ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

creased aridity in the southwest USA during the Middle
Holocene, and are comparable to thermoluminescence
ages on nearby soils underlying similar aged pavements.
These ages suggest a period of increased vesicular soil formation
and corresponding pavement development possibly
related to climatic changes. Although alluvial fan pavements
may have a more complex geomorphic history than
pavements forming on volcanic flows, similar trends in cosmogenic
dating and soil properties support the accretionary
mantle model.
MALCOLM G. ANDERSON & SUSAN M. BROOKS
Identification of climatic thresholds for slope failure:
application of physically-based models
Department of Geography, University of Bristol, University Road,
Bristol BS8 ISS, UK
Climatic triggering of slope instability has frequently been
identified by geomorphologists (Selby, 1982; Freeze, 1987).
From quantitative assessments of the climatic conditions
under which failures occur it has been possible to identify
both the intensity-duration thresholds for slope failure
(Caine, 1981) as well as the combination of antecedent
rainfall and storm intensity (Campbell, 1975). Most studies
which link climate and slope instability pay little attention
to the nature of the soil or regolith cover, but recent research
has indicated that soil hydrological behaviour can
be instrumental in dictating maximum stable slope angles
(Brooks & alii, 1993; Anderson & Brooks, 1995).
There are two dimensions to consider when examining the
interplay between climatic thresholds and soil hydrology.
Firstly, the relevant intensity-duration-frequency thresholds
will alter under progressive development and maturation
of soil profiles. In particular, horizon development
and profile differentiation can substantially alter the storm
type required to promote failure (Brooks & Richards,
1994). Secondly, little attention is given to downslope
enhancement of pore water pressures resulting from selective
movement of constituent soil particles downslope.
While investigation of the first element is possible using 1dimensional
modelling, the second aspect clearly requires
2-dimensional analysis.
Application of coupled soil hydrology-slope stability models
operating in 2-dimensions is now possible, and at a level
of detail relating to downslope changes in the soil cover.
With this facility it is possible to explore in greater detail
than previously, the climatic thresholds for promoting
failure. This paper, therefore, uses physically-based modelling
to consider the significance of lateral as well as vertical
variation in soil cover to the relationship between climate
and slope failure.
52
MARIE-FRANc;OISE ANDRE
Preglacial tors in Aurivaara (Swedish Lappland).
geomorphic pattern and glaciological implications
Blaise Pascal University, Laboratoire de Geographie Physique,
Upres A 1562 - CNRS 29, boulevard Gergovia
63037 Clermont-Ferrand Cedex, France
In previously glaciated nordic and arctic areas, tor-like features
are often interpreted as indicators of cold-based ice
conditions, whereas they were successfully used in
southern European mountains to map the Quaternary unglaciated
areas. New investigations in Swedish Lappland
lead to question the validity of such an antinomic view.
Fifteen syenite tor-like features were closely investigated
on the Aurivaara Plateau, west of Kiruna (68 0
N), a study
site which was discovered and signaled to the author by
Anders Rapp (University of Lund). The morphology of
weathering residuals was studied in detail as well as the distribution
and characteristics of glacial deposits and associated
smallscale landforms, Petrographical diagrams of erratics
scattered on tors and preliminary microscopic examination
of rock samples were carried out.
The finding of pink granite erratics and till material trapped
into joints up to two meters deep confirms the preglacial
age of the tors, which is also suggested by the deep
weathering of pyroxenes observed in thin sections of in situ
syenite. Moreover, erratics were also found in clitters
which fonned at the bottom of tors. These observations
confirm Rapp's hypothesis of a cold-based ice cover in Aurivaara
and are in accordance with the pre-Weichselian age
of the Pallentjakka blockfield established by this author
westwards in Swedish Lappland.
A wide range of morphologies was found among tors from
rock pillars still embedded in grus to ice-scoured flat tor
roots. This geomorphological variety appears to depend on
the position of the tors both in the topographical and the
glaciological patterns. As usual, the highest tors (up to 10
m) are found at the edge of the plateau, whilst landforms
have a poor vertical development (1""2 m) in the heart of
the plateau. At the same time, an asymmetric pattern is assumed
to derive from the direction of glacial fluxes over
the Aurivaara plateau: ice-scoured tor roots locally shaped
into roches moutonnes are found in the northeastern exposed
area, whereas rock pillars with spheroidal weathering,
mushroom-rocks and in situ grus characterize the southwestern
protected part of the plateau.
The Aurivaara plateau represents a smallscale expression
of the «classic» mosaic of scoured and not scoured landscapes
linked with contrasting basal thermal regimes
as shown in Greenland by Sugden (1974) and in Arctic Canada
by Dyke (1993). However, the interpretation
of the geomorphological pattern observed in Aurivaara is
complicated by the inding of erratics most probably belonging
to several glacial episodes, which requires a global
reconstruction of the «palimpsest glacial landscape»
(Kleman 1992).