MAGNETIC MAPPING OF PREHISTORIC ABORIGINAL
FIREPLACES AT BUNDA LAKE, BELARABON
STATION, WESTERN NEW SOUTH WALES
Theresa Bonhomme and John Stanley
T h e analysis and interpretation of surface sites continue t o present problems
for most Australian archaeologists.
Survey work especially in semi-arid
environments h a s been limited by problems associated with the erosional
history of the site and often a lack of visibility of artefact materials. The
visibility of surface materials and the identification of site boundaries is
dependent upon exposure, which is often gradual or patchy and therefore the
designation of a site boundary during survey is often arbitrary. Erosion
processes result in mixing of surface and sub-surface materials and a chrorlology
for these materials is often hard t o establish. The critical problems facing the
interpretation of many open sites cannot satisfactorily be solved by current
survey and analytical methods.
A t present exploratory open site excavation is considered time -consuming,
expensive and destructive.
In the political climate prevailing today in
Australian archaeology, additional problems face the researcher who wishes to
open up large areas of a site. Interested Aboriginal groups are tentative about
giving permission for large-scale exploratory excavation which may effectively
destroy a site. Therefore, in the application of heritage legislation. government
agencies are concerned to find ways in which current archaeological research
may be undertaken but a t the same time ensuring that a sarr~pleof eit tier a site
or a type of site is preserved for future changing research or heritage
requirements.
Clearly, methods which are non-destructive in their application but which
effectively delimit potential excavation areas are required in these
circumstances. This paper discusses the use of a non-destructive geophysical
method for locating buried features a t an open site a t Bunda Lake, in western
New South Wales. A caesium vapour magnetometer survey was undertaken t o
attempt t o solve problems of artefact association, site boundaries and site
chronology. The survey was directed towards locating magnetic anomalies
believed t o be associated with Aboriginal fireplaces.
A BRIEF HISTORY OF THE METHOD AND ITS APPLICATION
Magnetometers have been used with varying success a t archaeological sites in
Europe and in the United States.
T h e first proton magnetometer for
archaeological use was built for the specific purpose of locating Romano-British
pottery kilns, which were commonly constructed with clay. When fired, the
clay acquires permanent magnetisation and the kiln a s a whole produces a
magnetic field. When such a feature is 1-1.5 m below t h e det'ector the anomaly
strength which occurs is within the range of 100-200 gamma (Aitken 1974).
Such strong anomalies cannot be expected from smaller features.
Proton
magnetometers are also relatively slow when used t o cover large open areas and
therefore are most effective i n locating large-scale features. For these reasons
the magnetometer has had little application in the detection of features of
hunter-gatherer encampments especially in non-ceramic societies such as the
Australian hunter-gatherer groups.
There are, however, a number of features resulting from Aboriginal activity
which are conducive to detection and investigation by magnetorneter. These
include burials, mounds (such as those found in Victoria) and Aboriginal
fireplaces where clay material has been used in their construction.
An example of the use of the proton magnetometer in a n Australian context is
the survey of Aboriginal mounds at. Nyah Forest, Victoria (Elliot 1980). This
work will serve t o illustrate the survey and analysis problems associated with
the proton magnetometer. The aim of the survey was t o locate Aboriginal
cooking pits within a mound. Aborigines used balls of clay as heat retainers in
their cooking pits and the source of any magnetic field across the mound is
believed to result from the burnt clay which forms u p t o 50% of t h e core or
base of the mound (Elliot 1980:101).
A Geometrics 210G proton precession magnetometer was used t o record the
magnitude of the magnetic field at intervals of 0.5 m along lines 1 m a p a r t over
an area of 800 m2. Data processing required three stages: (a) correlation and
editing, (b) plotting and analysis of contour plotting for three-dimensional
modelling, and (c) modelling of the anomaly. Modelling can be completed by
hand or by computer. Elliot comments t h a t modelling is time consuming and
expensive and that all contour plots were assessed visually rather than
modelled. He does not explain his method. T h e results of the survey and
analysis were not available by the time of t h e 1978-79 field season a t Nyah. As
a result, although extensive excavation across a grid was undertaken the
anomaly location was not investigated and s o the results of the magnetic
survey were not tested archaeologically.
