizations

have

begun

to

emerge

as a

result

of

this

work,

it

is

surprising

that 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

vertical

displacement).

This situation indicates that

these kinds

of

experiments

will

have to be

repeated

many

times

for

reliable

generalizations

to be

drawn,

and that considerable work is

still

needed

before we are able to

apply

them to

archaeological

inference.

The

present paper reports

on six

experiments

designed

to

examine some

of

the

contradictory

results achieved

by previous

studies and to

explore aspects

of

trampling

processes

that have

received

little

attention

in

the literature. These include:

(a) patterns

of ceramic

breakage,

(b)

the influence

of

the

different

density

of various materials on

displacement,

and

(c)

the interaction between

trampling

and other

formation

processes.

MATERIALS AND

PROCEDURES

The six

experiments

are

labeled

TR-I

through

TR-VI and

were carried out

in

backyards

and

in

a

park

in

the

city

of Tucson.

A

summary description

of

them

is

presented

in

Table

1. The

next

three

sections offer

details about

the

trampled

substrate,

the materials

used,

and the

design

followed

in each case.

The

Substrate

Except

for

TR-III,

all

experiments

were

performed

on

dry,

highly

consolidated surfaces with no

vegetation

cover. TR-III

was carried out on the same sediments but five

hours after a

heavy

rain

in

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 trampling

impacts

the

archaeological

record:

texture and

penetrability.

A

grain-size analysis

of

the

sediments

in

the

trampled

areas showed that

according

to their texture

they

could be classified

as

muddy

gravels

(Folk

1980):

79

percent

gravel,

mostly

in

the

granule

fraction;

10

percent

sand;

11

percent

mud.

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

penetrability

in

kilograms per

square

centimeter

necessary

to stick

its

tip

into

the

ground.

This is the

only

technique

484

[Vol.

56,

No.

3,

1991]


4/22

Table 1.

Summary

of Features of

Each

Experiment.

TR-I

TR-II

TR-III TR-IV

Number of items 318

173

78

88

Materials used

bones,

lithics,

sherds

bones,

lithics,

sherds

bones,

lithics,

sherds

sherds

Size of

original

1

x

1

m

1

x

I m

1

x

1

m

.5

x

.5

m .

concentration

Wet/dry

dry

dry

wet

dry

Soil

penetrabilitya

2.49

>4.5

1.63

>4.5

Number

of cross-

1,500

800

800

100, 200, 300,

ings

400,

800

Variables

consid-

vertical-movement,

vertical and horizon-

vertical and

horizon-

fracture

ered

damage

tal movement

tal movement

a

Soil

penetrability

measured

in

kg/cm2.


5/22

AMERICAN

ANTIQUITY

Table

2. Size

Distribution

of Pieces Used

in

TR-I,

TR-II,

and

TR-III.

Size

Categorya

1 2

3 4

5

6

7

Total

TR-I

Bones

11 25

23

19 6 1

15

100

Sherds 25 30 26 19 6 1

0

107

Lithics 28

41 27 7 5 2 1 111

Total

64 96 76 45 17 4

16 318

TR-II

Bones

4

5 9 3

7

2 4 34

Sherds

2

12 1

3

11 14 9

12 73

Lithics

17 20

15 10

2 1

1 66

Total

23

37 37 24

23 12

17

173

TR-III

Bones

1 5

10 6 3 0

5

30

Sherds

0

7 7

1

3

2 2 22

Lithics

7

8 5 3

1

2

0 26

Total

8 20

22

10

7 4 7

78

a

C,7,t

=

_, lr-

mm

i

-

=

'11

m

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

precise

enough

to be taken as an absolute

measure of

penetrability

but rather as a relative estimation for

broad comparisons. Each locality was tested over an area of 5 m2 (10 points per

m2)

immediately

before and after the

experiments

in

order to assess variations

in

penetrability.

The means of

these

measurements 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

(fragments

of

mandible,

diaphysis

and articular

parts

of

long

bones,

and

vertebrae), fragments

of oak wood

and

brick,

and

sherds from the

following

five

types

of

pottery: (a) High-tempered

slabs made of

commercial

clay

(Westwood EM-207)

fired at 700?C for 30 minutes

(thickness

7

mm); (b)

small

Mexican low-fired

globular

vessel

(12

cm

high,

wall thickness 4.3-5.6

mm);

(c) large

Mexican low-

fired

globular

vessel

(40

cm

high,

wall thickness 4.8-7.4

mm); (d) biglobular (gourd-shaped)

Mexican

vessel (wall thickness 4.5-7.0 mm); (e) Italian high-fired flower pot (wall thickness 3.9-4.8 mm with

an 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

in

firing

temperature,

and therefore

the

grades

A

through

E

can be considered

a

rough

ordinal measure

of the

strength

of the

paste.

