Periglacial landforms
203
affected by both thermal and mechanical erosion. Using some river features of the
flood epochs, such as geometry, temperature and discharge, a one-dimensional model
was made to predict erosion in river banks with different ice content. The results
predicted by the model agree with the field observations made in the Lena River
(Costard et al., 2003).
Few studies have been devoted to the morphology of the valleys developed in
periglacial areas and most of the literature is focused on the problem of asymmetric
valleys. Quite a lot of information also exists about relict Pleistocene valleys developed
under past periglacial conditions from central and Eastern Europe. The asymmetric valleys
are those that have one of the slopes distinctly more steep than the opposite. The
asymmetry may be determined by lithological and structural factors and it is not restricted
to periglacial areas. In these regions the shaded slopes are the steepest ones (north-facing
slopes in the Northern Hemisphere). The earlier and longer thaw in the sun-facing slopes
favour gelifluction and mass movement processes in some cases. The detritus accumulated
at the foot of these slopes forces the fluvial currents to migrate towards the shaded margins
of the valleys, giving place to steeper slopes by undercutting. Besides, summer thawing
affects the permafrost in the shaded slopes to a lesser extent, thus contributing to
preserving their steeper gradients (Gravis, 1969; Czudek, 1973). A different explanation of
the asymmetric valleys is related to the preferential accumulation of snow in downwindoriented slopes (French, 1971). During the summer the mass wasting processes caused by
the meltwaters act preferentially on the downwind facing slopes, thus generating steeper
gradients.
The lower-order segments of mountain drainage basins may show different
morphologies. The U-shaped valleys (Tricart, 1967) with gentle longitudinal profiles
develop in sectors where the surface run-off has a limited morphologic influence. The
slope profiles have a convex-concave geometry because gelifluction is the dominant
processes in the transport of detritus. The bottoms of the U-shaped valleys are
commonly covered by non-sorted gelifluction deposits. These valleys are abundant in
extensive European regions that were affected by periglacial conditions during the
Pleistocene and are now mostly dry. Generally, the U-shaped valleys that are not
longer than 3 km frequently grade into flat-bottomed valleys. In Iceland the flatbottomed valleys are around 5 to 100 m wide, 5 to 20 m deep and have gradients
between 4 and 11% (Schunke, 1975). Their bottom is primarily composed of
nonsorted material derived from the slopes that has been subject to some longitudinal
fluvial transport. The link between the flat bottom and the slopes commonly shows a
concave geometry that indicates a limited capacity of the fluvial current to evacuate
the gelifluction deposits supplied from the slopes. In some cases the link between the
valley bottom and the slopes shows an angular geometry that reflects the effect of
the fluvial erosion at the foot of the slopes during high stage events (Tricart, 1967).
The flat-bottomed and U-shaped valleys are generally dry at the present time (Jahn,
1975). This circumstance seems to be related to the permeability and thickness of the
valley fill deposits that favour the percolation and subsurface flow of the water
through the alluvium. Nevertheless, it is important to take into account that dry
valleys may also develop in other contexts like karst regions regardless of the climatic
conditions.
203
affected by both thermal and mechanical erosion. Using some river features of the
flood epochs, such as geometry, temperature and discharge, a one-dimensional model
was made to predict erosion in river banks with different ice content. The results
predicted by the model agree with the field observations made in the Lena River
(Costard et al., 2003).
Few studies have been devoted to the morphology of the valleys developed in
periglacial areas and most of the literature is focused on the problem of asymmetric
valleys. Quite a lot of information also exists about relict Pleistocene valleys developed
under past periglacial conditions from central and Eastern Europe. The asymmetric valleys
are those that have one of the slopes distinctly more steep than the opposite. The
asymmetry may be determined by lithological and structural factors and it is not restricted
to periglacial areas. In these regions the shaded slopes are the steepest ones (north-facing
slopes in the Northern Hemisphere). The earlier and longer thaw in the sun-facing slopes
favour gelifluction and mass movement processes in some cases. The detritus accumulated
at the foot of these slopes forces the fluvial currents to migrate towards the shaded margins
of the valleys, giving place to steeper slopes by undercutting. Besides, summer thawing
affects the permafrost in the shaded slopes to a lesser extent, thus contributing to
preserving their steeper gradients (Gravis, 1969; Czudek, 1973). A different explanation of
the asymmetric valleys is related to the preferential accumulation of snow in downwindoriented slopes (French, 1971). During the summer the mass wasting processes caused by
the meltwaters act preferentially on the downwind facing slopes, thus generating steeper
gradients.
The lower-order segments of mountain drainage basins may show different
morphologies. The U-shaped valleys (Tricart, 1967) with gentle longitudinal profiles
develop in sectors where the surface run-off has a limited morphologic influence. The
slope profiles have a convex-concave geometry because gelifluction is the dominant
processes in the transport of detritus. The bottoms of the U-shaped valleys are
commonly covered by non-sorted gelifluction deposits. These valleys are abundant in
extensive European regions that were affected by periglacial conditions during the
Pleistocene and are now mostly dry. Generally, the U-shaped valleys that are not
longer than 3 km frequently grade into flat-bottomed valleys. In Iceland the flatbottomed valleys are around 5 to 100 m wide, 5 to 20 m deep and have gradients
between 4 and 11% (Schunke, 1975). Their bottom is primarily composed of
nonsorted material derived from the slopes that has been subject to some longitudinal
fluvial transport. The link between the flat bottom and the slopes commonly shows a
concave geometry that indicates a limited capacity of the fluvial current to evacuate
the gelifluction deposits supplied from the slopes. In some cases the link between the
valley bottom and the slopes shows an angular geometry that reflects the effect of
the fluvial erosion at the foot of the slopes during high stage events (Tricart, 1967).
The flat-bottomed and U-shaped valleys are generally dry at the present time (Jahn,
1975). This circumstance seems to be related to the permeability and thickness of the
valley fill deposits that favour the percolation and subsurface flow of the water
through the alluvium. Nevertheless, it is important to take into account that dry
valleys may also develop in other contexts like karst regions regardless of the climatic
conditions.