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Dating Rock Art

The major methodological limitation in rock art studies is that art assemblages can be difficult to date. However, chronological data is crucial to many types of analysis in which rock art evidence is integrated with other archaeological and environmental information. This section will briefly survey the range of dating techniques used in contemporary rock art studies. These fall into two broad categories:
a) Relative dating methods such as degree of weathering, superimposition analysis, stylistic analysis and inter-site patterning.
b) Absolute dating methods such as analysis on the basis of subjects depicted, consistent association with datable deposits, the dating of stratified deposits associated with rock art and the direct dating of the art itself.

RELATIVE DATING

Weathering

WeatheringOnce an engraving has been pecked or abraded into the rock, it is immediately subject to chemical and physical weathering. If this proceeds at a steady rate the degree of weathering can be used as an indicator of absolute age.

Differential weathering of art has also been used to suggest relative age differences (e.g. Lorblanchet 1992). This is based on the notion that a less weathered engraving will be younger than an engraving with greater signs of weathering. However, there are other factors which affect weathering rates, such as micro- environment, depth of engravings and so forth. Even so, there are some instances where significant differences in weathering clearly indicate significant differences in age.

Superimposition analysis

The basis of the method is stratigraphic in that a design occurring over, or through, another was executed later in time. Simple as the idea sounds on Superimpositionpaper, there are complications. For instance, an artist may deliberately superimpose motifs for ideological reasons (e.g. Lewis -Williams 1974). Moreover, there are a number of technical difficulties associated with the recording of superimpositions. For example, some colours are more intense than others and tend to come through overlying layers. Also, some colours may adhere badly on a pre-existing tint. It is also clear that in many studies totally inadequate data bases have been used to construct regional, or in some cases continental, rock art sequences without any regard for the methodological, statistical or interpretive problems involved in the analysis of superimpositioning.

Stylistic dating

Stylistic sequences are established on the basis of the differential weathering and superimposition of motifs as well as their formal attributes and consistent associations. This information is used to produce a chronology in which rock art styles are established in relation to other rock art styles. A classic stylistic sequence is that proposed by Leroi-Gourhan (1968) for European Upper Palaeolithic art.

In some circumstances, parts of the sequence may be anchored to absolute dates through the depiction of items of material culture dated in excavated contexts (e.g. Bronze Age tools at Val Camonica in Italy, see Anati 1976); the depiction of extinct fauna or contact items (e.g. Thylacines in Arnhem Land rock paintings, see Lewis 1977), or by relating changes in the art to environmental changes (e.g. the Arnhem Land rock art sequence, see Chaloupka's 1993 ).

Spatial analysis

At some sites differential weathering and superimpositions indicate that 'bursts' of artistic activity occurred over considerable time periods. In these cases intra-site patterning can reflect chronological patterning. By implication, there may be sites which were used for a short period only in which techniques, motifs and colours in use at one time have been 'stranded' by previously held cultural values determining site significance. If so, trends in the inter- site distribution of artistic variables can also provide evidence for sequence, assuming other determinants of such patterning can be monitored and taken into account (Morwood 1980).

ABSOLUTE DATING

Historical information

There are many instances in which rock art production has been observed and documented. This provides an absolute date for the art (e.g Chaloupka 1993: 238).

Mammoth motif

Subjects

Subjects depicted in rock art may permit an estimate of the maximum or minimum age for rock art production.

The portrayal of extinct fauna in art is one means by which minimum ages may be gauged (e.g. mammoth in the European Upper Palaeolithic, thylacines in northern Australia). Conversely, the appearance of 'introduced' items or animals provides maximum ages (e.g. European and Maccassan items in Arnhem Land, the bow and arrow in North America).

In some cases changes over time in the faunal assemblages represented in rock art assemblages -- Thylacine motifas distinguished by relative dating techniques -- can be related to patterns of dated environmental, economic or social changes, enabling the artistic sequence to be broadly dated. Examples include the Tassili region of the Sahara (e.g. Muzzolini 1986), the Somalian and Ethiopian regions of Africa (Brandt and Carder 1987) and Western Arnhem Land (Lewis 1988; Chaloupka 1993).