The

Experiments

During

the

experiments

attention

was

paid

to three different

aspects

of

trampling.

TR-I

through

TR-III

focused on horizontal

and vertical

movement and

general

damage

in

artifacts.

TR-IV

and

TR-V were

designed

to examine

patterns

of ceramic

breakage,

and

TR-VI

focused

on

the influence

of material

density

and

object

bulk ' on

horizontal

migration.

Accordingly,

three basic

designs

were followed.

TR-I,

TR-II

and

TR-III.

Before each

experiment

all items were

numbered and

weighed,

and

their maximum

length

was recorded

(Table

2).

The flakes

were

spray

painted

to facilitate

the

486

[Vol.

56,

No.

3,

1991]

I-

1L

C

fLL,rt

lll

i.

tal

C

I

I1-1-Ullllll,.L

-

j

I-+killllll,-t

-

-t I

-


6/22


7/22

AMERICAN

ANTIQUITY

Table 3. Number of Items of Each

Kind

of MaterialBuried

and

on the Surface

n

TR-I and TR-II.

Surface Subsurface

Totala

TR-I

Bones 48

(51.6%)

45

(48.4%)

93

Sherds 75

(40.8%)

109

(59.2%)

184

Lithics

27

(21.9%)

96

(78%)

123

150

(37.5%)

250

(62.5%)

400

TR-II

Bones 22

(66.7%)

11

(33.3%)

33

Sherds 179

(83.6%)

35

(16.4%)

214

Lithics 36

(51.4%)

34

(48.6%)

70

237

(74.8%)

80

(25.2%)

317

a

The differencesbetween the numbersof items recoveredand

those in the

original assemblages as described in Table 2 are due to the combined effects

of

loss and

breakage.

The

same

phenomenon

was recorded

by

Gifford-Gonzalez

et al.

(1985:808)

in their

experiment

on

loamy

soil,

and is familiar

to

ecologists (e.g.,

Liddle

1975;

Weaver and Dale

1978)

who conceive

trampling

both as an erosive

process

that increases the

depth

of the

paths

and as

a

compacting

process

that

increases the bulk

density

of the soil near the

surface. For a

given trampling agent

there

exists

a maximum stable

compaction

value that is a function of the

microstructure of the soil.

The

loose cover

is a more

dynamic

element

that is

likely

to

vary

in thickness

depending

not

only

on the

soil,

but also

on the

intensity

of

treadage,

slope,

and

patterns

of rainfall.

Trampling

after rain

(TR-III)

had different

effects. The

muddy

surface

was

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

them

were 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

implications

for the

interpretation

of

stratigraphic

sequences

and

chronology.

Most

of the studies

have been

carried out on loose

sandy

soils,

where artifacts

from the same

original

assemblage

have been

recovered

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

of

the items

found within the

first centimeter.

Existing

studies are

contradictory regarding

the

presence

of

a correlation

between

the

size,

weight,

or

density

of the artifacts and their

vertical

migration.

Villa and

Courtin

(1983:277)

worked

with

different

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 remain

near the level

where

they

were

placed.

Gifford-Gonzalez et

al.

(1985:811)

report

that

none

of the

attributes

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

displaced

downward. A similar correlation is apparent in Muckle's (1985:Table 16) trampling experiments

with

shell

on

a loam substrate.

In an

ethnoarchaeological

context,

Gifford

(1978:82)

previously

had

observed a

tendency

of smaller

objects

to be

trapped

in loose sand surfaces.

488

[Vol.

56,

No.

3,

1991]


8/22

REPORTS

Table

4.

TR-II:

T

Tests

for

Length/Weight

nd

Vertical

Migration.

Surface

Subsurface

Mean

s.d. Mean

s.d.

t

value

p

All items

Length

34.6

13.5

23.5

9.4 3.99 .000

Weight

4.9

4.6

1.9 2.5

7.48 .000

Bones

Length

54.7

21.5

26.3

9.6 5.24 .000

Weight

3.9 3.6

.7

.5 4.02 .001

Sherds

Length

30.9

12.3

23.5 9.2

4.09 .000

Weight

5.2

4.4 3.0

3.2 3.50

.001

Lithics

Length

34.6

13.5

23.5

9.4 3.99

.000

Weight

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.I

Under

dry

conditions,

this

corresponds

to the

thickness of the loose

top

layer

discussed

in

the

previous

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)

size

sorting

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 1,500

times,

the

top

level

averaged

1.5 cm and

contained 62.5

percent

of the artifacts.

