Erosion of archaeological sites: Results and implications of a site simulation model

Erosion of Archaeological Sites: Results and Implications of a Site Simulation Model

John Wainwright Department of Geography, King’s College London, Strand, London, WC2R 2LS, United Kingdom

A computer simulation model has been developed to investigate the postdepositional changes on archaeological sites due to soil erosion in a semiarid (Mediterranean) environment. Both soil matrix and archaeological artifact movement are accounted for by the model. The model is applied to a series of hypothetical site configurations on hillslopes, using archaeological structures, to observe the morphological changes due to slope form. The results show considerable variation in site preservation potential, relating to different initial spatial patterning of the site and initial slope form. The model predicts thresholds for the start of movement by artifacts, and suggests a correlation between the location of deposition and the potential for disturbance of the artifact record. Implica- tions are drawn from the results of the simulations, both in terms of archaeological theory and practice, and in terms of site conservation and management. The simulation method is recognized as a useful tool for the investigation of natural site formation processes. 0 1994 John Wiley & Sons, Inc.

INTRODUCTION The erosion of archaeological sites is widely recognized as a problem leading

to the partial or complete destruction of cultural remains, yet little systematic work has been carried out on the extent of the problem. Within the context of the archaeological record at a site, the need for greater understanding of erosive processes falls under two main headings. First, there is the application to the knowledge of site formation (Schiffer, 1987), relating to the differing degrees to which cultural interpretations may be placed upon artifact and context assemblages, and the confidence that may be placed upon these interpretations. Secondly, the erosion of sites within the landscape has major effects on the management of cultural resources, both in terms of site protection by active and passive means, as well as in terms of deciding which sites should be excavated from research and rescue viewpoints.

The requirements for the achievement of these strategies are twofold: the ability to assess the effects of erosion on the site sediment (assuming cultural material to be part of the sediment in this context), and secondly, to be able to project these assessments over varying timescales. These time scales are both in the past (retrodiction), in the assessment of deposit integrity, as well as in the future (prediction), in order to decide management strategy.

Geoarchaeology: An International Journal, Vol. 9, No. 3, 173-201 (1994) 0 1994 by John Wiley & Sons, Inc. CCC 0883-63531941030173-29

SITE EROSION SIMULATION MODEL

Advances in geomorphological methods in the last 20 years have led to the development of models based on physical principles to study the erosion problem in greater detail (Kirkby, 1987; Ahnert, 1987; Thornes, 1988). Simple applications have already been made to archaeological problems (Kirkby and Kirkby, 1976; Thornes and Gilman, 1983). However, a full application to problems of archaeological sediments-the displacement of both matrix and the artifacts within and on it-has not been previously attempted.

The potential for such an approach is investigated here by means of a simple model that has been specifically developed in an archaeological perspective for sites in a Mediterranean climatic setting. Using physically based descriptions of the erosive processes, it accounts for differences in the movement of the sediment matrix and artifactual material, differences in the hydrological characteristics of different archaeological contexts (Wainwright, 1992), and the presence of archaeological structures. This model is then used to produce a series of outcomes by varying the erosional parameters, in order to show the generic behavior of slopes with archaeological materials. The results are then discussed in terms of deposit integrity and resource management.

DESCRIPTION OF MODEL The archaeological hillslope erosion model (AHEM) has been developed

from a number of sources. It has a coupled structure, linking the processes of infiltration of rainwater and the corresponding generation of runoff by infiltration excess and saturated mechanisms, the routing of the overland flows generated, and the transport of sediment (Figure 1). Inputs required are the topography of the site, including any structures, the initial positions of artifacts, and the hydrological characteristics which have been shown to vary with the archaeological (morphological) context (Wainwright, 1992) and climatic parameters. The model outputs the resulting slope forms and artifact locations at preset intervals. The simulation results are then sub- jected to further analyses.

The infiltration component of the model uses the Scoging-Thornes approach (Thornes and Gilman, 1983), with the infiltration rate through time given using simple empirical relationships (Scoging and Thornes, 1979). Overland flow is generated in two ways. If rainfall exceeds the infiltration rate, then infiltration excess (Hortonian) overland flow is generated; its quantity is defined as the difference between the current rainfall and infiltration rates. Saturated overland flow occurs as the soil moisture storage capacity is reached. Subsurface flow effects have been ignored, as the model was designed to be applied to semiarid regions, where these processes are relatively insignificant.

Once generated, the overland flow is routed across the slope surface using a kinematic wave model (Dingman, 1984). The rate of flow is modified to account for variations in surface sediments including artifacts and the presence of vegetation.

174 VOL. 9, NO. 3


continuous cycle (-xi-)

of occurrence of storm event

1 calculate

storm event

I generate runoff I

I distribute w a t e q flow

transport selectivity : matrix : overland flow

splash particles :

allow burial

Figure 1. Structure of the archaeological hillslope erosion model (AHEM).

The overland flow generated in this way is then used to drive an erosion submodel. This model transports matrix and archaeological materials such as pottery using a single method, as presented in detail by Wainwright and Thornes (1991). The two parts of the model-sediment matrix and artifact transport-will be considered separately in relation to the application of the model to site form, and to the distributions of artifacts within the site, although interactions between the different sediments are still considered.

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL 175


APPLICATION TO SLOPE FORM This section of the model considers sediment movement deterministically at

a gross scale within hillslopes which form archaeological sites. An initial, stochastic description of the transport process is developed for uniformity with the treatment of artifact transport, and is then converted to a deterministic form by time averaging. Comparisons show a good agreement with other widely used overland flow transport functions summarized by Julien and Simons (1 985).

Soil creep is included in the model on a continuous level, assuming a rate directly proportional to the sine of the local slope (Armstrong, 1980). Failure of slopes is also incorporated very simply, again assuming an annual rate proportional to the slope, but with this rate only occurring above a given threshold. The failure process accounts for the collapse of the side-walls of ditches (Jewel1 and Dimbleby, 1966) and pits, as well as the form of collapse of hut walls. Deposition of materials in pits is commonly in a less dense form from that of the original soils, whereas material from hut walls will decrease in density on deposition, due to removal and decay of organic components, although the degree of change will depend on the original composition. These differences in density are accounted for in the model.

At the site scale, the model has been applied to a series of 12 hypothetical slope forms (Figure 2). These have been chosen to observe the full range of variability of the spatial distributions of archaeological features on a slope. As a simplification, these have been divided into two categories of “pits” and “walls”. In this implementation, various relative combinations of pits and walls have been used. Both straight and convexo-concave initial slope forms have been analyzed.

