Monts des Ksour" im Süd-Oranais / Nord-Algerien - StoneWatch

Wrong Dating of Rock Art?
Controversial preparation
of probes Coal and charcoal coating
on mineral surfaces.
Dr. GÜNTER PAUL - Frankfurter Allgemene Zeitung
Until a few years ago, there was only a poor indirect possibility
to determine times and periods at which the pictures
engraved in rocky surfaces all over the World had been
made. But recently a method was introduced allowing at
least to define a minimum age of so-called petroglyphs.
Only a few researchers make use of the new technology so
far. One of them, Ronald Dorn from the Department of
Geography at the Arizona State University, has now come
under fire. A group of scientists are expressing their reproach,
claiming that his probes do not really allow reasonable
dating.
The said technique relies on the phenomenon that on uncovered
rock a coating is gradually formed consisting mainly
of iron and manganese oxide but also of organic material
from plant residues. The bottom layer of the coating can
give information of how old the coating itself at least may
be. Carbon probes are analysed in accelerators to define the
carbon content whose C14 isotope concentration is important
for dating.
After testing probes of petroglyphs from a canyon in Northeast
Arizona, won by Dorn out of grooves of rock pictures
and sent to the University of Arizona for dating by Accelerator
Mass Spectoscopy, an inconsistency now attracted
public attention. W. Beck found in all of these probes two
unequal groups of black particles which seem to be coal or
charcoal. The coal particles turned out to be 28,000 and the
charcoal remainders only 4,000 years old ('Science', Vol.
CCLXXX, p. 2132). The same phenomenon was ascertained
in other laboratories on Dorn's test materials.
With probes supplied by independent mineralogists and not
prepared by Dorn the effect did not occur. Therefore Beck
and other researchers are now pointing out the possibility
that the preparation method customary in Tempe for material
to be dated is probably responsible for that failure. In
Tempe the probes are, besides other forms of enrichment,
treated with concentrated hydrochloric and hydrofluoric
acid. At any rate, the age of the probes would not be determinable
if using two components of different maturity, as
the result will turn out deviating, in dependence on the mixture.
The researcher in Tempe contradicts to the results of others
claiming that the treatment of the material would be complicated
and the knowledge about it not to be acquired in a
crash course. Consequently, the results of the check test are
not convincing for him ('Science', Vol. CCLXXX, p. 2135).
As for the rest, it has to be considered that charcoal dissolves
in an ambient medium of iron and manganese and the
particles envolved become smaller. With respect to this, it is
not correct to test only the smallest of the charcoal particles
as most of them would automatically be the oldest. Beck
and his assistants obviously had tested only the bigger ones.
The age deviation from the coal probes, it is argued, is only
so large because of this granular selection. In reality, the
material includes charcoal particles of very different age,
but only the oldest are decisive.
The correctness of his approach is verified, according to
Dorn, by the results of a so-called blind study in which he
took part together with A. Watchman. The latter is another
researcher who became publicly known in the field of
petroglyph dating. The two scientists were asked in 1995 to
evaluate the age of rock writing on the banks of the river
Côa in Northeast Portugal. The petroglyphs were endangered
for some time by a barrage project and would have been
inundated in case of realization of the project. In the meantime,
however, the barrage work was suspended and the
rock pictures are under protection since 1996 and now form
part of the newly founded 'Archeologic Park of Valle do
Côa.
There had been archeologic indications that the petroglyphs
in question are about 18,000 years old, a little older than the
prehistoric bison depictions of Altamira. Dorn and Watchman
independently of each other have now defined a totally
different age. Dorn arrived at a value of 4,100 years, with
an inaccuracy of 1,100 years, Watchman fixes a number of
4,600 years, with an uncertainty of 2,100 years. Dorn's opinion
is that the congruity of the two results is to be explained
merely by the fact that the evaluation methods used for
dating have been applied carefully. If both data had been
falsified neither of them would have presented a value
equally different from the presupposed 18,000 years.
For the time being, the controversy about Dorn's results is
undecided concerning the dating of petroglyphs and also of
artefacts made of stone on which, similar as on rocks, a
time-dependent suface layer is formed by nature. Among
other things, Dorn believes that some settlement relics in
North America dates back to a period before the Clovis
Culture, approximately 11,200 years ago. It was long assumed
to be the oldest culture level in America. In the meantime,
many scientists have changed their mind, stating that
at least the finds of Mesa Verde, South Chile would indicate
an older civilization on the continent. Analysis of the findings
has produced an age of 12,500 years. With other finding
sites apparently inhabited in periods earlier than Clovis
the dating results are vehemently disputed. Dorn also
participated in dating of rock writing in Australia. Here
again, disagreements on the findings have emerged still
waiting for clarification. Namely, the petroglyphs are said
to be older than all other sources of indication hitherto discovered
about the presence of man.
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