In TR-II

(>4.5

kg/cm2,

800

crossings),

the loose

layer

did not

exceed

1 cm

and included

only

25.2

percent

of the

recovered

assemblage (Table 3).

Size

sorting

is

apparent

when the

proportions

of

buried and unburied

pieces

of each kind of

material

are considered.

Lithics,

which included

smaller

(Table 2)

and flatter

items than sherds and

bones,

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

very

significant

(Table 4).

No such

sorting

occurred after

treadage

on

wet

ground.

The

proportions

of

surface/subsurface

artifacts

seem to

vary randomly

across

material

type

(Table 5). Moreover,

neither

length

nor

weight

render

statistically significant differences between objects recovered in various levels (Table 6). No

loose

cover is

developed

in

this

situation. The

materials, rather,

are

pushed

down

by

the feet

and

stuck

in

the

permeable

substrate.

If

the

surface is

penetrable

enough (ca.

2

kg/cm2

or

less),

no

sorting

occurs.

Trampling

under these

conditions will

tend to

fix in

their

initial

horizontal location

objects

of all

sizes

except

the

very large

ones.

Eventually,

once the soil

dries

and

hardens,

erosion will

develop

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

displacement

of

artifacts

trampled

on

wet and

dry

ubstrates. Under

dry

conditions

the artifacts tend

to act as

passive

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

protect

them

from

being

removed

during

maintenance.

489


9/22

AMERICAN

ANTIQUITY

Table 5. Number of Items of Each

Kind of MaterialBuried

and

on

the Surface in TR-III.

Surface Semiburied3

Subsurface Total

Bones

7

(25%)

13

(46%)

8

(29%)

28

Sherds 4

(17%)

17

(74%)

2

(8%)

23

Lithics

8

(31%)

16

(61%)

2

(8%)

26

Total 19

(25%)

46

(60%)

12

(15%)

77

a

Items that

penetrated

completely

into

the substrate

but

still were

partially

visible 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,

in

the case of

roofed

areas),

the

less-disturbed evidence

will

be

found

in

a thin

(20

mm

at

most),

loose

level

overlaying

a

hard,

compact,

and

probably

sterile one

(unless

previous

occupations

exist

in

the

site).

Holding

constant other

factors,

the artifacts recovered

in

that

upper

layer

should be

very

small

and could be

considered

primary

refuse.

2. If

the last

trampling period

took

place

under

wet

conditions,

items of all sizes will

be

found

embedded

in

a

relatively

hard

layer.

The

previous

discussion

also

illustrates the

complexity

of

the formation

of

living

floors. The

widely

shared notion that

intensively

occupied

surfaces are

hard

and

highly compacted

needs to be

treated

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 behavioral

evidence

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),

should

be employed to recover this information.

Horizontal

Displacement

Only

two

experimental

studies have searched for

patterns

in

the horizontal

displacement

of

trampled

artifacts.

Villa and Courtin

(1983:277)

observed that

the most

displaced pieces

are

light

while

the

heavy

pieces

moved little but there is no obvious linear correlation between

horizontal

displacement

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 between

length

and horizontal

migration

of flakes.

Table 6. TR-IIl: T Tests for Length/Weight and Vertical

Migration.

Surface

Subsurface

Mean s.d. Mean

s.d. t value

p

Bones

Length

37.4 8.8

35.9

19.0

.21 .84

Weight

2.7

1.8 1.8 2.0

.98

.34

Sherds

Length

54.7

29.1

26.0

7.1

1.87

.14

Weight

21.6

21.7 1.2

1.2

.51

.15

Lithics

Length

29.(

14.9

27.5 16.3

.12

.92

Weight

4.8

11.2

1.5 1.9

.78

.46

490

[Vol.

56,

No.

3,

1991]


10/22

REPORTS

Table 7.

Mean Horizontal

Migration

of Materials

in TR-II

and

TR-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 and

Pintar.

Although

the correlations

length/horizontal

movement

and

weight/horizontal

movement

are

positive

in all the

cases,

they

are not at all

significant

(range

of

r

values

=

.0884-.5545).

TR-III

produced

similar results. The

only

observed difference

is that materials

moved less

than

in

the

experiments

performed

on

dry

surfaces because

they

were

trapped

in

the substrate

from the

beginning

of the

process.