In 1976 Stanley and Green reported the use of a caesium magnetometer as a
met hod for defining anomalies associated wit h an experimental buried fireplace.
Theoretical principles and practical considerations indicated that, the caesium
vapour magnetometer when compared t o the proton magnetorneter would be
more efficient in locating small-scale targets, with weak magnetic fields, t h a t
are likely t o occur across Aboriginal sites.
Theory of the method
The burning of a campfire has two important effects on the magnetic properties
of the soil and any clay material used in the campfire construction. In the
reducing atmosphere of the fire, haematite (Fe204) in the soil and the clay
materials become reduced to the more' magnetic magnetite (Fe30,) and where
these minerals have been heated above their Curie temperature and then cooled
they acquire remanent magnetism (Barbetti and McElhinrley 1972; Tite and
Mullins 1971). The combination of these two effects result in a magnetic
anomaly associated with a campfire.
Experimental campfire observat,ions
indicate t h a t hearthstone/clay lump firing temperatures of 600°C are easily
obtained in open campfires. In the case of the experimental fire surface
temperatures of the clay lumps stabilised a t close t o 700°C for more than 1.5
hours after the fire had died down (Clark and Barbetti 1982). It is assumed
that, most Aboriginal campfires will reach the required temperature t o produce
a magnetic signal. By contrast natural grass fires d o not heat the soil t o
sufficient depth to produce any appreciable magnetic interference o r erasure of
a buried campfire anomaly.
The method
Data acquisition and automatic profile plotting of the anomaly signal was
accomplished using a caesium magnetometer hand-held sensor, and a vehicleborne d a t a processing system. The sensor was connected by coaxial cable to an
in-vehicle computer and signals were aut80mat.ically recorded on cassette tape
and concurrently plotted on an analogue chart.
Control tests
Since t h e effectivenes of t h e method was t o be tested, five small pilot surveys of
surface hearths were conducted.
An appreciation of the nature of the
associated anomaly signal was gained and the results were compared with the
anomaly signals recorded from experimental fireplaces (Stanley and Green
1976). Surface features such a s degraded termite mounds, eroded tracks and
surface iron pieces were also recorded t o ensure that a different signal from that
given by a fireplace would result. It should be noted that due t o the time
constraints placed on this experiment and its small-scale, such signal definitions
cannot be expected t o have more than a local relevance until further control
testing is undertaken. T h e results of the control tests suggested t h a t given the
conditions operating at Bunda Lake the differentiation of fireplaces from other
features was accurate.
On the basis of these tests, anomalies designated as fireplaces were identified in
t h e field by the following criteria: the signal peak strength should be less than
eight g a m m a strength; it should be no deeper than 15 cm, nor should it extend
over a n area greater than l m2. At an elevation of 150 mm for the hand-held
sensor the peak t o peak north/south anomaly signal was typically 30-45
nanotesla ( n T ) . There was a high intensity peak over the hearth and a low
intensity t o its immediate south (southern hemisphere) (Fig. l ) .
Figure 2
S t u d y a r e a , B u n d a Lake, western New South Wales
THE STUDY AREA AND THE PROBLEM
Bunda Lake is a small dry lake located on Belarabon Station 100 km southwest
of Cobar in western New South Wales (Fig.2). T h e lake is approximately 1.25
km long by 500 m wide and is bordered on its eastern margin by a small
lunette. During a field survey Ross (1981) located 33 surface sites consisting of
exposed termite hearths and scatters of stone artefacts around t h e lake margin.
Bonhomme located an additional 10 sites along Sandy Creek and away from
the lake.
