ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

tive, with a smaller portion of the bed mobilized, but with
longer transport distances. Travel distance of individual
clasts show a gamma distribution with both processes. The
average depth of burial of clasts after nival flood transport
and after sockeye spawning transport are nearly identical
(7 ern), although the depth of burial varies with each transport
event.
During the course of spawning, individual redds of approximately
0.3 m' are excavated. As spawning density increases,
redds become aligned forming a series of gravel
dunes. Finally, as the gravels are further excavated, a
uniform gravel sheet is produced. Sockeye salmon spawning
results in the filling in of pools and the channel
thalweg, and the widening of the effective channel. The intense
bioturbation by sockeye salmon results in selective
winnowing of fine sediments and increased stream bed
permeability. Salmon bioturbation probably increases normal
fluvial bedload transport by creating a loose packing
structure, and uniform bed which is easily disrupted as the
stream redevelops a thalweg and reexcavates pools during
the nival floods the following spring. The bedload transport
rate from sockeye salmon bioturbation reported here
may be significantly lower than under pristine conditions
since approximately 70 % of returning salmon are caught
in various fisheries.
ANDREW GOUDIE & HEATHER VILES
Rapid weathering of rock blocks by salt in the
Central Namib Desert
School of Geography, University of Oxford, Mansfield Road,
Oxford, OXl 3TB, U.K.
In the hyper-arid Central Namib Desert near Swakopmund,
Namibia, pre-weighed rock blocks were placed on
the desert surface in a range of locations to investigate the
rates and nature of weathering. After only two years, some
of the blocks had almost totally disintegrated, demonstrating
that this is a highly aggressive weathering environment.
Detailed geochemical, Xrd and Sem studies showed
that the degree of weathering was correlated with the
amount of salts that had been absorbed by the blocks suggesting
that salt weathering is the dominant weathering
process. Investigation of naturally weathered rock and fine
debris supported the hypothesis that salt weathering is a
dominant process, allied in places with the action of lichens.
The salts involved were primarily calcium sulphate
and sodium chloride. Among the factors responsible for
the rapid rate of weathering are proximity to the sea, the
frequent occurrence of wetting fogs, and the large amounts
of salts that have accumulated on the desert surface. Salt
weathering is a very potent force in coastal deserts and is
probably responsible for extensive planation and for the
production of large quantities of fine debris that are then
susceptible to wind and occasional water transport.
186
LAHCEN GOURARI 1 & MOUSSA BENIELLOUL 2
Erosion active et conditions morpho-bioclimatiques
et anthropiques dans Ie bassin synclinal d'Ain Nokrah
et son piedmont (Moyen Atlas, Maroc)
1 Departement de Geologie, Dhar EI Mahraz,
Universite Sidi Mohamed Ben Abdellah, 30000 Fes, Maroc
2 Departement de Ceographie, Dhar EI Mahraz,
Universite Sidi Mohamed Ben Abdellah, 30000 Fes, Maroc
Des systemes climatiques revolues ont laisse leurs traces
morphologiques dans Ie bassin synclinal d' Ain Nokrah.
Mais Ie climat actuel mediterraneen, subhumide ou humide
remodele activement ces formes heritees. Les conditions
de l'erosion actuelle sont dominees par Ie grand role
de l'action anthropique particulierement sur Ie manteau
forestier et les formations superficielles (de l'Holocene notamment).
Le fosse tectonique d' Almis du Guigou est une plaine de
climat semi-aride, qui forme Ie piedmont de la cr te anticlinale
failleede Tajda-Ben IJ et du causse du Guigou. C'est
une plaine d'inondation fluviatile, parsemee de depressions
lieesades systemes de levees entre des chenaux et de
depressions d' affaissements hydro-volcaniques de type volcano-karstique
et / ou de tassement (Nicod, 1987). Ce milieu,
particulierement riche en formes et formations heritees,
est Ie lieu d'etalement de cones de dejection caillouteux
mais aussi de depots des limons de crues en provenance
du bassin-versant d'Ain Nokrah principalement et
des carbonates et des sulfates par les eaux d' origine superficielle
et souterraine. Toutefois, la realisation d'un lit artificiel
au cours de l'oued Guigou risque d' accelerer l'erosion
dans Ie bassin .
II importe de connaitre aussi bien dans la montagne que
dans Ie bassin intramontagnard, les processus d' erosion
et leurs conditions morphologiques, bioclimatiques et
humaines.
STEFAN W. GRAB
Thermal regime for a thufa and its adjacent depression,
Mashai Valley, Lesotho
Department of Geography and Environmental Studies,
University of the Witwatersrand, Private Bag 3, 2050 WITS, South Africa
Few observations have been published on the thermal characteristics
of frost-induced mounds such as thufur. Accordingly,
a 12-month study on the thermal regime for a thufa
and its adjacent depression was undertaken in the Mashai
Valley (- 2950 m a.s.l.), Lesotho Highlands, during 1993/
94. An overview of the annual air and ground temperature
characteristics is presented. Seasonal ground temperatures,
the mean daily temperature range and soil temperature re-