Records of the Western Australian Museum 20: 415-429 (2002).

First dating of Pilbara petroglyphs

Robert G. Bednarik

International Federation of Rock Art Organisations, P.O. Box 216, CauIfield South, Victoria 3162, Australia

Abstract - The first attempts at scientific dating of petroglyphs in the Pilbararegion of Western Australia are reported. Using microerosion analysis, acalibration curve has been established on the basis of a series of engravedhistorical dates, which is then used to estimate the ages of severalpetroglyphs. The results indicate that some of these are of Pleistoceneantiquity. The consistency of the preliminary results of this project withobservations made elsewhere in Australia and abroad is noted.

THE DATING OF PETROGLYPHSRock art occurs in two forms: as the results of

additive processes, in the form of pictograms(paintings, stencils, drawings, beeswax figures); andas petroglyphs, i.e. the results of reductiveprocesses (pounding, abrading, engraving, pecking,drilling). This division immediately determines thefundamentally different approaches to rock artdating, or rather, to estimating its antiquity. Thesubstances added to the rock surface in the creationof pictograms provide the analyst with a variety ofdatable compounds that are of ages closelyresembling the time the art was produced. Forinstance, paint residues may contain remains oforganic binders or diluents (blood, saliva, orchidjuice etc.) or pigments (charcoal, cochineal, berryjuice etc.), as well as a great variety of incidentalinclusions (brush fibres, pollen, bark fragments etc.)that contain radiocarbon of adequate quantities topermit accelerator mass spectrometry (Cole andWatchman 1992; but cf. Ridges et al. 2000). The fine­grained mineral detritus removed in the productionof petroglyphs is, however, not likely ever to berecovered for analysis (cf. recent attempts by G.Susino to do so). Therefore, in order to datepetroglyphs directly we have to resort to analysinga physically related feature that either pre- or post­dates the art (lichen, accretionary skin or crust,biogenic deposit etc.), providing a minimum ormaximum age. In the language of Dunnell andRedhead (1988), such a procedure fails to focus onthe 'target event', which is the creation of thepetroglyph. Moreover, most of the methods usedare seriously impaired by inherent uncertainties.For instance, dating bulk carbon isotopeconcentrations in accretionary crusts may beseverely misleading, particularly if the crusts areopen systems (Bednarik 1979, 1994a; Nelson 1993;Dorn 1997). Thus, the dating of petroglyphs remainsone of the most intractable problems in archaeology.Indeed, of the methods proposed for petroglyph

dating, only microerosion analysis (Bednarik 1992)seeks to determine Dunnell and Redhead's (1988)'target event'. This paper presents the firstapplication of this technique in Australia, and thefirst scientific age estimates for Pilbara rock art.

Until the early 1980s, no inherently testablemethodology had become available for the ageestimation of rock art (Ward and Tuniz 2000).Previous dating claims had generally been based onnon-scientific criteria, and in the case of petroglyphsit had been sought to determine the ages from theirperceived iconographic content (i.e. what the artwas thought to depict), style or technique; fromspatial proximity to archaeological evidence (suchas occupation debris) and by excavation (usually ofpurported occupation deposits); or fromsuperimposition, patination and weathering.

Iconographic interpretation, however, is notfalsifiable in most instances, and stylistic constructs,while possibly comprising some valid elements, arenot a sound basis for dating rock art, as has beendemonstrated time and again in various continents(e.g. Macintosh 1977; Bahn and Lorblanchet 1993;Bednarik 1995a, b; Watchman 1995, 1996). Not onlyare such mental constructs of researchersuntestable, they tend to reflect modem Westernperception more than historically valid variablesinherent in the rock art. They are alsotaphonomically naive in that they ignore that thesurviving vestiges of any rock art are not randomsamples of living traditions (Bednarik 1994b), andthey tend to be formed without adequate recourseto archaeometry. The technique of petroglyphexecution, too, is rarely a reliable chronologicalmarker, and should not be cited as a dating criterionwithout substantial other evidence. Proximity tooccupation sites is almost irrelevant in the reliabledating of rock art, because rock art often occurs at'focal' sites within a landscape, as do occupationdebris, hence their co-occurrence is more likely tobe related to this factor than to contemporaneity.

416

Superimposition of motifs certainly provides sounddata for relative sequences, but is not alwaysreliably determined without the use of fieldmicroscopy and does not yield actual age estimates,so in rock art dating it is only useful as asupplementary method.

There are two basic approaches to dating rock artby excavation. The petroglyphs, either on a verticalwall or on horizontal bedrock, may have beencovered by subsequent sediment deposition(Rosenfeld et al. 1981; Steinbring et al. 1987; Crivelliet al. 1996), or detached and stratified fragments ofrock bearing petroglyphs may be excavated in anoccupation deposit (Hale and Tindale 1930;Mulvaney 1969: 176; Thackeray et al. 1981;Lorblanchet 1992; Fullagar et al. 1996). In all thesecases the dating of the sediment can only provideminimum ages for the petroglyphs, and it isdependent upon the validity of a chain ofunfalsifiable deductive arguments relating to thetaphonomy of both the excavated sediments and thedated material (e.g. charcoal). Moreover, andespecially in the case of rock art on vertical walls ordetached fragments, the minimum age securedfrom the sediment is likely to be very conservative.Also, archaeological claims about the dating ofsediments need to be viewed sceptically, as thereare numerous cases on record of incorrectly datedsediments related to petroglyphs. For instance atFariseu, Portugal, a colluviurn of probably less then17 years of age was claimed to be around 25 000years old (cf. Abreu and Bednarik 2000), and at}inmium, in the Keep River region of northernAustralia, a saprolithic sediment of the Holocenewas claimed to be over 170000 years old (cf.Roberts et al. 1998).

