636
Climatic Geomorphology
Pleistocene the wind action was more significant at intermediate latitudes than at high
ones, because large gradients existed at the boundaries of the ice sheets and vast plains
of till and sandar, which provide fine sediments, were developed. Finally, the fluvial
activity was more intense in intermediate latitudes due to a higher number of melting days.
In short, periglacial environments of intermediate latitudes during the Pleistocene period
were fundamentally different from the continental high latitudes, as pointed out above.
Therefore, the principal problem is to find precise indicators of past periglacial
conditions. Perhaps the most realistic and best trace is the presence of wedges of ice and
sand, which indicate permafrost. Landforms inherited from the thermokarst of the pingos
can be also considered as indicators of permafrost, as well as fossil rock glaciers. The
rest of the periglacial paleoforms are uncertain indicators of periglacial conditions
(French, 1996).
7. Relict periglacial landforms
French (1996) differentiated between the features related to the development of ancient
frozen soils in intermediate latitudes and those related to the existence of landforms
originated by the action of the frost. The identification of the latter depends on the
interpretation, whereas the features denoting ancient permafrost cannot be doubted.
Fossil wedges are relatively common and constitute the most important features for
recognizing ancient permafrost. They can constitute ice wedge casts filled by sediments
(Figure 7.4), which commonly form polygonal systems and sand wedges (Figure 7.12).
It is estimated that winter temperatures on the surface of active ice wedges vary between
- 15 and -20~ (P6w6, 1983). For this author, the mean annual temperature of the air
to produce ice wedge ranges from -6 to -8~
In central and western Europe they are
abundant in a wide band and the width and depth increase toward the south of the ice sheet
margin. This is understood as a intensification of the periglacial conditions, which seem to
be more severe (Poser, 1948). In North America wedge casts are located in the proximity
of the Laurentide ice sheet in a relatively narrow band, ranging in their age from 15,000 to
20,000 yr BP, according to several authors.
Ancient pingos can be recognized by a ramp, generally ring-shaped, that includes an
interior depression. This ramp is the difference from other depressions generated by the
thermokarst, as with the palsas. The height of the ramps depends on the size of the initial
pingo and varies between 0.5 and 5 m. The diameter of the depression ranges from
200 to 300 m. Remains of pingos occur mainly in western Europe and North America.
A classic example is the one studied by Pissart (1965) in the Ardennes of Belgium
(Figure 24.17). The irregular form and the interference among pingos show a complex
distribution, which is interpreted as pingos originating within an open system. The
date of most pingos in Europe and North America is very late glacial, approximately
10,000 to 12,000 yr B P.
Fossil rock glaciers, which can be located below the limit of present-day arboreous
vegetation, indicate a periglacial climate with permafrost in their origin. This reveals
the importance they have as paleoclimate indicators (Barsch, 1988).
Relict morphologies generated by frost are numerous, but only a few provide
unequivocal evidences of periglacial conditions. At intermediate latitudes commonly
Climatic Geomorphology
Pleistocene the wind action was more significant at intermediate latitudes than at high
ones, because large gradients existed at the boundaries of the ice sheets and vast plains
of till and sandar, which provide fine sediments, were developed. Finally, the fluvial
activity was more intense in intermediate latitudes due to a higher number of melting days.
In short, periglacial environments of intermediate latitudes during the Pleistocene period
were fundamentally different from the continental high latitudes, as pointed out above.
Therefore, the principal problem is to find precise indicators of past periglacial
conditions. Perhaps the most realistic and best trace is the presence of wedges of ice and
sand, which indicate permafrost. Landforms inherited from the thermokarst of the pingos
can be also considered as indicators of permafrost, as well as fossil rock glaciers. The
rest of the periglacial paleoforms are uncertain indicators of periglacial conditions
(French, 1996).
7. Relict periglacial landforms
French (1996) differentiated between the features related to the development of ancient
frozen soils in intermediate latitudes and those related to the existence of landforms
originated by the action of the frost. The identification of the latter depends on the
interpretation, whereas the features denoting ancient permafrost cannot be doubted.
Fossil wedges are relatively common and constitute the most important features for
recognizing ancient permafrost. They can constitute ice wedge casts filled by sediments
(Figure 7.4), which commonly form polygonal systems and sand wedges (Figure 7.12).
It is estimated that winter temperatures on the surface of active ice wedges vary between
- 15 and -20~ (P6w6, 1983). For this author, the mean annual temperature of the air
to produce ice wedge ranges from -6 to -8~
In central and western Europe they are
abundant in a wide band and the width and depth increase toward the south of the ice sheet
margin. This is understood as a intensification of the periglacial conditions, which seem to
be more severe (Poser, 1948). In North America wedge casts are located in the proximity
of the Laurentide ice sheet in a relatively narrow band, ranging in their age from 15,000 to
20,000 yr BP, according to several authors.
Ancient pingos can be recognized by a ramp, generally ring-shaped, that includes an
interior depression. This ramp is the difference from other depressions generated by the
thermokarst, as with the palsas. The height of the ramps depends on the size of the initial
pingo and varies between 0.5 and 5 m. The diameter of the depression ranges from
200 to 300 m. Remains of pingos occur mainly in western Europe and North America.
A classic example is the one studied by Pissart (1965) in the Ardennes of Belgium
(Figure 24.17). The irregular form and the interference among pingos show a complex
distribution, which is interpreted as pingos originating within an open system. The
date of most pingos in Europe and North America is very late glacial, approximately
10,000 to 12,000 yr B P.
Fossil rock glaciers, which can be located below the limit of present-day arboreous
vegetation, indicate a periglacial climate with permafrost in their origin. This reveals
the importance they have as paleoclimate indicators (Barsch, 1988).
Relict morphologies generated by frost are numerous, but only a few provide
unequivocal evidences of periglacial conditions. At intermediate latitudes commonly