ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

ock glaciers, the conditions of formation and maintenance
of permafrost not having been verified.
There was observation and description of the common
compressive and extensive surface fluidal texture, conical
pits and areas without flow structures. The surface texture
showed how, in some cases, the rock glacier is not the expression
of a single state of stress, but is the result of a series
of deformational events. A morphological convergence
with other alpine forms was observed. It was particularly
difficult to distinguish between protalus rock glaciers and
protalus ramparts, and also between moraine (valley floor)
rock glaciers and ablation morainic complexes. This leads
to the consideration that there is, as known from the literature,
an evolutionary continuity of form, due to periglacial
action, controlled by climatic and topographic conditions
between deposits of different origins (protalus ramparts,
ablation morainic complexes, talus slopes and debri cones)
and rock glaciers. By means of a morphological approach
there was estimation, in the field and also on topographical
maps, of the slope angles of the rock glaciers and of the detritic
slopes behind. From this comparison it emerged that
the rock glaciers have, due to their kinematic interior,
fronts which are steeper than their supply slopes. The protalus
ramparts, where they are not influenced by permafrost
phenomena, have external ridges of a steepness similar
to that of the back slopes and they are not characterized by
surface deformational structures. However, the partial collapse
and stabilization of some rock glaciers fronts, at
lower altitude, did not allow accurate observations making
it impossible to assess their features correctly. The distinction
between ablation till and rock glaciers was even more
problematic. The criterium used was that of recognizing
clear successions of concentric compression ridges, trenches
with a variable but more or less continuous tendency
and collapse stuctures due to internal ice melt. However,
some complex morphological situations were identified in
which the rock glaciers had developed both within frontal
morainic ridges, partly covering one another, and also starting
from these ridges, involving them in their deformational
movements. In some cases there was observation of valley
bottoms filled with detritic deposits apparently structured
in a tight succession of ridges stretching along the valley
axis. However there is no correspondence between this
apparent extensive fluidal texture and areas characterized
by the quite extensive compression ridges found in the
frontal part. Therefore these deposits have been interpreted
as ablation tills which have conserved structures transferred
from the glacial mass that they were covering. The
distinctive sedimentological features regarding rock glaciers
and protalus ramparts are the general characteristics
of the deposit, particularly the presence and distribution of
the matrix. Rock glaciers have an open work fabric, with a
clast-supported structure consisting of irregular blocks of
sizes from a decimeter to a meter and by fine sandy fractions
and coarse gravels; these are totally absent on the surfaces
of the detritic masses, are found mainly concentrated
at deeper levels and can be observed in correspondence
with the fronts and the external perimetric slopes distribu-
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ted in pockets, even if not uniformly. The classic zoned
structure of the frontal and lateral slopes of the main lobes
can often be found. The protalus ramparts also have a classupported
structure of angular blocks, but the matrix,
compared to that of the rock glaciers, is almost completely
absent in the vertical profile and constitutes the filling in
the spaces between the biggest blocks.
KEITH S. RICHARDS
Events in fluvial geomorphology; auxiliary hypotheses
and the normal science of magnitude and frequency
Department of Geography, University of Cambridge,
Cambridge CB2 3EN, U.K.
The introduction of the magnitude-frequency concept was
a significant event in fluvial geomorphology, and the concept
has underpinned interpretations of dominant or effective
discharge and its relationship to channel morphology
since 1960. However, it has been increasingly necessary to
bolster its role by a series of ad hoc auxiliary hypotheses
concerning the significance of flow process events, the
controls of sediment transport, and the adjustment of
channel form to sediment transport processes. For example:
- The duration and inter-arrival times of flow events (on
all time-scales), in addition to the magnitude of that event,
all interact to determine the transporting capacity of each
event. As a result, the potential effectiveness of flow events
cannot be judged solely by the product of their magnitude
and frequency.
- The actual effectiveness of a flow event in terms of work
done in transporting sediment is dependent as much on sediment
supply, both absolute and relative, as on transport
capacity.
- The morphology of river channels is a multivariate three-dimensional
phenomenon that reflects a wide range of
processes that are differentially dependent ·on flow events
of a particular frequency, and the event which has the most
lasting visual impact is not of critical importance for all
aspects of the morphology.
This paper will address these issues with examples that illustrate
the application of the magnitude-frequency' concept
over a range of space and time scales; from the analysis
of turbulent flow structures and their impact on instantaneous
sediment transport, through the determination of
effective or dominant discharge in relation to time-integrated
sediment yield, to the identification of relationships
between discharge and equilibrium hydraulic geometry,
and the analysis of flow and sediment transport as controls
of evolving channel morphology.
The aim will be to consider whether the auxiliary hypotheses
now burden the concept to such a degree that it