Indeed, of the traditional methods of estimatingthe ages of petroglyphs, only the study ofsubsequent patination and weathering promisessecure or testable data, and although it was perhapsthe first to be considered (Belzoni 1820: 360-61), ithas not been applied in a rigorous and systematicfashion up to the present time (but see Trendall1964). Instead of pursuing the development of ascientific methodology to date petroglyphs,archaeology has for the greater part of two centuriesopted for an 'archaeological methodology', whichconsists of non-quantifiable speculations,unexplained perception of invented styles and non­falsifiable propositions. The use of these untestableand thus non-scientific approaches continues to thepresent time, and is sometimes defended vigorouslyby archaeologists (Zilhao 1995; Rosenfeld and Smith1997).

The alternative approach is by 'direct dating', theuse of direct physical relationship of art and datingcriterion, and the presentation of falsifiablepropositions concerning this relationship for thepurpose of estimating rock art age (Bednarik 1996;

R.G. Bednarik

for a comprehensive critique of all rock art datingmethods, including those contributed by me, seeBednarik 2001: 111-137).

MICROEROSION ANALYSIS

Microerosion analysis seeks to estimate the actualage of petroglyphs. The rationale of this techniqueis that, after a new rock surface has been created, beit by natural or anthropic agents, it is subjected tochemical weathering processes. This appliesespecially in unsheltered locations, and it results incumulative effects that are a function of time,among other factors. While this is a fairlyself-evident principle, the difficulty in using theresults of such processes to estimate the age of arock surface is that our understanding of them, oftheir effectiveness on different component minerals,and of their susceptibility to environmental factorsremains limited (concerning typical rates ofsolution, cf. Acker and Bricker 1992; Busenberg andClemency 1976; Lin and Clemency 1981; Oxburgh etal. 1994; Rimstidt and Bames 1980; Williarnson andRimstidt 1994).

For the time-span we are concerned with in dat­ing petroglyphs (the last 50000 years), onlycomparatively erosion-resistant rock types are

Figure 1 Diagram depicting the laws of waneformation in a simplified form.

Dating of PiIbara petroglyphs 417

This leads to the prediction of [3, the angleexpressing the rate of wane development relative tosurface retreat:

The relationship wane width A versus age,irrespective of actual retreat, is ultimatelydetermined by the ratio a : [3, which must beestablished empirically. It follows that thedimensions A, r, z, and angles a and [3 in Figure 1are all related geometrically and algebraically, andthat the variables A, r, x, z and h are allproportionally equivalent, and increase linearlywith age. Of these, A is most easily measuredphysically in the field. It is therefore the variablepreferably used in micro-wane measurement.

In the field, the analyst scans the rock surfacemicroscopically to locate crystals that have beentruncated (either fractured by impact or truncatedby abrasion) by the event to be dated (e.g. thepetroglyph production). A statistically significant

suitable for microerosion analysis, because thosethat dissolve too fast are unable to preserve originalfracture surfaces for time-spans long enough to beof relevance. Sedimentary rocks, in particular,weather so fast that remnants of the surfacescreated at the time a petroglyph was made surviveonly for very short periods. So far, two differentmethods have been used. In one, the retreat of themore soluble component of a rock is measuredagainst a component that retreats at an extremelyslow rate. For instance the retreat of amorphoussilica cement in a heavily metamorphosed quartzitecan be measured against the crystalline quartzcomponent, or the alveolar retreat seen in schistoserocks can facilitate rough age estimates. However,the principal technique used so far is themeasurement of micro-wanes on fractured crystals(Bednarik 1992, 1993a). The 'radius' of wanes(strictly speaking, wanes are not equi-circular insection, but hyperbolic) increases as a linearfunction of age, as demonstrated by the geometry ofthe process. In wane formation, be it at themacroscopic or microscopic level (Figure 1), theratio h : r is constant for any angle a, irrespective ofdistance of retreat of the faces and the edge. Ratiox : z is a function of a, and for instance at a = 60°,x = 12. Dimension x can be expressed in algebraicfashion:

J[( Z )2 ]x- +Z2- tan 0.5cx

p= 2 sin - I ( r )x+h+r

(1)

(2)

sample of micro-wane widths along the edges ofsuch truncation surfaces is recorded and placed in acalibration curve. Age estimates are prefixed with acapital E, indicating that the result is erosionderived.