However, when the different materials are compared in terms of their mean horizontal migration

some trends arise

(Table 7).

Bones

moved more than

lithics, whereas,

at

least

in the

dry-trampling

case,

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

weigh

more. It also could be

argued

that

although length

is not a

predictor

of

horizontal

migration,

there

is still a weak

positive

correlation between both

variables. Since bone

assemblages

included more

large pieces

than lithic and ceramic

ones,

differences

in

size still could

be

responsible

for the

differences

in

mean

displacement.

Shape

is a third variable that

may

be reflected

in

these

figures.

Three

of the more

displaced

bones

were vertebrae

which,

by

their

very shape,

are more

likely

to be kicked

away

than flat elements

like

sherds or

flakes.

In

fact,

size and

shape

are

better conceived as a

single

attribute,

that can

be referred

to

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

(1.84

g/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 maximize

contrasts

in

density

and bulk.

Sixteen small sherds

(2.6-4.6

cm3,

maximum

length

=

32

mm)

were

included to facilitate

comparisons

with the

assemblages

used in

previous

cases.

All

pieces

were

scattered

along

a

heavily

used dirt

path

in

a

park

in

Tucson. The mean

displacement

recorded after

three

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 horizontal

movement between

both

materials

are

significant (t

=

1.90;

df

=

32;

one-tail

p

2

kg/m2),

objects

follow at least three

different

patterns

of horizontal

migration

according

to their size.

Very

small items

(50

cm3) are kicked

and

scuffed rather than trodden and therefore

will

move faster and more

systematically

to stable

positions

in the

marginal

zone. Less dense elements

will

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

492

[Vol.

56,

No.

3, 1991]


12/22

REPORTS

Table

9.

Breakage

ndex

(bx)

for Nine Ceramic

Assemblages

After DifferentAmounts

of

Trampling.

Number

of

Crossings

Cumulative

50

100

200 300

400 800

1,500

Index

Type

A TR-I

1.84 1.84

Type

A

TR-II

3.46 3.46

Type

B TR-V

4.00

1.22

1.23

6.00

Type

C

TR-I

1.36 1.36

Type

C

TR-II

2.05 2.05

Type

D TR-II

2.89

2.89

Type

D TR-III

1.45 1.45

Type

D TR-IV

2.01

1.17 1.08 1.24

1.11

3.18

Type

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 been

removed.

hremoved.hree

days

later

fragments

of all sizes had been cleaned

up,

but still the

larger

ones were

the most affected. On the other

hand,

none of the sherds

(all

of them smaller

than 4.6

cm3)

were

missing (Figure 1).

In

accordance with the

McKellar

Principle

(Schiffer

1987:62),

these observations

show that smaller items are left

behind

in

regularly

maintained

areas. From the

point

of

view of

spatial

analysis, they imply

that maintenance eliminates

part

of the noise that

trampling

introduces

in

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

present

section

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

slight

rounding

of

edges,

and

in

few cases

microchipping

and

delamination,

especially along

the

edges

of

polished

surfaces. But

breakage

is

certainly

the

most obvious kind of

damage.

To

facilitate

comparisons

among assemblages,

a

breakage

index

(bx

=

number

of

fragments

after

selected

trampling

episode

divided

by

number of sherds before

that

episode)

was

calculated for each

type

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

critical

role of

surface hardness

in

the

process

of fracture. The

three

assemblages

trampled

on

relatively

penetrable

soils

(A

and C

TR-I,

D

TR-III)

had a

breakage

index lower than

two,

even

though

the

sherds of TR-I were trampled twice as much; the rest of the assemblages (that were trodden on

surfaces

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

would

be 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 value

that,

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

original

fracture of the

vessels. Further

trampling

would result in

a slow

reduction of the modal value and

an

increasing

positive

skewness of the whole distribution.

When

the modal value reaches the smallest

size category, the whole curve would approximate a Poisson distribution. If this proposition is

correct,

it

could be a useful

device for

recognizing

archaeologically trampled

assemblages

and

perhaps

even for

determining

the relative amount of

treadage

that

occurred.

493


13/22

AMERICAN ANTIQUITY

50

*

TYPE

C

TR-I

N=28

40

-

TYPE

A

TR-I

N=80

30-

//

- -

-,

\

TYPE TR-ii

... / ....... \N=

..?'

*

\^

^^-

N=33

1

2

3 4

5

6

7

size 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.

494

[Vol.

56,

No.

3,

1991]


14/22

REPORTS

50

0

cross

100 cross

40

---_-

.......

