66
Climatic Geomorphology
profiles (Bakker, 1965; Thomas, 1994), is the presence of regolith of variable thickness
generated prior to the freezings. The existence of some regions of this material that are
easily eroded by the ice must be very carefully considered in the interpretation of glacial
erosion modelling and in the quantification of erosion by the ice. We consider that this
circumstance must be taken into account in the study of glacial sculpting in the Iberian
Central System, because the presence of thick weathering deposits is common in this area,
generated without a doubt in times prior to the Quaternary (Guti6rrez and Rodr/guez,
1978; Molina and Blanco, 1980).
In any case, fracturing of the rocky substratum, whether inherited or of glacial origin, is
a factor of extraordinary importance in glacial erosion, along with the availability of
preglacial loose material.
1.1.3. Evacuation of debris
When a glacier slides on its bed, the mobilization speed of particles depends
fundamentally on their size and forms and on the substrate roughness. For fragment
mobilization it is necessary for the traction force to exceed the frictional resistance
(Sugden and John, 1976). For the erosive action to be more effective it is also necessary to
evacuate the fragments contained in the substrate, generated by the previously indicated
processes. In this way the exposed rock can undergo glacial erosion mechanisms. It also
should be considered that a part of the debris export is made by subglacier melt waters
(Hallet, 1979; Drewry, 1986); this circumstance is significant in the case of temperate
glaciers and negligible in polar glaciers. Another form of bottom material mobilization in
temperate glaciers is by pressure of the ice mass on small size fragments and particles,
soaked in water and located at the rock-ice contact, flowing towards areas of lower
pressure.
2. Landform modelling by glacial erosion
The consequence of the action of glacial erosion processes is the generation of different
forms due to the action of ice masses. These are reflected in the classification of glacial
erosion forms of Table 3.1 (Sugden and John, 1976). It is difficult to carry out a
classification of this type, in which a certain dose of subjectivity becomes necessary. In
this classification, the three current process types are considered by distinguishing
unconfined areal ice flow, linear flow in rocky channels and a third process differentiation
corresponding to the interaction of glacial and periglacial activities. Another variable used
for the differentiation is based upon the height or depressed position of the resulting form.
Also as a classification norm the aligned or partially aligned morphology of the generated
form is used. All these criteria are located on the ordinate axis, and on the abscissa a
logarithmic size scale is placed.
As we will see, some of the forms are of doubtful interpretation, because strong
discrepancies exist if these have been generated by erosive action of ice or by subglacial
waters, or even by the combined action of specific processes of each one of these means,
intimately connected. It is therefore difficult in many cases to carry out a clear distinction
between glacial and fluvioglacial erosion.
Climatic Geomorphology
profiles (Bakker, 1965; Thomas, 1994), is the presence of regolith of variable thickness
generated prior to the freezings. The existence of some regions of this material that are
easily eroded by the ice must be very carefully considered in the interpretation of glacial
erosion modelling and in the quantification of erosion by the ice. We consider that this
circumstance must be taken into account in the study of glacial sculpting in the Iberian
Central System, because the presence of thick weathering deposits is common in this area,
generated without a doubt in times prior to the Quaternary (Guti6rrez and Rodr/guez,
1978; Molina and Blanco, 1980).
In any case, fracturing of the rocky substratum, whether inherited or of glacial origin, is
a factor of extraordinary importance in glacial erosion, along with the availability of
preglacial loose material.
1.1.3. Evacuation of debris
When a glacier slides on its bed, the mobilization speed of particles depends
fundamentally on their size and forms and on the substrate roughness. For fragment
mobilization it is necessary for the traction force to exceed the frictional resistance
(Sugden and John, 1976). For the erosive action to be more effective it is also necessary to
evacuate the fragments contained in the substrate, generated by the previously indicated
processes. In this way the exposed rock can undergo glacial erosion mechanisms. It also
should be considered that a part of the debris export is made by subglacier melt waters
(Hallet, 1979; Drewry, 1986); this circumstance is significant in the case of temperate
glaciers and negligible in polar glaciers. Another form of bottom material mobilization in
temperate glaciers is by pressure of the ice mass on small size fragments and particles,
soaked in water and located at the rock-ice contact, flowing towards areas of lower
pressure.
2. Landform modelling by glacial erosion
The consequence of the action of glacial erosion processes is the generation of different
forms due to the action of ice masses. These are reflected in the classification of glacial
erosion forms of Table 3.1 (Sugden and John, 1976). It is difficult to carry out a
classification of this type, in which a certain dose of subjectivity becomes necessary. In
this classification, the three current process types are considered by distinguishing
unconfined areal ice flow, linear flow in rocky channels and a third process differentiation
corresponding to the interaction of glacial and periglacial activities. Another variable used
for the differentiation is based upon the height or depressed position of the resulting form.
Also as a classification norm the aligned or partially aligned morphology of the generated
form is used. All these criteria are located on the ordinate axis, and on the abscissa a
logarithmic size scale is placed.
As we will see, some of the forms are of doubtful interpretation, because strong
discrepancies exist if these have been generated by erosive action of ice or by subglacial
waters, or even by the combined action of specific processes of each one of these means,
intimately connected. It is therefore difficult in many cases to carry out a clear distinction
between glacial and fluvioglacial erosion.