Horse motif

Stratified art

The most common method employed for the direct dating of rock art has been to use situations where art is 'stratified' in a dateable context. This can occur at the macro or the micro level.

Art occurs in macro-stratified contexts when it is covered by, or covers, dateable archaeological deposits. Depending on the nature of the association this can provide a maximum age (e.g. where pieces of decorated shelter wall have fallen and become incorporated in deposits) or a minimum age (e.g a panel of rock art covered by deposits). For instance, painted spalls and stones excavated at Apollo 11 Cave, Namibia, date as far back as 26,000 b.p. (Thackeray 1983: 24-5).

Desert varnish Rock art can also be 'micro-stratified' when it is covered by, or covers, mineral or biogenic coatings. Examples of such datable micro-deposits include-- desert varnish (e.g. Dragovich 1984; Clegg 1987); Oxalate crustssilica skins (e.g. Watchman 1985, 1996), oxalate crusts (e.g. Watchman 1993), secondary carbonate deposits (Bednarik 1985), mudwasp nests (Morwood et al 1994) and lichen (e.g. Joubert et al 1983). A range of dating techniques have been applied to these materials, including AMS radiometric dating of organics, CR dating of desert varnishes, OSL dating of quartz particles in mudwasp nests, and lichenometric dating of lichens over art.

Association

Rock art can be closely associated with deposits when an art site briefly used becomes sealed by rockfall or a build-up of deposits at the entrance. This has occurred at a number of the Upper Palaeolithic art sites in France (e.g. Fontanet in the Ariège, France).

There are other situations where the association between rock art and deposits is less secure, but where dates for the art can still be inferred with varying degrees of confidence, depending upon the exact circumstances. For instance, when specific types of art consistently occur with occupation deposits, or implements, of a certain age range, or with occupation of limited duration, it is often assumed that the art and the occupation are contemporary. Dates for the occupation (or other activities) are then assumed to date the associated art (e.g. Macintosh 1965). At present the oldest date for Australian rock art is based on this method: at Koonalda Cave on the Nullarbor Plain (South Australia), finger markings and abraded grooves on walls deep underground are probably associated with evidence for flint quarrying around 20,000 years ago (Maynard and Edwards 1971: 75-6).

Elsewhere consistent associations between dated pottery types and rock art at habitation sites has been used to infer age (e.g. Schaafsma 1985) as have ceramic/rock art stylistic affiliations.

Direct dating

In some cases it is possible to directly date the art itself. This can be done with pigments containing organic materials, such as charcoal, plant fibres, protein binders, as well as with beeswax figures (e.g. Cole et al 1995:155; Taçon 1996). However, some studies have shows that, with the small samples required for AMS dates, the question of provenance is crucial. For instance, McDonald et al (1990) recently obtained inconsistent radiocarbon dates, ranging from 6 085 bp to 29795 bp from charcoal taken from the same motif at a rock art site in the Sydney Basin, Australia. The important point to be drawn from this study is that micro-contamination may not be identified in those cases where only one sample is taken (McDonald 1996).

A. Watchman at workHowever, taking samples of rock art for direct dating is problematic since the sampling procedure necessarily damages the art to a certain extent. Moreover, it is not always possible to be certain in the field when sufficient organic material for dating purposes has been collected. The dilemma for archaeologists is the necessity to minimise sample sizes in order to protect the art versus the futility of collecting a sample which is too small to contain sufficient organic material for dating. This highlights the necessity for sampling to be undertaken only with expert technical assistance (Rosenfeld and Smith in press).

ROCK ART DATING TECHNOLOGIES

Recent advances in dating technologies have significantly increased the number of dating opportunities for rock art relative to standard radiocarbon dating. In particular, the development of the Accelerator Mass Spectrometer (AMS) radiometric dating means that milligrams of organic material can now be dated (van der Merwe et al 1987). Other 'new' techniques include Cation Ratio (CR), amino acid racemisation, Optically Stimulated Luminescence (OSL), lichenometry and micro-erosion.