The results for the different slope configurations with each parameter setting are assessed according to three factors. First, the long-term preservation of a structure is assessed, in terms of preservation (+I , partial preservation (01, or destruction (-). For a pit, preservation is taken as the survival of most of the structure’s form, whereas, for a wall, the criterion is defined as the survival of the immediately adjacent ground surface due to the natural collapse of the structure itself (Table 11). Secondly, the duration of survival of eroded structures is calculated; in other words, this relates to the length of time in years before their disappearance from the archaeological record (Table 111). Thirdly, the depth of burial in meters is given of surviving structures following all changes to structures (generally 5000 years for the long, and 2500-5000 years for the short slopes [Table IV]).

Results for Straight Slopes For the long slope, parameter settings a-c (Table I) compare the effects of

changing the overall erosion rate under constant rainfall-runoff relations. In the long term, this has no effect on the preservation of structures (Table 11).

176 VOL. 9, NO. 3


3. 4.

5.

Figure 2. Different forms of slope configuration, with pits and walls, used in the exploratory analysis of erosion using AHEM.

The pits are preserved throughout, although only partially on the upper slopes. Walls remain in the lower and middle sections (Figure 3). However, they tend to survive less well where they are associated with pits midslope, or where the wall is immediately upslope of the pit at the slope base. This result is related to the increased gradients and exposure to erosion in the higher flow conditions which are found at this location. Differences are visible, though, in the date of survival of structures (Table 111). As would be expected, the higher the erosion rate, the more rapid the removal of the structure. In the case of the isolated upslope wall, traces remain up to 5000 years under conditions of low erosion, but, with high erosion, this is reduced to only 3500 years. This would mean that whereas remains of up to Middle Neolithic age might be found in low erosion regimes, for high erosion, only Middle Bronze Age features are potentially retrievable (as the model has been developed for application in a southern French context, attributions to archaeological periods are relevant to this area). For the wall upslope of a pit, the corresponding reduction is from 2500 to 1000 years. Archaeologically, this relates to a reduction in visibility from Iron Age to Early Middle Ages. This implies that the survival of features



Table I. Combinations of parameter settings used in the exploratory model analysis. Parameter Slope Slope Erosion Rainfall Veg. Plastic Bulk Other Setting Length Shape Rate Rate Growth Flow Density

(m) Rate Coef."

a 60 b 60 C 60 d 60 e 60 f 60

Straight Low Medium Straight Medium Medium Straight High Medium Straight Low High Straight Low High Straight Low V. low

g 60

h 20 i 20 j 20 k 20 1 20 m 20 n 20 0 20 P 20 q 20 r 20 S 20 t 20 U 20 V 20 W 20 X 20

Y 20

2 20

aa 20

ab 20

Convexo- concave

Straight Straight Straight Straight Straight Straight Straight Straight Straight Straight Straight Straight Straight Straight Straight Straight Convexo-

concave Convexo-

concave Convexo-

concave Convexo-

concave

Convexo- concave

Low

Ex. low Ex. low Ex. low v. low v. low Low Low

Medium

High High High High High v . low Medium High High High

im High Im High

Low Low Low LOW Low

Low

Low

Low

Low

a Bulk density coefficient: relative 1

No No No No No No

No

No No No No No No No No No No No No No No Yes Yes No

Yes

Yes

Yes

Yes

Medium Medium Medium Medium Medium Medium

Medium

Low Low Medium High High High High Low Medium High Zero Low Medium Medium Medium Medium Medium

Medium

Medium

Medium

Medium

50 50 50 50 50 50

50

50 50 50 200 50 50 50 50 200

50 200 200 50 50 50 50 50

50

50

50

50

- - - -

No creep Fixed

base height -

- No creep - - - - - - - -

No creep No creep - - - - -

-

-

As y but high infilt. rates

As z but high infilt. rates

tage decrease in sediment density upon deposition in pit.

178 VOL. 9, NO. 3


+ l . l . + + l . l . l . l

+ I . l . + + l . l . l . l

+ I . l . + + l . l . l + l

+ l . l . + + l . l . l + l

+ 1 + 1 . 1 . 1 . 1 . 1 . 1

+ 1 . 1 . + + 1 . 1 . 1 . 1

+ l . l . + . l . l . l . l

+ I + l + . + l . l . l + l

+ l + l + l + l + l + l + l

+ + + + + I

+ + + + + I

+ I + + + I + . + + + I

+ . + + + I

+ + . + + + + . . + + I

+ I . + + I

+ .+ .+ . + + + + + I

+ + + + + I

+ .+ .+ .

+ l + . + . + . . l L I . l 0

l l I l 1 1 1 1 1 1 1 1 1 1

+ . + + + 1 + . . 1 . 1 . 1

+ + + + + . + l . l . l . l

+ . + + + . + . . l . l . l

+ . + + + . + . . 1 . 1 . 1

+ . + + + . + . . l . l . l



...........

500 years

15.0

In

a, c 13.0

E - 12.0 r, v

"I - onginal profile .- 10 years 50 wars

11.04 ---- _ _ _ _ _ _

v - 12.0 .- a, c

1000 years 2000 years 3000 years 4000 years 5000 years

11.0

...........

...................... 10.0

0 10 20 30 40 50 6 meters

Figure 3. Example of slope profile development under high erosion rate with 60-m-long, straight slope.

depends not only on the erosion rate, but on their relative distribution on the slope. The effect of increasing the erosion rate on the depth of burial (Table IV) is as would be expected, in that it increases at the slope base, but is reduced in the midslope section. An archaeological consequence of this is that in high erosion regimes, there may be a low visibility of sites, not because they are more eroded, but because they are more deeply buried.

In setting d, a low erosion rate is again used, but the amount of rainfall input is doubled. The result of this is not only an increased amount of erosion, but also a different distribution. In this case, all downslope structures are

180 VOL. 9, NO. 3

Tab

le 1

11. L

engt

h of

sur

viva

l in

year

s of

des

troy

ed st

ruct

ures

, usi

ng p

aram

eter

set

tings

def

ined

in T

able

I.

Slop

e Pa

ram

eter

Set

ting

'On

fig

a

b c

d e

f g

h i

j k

I m

n

o p

q r

s t

u v

w x

y z

aa

ab

1 2 3P

3w

4P

4w

5 6 7w

8W

9 10

llw

7P

8P

1lP

12P

3000

3000

3000

3000

3000

3000

4000

4000

4000

5000 4000

4000

5000

4000

1000

5000 4500 3500 3250 4750 1000

1000

3000 2500 2000 2000 3000 1000

inoo

10

20 5 5

800

5 10

3500 3000

400

2250

2500

I

5 3

10

50

400

30

4000

800

75

500

75

I750 1500 4000

3750 400

40

550

40

350

30

5

100

5

300

75

5 130

40

300

200

5 140

50

5

200

75

5 130

30

250

75

5 160

60

175

40

5 50

15

130

40

140

50

v, rn

--I n

130

100

rn

160

75

E 55

20

0

z

..