Visibility around the lake was patchy and artefacts were commonly found in
eroded arcas.
These areas called 'scalds7 are common in the semi-arid
rangelands arid result from cornpactior~of the clay-rich soil by sheep and cattle.
Such areas d o not generally recover but continue t o enlarge a s t h e 'scald' walls
retreat. Sparse artefact scatters also occurred on uneroded areas adjacent t o
the scalds. Presumably a s the scald walls retreated these materials would be
lowered t o the common compacted scald surface and the size of the 'site7 would
increase.
The scald walls were monitored over a period of two years (1981-82) and
art efa.rt loc.at,ions were noted. Field observations confirmed t h e retreat, of t h e
walls and also indicated that artefact movement was vertical rather than
horizontal, i.e. surface artefact material was lowered t o the scald surface. Even
after heavy rain resulting in small-scale fan and gully development across t h e
scald, there was only minimal movement of artefacts horizontally. Observation
of conjoin possibilities for artefacts across the surface helped support these
conclusions.
The geomorphic process.es operating a t the 'sites' made it impossible t o
determine if the greater numbers of artefacts on the scald resulted from the
mixing of artefacts as the sediments were eroded or if a sub-surface layer
representing an occupational unit had been revealed as overburden was eroded
away.
The surface scatter of artefacts on the uneroded areas appeared continuous
along the lunette. According t o the National Parks and Wildlife Service
guidelines the basic premise for classification a s a n open site where
classification is. based only on the presence of artefacts is t h a t there is more
than one artefact within a radius of 100 m. There should also be circumstances
suggesting that the unit of land designated as the site could be a locus of
activity. I t was relatively easy t o find two artefacts within I00 m of one
another, and under the National Parks and Wildlife Service classification t h e
entire 1 km of the lunette and most of t h e lake margin was a site and no intersite patternir~gcould usefully be explored.
A method was required t h a t would more satisfactorily define t h e limits of a site
and designate areas that might profitably be excavated t o determine artefact
concentrations through the deposit and t o locate hearths t h a t might yield
material for C1 4 dating.
The presence or absence of hearths was chosen a s the criterion for defining site
boundaries. It is not, suggested here t h a t hearths a r e representative of all
occupation areas nor are they considered the exclusive delimiting features of a
site. However, hearths of a particular constrort.ion (i.e. termite or clay Iumps)
have a magnetic property after firing and geophysical methods can be applird
t o their detection. The survey did not preclude the possibility that buried
occupation sites lacking hearth materials may exist.
In regard t o t h e
association of hearth and artefact materials in buried locations it, is also
acknowledged that erosion and deposition processes a r e more or less
continuous. It is possible for material t o be eroded and covered several times
resulting in mixing of artefact material. T h e association of artefacts and dated
hearth material may therefore be fortuitous. It is best t o approach such
material with caution while still believing that w t b can devise rr~ethodsfor
rcdising tht. archaeological potential of the material.
THE MAGNETOMETER SURVEY
Two grid areas (50 X 54 m and 50 X 38 m ) were surveyed. T h e vehicle-borne
recording system was located 50 m from the grid to avoid interference with t h e
sensor was hand-held and the grid was walked a t a
signal. The iriagr~et,orr~eter
slow continuous speed a t a line spacing of 500 mm. Samples were recorded
aulomatically a t intervals of 100 mm.
The grids were located adjacent to the scald (Bunda I) which contained visiblr
surface hearth material and artefact scatters. Each area took approximately
five hours t o map. It, is anticipated t h a t with increased familiarity with t h e
method the area could be mapped in considerably less time. As signals
indicating an anomaly were recorded, the survey was stopped and the location
Five anomalies were
was recorded and pinpointed with a wooden stake.
identified in each area.