The Cation Ratio dating of desert varnish is based on differences in the mobility of different chemical constituents of desert varnish with some cations like potassium (K+) and calcium (Ca+) leaching out of the varnish faster than others like Titanium (Ti+). If the cation-leaching curve can be calibrated using such techniques as K-Ar dating of basalt flows, tandem accelerator mass spectrometry radiocarbon dating of organic fractions, and ratios from surfaces of known age, the varnish can be dated to provide a minimum age for underlying engravings (Whitley and Dorn 1987; Nobbs and Dorn 1988). This dating method is still controversial at present.

Amino acid racemisation depends upon the presence of albuminous binders, such as blood or egg white, in paintings. The technique is based on the observation that the number of amino acids present in proteins decreases over time, which forms the basis of a decay curve using paint samples of known age. However, this technique is restricted to dating paints less than 1800 years old. Moreover, amino acid decay rates also are dependant upon micro-organisms and environmental conditions, which means that results from different regions are not directly comparable (Thackeray 1983: 22).

OSL measures the number of electrons trapped in micro-fissures in quartz grains. This is correlated with to the length of time that the quartz has been removed from sunlight, which 'bleaches' out any trapped electrons. This technique can be used to date mudwasp nests and termite tracks, which contain 'buried' quartz grains. When such nests or tracks cover, or are covered by rock art, this allows assessment of a maximum or minimum age for the art (Roberts 1996).

Lichenometry depends upon the development of a species-specific, lichen-growth curve by measuring lichen thallus diameters on dated rock surfaces: this can then be used to calculate the minimum age of rock surfaces by measuring the thallus diameters of the same lichen species growing on these surfaces (e.g. Joubert et al 1983).

Micro-erosion analysis uses the weathering of individual crystals in different rock types to determine the age of surfaces exposed when engravings were produced (Bednarik1992.) The technique requires calibration data for the specific type of crystal being studied (e.g. silica).

DISCUSSION

The number of examples of dated rock art is increasing rapidly. However, most dates have not been particularly informative because their sampling and analysis was not directed towards general questions. Clearly, there is a great range of possible dating techniques for rock art, and any particular body of such art will offer a unique combination of potential approaches. The most convincing dated art sequences are those based on a range of data and the complementary use of relative and absolute dating methods. A productive approach is to use relative methods to establish a large data base for sequential changes in rock art and to test and anchor this with a comparatively small number of absolute dates.

SELECTED REFERENCES

Anati, E. 1976. Evolution and Style in Camunian Rock Art. Archivi 6, Capo di Ponte.

Bednarik, R. 1985 Parietal finger markings in Australia. Bollettino del Centro Camuno di Studi Preistorici 22: 83-8.

Bednarik, R. 1992 A new method to date petroglyphs. Archaeometry 34 (2) : 279-291.

Brandl, E. 1973. Australian Aboriginal Paintings in Western and Central Arnhem Land. Australian Institute of Aboriginal Studies.

Brandt, S.A. and N. Carder. 1987. Pastoral rock art in the Horn of Africa: making sense of udder chaos. World Archaeology 19 (2): 194-213.

Chaloupka, G. 1993 Journey in Time. Reed Books, Sydney.

Clegg, J. 1987. Style and tradition at Sturt's Meadows. World Archaeology 19 (2): 236-55.

Cole, N. , A. Watchman and M.J. Morwood 1995 Chronology of Laura rock art. In M.J. Morwood and D.R. Hobbs (eds), Quinkan prehistory: the archaeology of Aboriginal art in S.E. Cape York Peninsula, Australia, pp. 146-60. Tempus 3, Queensland University Anthropology Museum, Brisbane.

Dragovich, D. 1984. Desert varnish as an age indicator for Aboriginal rock engravings: a review of problems and prospects. Archaeology in Oceania 19 (2):48-56 [SS].

Joubert, J.J., W.C. Kriel and D.C.J. Wessels. 1983. Lichenometry: its potential application to archaeology in southern Africa. The South African Archaeological Society Newsletter6 (1):1-2.