The method is not very precise at this early stage,because it has only a few calibration points in eachregion where it has been applied. The principalvariables in the solution process responsible formicroerosion are temperature, pH and moistureavailability. The first two are regarded asunimportant. Variations in mean annualtemperatures, even as far back as glacial peaks ofthe Pleistocene, are not thought to have been of amagnitude that would have affected solution ratesappreciably. Variations in pH back through timecan be assumed to have taken place, but they arejust as unlikely to have influenced solution rates. Inthe case of both amorphous silica and crystallinequartz, there is almost no change in solubility belowpH 9, and higher values would certainly not havebeen experienced in nearly every naturalenvironment. For alumina the effect is negligible inthe central region of the pH scale, which coincideswith most natural conditions. Precipitation certainlyvaried in the past, but it can be accounted for.Significant changes in moisture availability affectcomponent minerals differently, and should thus bedetectable by calibration of more than onecomponent mineral. Therefore it is preferable toapply the method to two or more differentcomponent minerals of the same surface, such asquartz and feldspar.

While rnicroerosion analysis is not thought to bevery accurate, it is probably more reliable than mostalternative methods of dating petroglyphs, and it iscertainly cheaper, simpler and more robust thanmost. It requires no laboratory facilities. Results canbe determined in the field, which may saveconsiderable effort necessitated by the need toreturn to a perhaps very remote site to obtainsupplementary data. The method provides not asingle result, but clusters of age-related values (themicro-wane widths) that can be converted intovarious statistical expressions - a luxury notavailable to all other dating methods currently used.Moreover, it is the only such method offering ameans of internal checking - that is, of checking thevalidity of the result without recourse to anothermethod (although luminescence dating has alimited feature of this type, i.e. the possibility ofchecking whether the uranium and thorium decaychains are in equilibrium). Finally, microerosionanalysis involves no removal of samples, or evencontact with the rock art, being a purely opticalmethod.

All these factors favour microerosion analysis.The valid technical arguments against the methodare: inadequate calibration curves, its limited

418

accuracy through its inherent coarseness, itsapplication is limited to rock types that preservecrystal surface features and have been continuouslyexposed to precipitation. These significantlimitations are outweighed by the benefits of themethod. The microerosion method by micro-wanemeasurement has been used on petroglyphs in sixblind tests: in Russia, Italy and Bolivia (Bednarik1992, 1993a, 1995b, 1997a, 2000a). Archaeologicalexpectations were matched in all but one case,where the results matched those of other scientificanalyses (Bednarik 1995b; Watchman 1995, 1996).Calibration curves are now available from LakeOnega (Russia), Vila Real (Portugal), Grosio (Italy),Qinghai (China; Tang 2000), Jubbah (Saudi Arabia)and eastern Pilbara (Australia). The technique hasalso been applied in India and South Africa. Themethod's practical time range on crystalline quartz,from perhaps 50 000 years BP to the present,renders it particularly suitable for rock art, verylittle of which can be expected to be in excess ofthat range. The perhaps most effective range (fromaround 10 000 years to about 1000 years) coincideswith the presumed age range of most petroglyphs.

PILBARA PETROGLYPHS

The rock art of the Pilbara region in north-westernAustralia (Figure 2) is reputed to be the world'slargest concentration of petroglyphs. This remained

R.G. Bednarik

unrecognised until the 1960s when the majority ofthe sites were located and first examined.Systematic scientific study of this massive corpusbegan with an expedition of the Western AustralianMuseum to Depuch Island in 1962 (Ride andNeumann 1964), followed by the survey work ofBruce Wright in the region from Roeburne to theUpper Yule River (Wright 1968, 1972). My ownwork, involving the finding and study of hundredsof sites, began in 1967 (Bednarik 1973) and is stillcontinuing into the 21st century.

However, the work conducted during the 1960swas not without precedents. The first recordedreports of any Pilbara rock art are those of Wickham(1843) and Stokes (1846: 166-77), recording the visitof Depuch Island by H.M.s. Beagle in 1840. The crewof that ship left at least three inscriptions behind.Captain Wickham took a particular interest in thepetroglyphs and his illustrations were republishedby Stokes. These authors, however, were probablynot the first Europeans to view the rock art. Muchearlier, in 1688, buccaneer William Dampier, afterwhom the Dampier Archipelago is named, visitedthe north-western coast and may well have seen rockart, but in his very brief account about Australia(under 2500 words) he makes no mention of it. Arecent discovery in the eastern Pilbara, however,suggests that a European of the 18th century,presumably living with indigenes, not only saw therock art but added his own designs (see below).

* NOtaI:IIe petrogIyph Site or canpIeX

* PaIntIng slte

o 100 200 km.....------'-._--'----"

Figure 2 Map shOWing the principal rock art sites of the Pilbara region, Western Australia,

e­N-

Dating of Pilbara petroglyphs

Much later, Pilbara petroglyphs are brieflymentioned by Richardson (1886), while Withnell(1901) dedicates a page to them. In 1939 theFrobenius Institute of Germany conducted briefexpeditions to three localities - Abydos, PortHedland and Depuch Island - which resulted insome preliminary descriptions (Fox 1939; Petri 1954;Petri and Schulz 1951). Davidson's visit in 1938-39is reflected in his opinions about similarities withother Australian rock art (Davidson 1952), but thefirst study of substance was the work of anotherGerman researcher, Father E. A. Worms, whoconducted field work in 1931 and in the early 1950s(Worms 1954). His observations, especially in theAbydos area, led him to suggest that manyanthropomorphous petroglyphs there with certaindistinctive features were connected with theKurangara cult, introduced from the east andoriginating in Amhem Land. This deduction is nolonger accepted today (McNickle 1985) and thefigures in question are now called 'Woodstockfigures'.

