Periglacial landforms
181
All these landforms develop more easily on slopes with scarce vegetation and on sunny
facing slopes where the thawing is favoured by the higher solar radiation. The gelifluction
deposits are usually poorly sorted and may show a crude stratification. The clasts are
commonly angular and show an oriented fabric with the major axis oriented parallel to the
direction of movement. These characteristics may be also occur in deposits affected by
gelifluction in other morphoclimatic regions. Those, however, that develop in periglacial
conditions are more angular and are scarcely affected by chemical weathering.
3.2. Cryoplanation terraces and cryopediments
These are low-angle, planated surfaces developed in periglacial regions. They have
received numerous descriptive and genetic names in the geomorphological literature,
especially in areas such as Siberia (goletz) and Alaska (altiplanation) where such
landforms are most numerous. Cryoplanation terraces develop in the upper and middle
part of slopes on hills and mountains up to 3000 m high. They show a terraced profile and
outcrops of bedrock and tors (rocky residual relied (Czudek, 1964) (Figure 8.11). These
terraces occur on slopes with gradients lower than 25 ~ and at the foot of small scarps.
The enlargement of the highest terrace may eventually give place to a planation surface in
the summit. The width of the terraces ranges from 5 m to more than 1 km, and the length
from 30 m to more than 10 km. The gradient varies between 1 and 14 ~ and the height of the
scarp may reach 50 m. The scarps show an abrupt link with the terraces and may have an
accumulation of snow. The terraces are covered by gelifluction deposits up to 3 m thick
derived from gelifraction processes (Priesnitz, 1988). These benches may show a structural
control and their origin is related to nivation processes. These involve gelifluction and
overland flow derived from snow melt waters that mobilise the particles produced by
congelifraction (Bryan, 1946). These processes cause the retreat of the scarp with the
consequent enlargement of the terraces and increase in their retention capacity of snow
(Demek, 1969). The presence of patterned ground on the surface of cryoplanation bench
treads and on pediments has often been raised as a potential indicator of contemporary
inactivity (Thorn and Hall, 2002). Excellent examples of cryoplanation terraces have been
recognised in the Sierra Nevada (Betic Cordillera, Spain) and in the Pyrenees (G6mez
Ortiz, 1996) (Figure 8.12).
Cryopediments are low-angle erosional surfaces developed at the foot of the slopes in
valley margins. Generally, only one level is recognised although in some cases several
stepped levels may occur. They have larger dimensions than the altiplanation or
cryoplanation terraces, reaching lengths up to several tens of kilometres. The slope of
the cryopediments varies between 1 and 12 ~ and shows a straight or slightly concave
longitudinal profile. They are frequently covered by a thin veneer (< 2 m) of detrital
material and the bedrock may crop out locally. Gelifluction landforms are frequent in
the proximal sectors of the cryopediments and patterned ground commonly occurs in the
distal areas (Priesnitz, 1988). Their origin is related to frost action in the source area
where the detritus, as on the cryoplanation terraces, is mobilised by gelifluction and
overland flow. The latter process is more active in the cryopediments and in
consequence the transport of particles is longer. In the distal sectors the particles are
transported by shallow braided channels. The continuous production and transport of
181
All these landforms develop more easily on slopes with scarce vegetation and on sunny
facing slopes where the thawing is favoured by the higher solar radiation. The gelifluction
deposits are usually poorly sorted and may show a crude stratification. The clasts are
commonly angular and show an oriented fabric with the major axis oriented parallel to the
direction of movement. These characteristics may be also occur in deposits affected by
gelifluction in other morphoclimatic regions. Those, however, that develop in periglacial
conditions are more angular and are scarcely affected by chemical weathering.
3.2. Cryoplanation terraces and cryopediments
These are low-angle, planated surfaces developed in periglacial regions. They have
received numerous descriptive and genetic names in the geomorphological literature,
especially in areas such as Siberia (goletz) and Alaska (altiplanation) where such
landforms are most numerous. Cryoplanation terraces develop in the upper and middle
part of slopes on hills and mountains up to 3000 m high. They show a terraced profile and
outcrops of bedrock and tors (rocky residual relied (Czudek, 1964) (Figure 8.11). These
terraces occur on slopes with gradients lower than 25 ~ and at the foot of small scarps.
The enlargement of the highest terrace may eventually give place to a planation surface in
the summit. The width of the terraces ranges from 5 m to more than 1 km, and the length
from 30 m to more than 10 km. The gradient varies between 1 and 14 ~ and the height of the
scarp may reach 50 m. The scarps show an abrupt link with the terraces and may have an
accumulation of snow. The terraces are covered by gelifluction deposits up to 3 m thick
derived from gelifraction processes (Priesnitz, 1988). These benches may show a structural
control and their origin is related to nivation processes. These involve gelifluction and
overland flow derived from snow melt waters that mobilise the particles produced by
congelifraction (Bryan, 1946). These processes cause the retreat of the scarp with the
consequent enlargement of the terraces and increase in their retention capacity of snow
(Demek, 1969). The presence of patterned ground on the surface of cryoplanation bench
treads and on pediments has often been raised as a potential indicator of contemporary
inactivity (Thorn and Hall, 2002). Excellent examples of cryoplanation terraces have been
recognised in the Sierra Nevada (Betic Cordillera, Spain) and in the Pyrenees (G6mez
Ortiz, 1996) (Figure 8.12).
Cryopediments are low-angle erosional surfaces developed at the foot of the slopes in
valley margins. Generally, only one level is recognised although in some cases several
stepped levels may occur. They have larger dimensions than the altiplanation or
cryoplanation terraces, reaching lengths up to several tens of kilometres. The slope of
the cryopediments varies between 1 and 12 ~ and shows a straight or slightly concave
longitudinal profile. They are frequently covered by a thin veneer (< 2 m) of detrital
material and the bedrock may crop out locally. Gelifluction landforms are frequent in
the proximal sectors of the cryopediments and patterned ground commonly occurs in the
distal areas (Priesnitz, 1988). Their origin is related to frost action in the source area
where the detritus, as on the cryoplanation terraces, is mobilised by gelifluction and
overland flow. The latter process is more active in the cryopediments and in
consequence the transport of particles is longer. In the distal sectors the particles are
transported by shallow braided channels. The continuous production and transport of