The stylistically Paleolithic petroglyphs
of the Côa valley (Portugal) are of Paleolithic age
A refutation of their <<direct dating>> to recent
times
João Zilhão
5.3. Implications that Watchman refuses to face
In Portugal the last 1700 years are fully historical, and written documentation
on art and religion is available for the entire period. In these circumstances,
the idea that thematically and stylistically Paleolithic art could have
been done in the Middle Ages or in subsequent centuries is simply ludicrous
and, in normal circumstances, its refutation would be a waste of time.
However, by categorically stating that a large majority of these petroglyphs
were actually done no more than one hundred years ago (Salema 1995), Watchman
aggravates his case. If that had been so, then several questions would have
to be answered, all of which Watchman refuses to consider:
Given that the grand- children and the great grand- children of the artists
would still be alive today, why would no memory of such activity have survived
the passing of only two or three generations?
Given that late nineteenth and early to mid-twentieth century engravings
are also known in the valley, their exact age being provided by the fact
that they are signed and dated, and by the nature of the representations
(castles, a train on a bridge, clocks, etc.), why would no such signing
and dating behavior have been associated with the stylistically Paleolithic
petroglyphs?
Given that the stylistically Paleolithic petroglyphs represent animals that
have been extinct in the area for thousands of years, how could the nineteenth
century millers who presumably would have engraved them have learned about
the existence and aspect of such animals?
Finally, how is it that the same millers were able to represent those species
following conventions that are typical of an art that, at that time, still
awaited rediscovery by modern science?
Not only does Watchman not answer these questions, he does not even realize
that his theory that the engravings were done last century by the millers
is a simple case of plagiarism. This theory was first presented more than
one month before Watchman came to the Côa, as a major feature in the
main news report of a Portuguese TV station. As they explained the day after,
that had been their way of respecting the April Fools tradition...
During the September 1995 International Rock Art Congress of Turin Watchman
elaborated this argument even further. In an interview with the Portuguese
news agency Lusa published by several Portuguese newspapers (cf. O Primeiro
de Janeiro, September 6, 1995), he stated: <<at Canada do Inferno
there are petroglyphs in areas where water mills for flour production and
even a nice sand beach used to exist [before flooding by the Pocinho dam]>>;
<<it is possible, therefore, that these engravings are no more than
20 to 50 years old and were made by the bathers for entertainment and amusement>>.
The spectacle of twentieth century bathers drawing extinct Pleistocene animals
is, however, one that most people will find difficult to contemplate...
It is in any case contradicted by historical evidence: letters and other
documents recently found in the Municipal Library of Mirandela (a nearby
town) demonstrate that the engravings in the Côa valley already existed
in 1939, when some were identified and described (but never published) by
a local doctor, José Silvério de Andrade (O Comércio
do Porto, July 16, 1995).
6. Watchman's OSL dating of river gravels
Watchman also used OSL (Optically Stimulated Luminescence) to date the river
gravels near Penascosa. The results he obtained indicated to him that those
gravels were only about 4000 to 6000 years old. From this he concluded that
the rocks and their engravings could not be of Paleolithic age: <<this
[OSL] study indicated that the engraved rocks were first exposed 4500 years
ago>> (Sá and Ferreira 1995).
In his presentation to the September 1995 International Rock Art Congress
in Turin, Watchman explicitly stated that the morphology of the valley indicated
that it represented a young, probably mid-Holocene, incision. The only reasoning
that, in this context, might conceivably have led him to derive, from the
OSL dating of the river gravels, a maximum age for the engraved rocks, is
this: since the gravels at the bottom of the valley are only 4000 to 6000
years old, such is the maximum age of the incision of the valley itself;
therefore, the petroglyphs could not possibly be of Paleolithic age, since
the rocks where they were executed had only been exposed (by river incision
of bedrock) in the mid-Holocene.