More detailed reports only began to appear in the1960s, first of the 1958 study by F. D. McCarthy onDepuch Island (1961), then his subsequentcomprehensive survey of the limestone ridge at PortHedland (1962), where he determined the presenceof about 7000 motifs, spread over a distance of morethan ten kilometres. Most of these figures were stillwell preserved when I first saw the site complex sixyears later, but by the end of the century only a fewhundred motifs remained. The massive industrialdevelopment at Port Hedland had resulted in thegradual deterioration of that rock art corpus. Thedestruction of rock art was of an even greater scaleon Burrup Peninsula, where I witnessed the loss ofmany thousands of motifs during the 1960s. The1962 expedition to Depuch Island by the WesternAustralian Museum (Ride and Neumann 1964) wasin fact prompted by a proposal to construct adeepwater port for loading iron ore on the island.The recommendations of the Museum teameffectively led to the abandonment of this plan, andto developing instead the loading facilities atDampier during the mid-1960s, where no surveywork had taken place prior to 1968. Abydos wasalso visited by Mountford (1968).

The principal limitations of all research efforts upto the mid-1960s were that they usually dealt withindividual sites or site complexes and were theresult, generally, of rather brief visits; they lackedin-depth consultation of Aboriginal informants,which has severely limited the amount of authenticethnographic information available about Pilbararock art; and until Trendall's (1964) work there wasno attempt to provide analytical data. Because ofthis piecemeal approach it was only with the workof Wright (1968, 1972) and myself during the 1960sthat the true magnitude of the Pilbara petroglyph

419

corpus began to become apparent. Wright was thefirst to conduct broadly based inter-site studies overa wide section of the region, and thus to define thisdistinctive rock art province. Indeed, hisquantitatively descriptive study, extending overtwo years and then intermittently into the early1970s, has not been bettered to the present time,even though much better funded projects have beenundertaken during the subsequent three decades.Wright kept interpretation of the rock art to aminimum and endeavoured to providecomprehensive initial descriptions, particularly ofthe significant and spectacular anthropomorphouscomponent of the region's rich iconography.Because his recordings were derived from ameticulous photographic record, on which the earlycomponent of the art tends to be invisible due torepatination, he only registered the more recenttechnological traditions in most instances, as hadalso been the 'case with all previous researchendeavours. Importantly, Wright was the first toconduct in-depth ethnographic research,particularly through the cultivation of personalrapport with indigenous elders in Roebourne.

My own study of Pilbara petroglyphs,commenced in 1967 (Bednarik 1973) and stillcontinuing (Bednarik 2ooob), differs from previouswork in the region in various aspects. It is a long­term project that initiated archaeological studies inthe region (Bednarik 1977), but focused onextracting scientific and ethnographic information(Bednarik 1979, 1998). I soon noticed theoccurrence of entirely re-patinated, almostinvisible petroglyphs of great antiquity, which hadbeen overlooked by previous investigators. If theyoccurred by themselves, they were very difficult tofind, and if they occurred with much more visiblerecent figures they had remained undetected. Theyled me to differentiate artistic traditions of greatlyvarying, but unknown, age. Motivated by wantingto place the rock art into an archaeological contextrendered it essential for me to focus particularlyon the question of the art's antiquity, because itcan only become an archaeologically meaningfulresource if its age is known - at leastapproximately.

Wright's painstaking site recording programinvolved close to 100 sites, and my addition ofseveral hundred more sites in the western Pilbara(about 570 sites just on the Burrup Peninsula)demonstrated that the Pilbara contains the largestknown assemblage of petroglyphs. Some of the moreenthusiastic estimates, such as the suggestion thatthere are 500000 petroglyphs just on Burrup(Lorblanchet 1986), are perhaps excessive. However,the number of petroglyphs now known in the Pilbarais at least in the hundreds of thousands, with perhapsin the order of 2000 sites known, many of whichnumber in excess of 1000 motifs.

420

Once the magnitude of this corpus becameapparent it attracted the interest of otherresearchers, and in particular the Dampier sitesbecame the subject of several studies (Virili 1977;Lorblanchet 1983, 1992; Vinnicombe 1987). Theseand other research projects were usually connectedwith industrial developments and were thereforeoften well funded, but they were also highly biasedin favour of specific localities and corporate orgovernmental preoccupations. Elsewhere in thePilbara, little further rock art research took place,although there are some exceptions (e.g. Palmer1975; Maynard 1980; McNickle 1985; Brown 1987).Since Wright's first tentative but inconclusive foraysinto the question of petroglyph antiquity, no hardevidence has become available. Concerning Burruprock art, Clarke (1978) stated his opinion that someof it must be in excess of 17000 years old, based onhis assumption that the patina found on it, whichhe thought to be desert varnish, formed during avery arid phase at about that time. However, muchof the patina is not of the varnish type, nor doesthat kind of accretionary deposit indicate aridconditions.

An attempt was made by Lorblanchet (1992) toarchaeologically date the petroglyphs at Gum TreeValley and Skew Valley on the western BurrupPeninsula. He obtained a series of radiocarbondates ranging in age from 7000 to c. 500 BP. Onlythe three youngest dates were secured fromcharcoal, all others are from marine shells. A dateof about 18 500 BP came from fragments of atrumpet shell (Syrinx aruanus) found on the presentland surface, some distance from the petroglyphs.