-,

*,;

.

200 cross

3..0

-

^ \

300 cross

400 cross

20-

-

800 cross

1 2 3 4

5

6 7

size

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

=

61-70

mm;

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

category

3,

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 and

1.357),

are

explained

readily by

their

originally

skewed distribution

which,

if

types B,

E

(TR-V)

and

D

(TR-IV)

are assumed to be

representative cases,

are

very unlikely

to occur

in

naturally

broken

pots. Furthermore,

if

the areas under

analyss

contain

secondary

refuse,

their

original

size distribution

will

tend to be skewed

negatively,

provided

that smaller

objects

are

more

likely

to

escape

maintenance

activities and be left behind

in

original activity

areas.

This

would

provide

an even

stronger

contrast

between

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

of

sherds

produced by

the initial

breakage

of whole

pots.

In

TR-IV,

after

just

100

crossings only

one

sherd

larger

than

71 mm is

left

(which

remains unbroken

throughout

the

process),

and the

general

curve

already

shows the characteristic

configuration. Changes

after this

first

stage

are much more

gradual.

This

process

is even clearer for

type

B TR-V

(Figure

5)

in

which no sherd exceeds 45

mm

in

length

after 50

crossings.

As

noted

previously, type

E

was a

high-fired

flower

pot

that had the

hardest

paste

among

the

vessels used.

Consequently,

it

experienced

the slowest size reduction

(Figure

6).

Several

fragments

corresponding

to

the reinforced

rim

of the vessel

remained

relatively large.

Unlike

type

B,

which

was

exposed

to the

same

trampling

conditions,

the mode of

the distribution for

type

E

did not reach

category

1

by

the

end of the

process.

The

response

of

this

type

suggests

that

particularly strong

wares-beyond

the

range

considered

in

this

study-probably

will

depart

from the trends described thus far.

Fragments

of

storage

vessels

with

extremely

thick walls or

ceramics fired at

exceptionally

high

temperatures

can be so

strong

that 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

its

size

distribution

is

found

to be

unimodal,

with the mode lower than 30

mm,

and

with no

fragments

495


15/22

AMERICAN ANTIQUITY

[Vol.

56,

No.

3,

1991]

0 cross

50 cross

100 cross

400 cross

-

-

-

-

1 2 3

4

5 6

7

size

categories

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

=

61-70

mm;

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

ceramic

material can be assumed approximately constant across samples

(i.e.,

they show a similar range of

internal

variability),

and no other cultural

formation

processes

are

acting

upon

this

dimension of

0 cross

50 cross

100

cross

400

cross

- -

-

-

1

2

3

4

5 6 7

size

categories

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

=

61-70

mm;

7

=

71 mm

or

more.

496

70

60

50

40

30

20

10

0

50

40

30

20

10


16/22

REPORTS

40

0+100

cr.

- k 0+300 cr.

~

/'

*\

0+800

cr.

10

-

0

'I

I

I

I

1

2

3 4

5

6

7

size

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

=

61-70

mm;

7

=

71

mm

or more.

the

material,

then the

degree

of

positive

skewness of the

distribution is a relative

indicator of the

amount of

trampling undergone

by

different

assemblages.

Two

objections

could be raised

against

these

generalizations.

First,

if

freshly

broken

assemblages

are mixed with

previously

trampled ones,

as

may

happen,

for

instance,

if a

path

crosses a

secondary

refuse

area,

would the

patterns

described above still be

recognizable?

Would it be

possible

to detect

the

presence

of both

kinds

of artifacts

in

the mixed

assemblage?

To answer these

questions,

the

figures

for some

trampled

and

untrampled

sets chosen at random

were combined

in

three

ways (type

D TR-IV

0+100

crossings; type

A TR-II

0

+

type

D TR-IV

300

crossings;

and

type

D

TR-II

0+type

D TR-IV

800

crossings)

and the distributions

represented

in

graphic

form for the

resulting

mixed

assemblages (Figure

7).

As can be

seen,

the

trampling

patterns

still are

perfectly

visible.

The

slightly

high

proportion

of

large pieces

shown

in

the curve

as a solid line could serve as an indicator of the

presence

of the

untrampled

set,

if

these sherds do

not

have

special

attributes

that would

give

them

higher breakage

resistance.

Another

process

that could

produce

a

similarly

skewed size distribution

in

the

assemblages

is

maintenance (Schiffer 1987:64), since the biggest artifacts would be retrieved and discarded in

secondary

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

in

rdual

primary

efusesidual

rimarprobably

will

remain.