12w

2500 2000 1000 1500 2250

1000

4 350

30

150

40

5 100

40

120

40

z c I-

v, rn

m

--I

II ii a v, I

c

Table N. De

pth

of b

uria

l in

met

ers o

f pre

serv

ed s

truc

ture

s, u

sing

par

amet

er s

etti

ngs

defin

ed in

Tab

le I

. Sl

ope

Para

met

er S

etti

ng

I-

Co

nf

ig

ab

cd

ef

gh

ij

kl

mn

op

qr

st

uv

wx

yz

aa

ab

5 a

0.4

0.4

0.6

0.4

0.4

0.7

0

0.5

0.8

0

z -

-

1 0

0.2

1.3

2 1.7

2.1

2.1

3p

0.6

0.8

2.1

3w

2.1

2.5

2.9

4p

1.8

1.8

2.3

4w

0.5

0.4

0.1

5 6

1.0

0.9

0.8

7p

00

-

7w

0.2

0 -

8p

0 -

0

8w

0.2 - -

9 10

--

-

llp

- - -

llw

- - -

12p

- - -

12w

- - -

__

_

_-

_ 0.

7 2.5

1.0

2.9

0.5

2.3

0.8

0 0 -

-

-

-

-

-

-

-

-

0.7

2.9

1.3

2.9

0.5 1.8

0.8

0.2

-

-

-

-

-

-

-

-

-

-

--

0.4

0.1

0.2

0.2

0.1 -

__

__

--

--

0.07 0.3

0.2

0.3

0.2

0.3

- 0.08 0.08 -

0.1

__

_

_ _

_

--

-

-

0.09

0.3

0.2 -

-

__

-

0.1

0.2

0.2

0.2

- 0.2

0.09

0.2 - -

__

_

_ -

-

0.05

0.2

0.09

-

-

0.4

0.5

0.5

0.5

0.5

0.5

0.4 - 0.6

0.6

0.5 -

0.3 - - -

__

_

_

__

_-

__

__

0.3 - 0.5

0.7

0.3 - 0.5

0.7

--

--

-

_-

-


preserved, with a greater depth of burial. However, in the upper slope and parts of the midslope, erosion occurs more rapidly, and extends further down the slope. At the top of the slope, survival rates are higher than those for low rainfall but high erosion conditions, whereas they are lower further down the slope. This result can be related to differential survival of the archaeological record on a regional scale.

The same parameter setting is repeated in e, except that no creep has been allowed under the walls even after their collapse (creep is always prevented in the model while the walls are still upstanding). This relates to the situation where a wall has a stone or dense, baked clay footing, preventing significant heave of the surface by thermal cycling. Although there is some increase in the depth of burial at the slope foot, the two major effects are that the midslope walls are less well preserved when in conjunction with pits and that walls remain for longer periods near the slope divide. The form of the surface is also significantly altered, in that the original wall locations are associated with steps in the slope. This may be compared with the development of features such as strip lynchets.

The final long, straight slope 0 observes the effects of having a fixed height slope base (Armstrong, 19871, corresponding to a supply-limited case with, for example, fluvial transport of material away from the slope foot. Even with a low erosion rate and a low runoff rate, this configuration leads to the eventual destruction of all structures on the slope. This process starts at the divide (after 1000 years), then works up from the slope base (3000 years) towards the midslope (4000 years). The individual location of structures in relation to the slope appears to cause this apparent anomaly.

For the short slope, the erosion of structures is sensitive to changes in erosion rate (settings t and u; e.g., Figure 4). At the low erosion rate, pits survive at any location on the slope. When the erosion rate is increased, however, the pits in the upper slope positions are only well preserved where there is the initial upslope protection of a wall. Walls at the base of the slope remain with low erosion rates, but all are removed at high rates. The short slope is almost more sensitive in terms of length of survival, with eroded features being removed after an average of 400 years under low erosion and 50 years with high erosion. The explanation of this is due to the fact that structures at the base of the slope are not rapidly protected with a mantle of sediment-the maximum burial of structures is 0.2 m after 2500 years-and that slope decline occurs much more rapidly. The shallow depth of burial is also related to the relatively lower quantities of sediment, larger proportions of which are required to form the pit fills.

These parameters are again used in settings v and w, but here the vegetation interactions are included. Here, the erosional patterns are substantially altered. In fact, the presence of vegetation, which is usually taken to provide protection, has the opposite effect on the low erosion slope, causing more erosion on the upper parts of the slope, with structures disappearing more rapidly. In



11.5

11.0 k?

k. 10.5

W 0, c

r' 0) W c .-

10.0

9.5

11.5

11 .o - t?

k 10.5

al W c

r' 0 al c .-

10.0

9.5

11.5

11.0 t?

k. 10.5

W W c

r: 0) W c .-

10.0

- original profile 1 year 5 years 10 years 20 years

---- _ _ _ _ _ _ ........... .....................

I I I 5 10 15

meters

- original profile

---- 50 years 1 w years i 50 years 200 years

_ _ _ - - - ........... .....................

I 5 10 15

meters

11.0 ""j 0) W c .-

10.0

9.5 i 9.5

- original profile 500 years 700 years 850 years

---- - - - - - - ...........

5 10 meters

1'5

Figure 4. Example of slope profile development under low erosion rate with 20-m-long, straight slope.


the case of the high erosion regime, however, there is a net improvement, with more structures surviving and for longer periods. On the other hand, in both regimes, vegetation growth acts to trap sediment and cause deeper burial of remaining structures. An implication of this result is that revegetation cannot be used simply as a means of stabilizing sites for their protection and that other factors such as slope form and soil type need to be taken into account.

The aim of the variations in parameter settings h-s is to observe the small scale dynamics of the erosion of structures under various conditions. This allows the determination of microstratigraphic details where they might not be immediately apparent from the soil texture. Alternatively, where stratigraphic details are visible, it is possible using this technique to give more accurate dating evidence, within a relative time scale. For short time spans, this has greater potential than radiocarbon dating, as even where enough datable material and the resources to pay for dates exists, problems of calibration or determination errors do not come into play.

The effects of modifying four parameters on stratigraphies will be discussed in turn. These are the erosion rate, the rainfall causing runoff, the collapse coefficient (plastic flow), and the bulk density coefficient for pit fills. By varying the environmental parameters, the variability in the extreme case of the natural decay of structures, with no human involvement, may be observed.

Changes in the erosion rate can be seen to affect the stratigraphy associated with pit and wall structures (Figure 5). For the extremely low erosion rate (setting h), the early fill can be seen to be relatively thin, in the order of 25 cm for the first 10 years, increasing to a total of 50 cm after 50 years, and 67 cm after 100 years. This gradual decrease in the accumulation of sediment with time is due to the early importance of pit sidewall collapse as the dominant process of infilling, which can be seen by the enlargement of the pit mouth. Following this, deposition from overland flow is more important. The final 7 cm of fill correspondingly take a further 400 years, so that the representative stratigraphy of the upper part is extremely compressed relative to the basal fill. With a hundredfold increase in the erosion rate (setting o), the first 10 years’ fill is 35 cm thick, with 67 cm after 50 years. This early increase has three consequences. First, the upper part of the stratigraphy is more compressed. Second, more of the pit profile is preserved, although this is only on a scale of approximately 2 cm. Third, the enlargement of the pit mouth is also more accentuated, and is preserved from further erosion. Pit fills always tend to be asymmetrical, with the greater depth on the upslope side. This uneven filling may be compared with the experimental earthwork at Overton Down, although here the asymmetry was interpreted as being due to slope aspect (Jewel1 and Dimbleby, 1966). Difference in process rates due to aspect have not been included in this model, but have, for example, been incorporated in the erosion models of Kirkby et al. (1990).