R.G. Bednarik

On the basis of this rather slender evidence,Lorblanchet constructed a chronology of successivepetroglyph traditions extending back over 18 000years (Lorblanchet 1992: Figures 20, 21), which isunlikely to be valid since it relies on a single shelldate. Moreover, the Burrup would then have beenover 100 km from the sea shore and ratherinhospitable - the consequence of full glacialaridity.

In the Pilbara, the most likely place to findPleistocene occupation debris is inland, along rivercourses, at waterholes, soaks, springs and inrockshelters, especially in areas of relativelyimpermeable rock, particularly granites, whichallow aquifers to remain close to the surfacethroughout the year. Much of the eastern Pilbarafits this description. Despite its arid ecology, thereare reliable supplies of permanent water close tothe surface, and aquifers are sometimes exposedeven in quite flat areas. Morphologically, thepiedmont region is dominated by roughly conicalboulder piles which in some areas occur in largegroups and are generally of granite facies (Figure3). Sub-parallel dolerite dykes occur locally and canbe hundreds of kilometres long. Drainage of theoccasionally cyclonic and always unpredictablerainfall is effected via substantial river systems,most of which carry little or no water for much ofthe year. Rock art is found at hundreds of the manyboulder piles, and even the most cursoryexamination reveals the presence of many traditionsseparated by great time spans. The earliercomponents of these rich sequences of petroglyphscould well be of Pleistocene age.

Figure 3 Typical granite boulder pile of the eastern Pilbara. This is Spear Hill site 7, the principal calibration site inthis project.

Dating of Pilbara petroglyphs

ESTABLISHING A CALIBRATION CURVE

Twenty-two years after commencing my researchin the Pilbara, my development of the microerosionmethod in 1989 seemed to render Pilbarapetroglyphs datable at last. Unfortunately, inAustralia, and most especially in the remote andvery thinly populated north-west of the continent,there are no rock surfaces of historically knownages available to establish calibration curves. Thegreat wealth of rock-made structures, quarries,gravestones and glacial abrasions in Eurasia whoseage is either known precisely or can be ascertainedwith reasonable accuracy renders this region farmore amenable to this method. In July 2000 Imanaged to locate a large series of engraved dateson one of the four major granite facies of the easternPilbara, which provided the means of securingcalibration values for this important rock art region(Figure 4).

While surfaces of historical structures (Romanbridges, Buddhist inscriptions) and glacial striaefrom the end of the last stadial have been utilised inEurasia, dated inscriptions of the last two centuriesseem to be the only available option in Australia. Ihad already used dated inscriptions very profitably

Figure 4 Large granite boulder at the peak of SpearHill site 7, bearing numerous inscriptions.

421

Figure 5 Historical petroglyphs, apparently of 1771,eastern Pilbara.

abroad, but in the case of Australia, the exceedinglyshort time range covered by such dates wouldintroduce a higher level of imprecision. However,the potential imprecision imposed by an aridclimate could be considerably greater than that ofvery short-range calibration values.

The series of dated inscriptions located in thePilbara includes an apparently authentic examplefrom 1771, which is part of the earliest currentlypublished non-indigenous rock art composition yetfound in Australia (Bednarik 2000b; I emphasisethat I know of two earlier European inscriptions). Itconsists of four motifs, two of which show limitedevidence of the application of a metal implementprior to completion with a pounding stone (Figure5). The panel is still being subjected to furtherresearch, and this work may lead to thedevelopment of a calibration curve for olivine orpyroxene. It was engraved on a basaltic dolerite,free of quartz and feldspar, and therefore cannot beutilised for microerosion analysis until one of itsconstituent minerals has been subjected to acalibration study.

The remaining engraved dates recentlydiscovered in the eastern Pilbara and examined forconstructing a calibration curve are all located on

422 R.G. Bednarik

D07

Do'

D05

Do'

"30 11120 11110 1IlOO 1180 1180 ..

Figure 6 Microerosion calibration curve for quartz, Spear Hill rock art complex.

Table 1 Microerosion calibration values (in microns)from micro-wanes on eight engraved dates atSpear Hill, Pilbara, for crystalline quartz.

AGL granite, a well foliated, fine to medium­grained biotite adamellite representingremobilised older granitic rocks. Eight datesranging from 1881 to 1997 were surveyed with aspecially adapted field binocular microscope. Sixlie near the peak of Spear Hill site 7, one on thehill's western flank and one on nearby Spear Hillsite 9 (McNickle 1985). Six of these inscriptionsprovided quantifiable micro-wane width readingsfrom 90° fracture edges on crystalline quartz (seeBednarik 1992, 1993a), with samples ranging fromfour to thirty-two measurements per inscribeddate (Table 1).

