Archaeologia Islandica - 01.01.2002, Page 14

Timothy J. Horsley & Stephen J. Dockrill
limitations of the methods, and the con-
ditions that make a site suitable for sur-
vey work.
Iceland presents a particular set of geo-
morphological and archaeological prob-
lems for the case of geophysical prospec-
tion, and it was the aim of this project to
evaluate the success of such methods for
the location and interpretation of buried
archaeology.
This research has investigated what,
at present, appear to be the most impor-
tant of these limiting factors for Icelandic
geophysics: soils, geology, geomorphol-
ogy and archaeology, although it is a
combination of these and more which
produce the complex result obtained.
Igneous geology and tephra
The principle constraint identifíed with
archaeological prospection in Iceland is
the nature of the geology of the island:
being volcanic and hence igneous, this
will have a effect on magnetic surveys
undertaken over such bedrock (Clark
1990, 92-4; David 1995, 10).
A serious complicating factor with
igneous material is the presence of a ther-
moremanent magnetisation, acquired
when the rock first cooled (Burger 1992,
412, 438; Clark 1990, 92). This geologi-
cal thermoremanance will produce an
intense response, far greater than that due
to archaeological deposits.
Igneous rock will also be present in
glacial erratic material and, having been
displaced, the magnetic directions of the
rocks are randomly jumbled, which can
produce 'noisy' background signals that
obscure archaeological anomalies (Clark
1990, 94).
However, prior to this study no mag-
netometer data has been systematically
collected over archaeological sites in
Iceland. Little work in general has been
directed into the effect of igneous parent
material on archaeological prospection
anywhere, and to date there have been no
comprehensive studies into the eífects of
tephra deposits on magnetic surveys.
Geomorphic processes in Iceland
When compared to Britain a number of
unusual geomorphic processes are active
in Iceland that have implications for
archaeological prospection. This field-
work has revealed that for the results of
geophysical surveys, geomorphological
influences are as important as geological
features.
When the soil temperature drops
below 0°C, the transformation of soil
water into ice, results in a marked
increase in the overall soil volume. The
resulting stresses bring about fragmenta-
tion, compaction and deformation of the
soil constituents. These mechanical dis-
turbances generate specific features
whose nature and degree of development
are related to the intensity of the frost, the
water content and the characteristics of
the soil materials (e.g. soil texture and
porosity) (Courty et al. 1989, 160). It is
clear that several periglacial processes
tend to separate coarser and finer parti-
cles in soils, and repetitive freezing
favours the fragmentation of coarse ele-
ments; after some time the initial charac-
teristics of archaeological soils may have
been irreversibly altered (Ibid. 1989,
161; Bird 1974, 720).
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