In order to demonstrate without excavaiion that the anomalies were hearth
features an addit)ional test was run. A close grid, i.e. with closer line spacings,
was established over a suspected 'hearth'. After reviewing the printout d a t a
and comparing i t with sample runs over known surface hearths in the scald it
was apparent that the anomalies were likely t o be hearth features. Figure l
shows three-dimensional modelling of a survey over a known hearth a t Bunda
Lake.
The survey demonstrated that the.anomalies did not occur more than 30 m
from the original surface site of the Bunda I scald in either survey area. Figure
3 shows the location of known surface hearths in relation t o the scald wall, and
the survey area (50 X 54 m). The buried anomalies are aligned in a continuous
pattern with the surface hearths. A total of 10 features (buried anomalies and
surface hearths) occurred over an area 45 X 20 m. This result suggests t h a t t h e
method effectively defines the extent of the occurrence of hearths and so
0 Surface hearths
AExcavation trench
&.Inexcavated anomaly
Figure 3
-
0
Edge of scald
----Area
of magnetic survey
10
20
30
Mstrer
B u n d a 1. Location of surface h e a r t h s , m a g n e t i c
s u r v e y a r e a , a n d trial trenches
delimits site boundaries where hearths a r e used t o define a site. This meant
t h a t a loci of a p p a r e n t activity had been defined and a comparative area for
excavation locat,ed, in a n area of otherwise continuous artefact scatter
averaging 1.26 artefacts/m2.
EXCAVATION
In order t o d e m o n s t r a t e t h a t t h e anomalies were of Aboriginal origin the survey
was later followed by a limited excavation. One area (50 X 54 m ) containing
five anomalies w a s chosen for testing. T h e a r e a was chosen because it was least
disturbed by erosion a n d had three anomalies within a 6 X 6 m a r e a adjacent t o
t h e scald wall. F o u r anomalies were excavated over t w o field sessions. O u r
permit allowed only for test excavation in the imnlediate vicinity of the
anomaly n o t for a n excavation p o c e d u r e t h a t would explore the spatial
patterns of art,efact material and the anomalies in relation t o one another.
RESULTS AND DISCUSSION
T h e first anomaly excavated ( H 1 ) was a hearth of distinctive form and content
located a t a depth of 20-25 cm below the surface. The hearth was composed of
clay lumps 5-8 cm in diameter extending over an area 30 cm2 t o a depth of 10
cm. Large concentrations of charcoal were found around and beneath the clay
lumps. In Spit 5 (20-25 cm) 24 gm of charcoal were collected, while Spits 3 and
4 had only 7 gm.
Immediately below the hearth the deposit was red-brown sand with only fine
flecks of charcoal ( 3 gm) occurring. Before the clay lumps were removed they
were recorded in their original position for palaeomagnetic studies. The results
of this work are reported elsewhere (Barton and Barbetti 1982).
Charcoal concentrations did not occur
H2. Large charcoal fragments occurred
cm. At 38 crn large clay nodules with
recovered. T h c feature extended over a
until a depth of 30 cm was reached in
around clay lumps over an area 10 X 20
charcoal pieces embedded in them were
depth of approximately 10 cm.
A third anomaly ( H 3 ) contained a localised concentration of charcoal near the
centre of the trench a t a depth of between 28-30 cm. No other evidence of
firing was apparent.
In the fourth test pit ( H 4 ) charcoal concentrations began a t 20 cm and a t 25 cm
nodules of clay were found. Between a depth of 25-30 cm charcoal arld clay
lurnps werc concentrated over an area of 50 X 50 cm. The form of the feature
was more in keeping with surface hearths that had been eroded and scattered
over a wider area.
Heart h features had been recognised by their structure and contents
(charcoal/ash and clay /termi te lumps). These characteristics were derived
from previous experience with surface hearths and from descriptions of
Aboriginal hearths in the literature (Hayden 1979).
T h e four anomalies excavated showed a n unexpected range in the structure and
contents of t h e fireplaces. Anomaly H, had a characteristic hearth form. It did
not contain termite lumps but had hardened clay balls. The other anomalies
ranged from a scatteredldisturbed hearth form to a localised patch of charcoal.