The calibration curve derived from these values(Figure 6) provides a reasonably distinctive trend,although the values recorded for the 1964 date arenot a good fit. However, only four determinationswere possible which is statistically inadequate. The

remaining values fit well, but the resulting curvediffers considerably from those secured previouslyfrom temperate to sub-Arctic sites. This wascertainly expected, and the difference is not even asgreat as one might have predicted, considering themuch greater difference in precipitation (cf.Bednarik 1992, 1997a). However, erosion rates in anarid climate may well be determined not so muchby annual precipitation, but by the relative durationof surface moisture, which is determined by thelength of rainfall periods and evaporation rates. Inthat sense, the Pilbara with its cyclonicprecipitation, very low relative air humidity andsparse vegetation is probably close to the end of thespectrum of mineral solution rates in commonnatural environments. The ubiquitous iron-richaccretions of the entire region, which havepreviously been interpreted as autochthonousconversion products, are another result of theseconditions, being largely, though certainly notentirely, the outcome of inadequate flushing ofdissolved minerals.

Two other points need to be emphasised. First,this calibration curve lacks in precision for anumber of reasons, especially the short range of thecalibration values it is based on. Second, itsreliability could be backed up by a feldspar curve.This first determination for the Australian Pilbara istherefore best regarded as preliminary, subject torefinement and testing, and its purpose is merely toexplore the time depth represented by Pilbara rockart and to set some of the parameters within whichfuture datings of Pilbara petroglyphs are to beconducted.

0.2330.237

Meanwidth

Max.width

Min.width

15 0.1 0.48 02 Q3

Micro-wanes are slightly <0.1 micronMicro-wanes too small to measure effectively

4 0.2 0.3 0.27532 0.2 0.4 0.2598 0.4 0.7 0.5378 0.3 0.7 0.412

Date No. ofdeterminations

19381941-A

198019971964

1941-B18811917

Dating of Pilbara petroglyphs

THE FIRST DATING OF PILBARAPETROGLYPHS

The Spear Hill calibration curve (Figure 6) hasbeen applied to selected petroglyphs at threegranite boulder piles: Woodstock 65B, Spear Hill 7and Spear Hill 9 (Figure 2). In each case, fracturedquartz crystals were located that had remained freeof accretionary mineral deposits and possessededge angles between 85° and 95°. The micro-wanesdeveloped on these edges were then measured inaccordance with standard microerosion analysis(Bednarik 1992, 1993a). Not every motif on graniteis suitable for this kind of analysis, many are soextensively covered by accretionary mineral matterthat weathering processes have been retarded. Suchsurfaces are unsuitable for microerosion analysis.However, the deposits are often sufficientlydiscontinuous, even on repatinated motifs that areof macroscopically even colour, that microerosionanalysis is possible.

The ubiquitous reddish-brown patination of theentire Pilbara region is generally neither an intrinsicalteration product nor entirely an accretion, it is acombination of the two. On the dolerites, themagnetite component takes well under a centuryfor an initial crust of iron oxides and hydroxides toform in situ, but on the granites there is only limitedinherent potential for the development of alterationproducts. The accretionary matter is selectivelydeposited, sometimes its distribution is determinedby aeolian factors, and its microscopic morphologyis closely related to whether the support surface isvertical or horizontal. Distinctively 'laced' or'terraced' morphologies are common. Underadequate magnification (>60x), the accretionarymatter reveals a diversity of airborne materials,including widely transported mineral grains andcharcoal detritus, and this is caked together mostlyby iron salts and amorphous silica. These depositswere found to form fairly consistently as a functionof time. An incipient film becomes evident after asurface has been exposed for 30-40 years, and afterabout 100 years, the patchy deposit reaches athickness locally of 30-50 microns (pm). Thethickness of the laced accretionary deposits in partof the above-mentioned 1771 inscription reaches100-150 pm, which is crucial in assessing theauthenticity of this composition. However, ongranite such deposits remain often so discontinuousthat fracture surfaces that have remained exposedcan usually be found on all but the very oldestmotifs.

Woodstock 65BWoodstock site 65B is located near the abandoned

Abydos homestead, on AGM granite, a fine tocoarse, even-grained biotite adamellite, biotitegranodiosite and, less commonly, biotite tonalite,well foliated and often gneissic. The hill is a typical

423

conical boulder pile, rising about 80 m above thepediplain. It bears an estimated 2000 petroglyphs,which are scattered in several concentrationsaround its slopes, with a notable occurrence of veryelaborate and detailed anthropomorphs found nearthe summit. Large boulders form a distinctiveshoulder on the western slope, creating an area oflevel ground where petroglyphs of very differentages occur in close proximity, even on the samepanels. They include circle motifs of a quite specificgenre thought to be of the Pleistocene near thesouthern coast of Australia (Bednarik 1990). Theupper surface of a small elongate boulder bearsseveral deep impact scars (Munsell lOR-4.5/7) andan almost fully repatinated pounded circle, togetherwith faint linear marks that may also be of formercircles. Twenty quartz micro-wane widths (A) weremeasured in one of the impact scars (Table 2),yielding a mean value of 17.25 pm (range 1D-30pm). On the newly constructed calibration curve(Figure 6), this corresponds to an estimated age ofE3670 (+2713 -1543) years BP. The 'E' in front of theage estimate indicates that it is derived frommicroerosion analysis.

The immediately adjacent circle was alsoexamined (Munsell 7.5R-4/6) and an area ofsuitable accretion-free quartz edges was located.Measurements of their micro-wanes yielded a meanwidth of 125.74 pm (range 11D-180 pm, N = 14),which corresponds to E26753 (+11 545 -3349) BP.Thus, the lightly-patinated impact scars, whichseem to be a more recent reaction to the earliercircle, are significantly younger than the circle.