Leroi-Gourhan, A. 1968. The Art of Prehistoric Man in Europe, Thames and Hudson.

Lewis, D. 1977 More striped designs in Arnhem Land rock paintings. Archaeology and Physical Anthropology in Oceania 12:98-111.

Lewis, D. 1988. The rock paintings of Arnhem Land, Australia. BAR International Series 415

Lewis-Williams, J.D. 1974 Superimpositioning in a sample of rock paintings from the Barkly East district. South African Archaeological Bulletin 29: 93-103.

Lorblanchet, M. 1992 The rock engravings of Gum Tree Valley and Skew Valley, Dampier, Western Australia: chronology and function of the sites. In J. McDonald and I.P. Haskovic (eds) State of the art: regional rock art studies in Australia and Melanesia. AURA Publication No. 6: 39-59.

McDonald, J., K. Officer, T. Jull, D. Donahue, J. Head and B. Ford. 1990. Investigating C14 AMS: dating prehistoric rock art in the Sydney Sandstone Basin, Australia. Rock Art Research 7 (2):83-92.

McDonald, J. 1996 AMX dating charcoal drawings in the Sydney region: results and issues. Paper presented to the First Australian Workshop on Rock Picture Dating, Sydney.

Macintosh, N.W.G. 1965. Dingo and horned anthropomorph in an Aboriginal rock shelter. Oceania 36: 85-101.

Maynard, L. and R. Edwards. 1971. Wall markings. In R.V.S. Wright (ed.) Archaeology of the Gallus Site, Koonalda Cave. A.I.A.S.

Morwood, M.J. 1980. Time, space and prehistoric art: a principle components analysis. Archaeology and Physical Anthropology in Oceania 15 (2): 98-109.

Morwood, M.J., G.L. Walsh and A. Watchman 1994. The dating potential of rock art in the Kimberley, N.W. Australia. Rock Art Research 11 (2): 79-87.

Muzzolini, A. 1986 L'art rupestre prehistoique des massifs centraux sahariens. BAR International Series No. 318, Oxford.

Nobbs, M.F. and R.I. Dorn 1988 Age determinations for rock varnish formation within petroglyphs: cation- ratio dating of 24 motifs from the Olary region, South Australia. Rock Art Research 5 (2):108-146.

Roberts, R., 1996 Preliminary investigations using optically-stimulated luminescence to date fossil mud- wasp nests associated with rock pictures in the Kimberley. Paper presented to the First Australian Workshop on Rock Picture Dating, Sydney.

Rosenfeld, A and C. Smith in press Recent developments in radiocarbon and stylistic methods of dating rock art. Antiquity.

Schaafsma, P. 1985. Form, content and function: theory and method in North American Rock Art Studies. In Michael B. Schiffer (ed.), Advances in Archaeological Method and Theory, Vol. 8, Academic Press: 237-77.

Taçon, P. 1996 Dating beeswax Ð view from central Arnhem Land. Paper presented to theFirst Australian Workshop on Rock Picture Dating, Sydney.

Thackeray, A.I. 1983. Dating the rock art of southern Africa. In J.D. Lewis-Williams (ed.) New Approaches to Southern African Rock Art. The South African Archaeological Society Goodwin Series 4: 21-6.

Van der Merwe, J. Sealy, and R. Yates. 1987. First accelerator carbon-14 date for pigment from a rock painting. South African Journal of Science 83: 56-7.

Watchman, A.L. 1985. Mineralogical analysis of silica skins covering rock art. In R. Jones (ed.) Archaeological research in Kakadu National Park. Australian National Parks and Wildlife Service. Special Publication 13: 281-90.

Watchman, A. 1993 Perspectives and potentials for absolute dating rock paintings. Antiquity 67: 58-65.

Watchman, A. 1996 A review of the theory and assumptions in the AMS dating of the Foz Coa petroglyphs, Portugal. Rock Art Research 13 (1): 21-30.

Whitley, D.S. and R.I. Dorn 1987. Rock art chronology in eastern California. World Archaeology 19(2): 150-64.