For a wall under the extremely low erosion rate, the remaining stratigraphy is severely truncated on the upslope side, with only a thin lens containing



oriainal Drofile

a. 0

original profile r - - - - I I I--" I

Figure 5. Effects of different erosion rates on the stratigraphy relating to archaeological structures: (a and c) low erosion rate; (b and d) high erosion rate.

material deposited in the first 10-50 years. On the downslope side, however, quite a substantial talus develops, due to the initial protection afforded by the structure. The thickness of the talus deposits on this side again decreases through time, with the greatest rate during the first 10 years. With the higher erosion rate, an even smaller lens is preserved upslope, but there is a greater resolution in the downslope talus. The formation of this talus can be compared with the empirical models of McIntosh (1974, 1977) and Kirkby and Kirkby (1976). With increasing erosion rates, the effect on walls is to introduce greater asymmetry of deposits, with the upslope components completely removed at higher erosion rates.

In parameter settings m and n, the rainfall rate is increased from 0.03 to 0.06 m per event. In the evolution of pit fills, this initially causes no difference (Figure 6). However, after 50 years, the overland flow erosion again becomes predominant over plastic flow, leading to a depth of fill which is 2 cm greater at the higher rainfall rate. This difference continues and also causes the upper part of the profile to be wider in this case.

A more marked change is visible with the wall sediments. At the low rainfall rate, the pattern is similar to that already described, with very little upslope deposits remaining. The rapid early erosion caused by the higher rainfall, leads

186 VOL. 9, NO. 3


original profile > surface on excavation

abandonment

r - - - - I I I I I

I I I I I I I I I I I

I I I I I -- I -1

I I I

I -- I surface on excavation

d. Figure 6. Effects of different rainfall rates on the stratigraphy relating to archaeological structures: (a and c ) rainfall events of 30 mm; (b and d) rainfall events of 60 mm.

to a more substantial talus on the upslope side of the wall. Sediments up to 200 years following the start of the process are relatively well preserved, and the downslope talus is also significantly larger. The consequence of this is the formation of a mound above the position of the wall, which can be compared with the results of Kirkby and Kirkby (1976). This provides another example of rapid erosion actually leading to the preservation of features where slower rates cause their eventual destruction.

Changing the value of the plastic flow coefficient corresponds to physical differences in the structure. For a pit, the lower value (setting 0) relates to side walls which are well consolidated, for example, those in clayey or other cohesive sediments. Conversely, pits in noncohesive soils will have higher values (p). The results of this are very marked (Figure 7). The 10-year fill in the first case being 33 cm, whereas this increases to 55 cm in the latter. Again this leads to a relative compression of the later stratigraphy in the case of noncohesive sediments. Paradoxically, the remaining pit profile is also more representative of the original in this case, as less erosion of the mouth can take place in the later stages of erosion.

The physical differences for the walls may relate for example to that between a baked clay and a lighter daub structure respectively. This distinction is again



original profile \ surface on excavation

abandonment

original profile r - - - - I I I

I I I I I I

I I I I I I 1

I surface on excavation

C.

I”“ I I I

I I I

I I I I

I I I

Figure 7. Effects of different plastic flow rates on the stratigraphy relating to archaeological structures: (a and c) low rate; (b and d) high rate.

important. In the former case, a wider range of stratigraphy is preserved, while all the upslope sediments are removed in the latter case. This is due to the fact that the structure itself provides little protection against erosion in this case.

Sediment characteristics are also reflected in the coefficient for density change following deposition in a pit. By doubling the density increase, for example reflecting a coarser sediment (Figure 8), the initial fill is deeper, and this again leads to the better preservation of the original form.

Results for Convexo-Concave Slopes Figure 9 gives an example of the results of erosion on a go-m-long, con-

vexo-concave slope (setting g). The principal difference from the straight slope is that the structures tend to be much less well preserved. This is especially the case with walls which are eroded very rapidly due to the combinations of local slope steepening due to the slope form and the presence of the structures themselves. Locally accentuated erosion near the foot of the walls may be compared to erosion in “rain gullies” formed by water running off roofs where present. In the case of convexo-concave slopes, this acts to undercut the down-

188 VOL. 9, NO. 3


original profile

7 surface on excavation

rewoeed colluvium

ingly dense fill

loose initial fill

original profile

surface on excavation 00 years -

colluvium ponded sediments denser fill

loose initial fill

Figure 8. Effects of different bulk density changes following deposition on the stratigraphy of pit structures. Upper: 50% increase in density; lower: 200% increase in density.

slope side very rapidly, leading to the destruction of the structures much sooner than for the straight slope. Where pits are the furthest downslope structures (slope configurations 1 and 3), the depth of burial increases, but otherwise preserved structures are much less well protected by overburden.

For the short slope, this pattern is closely repeated (setting x replicates t, but with a convexo-concave slope form). Apart from the isolated pit at the slope base, no structures are buried, and all walls are eroded very rapidly. The effects of vegetation growth (y and z) are much more pronounced on this type of slope, especially in preventing the undercutting of walls towards the slope foot. Features are preserved for longer, although not for an archaeologically significant length of time. Again, the vegetation can be seen to be an effective sediment trap, causing deeper and more widespread burial of structures.

Finally, settings aa and ab show the effects of raising the values of the hydrological parameters on average by a factor of 4. This has only minor effects in the preservation of structures. Paradoxically, the pits in slope configuration 12 are less well preserved, probably due to the lack of sediments leading to early burial. Some features do last longer on the higher parts of the slope and



14.0 -I I? $ 13.0 E

.p 12.0 = i r

ll.O i - original profile

1 year 5 years 10 years 20 years

---- _ - - - - - ........... ......................

I I I I I I

0 10 20 30 40 50 I meters

14.0 - I? $ 13.0 E

E - .p 12.0 r

11.0

4.0

(I) L

$ 13.0 E

I - .p 12.0 r

11.0

- original profile 50 years loo years I 50 years 200 years

---- - - - - - - ...........