Two metres to the south of this boulder, a well­rounded large boulder bears an only faintlypatinated male anthropomorph. The quartzcleavage faces in this petroglyph appeared quitefresh under magnification. Analysis of their ",90°edges produced a mean value of 2.0 pm (range 1-4pm, N = 10). Consequently this typical Woodstockfigure is only E425 (+426 -212) years old (Table 2).

Some 10 m to the east of this motif is a large flatboulder, about 6 m long. Most of its horizontalupper surface has experienced massive laminarexfoliation of 3-5 cm thickness, but near its northernend, one square metre of patinated crust is stillpresent. It bears the remains of a design of whichfour circles and an elongate, bisected outline in theKarake style (Aslin and Bednarik 1984) remainvisible (Figure 7). This is heavily weathered(Munsell lOR-4/5, on lOR-3/5 background), andthere is a patch of abraded area on the survivingpanel. Fourteen micro-wane widths measured onthe east side of the largest circle yielded a meanvalue of 91.07 pm (range 75-125 pm), suggestingthat the design is E19 376 (+7219 -3419) years old.The ground area, about 70 cm across (Munsell 5YR­8/4, but very speckled), is significantly younger. Itis about the same age as the Woodstock figure just

424 R.G. Bednarik

Figure 7 Circle petroglyphs of the Karake style, estimated to be just under 20 000 years old.

Spear Hill complexTwo petroglyphs were examined at sites of the

Spear Hill complex (for a detailed description ofthis extensive complex of about forty boulder piles,see McNickle 1985). The '1917' date included in thecalibration curve, located at site No. 9, is just

crossed or bisected circles on the north-west side ofthe boulder appear to be of intermediate age.Although the cupules are not datable by the methodused here, there can be little doubt that they are theearliest component of the rock art present. Theiralveolar erosion patterns are well in excess of thecondition observed in the c. 27 ODO-year-old motif.This observation of the precedence of cupules isconsistent at many sites in the region, andelsewhere in Australia and the world (Bednarik1994c).

mentioned: micro-wane sizes of about 2 ~ areconsistent, although no quantitative count wasattempted.

Some of the most prominent petroglyphs at site65B occur about 30 m north of the locality so farconsidered, on a highly visible dark boulder. Thenumerous motifs include female Woodstock figures.One of them, with distinctly S-shaped torso, exhibitswane widths of 10-15~, and is therefore betweenabout E2130 and E3190 years old. The south-westwall of the large boulder bears at least 82 cupules,typically of 25-40 mm diameter and 5-10 mm deep.Unfortunately they are so heavily coated byaccretionary deposits that microerosion analysiscould not be attempted. They appear to havecontinued onto the upper surface of the rock, andthere could be taphonomic selection based onrelative orientation evident. Several Karake-style

Table 2 Quartz microerosion data from seven petroglyphs, at Woodstock 658 and Spear Hill 7 and 9, eastern Pilbara.Micro-wane dimensions in microns.

Motif Wanes Min. A Max.A Mean A Age, years Tolerance

FemaleSH7 No measurements taken c. E350Male 658 10 1 4 2.00 E425 +426, -212

Anthrop. SH9 12 3 5 4.25 E904 +160, -266Female 658 Micro-wanes range from 10-15 microns E2127-3191

Scar 658 20 10 30 17.25 E3670 +2713, -1543Circle 658 14 75 125 91.07 E19376 +7219, -3419Circle 658 14 110 180 125.74 E26753 +11 545, -3349

Dating of Pilbara petroglyphs

Figure 8 Pounded date '1917' superimposed over anthropomorphous petroglyph of about 900 years age.

425

marginally superimposed over a non-Woodstockanthropomorph (Figure 8). The latter provided aseries of twelve micro-wane width measurementswith a mean value of 4.25 pm (range 3.~5.0 pm).This corresponds to an estimated age of E904 (+160-266) years BP. At the western base of site 7, aprominent group of three female anthropomorphsincludes one particularly recent example. Althoughno quantitative data were collected from it, itappears to be about three times as old as the nearby'1881' date, having been made about E350 years ago(Table 2).

DISCUSSION

Although Pilbara petroglyphs, long thought to beof great antiquity, have attracted European interestfor at least 160 years, until now their age hasremained entirely conjectural. This paper has,however, demonstrated a standardised methodcapable of routinely yielding credible age estimatesfor individual motifs in the granite-dominatedeastern Pilbara. This development was facilitated byexploiting geochemistry and micro-geomorphologyto date rock art directly, based on the theory ofmicro-wane formation (Bednarik 1992). Thediscovery of engraved historical dates (Bednarik2000b) has now made it possible to create the firstmicroerosion calibration curve for the Pilbara(Figure 6) and use it to estimate the ages of older

petroglyphs (Figure 9). The motifs analysedcomprise a fairly random selection, although therewas perhaps some bias in favour of very young andvery old examples, to acquire an initial appreciationof the time depth represented by a few of the rockart sites in the Pilbara.

