ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

temporary (Svalbard/Norway) and ancient (Scotland) glaciated
sites where subglacial till has been injected into bedrock
fractures and joints.
Joints and stress-induced fractures in bedrock are the key
to the erosion of material beneath both ice-rock beds and
ice-till-rock beds, because they represent avenues for potential
water movement and thus till injection. If fines in
the till are abundant they may be carried into the joints and
fractures by water movement. Under conditions of over
pressuring, hydrofracturing of the bedrock with associated
injection of till may occur. This movement of till into the
bedrock joints effectively separates a rock fragment from
the bed and results in a reduction of its buoyant weight and
inertia. Joint cohesion and the coefficient of static friction
are also reduced. The removal of the fragment (clast) now
requires a much lower shear stress. This maybe applied through
stiffening of the till due to water pressure reductions
or due to thinning of the till layer with subsequent ice-rock
contact occurring. Such water pressure and till layer fluctuations
are possible beneath large areas of the glacier bed.
We introduce an important new process in subglacial erosion
studies. Previously it has been assumed that the presence
of a deforming till layer precludes quarrying. However
it now appears that quarrying may be very effective in
the presence of a thin or patchy deforming till in association
with a well jointed/fractured hard rock bed.
A. REBEIRO-HARGRAVE
Large scale modelling of drainage evolution in
tectonically active asymmetric intermontane basins
using cellular automata
Department of Geography, King's College London, Strand,
London WC2R2LS, U.K.
In many orogenic regions undergoing compressional and
extensional tectonics it is common to find characteristic
drainage patterns that appear to be self organised. This is
observed in asymmetric intermontane basins where axial
drainage pattern is sensitive to regional tilting. Identified
as half-graben basins, uplift occurs preferentially at the
boundary fault and results in the hanging wall and drainage
sloping regionally towards the active fault. There has
been limited numerical research in drainage evolution
within basins subjected to preferential uplift and corresponding
regional tilting. This paper reports the results of
cellular automata simulations which indicates how drainage
evolution may respond to tectonic stresses under semiarid
climatic forcing regimes. Specifically, it will highlight
the difference between large-scale self organisation and
small-scale chaotic channel propagation based on regional
tilting.
An emergent cellular automata algorithm is routed on various
digital terrain models which represents the half-gra-
ben morphology at different stages of uplift. The hanging
wall tilt ranges from a minimum regional slope 0.5 percent
to a maximum slope of 3.2 percent. The local rules are governed
by rock strength and permeability parameterized
by field measurements taken from the Neogene Guadix
Basin , Southern Spain. The density of erosion cells is based
on the spatial and temporal distribution of climatic
events in semi-arid environment.
After 500 thousand iteration years, results show that with a
minimum regional slopes there is an increase of entropy ­
the tendency for chaotic drainage pattern, whilst for the
maximum slope there is a decrease in entropy suggesting
self organisation. This is interpreted as low regional slopes
characterised with high chaotic drainage density and tectonically
inactive, and high regional tilting with self organised
long channels and tectonically active. Field observations
tend to support this hypothesis with the slowly uplifting
regions characterised by short, wide bedrock channels
and the opposing tectonically inert sloping pediment with
long deeply incised bedrock channels.
DENISE J. REED
Hurricane impacts on microtidal marshes
in the subsiding Mississippi Delta Plain:
destructive or beneficial?
Louisiana Universities Marine Consortium, 8124 HWy. 56,
Chauvin LA 70344, U.S.A.
The coastal marshes of Louisiana are subjected to high rates
of sea-level rise (> 1 cm/yr) because of subsidence of
Holocene deltaic sediments. Their microtidal nature
means there is little tidal energy to transport sediments into
the marshes to offset this subsidence. Several studies have
suggested that elevated water levels associated with
frontal passage across the coastal zone are more important
than daily tides in delivering sediments. Hurricane impacts
are of greater magnitude but less frequency than cold
fronts and studies of recent hurricanes demonstrate that
they can be either beneficial or .destructive depending
upon the type of coastal marsh impacted.
The salt marshes of the Mississippi Delta Plain require mineral
sediment inputs both to keep pace with subsidence
and to counter the potentially toxic effects of sulfide buildup
within the marsh soils. Evidence from a number of tropical
storms and hurricanes confirms that these events can
result in significant sediment deposits in the marshes (several
em in thickness) which appear to benefit both vegetative
growth and marsh sustainability in the face of sea-level
rise. The longevity of the effect, however, seems to be dependent
on the nature of the storm passage including its
direction at landfall, speed of movement and the amount
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