1 I I I I I 1 10 20 30 40 50 f

meters

\ \ \ \ \ \ \

- original profile ---- 500 yean

700 years 850 years

- - - - - - ...........

r I I I I I 0 10 20 30 40 50 f

meters

Figure 9. Slope profile development under low erosion rate with 60-m-long, convexo-concave slope.


especially with high rates of erosion, but again this is likely to be archaeologically insignificant. For the lower slope, there are no differences in survival times. Where there is a difference, however, is in the depth of burial. This difference is also variable according to slope position-the midslope buried features are found at the same depth, whereas isolated structures at the slope base are less well buried. On the other hand, the downslope components of the paired structures in slope configurations 3 and 4 are much more deeply buried (except for the wall in configuration 3 at low erosion rates, which is slightly less deeply covered).

Thus, it may be concluded that sites originally located on convexo-concave slopes are much more prone to destruction by erosion than those on straight slopes, although there are some instances, for example, with vegetation growth, where this is not consistently the case. Those sites situated on convexo-concave slopes where vegetation regrowth is very slow or does not occur (for example, due to various forms of anthropogenic pollution) will be especially poorly represented in the archaeological record. From a site management perspective, it is thus vital to revegetate this type of slope, should it become denuded, when it is known to cover archaeological materials.

APPLICATION TO ARTIFACT MOVEMENT Artifact movement cannot be considered in the same way as the sediment

matrix movement, because the transport of individual particles is of central importance. Using a bulk sediment transport rate is not therefore useful. Experiments have demonstrated the variability of entrainment and distances moved by artifacts in overland flow (Wainwright and Thornes, 1991). Therefore, stochastic models are more suitable in this case. This model used for artifact movement is based on a queue-type analogy, using random variables to determine the periods during flow in which no movement occurs (a “waiting” time), and the amount of movement that occurs at the end of this waiting period (or “amount of service” in the queue analogy). This type of model is commonly used in the geomorphological and engineering literature for sediments in river channels (Einstein, 1942; Grigg, 1970; Hubbell and Sayre, 1964; Hung and Shen, 1971; Sayre and Conover, 1967; Yang and Sayre, 19711, and the experiments cited above have demonstrated its applicability to hillslopes. The random variables have been shown to be drawn from gamma distributions, with parameters dependent on the grain size of the particle. For simplicity, all particles are here assumed to be pottery, although there is no reason that bone and other materials could not also be included in further simulations (cf. the taphonomic experiments of Hanson, 1980).

Both qualitative and quantitative analyses of the results of modeling artifact movement over long time periods may be carried out. Qualitatively, the movement of artifacts is observed in relation to their positions both on the slope and to structures, which are distributed according to Figure 2. In the quantitative analysis, a simple measure of spatial distribution is used. Two measures based



on nearest-neighbor distances are utilized-the Clark and Evans statistic, which represents the ratio of the difference between mean and expected distances between object within a given area to the standard deviation of the expected number, and the spatial randomness ratio, which is the ratio of observed to expected mean nearest-neighbor distances (Hodder and Orton, 1976). These measures have been used because of the simplicity of the nearest-neighbor technique. Its main fault is the sensitivity of the result to the area included in the analysis, which can often be a problem in archaeological applications because the boundaries of this area are usually unknown (Donnelly, 1978). However, this factor is not a problem here, because the boundaries are known, fixed, and enclose a comparable population of artifacts.

The analysis has been carried out on both straight and convexo-concave slopes, at rainfall intensities of 4, 10, 40, and 80 mm in a 1-hr-long storm event. Particles of different sizes (1-2 cm, 2-4 cm, 4-6 cm, 6-8 cm, and 8-10 cm) were distributed evenly across the slope in each case in order to test the localized nature of movement.

At the lowest rainfall level (4 mm), virtually no movement occurs on either straight or convexo-concave slopes. This is because the overland flow generated by this rainfall even at very low infiltration rates is not competent enough to cause significant entrainment. Any movement is thus restricted to one or two particles of the lowest size class (<2 cm), and the Clark and Evans statistic and randomness ratio are not significantly modified. The same pattern is repeated for an event of 10 mm.

When rainfall increases to 40 mm, however, significant amounts of movement take place. This movement seems to be especially concentrated around the structures (Figure lo), and tends to occur in two phases. This is initially a relatively large amount of movement, which is reflected by a rapid jump in both the Clark and Evans statistic and the randomness ratio (Figure 11). According to these statistics, this shows an increase in the ordering of the particle distribution. None of the model runs conducted showed a decrease in randomness through time. In the succeeding events there is little further movement, due to the effects of the renewal and vegetation components of the model, as well as particle burial. Final readjustment of the surface leads to more movement as particles become re-exposed, again in the area of the structures. This movement ends after about 200 years (Figure 10). Its consequences, however, are important archaeologically, in that it appears that the preferen- tial transport of particles occurs in association with structures, so that contextual assumptions in these areas are paradoxically likely to be weakest. The two-phase nature of the movement can also lead to the development of “false” layering in the stratigraphy associated with structures. Little or no movement happens in the open areas near the slope divide or foot.

On the convexo-concave slope, the same pattern of initial particle redistribution can be seen. Rather than reaching an asymptote, this slope form shows a continual evolution of the transport of particles, which is again concentrated

192 VOL. 9, NO. 3


3.0 - t? a, z 2.6- 2.2 -

- - - X

5 10 15 meters

0 0 0 0 0 0 A A A A A A A A A A ' A A O A A A A A

0 0 - 0 0 0 o o o o o

0 0 0 0 0 0 0 0 o o o A o 0 0 0 0 0 0 + + + + + + + + + + + + + + + + + +

o n o n n ~ n n n n n o n n n n n

3.0 - - t? a, 2.6- - -

X 2.2 -

0 0 0 0 0 0 0 O O - 0 0 0 0 0 0 0

A A A A A A A A A O A A O A A A A A A A 0 0 0 0 0 0 0 o o 0 0 0 0 0 0 0 0 0

+ + + + + + + + + + + + + + + + + + a n o o o o n o n n n n O O O ~ O

t? 3.0 - -

a, 2.6- -

GEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL

0

~ 0 0 0 0 0 0 0 O 0 0 0 0 0 0

0 0 0 0 0 0 0 g o o A o o 0 0 0 0 0

0

A A A A A A A A A O A A O A A A A A

193

t? 3.0 - -

a,

Y - E 2.6-

X 2.2 -

0 - 0 0 0 0 0 0 0 O 0 0 0 0 0 0

0 0 0 0 0 0 0 g o o A o o 0 0 0 0 0

0

A A A A A A A A A O A A ~ A A A A A

+ + + + + + + + + + + + + + + + + + o n n o o n n n u n o n n o n n n


................................................................................................

2 4 j 0 2 )../ I L '

I I

0 500 1000 1500 2000 2500 I I I I

time (years)

................................................................................................