Second,

in

trampled assemblages

not

subjected to

maintenance,

one should find a consistent

proportion

of

big

pieces

of other kinds of materials more

resistant to

fracture

(e.g.,

bone,

lithics).

It

should be

recalled, however,

that these

large

items

may

have

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

sherds

is

random,

it is

possible-though

improbable-that

such distributions fall within the

specifications

for

trampled

material.

Type

C

(TR-I), represented by

the solid

line

in

Figure

2,

would be a case

in

point.

Although

it

does not

represent

any

natural

breakage,

if

such a distribution

was found it

would be interpreted erroneously as a trampled assemblage.

To

provide

an additional

control,

it was

postulated

that there should be a correlation between

the

mean size of

the sherds and

their size variability (as

measured

by

the standard deviation

of

the

497


17/22

AMERICANNTIQUITY

30

before

trampling

25-

A

A

25

A

after

0

-

trampling

._

20-

>

-

4

)

10-

5-

10

20

30 40 50

60 70

Mean Size

(maximum

length)

Figure

8. Size and size

variability

for

eight

ceramic

assemblages

before and after different amounts

of

trampling.

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

variables

in

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

through

he

diagram

(Pearson's

r

=

.4938),

while those

corresponding

to

trampled assemblages

are

aligned

in

a

regular

pattern (r = .9271) indicated by the regression line in the graph. The trajectory of individual

assemblages through

increasing

treadage

show even

higher

correlations

(r

=

.99 for

types

D TR-IV

and

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

extremely

heterogenous,

it is

likely

that the

pattern

will

hold

in

any archaeological

situation

regardless

of the

internal

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

expression

of the

regression

shown

in

Figure

8

(y

=

bx

+

a)

should

predict

its standard deviation

from its

mean size.

In

other

words,

the

expression

S

=

.4686

X

-

3.283,

where

5S

=

standard deviation

of

length,

and

X

=

mean

length,

should hold

with

a

margin

of error

of

?

1.19

(measurements

taken

in

mm),

corresponding

to the standard error

of

the

regression

line.

The

range

of values

predicted

Table 10.

PredictedandActualStandard

Deviations of

Length

for

Nine Ceramic

Assemblages

Before

Trampling.

Assemblage

Mean

(mm)

Predicted S Actual S

A TR-I

29.59 9.39-11.77 12.28

A TR-II

41.63

15.03-17.41 18.79

B TR-V

42.29 15.34-17.72

25.60

C

TR-I

33.86

11.39-13.77 9.08

C

TR-II

54.00

20.83-23.21

13.76

D TR-II

59.33

23.33-25.71

23.85

D TR-III

41.36

14.91-17.29

18.58

D TR-IV

40.86

14.67-17.05

21.16

E TR-V

45.73

16.96-19.34

24.09

498

[Vol.

56,

No.

3,

1991]


18/22

REPORTS

25

3

ass.

trampled

20

-

1

ass.

trmp.

? + 1

untrmp.

C

-

A

'>

A

o

....-

.

3

ass.

qa

15

C)

o ,,--

untrampled

^

-U

*e

---'

o

-~0

5-^

-6^

0

0

i

15

20

25 30 35

40

45

Mean Size

(maximum

length)

Figure

9. Size and size

variability

for three

hypothetical

ceramic

assemblages combining

trampled

and

untrampled

sherds.

The line

represents

the

pattern

obtained from the

eight assemblages

with various

amounts

of

trampling

(small circles) (see Figure 8).

by

this

procedure

and

the actual ones for the standard

deviation of the

nine

assemblages

used

in

this

study

before

trampling

are

displayed

in

Table

10. Most values of S

fall,

as

expected,

out of

the

predicted

range

for

trampled assemblages, including

C

TR-I

that could have

been

mistaken as

trampled following

the size-distribution

procedure.

The

only

exception

is

D

TR-II, that,

in

any

event,

can never be mistaken as trodden if its size distribution-35

percent

of the sherds

larger

than

71

mm-is

considered

(Figure

2,

long

dashed

line).

Finally, Figure

9 shows the

ability

of the

size-variability procedure

for

detecting

assemblages

containing

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

(small

circles).

As

expected,

the solid

circle

(three

trampled

sets

added)

falls

close to the

line,

within the

range

of

variability

predicted

for trodden materials. The combination of one

trampled

and one

untrampled

assemblages

(solid triangle)

is situated out of this

range,

and three

untrampled

sets

together

fall even farther

away

from the line.

