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

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analysis of other cosmogenic nuclides, particularly 10 Be and
26 Al. The dating of rocky surfaces by measuring cosmogenic
nuclides was in fact inaugurated only after developing
the accelerator mass spectometry.
Beginning in the eighties, there has been a whole series of
attempts to date organic material found under weathering
cortex layers and in-between, of arid zones, with the help of
the 14 C-AMS method. Probe examples in this case are artefacts
covered with desert varnish (Dorn et al.,1986) and
geoglyphs (von Werlhof et al., 1995) from the southwest
territories of North America. The organic components are
derived from bacteria, plant detritus and also from alga and
lichen colonies out of the bottom layer of desert varnish so
that at least once being tested strictly methodically, this
substratum should be capable to deliver some reliable information
about its subaerial metamorphosis as well as its 'status
ante quem' and also, vice versa, as to how much time
has elapsed from the date when the overlaying desert varnish
came into existence. The desert varnish probes require
several steps of preparing and refining before they can
undergo 14 C analysis testing in an AMS laboratory. Recently
it has been doubted that the 14 C method, not as a valuable
analytic methodology, to be true, but as to its applicability
to the age dating of desert varnish (Beck, et al., 1998). The
doubtfulness arose from refined weathering cortex probes
of petroglyphs in which two different types of carbon-containing
particles had been visually detected, one consisting
of sub-bituminous coal and the other of charcoal. These two
types delivered very different results of 14 C aging, and depending
on their mixing ratio, various apparant and arbitrary
14 C ages are the answer. Obviously it is not a problem of
the actual 14 C analytics but of the organic material from
weathering cortex to be analyzed. If its nature, origin and
formation is not clearly worked out beforehand, its 14 C age
is of no chronologic indicative value for evaluation of surfaces
(Watchman, 2000).
The cosmogenic nuclides 10 Be and 26 Al produced in situ on
rocky surfaces are normally tested together. They are capable
to indicate surface exposure ages in a range of one
thousand up to several millions of years. For dating, almost
only quartz-containig minerals are used. The sufaces must
not show traces of erosion. They must be obtained from a
definite depth, with best results from only a few centimeters
under the earth. Promissing experience has been made
with respect to probes from moraines, glacier abrasions,
earthquake-born and vulcanic rock (Nishiizumi et al., 1989;
Bierman/Gillespie/Caffee, 1995). In spite of their archeologically
interesting capabilities, these nuclides have not been
optimally made use of so far.
The noble gas nuclides 3 He and 21 Ne cosmogenerated in situ
are stable and are generally used together for surface dating.
With the help of the 3He method, exposure ages of olivine-containig
basalts and high-grade quartz minerals can
be determined. The chronology ranges from 250 to one million
years. Probe extraction from the soil should be of a few
centimeters and clearly defined depth under non-eroded
surfaces. Applicability examples are basalt minerals of the
island of RÈunion (Staudacher/Allegre, 1993) and moraine
blocks from the Antarctic Continent (Brook et al., 1995).
With these nuclides too, the archeological application possibilities
are still not exhausted to the utmost.
Luminescence
In rocks there is always a weak level of ionizing radiation
present originating from natural radioactivity of elements
like sodium, uranium and thorium as well as, to a minor
part, deriving from cosmic rays. The ionizing radiation causes
damages to rocky minerals, especially in quartzes and
feldspars, which accumulate in the course of time. The intensity
of the radiation damage can be determined in form
of luminescence. Luminescence is a sort of "cold" glowing
fed by the energy inherent in the radiation damage. For triggering
the luminescent effect, the mineral has to be stimulated
thermically and optically. Depending on the stimulation
type, we distinguish between thermoluminescence (TL) and
optically stimulated luminescence (OSL). The al-titude of
the luminescence signal will therefore be a measure of the
age of the mineral or, more precisely, of its irradiation time.
For evaluation of the age, the dosis power has to be known
additionally, i.e. the radiation intensity acting on the mineral
per time unit.
For dating rocky surfaces it is necessary that a latent luminescence
signal already existing in the mineral be extinguished
by exposing it to light. A few minutes of daylight is
sufficient. If the exposure is followed by redarkening the
luminescence signal is restored afresh and its new intensity
can now serve as determinator of the time elapsed since the
last exposure. Rock surfaces formerly exposed to light we
can find on archeologic stone devices, covered rock walls
and stone blocks in buildings. In all of these cases it should
be principally possible to date the age of the last exposure
to light. If the creation of the stone surface is only within a
neglectible short interval before its black-out, then the surface
age can be evaluated by the luminescence method (fig.
3). This should be realizable with most of stone construction
elements, otherwise the luminescence age results in a
deadline 'ante quem' for the making of the surface. Limespar
from the marble of the Apollo Temple in Delphi, Greece,
already served for testing the TL dating technique (Liritzes
et al., 1997). But still it is a long way to an experienced
routine dating of exposed surfaces. Anyway, basic physical
research work on feldspars has proved that the OSL method
conceals great capabilities of surface dating on granite rock
minerals (Habermann, 2000).
Lichenometry
Freshly exposed rock surfaces are normally invaded by
lichens. The individual organisms, in most cases of round
shape, grow constantly in the course of time so that the largest
in diameter, that is to say the first of the colonization,
reflect the duration of the suface's exposure (Berschel,
1950). The growth velocity depends on many factors, as
there are the race of the lichen, lithology, surface roughness,
temperature, moisture, and microclimate (Bull/Brandon,
1998; Evans/Archer/Wilson, 1999). In order to determine
a numeric age, the growth velocity has to be defined
on a basis of independent aging data, as historic information,
dendrochronology, and 14 C figures. This type of calibration
will always be confined to restricted areas, in view of
the aforementioned influence factors. The age range capable
for dating covers approximately the last 1,000 years.
On favourable conditions, the dating can be as exact as ±10
years.
The lichenometric dating can be adopted preferably for silicate
and, more rarely, for calcarious surfaces which must be
uncovered in a relatively quick action. Surfaces of this kind
are to be found on boulders brought down by landslips and
rockslides, moraines and debris slopes, but also on natural
and manmade breakoffs and slabs. Parallel to geomorphologic
and paleoseismologic applications, lichenometry is potentially
suitable also for archeology.