The arid region piedmonts: glacis and alluvial fans
349
gradually diminish by infiltration. Most typical pediment channels have a braided pattern.
They are shallow channels subject to significant switching during extreme floods. During
periods of base-level fall, channel entrenchment can also propagate to distal locations
giving rise to subparallel drainage patterns (Cooke and Warren, 1973). In most cases,
pediments are planation surfaces directly carved in the bedrock, but mantled by a thin and
discontinuous alluvial cover. In particular cases some small relief emerges from the glacis
surface as residual landforms denoted by the French authors as "chicots" (Figure 15.7).
1.2. The role of geology and climate in pediment development
As aforementioned, some French authors differentiate between glacis developed on soft or
in hard rocks. This division attempts to emphasize the great influence exerted by lithology
and geological structure on glacis location and evolution. When pediments develop on
granites, or crystalline bedrock, they usually present a sharp knick-point attributed by
some Australian authors (Mabbutt, 1966; Twidale, 1967) to the occurrence of a more
intense weathering in the piedmont junction zone. As chemical weathering processes are
very limited in arid zones by water availability, the occurrence of sharp knick-points is
attributed to previous humid periods occurring during Quaternary, or even Tertiary times.
In the south of Arizona, where schists and basalts are the dominant bedrock lithology,
knick-point angles are significant (Kirkby and Kirkby, 1974). On the contrary, in more
erodible substrates constituted by several types of weak rocks, the transition between the
backing hillslope and the pediment surface is gradual, but marked by a large concave
knick-zone (Dohrenwend, 1994; Oberlander, 1997b). This latter is the case of the mantled
pediments developed in the Ebro Depression of Spain and throughout the Mediterranean
littoral.
The recent tectonic history of a region (i.e. neotectonic activity) exerts a primary
influence on pediment development. Tectonically stable areas are appropriate zones for
the initiation and development of glacis, which in addition are characterized by sinuous
mountain fronts. On the other hand, where tectonic activity is relevant, mountain fronts are
linear and alluvial-fan development prevails (Bull, 1977; Bull and McFadden, 1977). All
the published observations indicate that tectonic quiescence (crustal stability) is a very
important factor for pediment development.
In arid zones, pediment development is influenced by water availability for surface runoff. In hyperarid areas (i.e. Atacama, Central Sahara) the deficient rainfall gives rise to the
nearly absent overland flow erosion (erosion paralysis of Oberlander, 1997b), and therefore
the existing pediments should derive from previous wet (pluvial) periods. Climatic changes
also affect the size and extent of the vegetation cover on the pediment surface, and
consequently to the relative interaction of the working erosion and depositional processes.
The role of climatic changes and their impact in pediment development is analysed in detail
in Section 15.1.5 (pediment evolution).
1.3. Dominant processes in pediment development
The processes involved in the generation of overland flows are may be the more important
ones in desert areas. Because the arid regions are specifically defined by a permanent
349
gradually diminish by infiltration. Most typical pediment channels have a braided pattern.
They are shallow channels subject to significant switching during extreme floods. During
periods of base-level fall, channel entrenchment can also propagate to distal locations
giving rise to subparallel drainage patterns (Cooke and Warren, 1973). In most cases,
pediments are planation surfaces directly carved in the bedrock, but mantled by a thin and
discontinuous alluvial cover. In particular cases some small relief emerges from the glacis
surface as residual landforms denoted by the French authors as "chicots" (Figure 15.7).
1.2. The role of geology and climate in pediment development
As aforementioned, some French authors differentiate between glacis developed on soft or
in hard rocks. This division attempts to emphasize the great influence exerted by lithology
and geological structure on glacis location and evolution. When pediments develop on
granites, or crystalline bedrock, they usually present a sharp knick-point attributed by
some Australian authors (Mabbutt, 1966; Twidale, 1967) to the occurrence of a more
intense weathering in the piedmont junction zone. As chemical weathering processes are
very limited in arid zones by water availability, the occurrence of sharp knick-points is
attributed to previous humid periods occurring during Quaternary, or even Tertiary times.
In the south of Arizona, where schists and basalts are the dominant bedrock lithology,
knick-point angles are significant (Kirkby and Kirkby, 1974). On the contrary, in more
erodible substrates constituted by several types of weak rocks, the transition between the
backing hillslope and the pediment surface is gradual, but marked by a large concave
knick-zone (Dohrenwend, 1994; Oberlander, 1997b). This latter is the case of the mantled
pediments developed in the Ebro Depression of Spain and throughout the Mediterranean
littoral.
The recent tectonic history of a region (i.e. neotectonic activity) exerts a primary
influence on pediment development. Tectonically stable areas are appropriate zones for
the initiation and development of glacis, which in addition are characterized by sinuous
mountain fronts. On the other hand, where tectonic activity is relevant, mountain fronts are
linear and alluvial-fan development prevails (Bull, 1977; Bull and McFadden, 1977). All
the published observations indicate that tectonic quiescence (crustal stability) is a very
important factor for pediment development.
In arid zones, pediment development is influenced by water availability for surface runoff. In hyperarid areas (i.e. Atacama, Central Sahara) the deficient rainfall gives rise to the
nearly absent overland flow erosion (erosion paralysis of Oberlander, 1997b), and therefore
the existing pediments should derive from previous wet (pluvial) periods. Climatic changes
also affect the size and extent of the vegetation cover on the pediment surface, and
consequently to the relative interaction of the working erosion and depositional processes.
The role of climatic changes and their impact in pediment development is analysed in detail
in Section 15.1.5 (pediment evolution).
1.3. Dominant processes in pediment development
The processes involved in the generation of overland flows are may be the more important
ones in desert areas. Because the arid regions are specifically defined by a permanent