E 1.4 8 1.3 5 1.2

0 500 1000 1500 2000 2500 time (years)

Figure 11. Results of first and second order nearest neighbor analyses of artifact distributions as illustrated in Figure 10. Upper: Clark and Evans statistic. Lower: Randomness ratio. Solid line-first order; dash-second order.

about the central sections of the slope, and is related to the specific pattern of slope evolution, with upslope migration of a locally steepened section cutting back and exposing particles for transport. Comparisons between movement under conditions of low and high erosion rates shows little difference initially, but a slight increase in the later phases, due to the increased rate of particle exposure at the higher level of erosion.

As the rainfall rate goes up to 80 mm, the amounts of particle movement also increase, but the pattern remains spatially the same. A consequence of this is that after an initial sharp rise in the randomness ratio, there is a slight drop to a more or less constant value.

Finally, the length of the slope is important in the redistribution of particles. This is especially the case where large areas of the slope are unbroken by structures, leading to the development of higher overland flow discharges (and usually rilling, although the model here has not been developed to demonstrate this). The example shown (Figure 12) is for a 40 mm event on a 60-m-long convexo-concave slope, and demonstrates distinct spatial patterning. Near the slope divide, the largest particles are unmoved, although, slightly further downslope, there is transport of all particle sizes. These are associated again with the structures here, forming distinct concentrations where they have been trapped, both on the upslope side of the wall and in the pit. In the midslope section, all particles are removed from the central area of the slope (that is, the area between values of x of 2 and 3 m, which is the modeled part of

194 VOL. 9, NO. 3


0

0 + 4 # A b 3 + ? & @ $&A& o o

- + 0 % ~ 08 ++@@A & $g+%&A+@;o*

& + O A - - X

1.0- 0 0

A

$ 15.0 5 14.0 E - 13.0 g 12.0 '5 11.0 c

- - onginal profile eroded profile -

I I I 0 10 20 30 40 50 60

...........

meters

the slope-particles cease to be moved further once they are moved into the boundary areas). At the foot of the slope, there is a great concentration of these redistributed particles.

To summarize, it can be seen that particle movement begins to occur on semiarid slopes a t a threshold rainfall event of about 40 mm. This amount is not uncommon in the Mediterranean region under consideration here (Wain- wright, in press a; Garnier, 19741, and is not infrequent in other environments (Boardman, 1992). Initially, there is little difference in movement, where it occurs, between straight and convexo-concave slopes, or between low and high erosion regimes. Differences do become marked, however, on the scale of 1000 years or more. Greater transport tends to occur on convexo-concave slopes, and this is especially marked with increases in slope length.

IMPLICATIONS

Implications for Archaeological Theory and Practice Several significant implications for archaeological theory and practice can

be derived from the results presented here. It has been shown experimentally that overland flow is important in the transport of artifacts. In semiarid, sparsely vegetated environments, runoff is frequently generated at sufficient



rates to cause considerable disruption of the distribution of artifacts on a site surface. Using AHEM, it was demonstrated that the link between the erosion of artifacts and the spatial pattern of archaeological structures is very close. In fact, in certain cases, the presence of the structure leads to a total disruption of the spatial integrity of the site immediately adjacent to it. The relevance of this is that contextual definitions and the placing of artifacts in spatially defined contexts due to their association(s) with structures are almost always insecure in erosionally active areas. Because the contexts of such artifacts relate solely to the erosional processes, parts of sites thus become “blurred” from the perspective of making cultural interpretations. Although the modeling process may be able to give information on which areas of sites these are, it is not possible to back-project to give a definitive picture of the abandonment phase of a site. Thus, in searching for spatial patterning on a site, techniques which treat the site as a single unity are likely to fail in producing culturally meaningful results. Qualitative elements need to be introduced which can be used in combination with simulation in testing various hypotheses, although in many cases it may prove impossible to make any reasonable reconstructions of a site, for example, if contexts have been completely removed, or if subsequent occupations have modified the original site in unforeseen ways. There needs to be a discursive element in practice between the hypothesized past conditions and the material remains as represented in the archaeological record.

Further, the exploratory model analysis shows that the spatial preservation of a site is directly controlled by the spatial distribution of structures in it. Under certain site configurations, some structures can be completely destroyed whereas others are well preserved, even in quite close proximity of each other. This tendency increases through time, so that older sites become progressively unrepresentative of the initial structures, with the original contexts again never being entirely reconstructable.

Therefore, because all archaeology must be based on contextual associations (once the notion of “artifact” is applied, there is an implicit assumption that this cannot be an object in itself, with no external referents), we must accept that the archaeological record as excavated is almost never (at least in this environment) fully representative of the past. In the debate between Binford (1981) and Schiffer (1985) on the “Pompeii Premise,” the search for miniature “fossilized” Pompeiis was criticized. However, the results here suggest that, for a full understanding of the archaeological record through its inherent spatial structure, certain numbers of this type of site need to be identified. This may only be possible though following excavation of the site. The implication is that different sites will best represent various types of data ranging from those that contain intact features and can thus be used to interpret (micro-)social factors, to those that only provide bulked cultural or economic assemblages, or limited evidence on certain structures. That is not to say that only sites belonging to the former category should be excavated, as the others provide data that are necessary in constructing a full prehistory. In many cases, this

196 VOL. 9, NO. 3


has been a pragmatic choice in the ongoing practice of archaeology. However, the current trend in the development of a “social archaeology” has largely been a reaction to this viewpoint. The problem with this is that using data which does not always come from primary sites (the “Pompeiis”), with a narrow contextual viewpoint, means their results are always bound to be distorted. The further development of a social archaeology must be in conjunction with the search for and excavation of this type of site, which also implies a need for greater resources to be sought for this form of archaeology. Whereas funding may often be available by public and commercial means because of the nature of the site, this perspective is not one frequently pursued by this school. This apparent contradiction needs to be addressed as one of the immediate and vital steps in the development of archaeology as a discipline.

A further implication regards the formation of the archaeological archive, in this sense defined as the continued movement between depositional, excavated, and discursive contexts of archaeological data. Because single sites will more often than not only represent limited aspects of the initial settlement, they are thus limited in the historical, social, and economic data they can provide. Therefore, the development of the archive can only ever take place in a piecemeal fashion. Major reorientations of our understanding of the past can only therefore take place through comparative work, rather than work on particular sites, although these may often be catalysts. Ongoing syntheses away from the site report format are thus ofprimary importance in archaeological practice. This viewpoint is traditionally eschewed in archaeology, especially among continental practitioners. While being both archaeologist and prehisto- rian undoubtedly helps in the appreciation of the validity of data and their interpretation, a certain amount of specialization is vital in the continuation of a progressive archaeology.

Finally, it has been demonstrated that the use of simulation techniques is a useful heuristic technique in the interpretation of the archaeological record. This is not solely restricted to a philosophical viewpoint using hypothetico- deductive methods, as with the New Archaeologists, but also allows the addition of further levels of analysis to structuralist and hermeneutic approaches. This use of the technique is entirely justified in the study of natural formation processes.