This is, however, utterly absurd. Wherever data on valley incision have
been obtained for the Portuguese Quaternary, as is the case in Estremadura,
the littoral region of central Portugal, they show that the last glacial
maximum valley bottoms were lower than at present (Marks et al. 1994)! Furthermore,
the torrential regime of the Côa implies that most sediment carried
by the river tends to be transported downstream to the Douro, and very little
can be found, even today, covering its rocky river bottom. If Watchman's
OSL dates were to be confirmed by future research, they would simply indicate
that the terrace at Penascosa dated to the mid-Holocene. It is obvious,
however, that dating the fill of the valley only provides a minimum age
for the valley incision itself, which, given our current knowledge of the
Portuguese Quaternary, in all likelihood significantly predates the Upper
Paleolithic.
The absurd nature of Watchman's reasoning is demonstrated by the results
of the archaeological excavations carried out at Penascosa in August 1995
at the base of the engraved panels. The upper part of the gravels, which
were buried under ca. 1 m of sandy deposits, contained iron horse shoes
and iron horse shoe nails used early this century. This indicates that those
gravels may be related to the changes in the regime of the river that resulted
from the construction, in the early 1900's, of the water mill (and associated
damming facilities) whose ruins are still visible at Penascosa. If the age
of the gravels indicated the age of the incision, then the Côa would
have excavated its valley only in the last few decades!
Archaeological survey also undertaken in August has in any case allowed
the discovery of two Upper Paleolithic camp sites located on Pleistocene
deposits some 10-20 m above the present valley bottom (see below). This
suffices to demonstrate that the valley incision is indeed of pre-Holocene
age, contrary to Watchman's opinion, which, it should be stressed, was not
substantiated by a single piece of geological or geomorphological evidence.
7. Bednarik's microerosion dating
Robert Bednarik is supposed to have used the microerosion direct dating
technique, according to which none of the stylistically Paleolithic engravings
could be older than ca. 6500 BP, and most were done only some 3000 years
ago.
Microerosion was theoretically presented as a method of direct dating of
petroglyphs in a 1992 issue of Archaeometry (Bednarik 1992a). It is based
on the concept that there is a relation between the degree of erosion (measured
through observation with a binocular microscope) suffered by some rock components
(quartz grains, for instance) exposed by the engraving and the amount of
time elapsed since the engraving was executed. The only concrete application
so far published, however, is that of Besov Nos, Lake Onega, Russia (Bednarik
1992b). Moreover, no true <<blind test>> of this technique was
ever undertaken. That is, no experiment in dating an engraving whose age
was unequivocally established but unknown to the practitioner of the method
has so far been reported. And the same is true for cases of replication
of the same results by different practitioners in similar <<blind
test>> conditions. Until these two conditions are met, microerosion
should only be considered, at best, as an interesting possibility, but not
as a dating method, not even at an experimental level. In any case, as Andrée
Rosenfeld (in litteris, July 28, 1995) stated, microerosion <<was
developed by geographers in the UK to measure the erosion rate of large
natural rock surfaces -- it depends on taking numerous readings and was
intended to arrive at an average rate for extensive areas. It was not designed
to allow for the many uncontrollable variables that may operate at any one
spot on a rock surface -- as e.g. in a petroglyph>>.
Even if one agrees to discard these objections and accept that using Bednarik's
variety of microerosion analysis for petroglyph dating may not be such a
bad idea after all, one is faced with the fact that, according to Bednarik
himself, <<schist and other rocks of low metamorphism (slate, phyllite)
are not well suited for microerosion analysis>>. There are many reasons
why this is so, but one is readily apparent: the mineral components of these
rocks are of microscopic size and the observation of microerosion features
therefore impractical. However, <<there are a few petroglyphs on granite
in the Côa valley, which is an ideal rock for this method, but they
are of difficult access and time did not permit me [him] to see them>>
(Bednarik 1995b). In short, Bednarik could have used his method on what,
in his own terms, would be suitable rocks (the granites bearing stylistically
Neolithic petroglyphs), but chose instead to use it on the unsuitable ones
(the schists bearing the stylistically Paleolithic engravings).
