Climatic geomorphology
25
(selva, savannah, maritime and boreal regions of Peltier). In these latter zones most of
the landscapes may be considered of doubtful designation (Stoddart, 1969b). Another
fact analysed by Twidale and Lageat (1994) is related to the climatically triggered
processes and to the shared mechanisms operating in the different climatic regions. In
this sense, landforms linked to fluvial erosion and sedimentation can be observed all over
the climatic regions (gulling, gorges, meanders, alluvial fans, braided channels, etc.).
Tafonis, gnammas and weathering pits can be recognized in the wide variety of climatic
zones, and patterned soils (grounds) in periglacial and arid zones represent a problem of
landform convergence or "equifinality" throughout different climatic environments
(Birot, 1955). In these cases, the activity of different processes, and with different
intensity, eventually give rise to similar morphologies. Consequently, these authors
admitted that the impact of climate has been largely overvalued, but did not dispute that
it is a key influence. Also they assessed critically the establishment of a humid tropical
region on the basis of the abundance of mass movement and etched surfaces, because
these morphologic features also develop in other climatic zones.
4. The zonal concept in climatic geomorphology
From a planetary perspective, the variations occurring at the lithosphere-atmosphere
contact may be considered as zonal, and they form wide bands from the poles to the
equator derived from the different amounts of both precipitation and insolation
(Derbyshire, 1976a,b). The zonal concept has been previously used, in other branches
of the physical geography, such as climatology, pedology and biogeography. The subdivision of the continental zones into climatically featured edaphic regions was first
proposed by Dokuchayev (1883), who distinguished zonal, azonal and intrazonal soils.
The first ones are well developed soils that differ according to the ecological-climatic area
in which they were formed. The azonal soils are poorly developed, with many features
close to those present in the host rock. Finally, the intrazonal soils display good
development, but they are not well drained or are affected by salinity, and so forth.
The work of Troll (1944) on periglacial micro-landforms is considered to be an
introduction of the zonation concept in climatic geomorphology. The term zonal applies
to those processes or events in which geographical distribution occurs in latitudinal
bands parallel to the Equator, such as the equatorial forests, coral reefs and inland ice
caps. Each one of these environments constitutes a morphoclimatic area (Tricart and
Cailleux, 1965). The term azonal refers to those processes with a world-wide geographic
distribution such as fluvial, aeolian and coastal environments. Finally, the term extrazonal is used to denote some processes that, even though characteristic of a certain
environment, can also locally occur in other areas, as in the case of the periglacial and
glacial processes and landforms occurring in low latitude mountain systems.
The mountain areas display a characteristic climatic zonation or altitudinal gradient.
In large mountains located at low latitudes we can find, in a transect from the bottom to
the summit, the following climatic sequence: temperate, periglacial and, lastly, glacial
environments. In each altitudinal step the landscape is controlled by a specific morphoclimatic zone (Biidel, 1948). This climatic altitudinal gradient is due to the variations
of temperature and precipitation with elevation. Temperature diminishes gradually with
25
(selva, savannah, maritime and boreal regions of Peltier). In these latter zones most of
the landscapes may be considered of doubtful designation (Stoddart, 1969b). Another
fact analysed by Twidale and Lageat (1994) is related to the climatically triggered
processes and to the shared mechanisms operating in the different climatic regions. In
this sense, landforms linked to fluvial erosion and sedimentation can be observed all over
the climatic regions (gulling, gorges, meanders, alluvial fans, braided channels, etc.).
Tafonis, gnammas and weathering pits can be recognized in the wide variety of climatic
zones, and patterned soils (grounds) in periglacial and arid zones represent a problem of
landform convergence or "equifinality" throughout different climatic environments
(Birot, 1955). In these cases, the activity of different processes, and with different
intensity, eventually give rise to similar morphologies. Consequently, these authors
admitted that the impact of climate has been largely overvalued, but did not dispute that
it is a key influence. Also they assessed critically the establishment of a humid tropical
region on the basis of the abundance of mass movement and etched surfaces, because
these morphologic features also develop in other climatic zones.
4. The zonal concept in climatic geomorphology
From a planetary perspective, the variations occurring at the lithosphere-atmosphere
contact may be considered as zonal, and they form wide bands from the poles to the
equator derived from the different amounts of both precipitation and insolation
(Derbyshire, 1976a,b). The zonal concept has been previously used, in other branches
of the physical geography, such as climatology, pedology and biogeography. The subdivision of the continental zones into climatically featured edaphic regions was first
proposed by Dokuchayev (1883), who distinguished zonal, azonal and intrazonal soils.
The first ones are well developed soils that differ according to the ecological-climatic area
in which they were formed. The azonal soils are poorly developed, with many features
close to those present in the host rock. Finally, the intrazonal soils display good
development, but they are not well drained or are affected by salinity, and so forth.
The work of Troll (1944) on periglacial micro-landforms is considered to be an
introduction of the zonation concept in climatic geomorphology. The term zonal applies
to those processes or events in which geographical distribution occurs in latitudinal
bands parallel to the Equator, such as the equatorial forests, coral reefs and inland ice
caps. Each one of these environments constitutes a morphoclimatic area (Tricart and
Cailleux, 1965). The term azonal refers to those processes with a world-wide geographic
distribution such as fluvial, aeolian and coastal environments. Finally, the term extrazonal is used to denote some processes that, even though characteristic of a certain
environment, can also locally occur in other areas, as in the case of the periglacial and
glacial processes and landforms occurring in low latitude mountain systems.
The mountain areas display a characteristic climatic zonation or altitudinal gradient.
In large mountains located at low latitudes we can find, in a transect from the bottom to
the summit, the following climatic sequence: temperate, periglacial and, lastly, glacial
environments. In each altitudinal step the landscape is controlled by a specific morphoclimatic zone (Biidel, 1948). This climatic altitudinal gradient is due to the variations
of temperature and precipitation with elevation. Temperature diminishes gradually with