Implications for Site Conservation and Management The use of simulation can also be shown to be useful in the practice of site

conservation and management. A site that is under threat may be simulated using various scenarios ranging from leaving the site untouched to modifying it to different extents. Simulation may then be used positively in order to explore and assess the alternatives that will result in the best preservation of the site in the long term. An advantage is that it can anticipate unintentional side effects of measures taken to prevent erosion. These have been found to be a particular problem in various site management schemes undertaken in the



United States (J. Hester, personal communication, 1989; Mathewson, 1989). Two simple examples can be shown from the above analyses. First, vegetation is often used in an attempt to stabilize the ground surface against erosion. In certain cases, however, the effect of the addition of vegetation is merely to alter the spatial pattern of erosion on the site. With low erosion rates, paradoxically, this leads to an increased rate of removal of the upslope parts of a site, together with their structures and artifacts. Secondly, site burial has also been suggested as a means of long-term preservation (Mathewson and Gonzalez, 1988a, 1988b, Mathewson, 1989). Modeling of convexo-concave slopes suggests that if a site at the slope foot were buried, this would lead to accelerated erosion upslope due to the local slope steepening immediately above the buried area. If the extent of the site had been underestimated, as is always possible with surface survey, especially if downslope movement of artifacts at the surface (due to water erosion, ploughing, or other processes) is not considered, this action could lead to damage of parts of the site which were not previously under threat.

Where there is a proposal for site preservation, then various scenarios should be developed. These should include changing environmental variables, notably climate, for a complete analysis. It should then be possible to calculate the most cost-effective measure, balancing the factors of the length of proposed preservation, the cost of the different measures and of increased future costs of excavation in light of the modifications used. In certain cases it will be doubtless found that there is no effective conservation or management procedure, leaving a choice between excavation or abandoning the site to be destroyed.

The simulation process may also be taken a step further, in the modeling of the overall cultural resource. Parameters could be collected to allow simulation on a regional scale. Either by developing hypotheses on the distributon of sites in a landscape, or by using prior data on this, for example, from field survey (more realistically by using a combination of both), it would then be possible to show the effects of erosion on sites in different environmental settings. From the results presented above, an effective working hypothesis is of the systematic removal of older parts of the archaeological record. Those sites that are most vulnerable could be targeted for excavation in order to maximize the archaeological resource and archive most efficiently. In certain cases, this may come into direct conflict with the suggestion made above of the need to find primary sites. Other factors will thus need to be used in the light of the scarce resources that are likely to continue to be available for excavation.

The results from simulations can also be used in syntheses. For example, a procedure to suggest that part of the archaeological record was missing due to erosion could start with a model on the landscape scale. The timing and duration of erosion could then be evaluated in the light of ancillary environmental data to see which parts of the record may have been eroded. A simple case study has been carried out on the Early Bronze Age of Southwest France

1 98 VOL. 9, NO. 3


(Wainwright, 1991, in press a), to demonstrate the potential of this approach. Although this would often mean arguing from negative evidence, it would be possible in certain cases to find corroborative evidence, for example, in adjacent, deep alluvial, or lacustrine cores. However, due to the complexity of the hillslope-channel system, especially with reference to the poorly coupled nature of many dryland slopes and rivers, such corroboration may prove unfeasible in many settings. In this instance, simulation provides the only technique for developing an understanding of the processes.

CONCLUSIONS The development of the site simulation model from theory and experimental

data shows the importance of erosion processes to the distribution and preservation of archaeological sites and their contents in a Mediterranean setting. A simple, two-dimensional case has been presented here using “hypothetical” sites to demonstrate generic principles. However, a further application is to simulate the three-dimensional effects on actual archaeological sites, as has been described elsewhere (Wainwright, 1992, in press a, in press b). The limita- tions at this stage are largely related to problems of computing time, which increase exponentially with increasing site size.

Archaeologically , four principal developments present themselves from this study on erosion of sites by overland flow. Further research on the causative mechanisms leading to the links between archaeological context and hydrological parameters would enable better prediction of the latter when modeling sites, especially when field measurement is not possible. Experimental work on such pedogenesis would be the ideal method, although it is unlikely that sufficient resources should be available on the necessary scale. Of course, these data would also be applicable to other archaeological techniques, most notably site surveying by soil resistivity and other remote sensing methods.

Secondly, this article has concentrated on semiarid environments. Although the effects of erosion should be similar under other regimes, the varying roles of other environmental factors and their interactions may lead to significant differences. Thus an important step forward would be to parameterize and run the model in other environmental conditions in order to check where overland flow is and is not important as a natural site formation process.

The modeling perspective has been shown to be a useful technique in the interpretation of natural formation processes. This approach could therefore be extended to all nine of Wood and Johnson’s (1978) categories. Among the most fruitful geomorphic regimes are likely to be alluvial environments and lacustrine environments with the effects of seasonal or irregular flood events; coastal environments, especially in the light of past and present changes in sea level, gravity processes, particularly soil creep and slope stability, which were shown above to be important in the sedimentation of archaeological structures, and zones of fluctuating water table heights, in relation to the potential for differential preservation of the remains in a buried site.



Finally, as outlined above, there is the potential for assessing the cultural resources on a regional scale. This should provide further important data, which will allow better synthesis of the existing archaeological archive, and will allow additions to be made to it in the most effective manner.

The research upon which this article is based was funded partly by the British Academy and by the University of Bristol. Helpful comments and advice were provided by Richard Harrison, John Thornes, Jean Gasc6, and Laurent Carozza.

REFERENCES Ahnert, F. (1987). Approaches to Dynamic Equilibrium in Theoretical Simulations of Slope Devel-

opment. Earth Surface Processes and Landforms 12,3-15. Armstrong, A.D. (1980). Simulated Soil Development Sequences in a Three-Dimensional Context.

Earth Surface Processes and Landforms 5,265-270. Armstrong, A.D. (1987). Slopes, Boundary Conditions and the Development of Convexo-concave

Forms-Some Numerical Experiments. Earth Surface Processes and Landforms 12, 17-30. Binford, L. R. (1981). Behavioural Archaeology and the “Pompeii Premise”. Journal of Anthropo-

logical Research 37(3), 195-208. Boardman, J. (1992). Current Erosion on the South Downs: Implications for the Past. In M.

Bell and J. Boardman, Eds., Past and Present Soil Erosion. Archaeological and Geographical Perspectives, pp. 9-20. Oxford: Oxbow.

Dingman, S. L. (1984). Fluvial Hydrology. New York: Freeman. Donnelly, K.P. (1978). Simulations to Determine the Variance and Edge Effect of Total Nearest-

Neighbour Distance. In 1. Hodder, Ed., Simulation Studies in Archaeology, pp. 91-96. Cam- bridge: Cambridge University Press.

Einstein, H.A. (1942). Formulae for the Transportation of Bedload. Transactions of the American Society of Civil Engineers 107,561-577.