Another precondition for the applicability of Bednarik's technique would
be the availability of a locally valid calibration curve, that is, the establishment
of certain parameters derived from engravings whose age is unequivocally
established. In Bednarik's own words, <<in a practical application
of dating a geomorphic or petroglyph surface through assessing microerosion
phenomena it is requisite to determine the rate of wane development, initially
by establishing a calibration curve for surfaces of known, or approximately
known, ages. These rates may vary in different climates, rock types, and
even mineral compositions>> (Bednarik 1992a). That is, <<without
locally established calibration curves this method is of poor accuracy>>
(Bednarik 1995b). Since dated inscriptions are known in the Côa, one
might be led to think that such a calibration curve would not be difficult
to establish. As regards such historical petroglyphs, however, Bednarik
(1995b) states that <<the extreme anisotropism of the rock would render
it difficult to accept the engraved date for calibration purposes, which
applies also to historical inscriptions I observed elsewhere in the region>>.
If the rocks are not appropriate, and if there is no locally established
calibration curve, how can Bednarik legitimately claim that he applied microerosion
dating? And how, then, does he arrive at an age estimate? As for the last
question, the answer is very simple. His conclusion that <<among the
figures I have seen, none can be older than 6000 to 8000 years at the most>>,
derives from the maximum age he gives to one of the figures in the main
panel of Canada do Inferno, which <<would be between 4500 and 8500
years old, with the highest probability at about 6500 years BP>>.
This, in turn, derives from the <<experimental>> application
of the Lake Onega calibration curve, <<ignoring the differences in
climate and lithology>>, to the pattern of measurements made by Bednarik
on this figure. According to him, such an application <<does not provide
us with a true age of the motif but does offer a fairly reliable indication
of magnitude of age for this figure>> (all quotes from Bednarik 1995b).
Even if one would find it intellectually challenging to play Bednarik's
game, it should by now be well apparent how absurd the whole thing is: a
reliable indication of magnitude of the age of the Côa slate petroglyphs
based on a calibration curve for granites from Russia! More interesting,
however, is that the application of the same curve to a quartz vein in panel
6 of the Penascosa site (Fig. 4) <<would
imply an exposure age of the panel of perhaps 30,000 years>> (Bednarik
1995b)! And why would 6500 years be a reliable maximum age, and 30,000 years
a non-reliable one? Because the latter <<seems to high too me in view
of the absence of gelifraction damage>>, and because the stratigraphy
and differential microerosion of abraded and pecked figures would imply
that the pecked ones <<would have to be several times as old>>,
something impossible to accept on the basis that <<we have no indication
of an iconic rock art tradition in Europe from the Middle Paleolithic, which
such an interpretation would stipulate>> (all quotes from Bednarik
1995b).
The preceding paragraph immediately brings up several questions. For instance,
why would the differential microerosion between abraded and pecked figures
imply that a large amount of time would separate both kinds of motifs? Why
is this difference not just a byproduct of the different engraving techniques
used in each case, and of the resulting differential exposure to weathering
agents? Where are the studies proving that weathering (or, for that matter,
accumulation of crusts, or the formation of patinas) is a linear function
of time? On the other hand, why should gelifraction be expected in the Côa
valley? Incidentally, it should be noted that the reasoning regarding frost-weathering
is nothing but a restatement, almost 120 years later, of the arguments put
forward by some nineteenth century French positivists to sustain that Altamira
was a fraud (the art could not possibly be that old because it simply would
not have been able to survive for so long)!
Besides frost-weathering, Bednarik (1995b) also invokes, en passant, a series
of other geological and archaeological, as well as stylistic, arguments,
which he considers as <<still more serious objections to the Paleolithic
antiquity of the Côa art>>. Given the fact that the two maximum
age estimates provided by the direct dating technique are so far apart,
it must be considered that it is these arguments that, in the last instance,
enable him to decide which is the <<good>> one and which is
the <<bad>> one. In other words, since Bednarik's microerosion
analyses of the Côa petroglyphs violated every single methodological
requirement he himself had established in his own theoretical papers on
the issue, it is such contextual arguments that must be considered as the
real logical foundation of his dating of the stylistically Paleolithic figures
to post-Paleolithic times. Let us now proceed to examine how sound such
arguments are.
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