Given the tendency of archaeologists tomisinterpret or over-interpret dating evidence(Bednarik 1996; Watchman 1999), the datapresented in Figure 9 require qualification:1. They do not constitute secure and precise

datings. Substantial tolerances are attached toeach age, reflecting the spread of the primarydata. The true ages of the motifs dated do notnecessarily lie within the tolerance values,although this is highly probable.

2. The reliability of each result is largelydependent on the number of micro-wanemeasurements made.

3. The calibration curve the age estimates listed inTable 2 are based on is tentative and may needto be refined, although there is little prospectfor such refinement in Australia. It may comefrom comparative data from similar aridregions in other continents. This possibility iscurrently under investigation.

4. To obtain reliable ages by microerosion analysistwo or more calibration curves from two ormore minerals are desirable. Therefore, acalibration curve for feldspar should be

426 R.G. Bednarik

..175 ~..•

~'l5

150 Q.

~'ll

125! V

........i,..;'

,,;'",,;'"

,,;'"V ..

100 ,,;'":3,;

........ ,;' :!1~ ,,;'"

u

(/) V75~ t .. ........'"., ........... i ~

........~ ........

50 ~ t ~ a l---'"........

III I v vv

I~I...--' I...--'

25 l---'"Jl-rr ! v H

•..ka I 10 15 20 Z5 28

Figure 9 Age estimates of six petroglyphs of the eastern Pilbara, based on microerosion indices.

established for the Pilbara to render the ages inTable 2 more reliable and precise.

5. Crystalline quartz occurs in different forms.While their solution characteristics are unlikelyto differ sufficiently to affect the rather coarseresolution of the method described above, thisassumption should be tested by analysingsurfaces of known age but different quartztypes.

6. Much Pilbara rock art occurs on plutonic orextrusive igneous rocks such as gabbro, doleriteand basalt, making analysis of the microerosionbehaviour of pyroxene, augite and olivine veryuseful for an expansion of the dating programdescribed above. I plan to attempt this shortly.

7. The preliminary dates in Table 2 cannot be usedto interpret archaeological traditions,occupation duration, or any of the other typesof archaeological constructs often extractedfrom rock art. The few determinations nowavailable tell us nothing about populationdensities, artistic trends, 'styles' etc. The rockart of the Pilbara may yield much older dates indue course, for instance from cupules.

It is likely that adverse climatic conditions duringthe Last Glacial Maximum, 20000 - 15 000 BP,depopulated ecologically marginal regions, such asmuch of the Pilbara. This does appear to bereflected in its rock art, which indicates a lengthyperiod of very little, perhaps no, petroglyphproduction during the final Pleistocene, from theLGM to the establishment of present sea level in theearly Holocene. The few age estimates presented

here support such a scenario, but it requiresextensive testing, through excavation and rock artdating. Hundreds of randomly selected rock artmotifs need to be dated. Fortunately, themethodology described here could secure fairlyreliable age estimations from numerous motifs andsites during quite brief periods of fieldwork.

This analysis has clearly demonstrated thepresence of Pleistocene rock art in the Pilbara.Indeed, it suggests that petroglyphs of suchantiquity occur commonly, because deeplyrepatinated, entirely non-iconographic motifssuch as cupules and some linear arrangementsaccount for >20 % of the region's rock art motifs.The ubiquity of Pleistocene rock art in the Pilbarahas long been suspected, but until now theantiquity of the many thousands of known motifswas purely speculative. This research suggeststhat the Pilbara comprises not only the largestregional concentration of petroglyphs, it may alsopossess the world's largest surviving corpus ofPleistocene art, far larger than that in the caves ofsouth-west Europe and older than any rock artknown in the Americas or Africa. Older rock artdoes occur in Asia (Bednarik 1994c), but little isknown about its extent. In Australia, Pleistocenerock art occurs in caves along the southern coast(Bednarik 1990) and in various northern andcentral regions (Bednarik 1993b), but theseoccurrences do not rival those in the Pilbaranumerically. Finally, while rock paintings dooccur in the Pilbara, they are rare and none islikely to be of Pleistocene age.

Dating of Pilbara petroglyphs

I suspect that the Pleistocene rock art tradition ofAustralia was brought to the continent by seafarerswho arrived most probably from Timor or Roti(Bednarik 1997b, 1999; Bednarik and Kuckenburg1999), about 60 000 years ago. It derives fromearlier, but very similar rock art traditions of theAcheulian in southern Asia (Bednarik 1993c,1994c), and resembles that of Europe's Mousterian(Peyrony 1934: 33-36, Figure 33). While Pilbararock art does not document the beginnings ofpalaeoart, it is a manifestation of a magnitudeunequalled elsewhere. I hope that this paper leadsto a balanced reconsideration of its importance andprotection.

ACKNOWLEDGEMENTS

I thank the most senior traditional custodian ofthe Woodstock-Abydos region, Gordon Pontroy, forgiving me permission to study and record theprincipal corpus of rock art considered in thispaper, and for sharing with me some of hisknowledge about the traditional meanings of thepetroglyphs at site 65B. Thanks are also due to JulieDrew, Dr Jorg Hansen, Horst Jessen, Megan Lewis,Wolfgang Losel and Dr Anthony Manhire, forfruitful discussions in the field; and especially toNicholas Rothwell, for organising a return trip tothe region in November 2000 to complete relevantobservations.

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Manuscript received 24 December 2000; accepted 27 May2001.