All features had givcn a distinctive magnetic signal. An explanation for the
lack of distinctive form despite the similarities in signals, especially for H3 is
proposed. As the hearth burns, the soil sub-surface, which has a high clay
content, is baked and acquires remanent magnetism as does the clay fireplace
material. Surface hearths observed in the scald demonstrate that erosion
results in charcoal and clay material being dispersed over a wide area until
little of the original hearth form remains. It may be that the magnet,ometer
locates 'ghost features', t h a t is, . t h e baked sub-surfaces not discernible in
excavation. These features represent the invisible evidence of a firing event and
remain even after the hearth material has been scattered. A hearth quickly
buried after firing is more likely to retain its structure a s long a s it remains
buried. The condition of a hearth cannot be seen, however, a s an indicator of
time a s there is no way t o control for the geomorphic processes operating at the
site.
Four radiocarbon dates were obtained from surface hearths around t h e lake
margin. One surface hearth (TPIO) a t the Bunda 1 site was dated t o 940 f 80
B P (SUA-1625). Two dates came from surface hearths a t a n open site located
approximately 1.5 km north of Bunda I. At this site, Bunda 11, a total of 56
hearths were recorded during survey in a grid area of 3000 m'. T h e contents
and the structure of these surface hearths could be divided into four types: (a)
those. made with termite nest lumps, (b) hearths with termite lumps and
sandstone hearth stones, (c) sandstone and charcoal mpunds, and ( d ) one
heart h cornposed of charcoal and burnt egg shell. One saridstone and charcoal
mound was dated t o 760 f 70 RP (SlJA-1627) and a termite lump hearth t o
1020 f 70 B P (SlJA-1626). On the western side of the lake a termite lump
hearth ( A M I ) was dated to 1800
140 B P (SUA-1628).
*
The excavat.ed hearth ( H , ) a t Runda I had a d a t e of 3830 k 110 BP
(SliA-1948). It is interesting t o note t h a t in age, structure and component
materials the excavated material from the Burida I site is different from surface
hearths surveyed and dated around the lake. This raises questions concerning
the possibility of changes in the structure and function of hearths over time but
t-hc scale of radiocarbon dating required t o establish a chronology a t Bunda
Lake to investigate (.hew questions was beyond the scope of the project.
CONCLUSION
If the use of magnetirs is t o be viable in the mapping of archaeological targets
with dimensions of only 1 m' it is important t h a t d a t a acquisition be rapid and
automatic so large areas can be surveyed efficiently. In this survey the ca.esium
magnetometer has been shown t o have advantages over earlier techniques. T h e
survey effectively delimited the extent of hearth occurrence away from the
exposed site area. A site boundary based on t.hr location of the hearths was
then extended beyond visible surface material. T h e method was rapid and
efficient and visual printout in the field allowed for spot checks of d a t a and
correction to be made without delay. The potential areas for excavation were
designated and the material recovered yielded d a t a for C14 d a t i n g and for
palacomagnetic studies. Artefact matedal recovered in association with dated
hearth could be assigned a relative age.
At Bunda 1 some stratigraphic and chronological control over surface and
excavation materials was achieved. An early Aboriginal presence a t the lake a t
3830 B P was determined, and a range of hearth structure was described.
The usefulness of the caesium magnetometer technique in the archaeological
work a t Bunda Lake leads us t o conclude t h a t this technique is a potentially
rapid, non-destructive method for locating buried features across uneroded
areas suspected of containing Aboriginal materials.
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30,000 years BP. N a t u r e 239:327-30
Barton, C.E. a n d Rarbetti. M. 1982 Geomagnetic swular variation from recent
lake sedin~ents, ancient fireplaces and historical monuments in
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In
Clark, P. and M. Barbetti 1982 Fires, hearths and palaeomagnetism.
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St arrley,
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The Faculties
Australian National University