Garnier, M. (1974). Longues Sdries de Mdsures de Prdcipitations en France. Zone 4 (Mdditerra- ndenne). MBmorial de la MBtBorologie Nationale 53, no. 4. Paris: La MBMorologie Nationale.

Grigg, N.S. (1970). Motion of Single Particles in Alluvial Channels. Proceedings of the American Society of Civil Engineers, Journal of the Hydraulics Division 96, 2501-2518.

Hanson, C.B. (1980). Fluvial Taphonomic Processes: Models and Experiments. In A.K. Behrens- meyer and A.P. Hill, Eds., Fossils in the Making, pp. 156-181. Chicago: University of Chicago Press.

Hodder, I., and Orton, C. (1976). Spatial Analysis in Archaeology. Cambridge: Cambridge Univer- sity Press.

Hubbell, D.W., and Sayre, W.W. (1964). Sand Transport Studies with Radioactive Tracers. Proceed- ings of the American Society of Civil Engineers, Journal of the Hydraulics Division 90,39-68.

Hung, C.H., and Shen, H.W. (1971). Research in Stochastic Models for Bed-Load Transport. In H.W. Shen, Ed., River Mechanics. Vol. 11, pp. B1-47. Fort Collins, Colorado: H.W. Shen.

Jewell, P.A., and Dimbleby, G.W. (1966). The Experimental Earthwork on Overton Down, Wilt- shire, England The First Four Years. Proceedings of the Prehistoric Society 32, 313-342.

Julien, P.Y., and Simons, D.B. (1985). Sediment Transport Capacity of Overland Flow. Transactions of the American Society of Agricultural Engineers 28,755-762.

Kirkby, A.V., and Kirkby, M.J. (1976). Geomorphic Processes and the Surface Survey of Archaeo- logical Sites in Semi-Arid Areas. In D.A. Davidson and M.L. Shackley, Eds., Geoarchaeology. Earth Science and the Past, pp. 229-253. London: Duckworth.

Kirkby, M.J. (1987). Models in Physical Geography. In M.J. Clark, K.J. Gregory, and A.M. Gurnell, Eds., Horizons in Phykical Geography, pp. 47-61. Basingstoke: MacMillan.

Kirkby, M.J., Atkinson, K., and Lockwood, J. (1990). Aspect, Vegetation Cover and Erosion on

200 VOL. 9. NO. 3


Semi-arid Hillslopes. In J.B. Thornes, Ed., Vegetation and Erosion, pp. 25-39. Chichester: John Wiley and Sons.

Mathewson, C.C., Ed., (1989). Interdisciplinary Workshop on the Physical-Chemical-Biological Processes Affecting Archaeological Sites. Contract Report EL-89-1. Vicksburg, Mississippi: U.S. Army Engineer Waterways Experiment Station.

Mathewson, C.C., and Gonzalez, T. (1988a). Burial of Archaeological Sites for Protection and Preservation. I n Proceedings, 24th Annual Symposium on Engineering Geology and Soils Engi- neering, Coeur d’Alene, Idaho, pp. 443-452. Coeur d’Alene: Idaho State University.

Mathewson, C.C., and Gonzalez, T. (1988b). Protection and Preservation of Archaeological Sites Through Burial. In P.G. Marinatos and G.C. Koukis, Eds., The Engineering Geology ofAncient Works, Monuments and Historical Sites. Preservation and Protection, pp. 519-526, Rotterdam: A.A. Balkema.

McIntosh, R.J. (1974). Archaeology and Mud Wall Decay in a West African Village. World Archaeol-

McIntosh, R.J. (1977). The Excavation of Mud Structures: An Experiment from West Africa. World Archaeology 9, 185-99.

Sayre, W.M., and Conover, W.J. (1967). General Two-Dimensional Stochastic Model for the Trans- port and Dispersion of Bed-Material Sediment Particles. In International Association for Hy- draulic Research, 12th Congress, Fort Collins, Colorado, Vol. 2, pp. 88-95.

Schiffer, M.B. (1985). Is There a “Pompeii Premise” in Archaeology? Journal of Anthropological Research 41, 18-41.

Schiffer, M.B. (1987). Formation Processes of the Archaeological Record. Albuquerque: University of New Mexico Press.

Scoging, H.M., and Thornes, J.B. (1979). Infiltration Characteristics in a Semi-Arid Environment. In The Hydrology ofAreas ofLow Precipitation. Proceedings of the Canberra Symposium, IAHS- AISH Publication No. 128. pp. 156-67.

Thornes, J.B. (1988). Erosional Equilibria Under Grazing. In J. Bintliff, D. Davidson and E. Grant, Eds., Conceptual Issues in Environmental Archaeology, pp. 193-210. Edinburgh: Edinburgh University Press.

Thornes, J.B., and Gilman, A. (1983). Potential and Actual Erosion Around Archaeological Sites in South-East Spain. In J . de Ploey, Ed., Rainfall Simulation, Runofand Soil Erosion. Catena Supplement 4, pp. 91-114. Braunschweig: Catena.

Wainwright, J. (1991). Implications of Past Land Degradation: The Case of Southwest France. Annales Geophysicae S(Supp1. ), 584.

Wainwright, J. (1992). Assessing the Impact of Erosion on Semi-Arid Archaeological Sites. I n M. Bell and J. Boardman, Eds., Past and Present Soil Erosion: Archaeological and Geographical Perspectiues, pp. 228-241. Oxford: Oxbow.

Wainwright, J. (in press a). Anthropogenic Factors in the Degradation of Semi-Arid Regions: A Prehistoric Case Study in Southern France. I n A.C. Millington and K. Pye, Eds., Effects of Environmental Change in Drylands, pp. 285;304. Chichester: John Wiley and Sons.

Wainwright, J . (in press b). La Constitution et I’Evolution des Nappes ArchBologiques. I n J . Gascb, L. Carozza, S. Fry, R. Fry, and J. Wainwright, Eds., Le Laouret et la Montagne d’Alaric a la Fin de 1’Age du Bronze. Paris: CNRS.

Wainwright, J., and Thornes, J.B. (1991). Computer and Hardware Simulation of Archaeological Sediment Transport on Hillslopes. I n K. Lockyear and S. Rahtz, Eds., Computer Applications and Quantitative Techniques in Archaeology, 1990, pp. 183-194. BAR International Series 565. Oxford: British Archaeological Reports.

Wood, W.R., and Johnson, D. L. (1978). A Survey of Disturbance Processes in Archaeological Site Formation. In M.B. Schiffer, Ed., Advances in Archaeological Method and Theory, Vol. 1, pp. 315-381. New York: Academic Press.

Yang, C.T., and Sayre, W.W. (1971). Stochastic Model for Sand Dispersion. Proceedings of the American Society of Ciuil Engineers, Journal of the Hydraulics Division 97, 265-288.

Ogy 6, 154-71.

Received December 6, 1993 Accepted for publication January 15, 1994


Documents

Erosion of archaeological sites: Results and implications of a site simulation model