Consequently,

it is

suggested

that the

application

of both

procedures,

size distribution

and size

variability, by using

two different

dimensions of the

data,

can discriminate with a

high

degree

of

confidence between trampled and untrampled sherd samples, and even establish relative amounts

of

treadage (or activity)

on the material

if

certain conditions are met.

Lithics. The three kinds of

damage

considered

here

(breakage, microflaking,

and

abrasion)

were

observed on lithics. Table

11

shows the number of

pieces

representing

the first two

alterations in

Table 11. Number of

Flakes

Showing Breakage

and

Microflaking.

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

499


19/22

AMERICAN

ANTIQUITY

TR-I,

TR-II,

and TR-III.

As

noted

by

Gifford-Gonzalez et

al.

(1985:813), breakage

is more

frequent

on

harder surfaces

(TR-II).

Artifacts

in

TR-II were

more

damaged

even

though

artifacts

in TR-I

were

trampled

twice as

much. Lithics

trampled

on a

wet substrate

(the

most

permeable)

were

the

least

damaged.

Abrasion was

especially

severe on

prominent parts

of the

flakes,

such as

percussion

bulbs and

dorsal

ridges.

There are

considerable

differences in the

literature

concerning

the

type

of

edge damage

produced

by

trampling. Working

with

obsidian

flakes,

Tringham

et al.

(1974:113),

who

performed

the

first

experiment

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 surface

opposite

to

the

trampler;

and

they

lack

fixed

orientation

or

size,

but

are characterized

by

marked

elongation.

A later

experiment

carried out

by

Flenniken and

Haggerty (1979)

contradicts

these criteria.

They

found that out of 157 flakes

(37 percent)

that underwent

edge

modification

during

treadage,

56

(13

percent)

could be classified

as

tools,

and

their

edges

were

remarkably

similar to used

ones.

They

pointed

to the absence of

polish

as the

only

definitive indicator

of

trampling damage

as

opposed

to

use.

On the other

hand,

Gifford-Gonzalez

et al.'s

(1985)

and

Pryor's

(1988)

studies

agree

with

Tringham

et

al.

concerning

the

sparseness

of

the

scars

along

the

edges,

but these scars

were not

elongated

and

originated

on both surfaces

of the artifacts.

The results of the

experiments

reported

here

are

in

general

agreement

with the

conclusions

of the

latter 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

be

identified,

except

for

a trend

of

larger

scars

to

occur

on

steeper

edges.

However,

six or seven

pieces

from

the

dry-trampled assemblages

depart

from this

general

trend.

They

show rows of

continuous

parallel

scars

along

one

or more

edges

that

could

be

mistaken

easily

for intentional

retouch.

CONCLUSIONS

Several 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

integrated

to outline

sets of traces

in the

archaeological

record

that

can

help

to

identify

trampled

contexts.

Of

course,

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

on

hard-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

the

trampled

area

where

they

will

stay

stationary

unless

affected

by

other

factors.

Small and

medium

size

objects

will move

randomly

within the

traffic

zone,

blurring previous

patterns

in their

horizontal

arrangement that might have existed

within the

rampled

area.

Only very

small items

will

remain

in

their

original

location

by being

absorbed

in

he

riginal

substrate.

In other

words,

even

moderately

trampled

areas

will

be

composed

of

a

marginal

zone characterized

by

a

high

proportion

of

bulky

artifacts,

and a

traffic

zone

with

small-

and medium-size

items

randomly

scattered

and

very

small

ones buried

close

to

their

original

spot

of

deposition.

In this

initial

stage

the

sherds

already

will exhibit the

typical

relation

between

mean and

standard

deviation

of

size

and

will

adopt

a size

frequency

distribution

that

resembles

a

bell-shaped

curve

or

a

Poisson

distribution,

depending

on

the

strength

of the

paste,

the thickness of

the

sherds,

and

their

curvature.

Few

lithics

will break

and

present

isolated

and

randomly

distributed

flake

scars

along

their

edges.

A

few

damaged

edges

may

mimic

retouching.

If

trampling

continues

and the

area

is

not

cleaned,

the

original

pattern,

preserved

by

the

small

pieces initially

trapped

in

the

substrate,

will

be

obscured

increasingly

by

the

absorption

of

new

small pieces produced by the fracture of objects after having been displaced horizontally. On the

other

hand,

medium-size

items

that are

unlikely

to

be

trapped

in the substrate

will reach

gradually

stable

positions

in

the

marginal

zone.

500

[Vol.

56,

No.

