170
Climatic Geomorphology
The combined effect of the eolian sedimentation of snow and mineral particles generate
nivo-eolian deposits (Koster and Dijkmans, 1988). As in ice masses, its differential
melting produces small hillocks and depressions that may disappear from one year to
another. The most prominent depositional features, however, correspond to sand dunes
and especially to unstratified, wind-blown accumulations of silt particles called loess
(P~w6, 1955; P~w~ et al., 1995) (Figure 7.22). These are supplied by dust clouds and may
be deposited at great distances from their source. Loesses are massive and very well-sorted
deposits with 50 to 60% of the particles between 0.01 and 0.05 mm (silt) and the rest of
sand and clay size. They generally have a gray colour that turns into a yellowish tone with
weathering. Gelifluction structures, cryoturbations and ice-wedge casts may be common.
During the Pleistocene, extensive loess accumulations were formed around the southern
margins of the North American and Eurasian ice sheets. There is, however, also loess
generated in nonperiglacial environments such as those occurring in tropical and
midlatitude desert areas. The dominant wind and atmospheric conditions (isobars) of past
periods can be inferred with paleocurrents obtained from the internal structures of old
eolian deposits (Poser, 1950) (Figure 7.23).
Climatic Geomorphology
The combined effect of the eolian sedimentation of snow and mineral particles generate
nivo-eolian deposits (Koster and Dijkmans, 1988). As in ice masses, its differential
melting produces small hillocks and depressions that may disappear from one year to
another. The most prominent depositional features, however, correspond to sand dunes
and especially to unstratified, wind-blown accumulations of silt particles called loess
(P~w6, 1955; P~w~ et al., 1995) (Figure 7.22). These are supplied by dust clouds and may
be deposited at great distances from their source. Loesses are massive and very well-sorted
deposits with 50 to 60% of the particles between 0.01 and 0.05 mm (silt) and the rest of
sand and clay size. They generally have a gray colour that turns into a yellowish tone with
weathering. Gelifluction structures, cryoturbations and ice-wedge casts may be common.
During the Pleistocene, extensive loess accumulations were formed around the southern
margins of the North American and Eurasian ice sheets. There is, however, also loess
generated in nonperiglacial environments such as those occurring in tropical and
midlatitude desert areas. The dominant wind and atmospheric conditions (isobars) of past
periods can be inferred with paleocurrents obtained from the internal structures of old
eolian deposits (Poser, 1950) (Figure 7.23).