TRACCE no. 3 – by A. Arcà, A. Fossati
If the battle for saving Côa Valley petroglyphs against the dam has been successful, another “fight” is now enlightened: the dispute between archaeologists who (unanimously) sustain the Palaeolithic age of Côa engraved rocks, and direct-dating researchers, trying to demonstrate their (very) recent antiquity.
Direct dating results at Coa has yet been (sometimes firmly) criticized by C. Züchner (1), Clottes-Lorblanchet-Beltran (2), Zilhão (3), Arcà (4). During NEWS ‘95, the international rock art congress (and also IFRAO meeting) held in Turin, it was clearly pointed out that each “new” research method should work in a close relation with other parallel approaches, here mainly including the archaeological one. Each attempt to separate rock art research (which we would like to call “Rupestrian Archaeology”) from archaeology is simply unacceptable.
We can read by L. Loendorf (5) about cation-ratio varnish dating: “…under the direction of an archaeologist, more than half of the samples were collected to test the validity of the cation-ratio dating method. Most of these test samples were selected to measure the numerical age of a petroglyph for which there was good relative evidence as to its age…”.
So called blind-tests conducted at Côa have been conducted with different rules, assuming “a priori” their validity, completely unrelated with any other local archaeological research. We can find at Côa many well-known historical engravings: why not date them as comparison? The same “blind” method was applied in Valtellina (Italy), conducting microerosion analysis without asking complete information to the rock-art research team who worked there for five years, ignoring that the rock was covered by moss until 20 years ago and ignoring that the rock was strongly washed with chemical substances 10 years ago by the local researchers.
The central point of the microerosion method is micro-wane formation and its optical (100x microscope) measurement. We can find a detailed explication in Archaeometry 34 (6). In fact “it is based on the principle that for relatively stable rock surfaces microscopic erosion phenomena provide a measure of their relative antiquity”. It “postulates that these laws of wane formation are universal, they apply macroscopically as well as microscopically (on crystals…)”. Correcting with more measurement and geometrical relationships the Cernohouz and Sole method (1966, which measured the retreat from the supposed “fresh” edge of a cristal to the actual edge), and knowing the weathering rate, it is possible to obtain the period of exposition of a (mostly quartz) crystal truncated by the engraving activity.
But at Côa “only a minimal number of wane-width determinations (14) could be secured“ (-7-, p. 879) , because schist (dominant in many engraved surfaces) provides “a comparatively poor medium for micro-wane width determination“ (-7-, p. 879)
The oldest date assured by the micro-erosion method (at Canada do Inferno) was “about E6500+-2000 BP” (-7-, p. 879), it means 8500-4500 BP: not a very defined range, indeed. The older date, taken as a minimum age, could anyway comply with an Epi-Palaeolithic attribution.
But the problem is: how can environmental conditions affect the wane formation?
“…at this initial stage in the development of the new methodology, calibration can only be considered valid for the geology and climate of the region where it was obtained. One needs to consider several possibilities here, such as vegetational changes, oscillations of climate and environmental pH, or whether a site may have been covered by sediment” (Archaeometry pag. 288,-6-).
And more, about Côa experience: “…to satisfy the requirements of the microerosion dating method, at least two regional calibration curves would be obligatory. Regrettably, there was not enough time available to determine locally precise multiple calibration curves” (-8-, p. 820) “no micro-erosion calibration curves are available for Portugal…the quartz calibration curve from Lake Onega was used merely to acquire an approximate age estimate (-7-, p. 879) .
At Côa Valley (Portugal) the calibration curve of Onega Lake (Russia) was applied, the only available in Europe! There was not enough time available to determine a local calibration curve! But we can see in a picture (Archaeometry) that Onega Lake engravings have been executed on a horizontal surface. It is also possible that some Onega Lake surfaces have been sometimes submerged. At the opposite side Coa Valley surfaces are vertical, often half-shielded. We know very well that there is a huge difference in weathering from vertical and horizontal surface: it is sufficient to compare petroglyphs of the same age executed in these different positions. The ones on vertical surface always seem to be executed few years ago… We know also that at Coa “the regional erosion rate is relatively slow” and that “the relatively resistant nature of the rock and the low hydraulic gradient of the trunk river indicates that erosion rates may be low and the landscape relatively stable” (9).
We will reserve another space in the next TRACCE issues for AMS debate: we can’t resume it here. For the moment it is sufficient to say that Chlorine-36 surface exposure dating (by R. Flinsch and Fred M. Phillips) confirms that rock panels age “range from 40 to 272 ka, indicating that the panels were extant during Paleolithic time” (9). Why silica skin earlier dates are only 4000 BP? And also it is not correct to assure that “several blind tests, using a variety of methods, all produced identical results” (-8-, p. 880). We will conclude with some words about cation-ratio method (-6-, p. 280, Bednarik reporting Dorn’s considerations): “it remains an experimental method, he has to revise calibration curves and the method ‘will always be a weaker sister’ of other approaches”. We think that we can better apply these considerations to the microerosion method.
And about stylistic analysis (we hope there will be space to better discuss this point), we can find in rock art research many relationships between archaeological well-dated objects and traced figures. And we can find many superimpositions between earlier and more recent figures. Is this a merely “stylistic” method? Each figure is a cultural expression. There a meaning beyond it. There is an iconographic environment that we can’t ignore. Each iconographic environment is strictly related to its mother culture. It’s a human, a cultural law, and in this sense is scientific and archaeologic. If Côa Valley engraved big animals are not Palaeolithic (or Epi-Palaeolithic), we must suppose either a hidden “Palaeolithic-like” culture has survived in this region till a few centuries ago, or that a big tribe of Rock Art counterfeiter worked at Côa (at also at Siega Verde, Domingo Garcia). It’s hardly allowable. It’s more admissible that it’s better to have a well tested method before claiming its worldwide qualities.
BIBLIO (Arcà – Fossati, style)
- ZÜCHNER C., 1995. Some comments on the rock art of Foz Côa(Portugal), “INORA”, no. 12, pp. 18-19
- CLOTTES J. – LORBLANCHET M. – BELTRÁN A., 1995. Are the Foz Côa engravings actually holocene?, “INORA”, no. 12, pp. 19-21
- ZILHÃO J., 1995. The age of the Côa Valley (Portugal) rock-art: validation of archaeological dating to the Palaeolithic and refutation of “scientific” dating to historic or proto-historic times, “ANTIQUITY”, 69, pp. 883-901
- ARCÀ A., 1996. Direct dating: does it work? The Côa Valley experience, “TRACCE”, no. 1, [online]. Available:index.html#dat
- LOENDORF L.L., 1991. Cation-ratio varnish dating and petroglyph chronology in southeastern Colorado, “ANTIQUITY”, 64, pp. 246-255
- BEDNARIK R. G, 1992. A new method to date petroglyphs, “Archaeometry”, 34, pp. 279-291
- BEDNARIK, R. G. 1995. The Côa petroglyphs: an obituary to the stylistic dating of Palaeolithic rock-art. Antiquity 69: 877-883.
- BEDNARIK R. G., 1995. Refutation of stylistic constructs in Palaeolithic rock art, “Comptes rendus de l’Académie de Sciences”, 321 (série IIa, no. 9), pp. 817-821, Paris
- FLINSCH M.R – PHILLIPS F.M., 1995. Chlorine-36 Surface Exposure Dating of Petroglyphs in the Foz Côa Valley, Portugal: A Role for Cosmogenic Nuclide Dating in a Cultural Resource Controversy, [online].