3,

1991]


20/22

REPORTS

Thus,

while

original

fine-grained

horizontal

configurations

within

the

traffic

zone

will

be

no

longer recognizable,

the contrast between traffic and

marginal

zones

will

be

stronger.

The

former

will

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).

The

latter

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

many

small

artifacts and microartifacts and

an

extremely compacted,

sterile bottom

layer.

The

latter

usually

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

also

can

be

recognized

in

the

archaeological

record

(see

Koike

1987).

In

addition,

the

whole

assemblage

should show severe

damage--randomly

scarred

edges

and

abraded

ridges

in

most

flakes,

and

rounded, microchipped,

and delaminated

edges

in

sherds.

The

size

frequency

distribution of sherds should be

extremely

skewed

as

well.

When

applying

these

generalizations

to

archaeological

cases

it

should be

kept

in mind

that

every

deposit is the result of multiple formation processes, including human activities and the action of

environmental

factors. Sometimes the material effects of

these

processes overlap,

this

is,

different

processes

can

produce

similar

traces.

For

instance,

ethnoarchaeological

studies have demonstrated

that horizontal

size

sorting

also can result

from

the

spatial

organization

of activities themselves and

corresponding disposal

modes

(Binford

1978;

O'Connell

1987),

or

cleaning (DeBoer

and

Lathrap

1979;

Simms

1988).

When

inferring trampling

in

archaeological

cases, therefore,

one should consider

as

many

traces

as

possible,

as well

as relevant

independent data,

in

order to differentiate it from

other

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

of

trampling

itself. For

example,

as has

repeatedly

been

demonstrated,

maintenance

operates

selectively

on

larger

items.

If

an

intensively

trampled

area

is

frequently

cleaned,

clear

depositional patterns

are

likely

to be

preserved

in

the

distribution of small artifacts

in

the traffic zone.

Bigger objects,

therefore

more

displaced,

will

be

cleaned

up

systematically

and

will

not remain

long

enough

to be

randomly

displaced

and

fractured

in

different

locations,

contributing

additional

small

artifacts that would

obscure

existing patterns

reflected

in

this size

fraction. On the

other

hand,

in

rarely

maintained

areas the

contrast between

marginal

and

traffic zones

will

be

stronger,

and

damage

will

be more

severe because

artifacts

will

be

exposed

longer

to

treadage.

Differences

in

environmental

conditions,

or

in

exposure

to such

conditions,

can

effect variations

in

these

patterns

as well.

Rain,

for

instance,

generates

a

muddy

and

very penetrable

surface that

will

trap

all

artifacts

regardless

of

their size.

This will

prevent

the

objects

from

moving

horizontally

and

will

reduce

damage.

However,

once

the surface dries

and

treadage

erosion

generates

the loose

cover

again,

larger

items

will

be

released,

start

moving

horizontally,

and

be

exposed

to

retrieval

during

cleaning

activities,

while smaller

ones

will

remain

embedded

in

the

top

layer.

If rains are

very frequent and evenly distributed throughout the year, very little horizontal movement (and

postdepositional

patterning)

will

take

place

in

unroofed

trampled

areas.

Even

in

spaces

regularly

cleaned, compact

occupation

floors

will

be found with

artifacts

of various sizes embedded and

damage

will be

less

severe. On the other

hand,

if

the

amount

of

rainfall

is small and

unevenly

distributed

during

the

year (as

is often the

case

in

semiarid

environments),

trampling

patterns

will

be

much

clearer,

more so

if

the surfaces were

buried or

abandoned after a

long

period

of

dry

treadage.

Future

studies can

define variations

in

the

patterns

discussed above when

treadage

occurs

under

different

conditions.

This

basic

understanding gained through experimentation,

together

with

relevant

ethnoarchaeological

observations,

can

provide

criteria

applicable

to the

archaeological

identification of

trampling,

and hence contribute to assess

the

potential

of

particular

contexts for

various

kinds of

behavioral inference.

Acknowledgments.

The

research

reported

on here

has been

supported

by

the

Laboratory

of

Traditional

Technology, Department

of

Anthropology,

University

of

Arizona,

Tucson. I

want

to

express my gratitude

to

501


21/22

AMERICAN

ANTIQUITY

several

persons

that

help

me

during

the work. Michael Schiffer

gave

me access to the lab and made

insightful

comments on drafts of the

paper.

William

Walker,

James

Spicer,

and Nieves Zedeniio ent their

feet and shared

generously

their time and ideas.

James

Skibo,

Chuck

Bollong,

William

Walker,

and the

editorial staff ofAmerican

Antiquity 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.

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