350
Climatic Geomorphology
deficit of water (Tricart, 1969), however, the previous statement may be considered a
contradiction. Nevertheless, the typical desert rainfall events are rare high-magnitude
storm events during which energetic surface run-off processes (arroyada) are generated.
Overland flows can be unconfined generating sheet-floods, but eventually can be confined
in gullies.
In regions of tectonic stability, the pediment surface can be divided into a proximal
erosive area, an intermediate transport area, and a distal depositional area (Johnson,
1932b). During episodes of increasing sediment supply these different areas shift upward to
the proximal zone, and conversely when it diminishes, these different pediment segments
move downwards to distal areas (Cooke and Mason, 1973). The erosive and transport areas
are characterized by the occurrence of mobile shallow braided channels but also by sheetflow processes. In these two zones of the glacis, we can observe former pediment surfaces
slightly elevated over the functional ones. These are relict landforms in which the
dominant geomorphic processes are surface and subsurface weathering, eolian reworking,
development of desert pavements with varnish or special soils such as pedocals.
For some authors (mainly Australian ones), the subsurface weathering plays an
important role in pediment development, mainly on those generated on crystalline bedrock
(Ruxton, 1958; Mabbutt, 1966, 1977; Twidale, 1967, 1983). The upper regolith cover can
be easily removed by overland flows. This fact facilitates the exposure of the basal surface
of weathering, generating an etched plain or etch surface. This kind of process is common
in savannah areas, but they also occur in the semi-arid environment. The zones subjected
to deeper weathering are localized at the piedmont junction along the contact between
the bedrock and the regolith, which constitutes a linear weakness zone.
In softer bedrock the overland flows have higher erosive power. In the case of
calcareous rocks, planation is favoured and assisted by karstic corrosion (Nicod, 1992).
In the Iberian Range of Central Spain, mainly constituted of calcareous rocks, pediment
surfaces have an excellent development, giving rise to extensive pediplains.
The study of micro-topographic changes, carried out in small experimental parcels by
Schumm (1962), provides very valuable data about the variations of the micro-pediments
over an 8 year time-series. They were monitored with erosion pins in the gentle hillslopes
developed on the siltstones of the Badland National Monument, South Dakota. The micropediment surfaces underwent a progressive lowering that increased with their slope
gradient, and the pediment junction experienced parallel retreat. On the other hand, in some
of the cases, active deposition took place in the distal areas (Figure 15.8). In that paper,
Schumm pointed out that the formation of such miniature pediments did not have to be
necessarily similar to the real case. Chorley et al. (1984), however, indicated that this
process may occur in many places at larger scales, suggesting that the analysis of badlands
can improve our understanding about the development of some type of pediments.
1.4. Hypothesis on the origin of pediments
Pediment surfaces occur preferentially in zones of low-relief mountain catchments.
When the catchments are more modest, the bigger will be the surface area occupied by
pediments. The main proposed hypothesis for pediment origin is based on the explanation
of those processes governing slope degradation and piedmont generation. The different
Climatic Geomorphology
deficit of water (Tricart, 1969), however, the previous statement may be considered a
contradiction. Nevertheless, the typical desert rainfall events are rare high-magnitude
storm events during which energetic surface run-off processes (arroyada) are generated.
Overland flows can be unconfined generating sheet-floods, but eventually can be confined
in gullies.
In regions of tectonic stability, the pediment surface can be divided into a proximal
erosive area, an intermediate transport area, and a distal depositional area (Johnson,
1932b). During episodes of increasing sediment supply these different areas shift upward to
the proximal zone, and conversely when it diminishes, these different pediment segments
move downwards to distal areas (Cooke and Mason, 1973). The erosive and transport areas
are characterized by the occurrence of mobile shallow braided channels but also by sheetflow processes. In these two zones of the glacis, we can observe former pediment surfaces
slightly elevated over the functional ones. These are relict landforms in which the
dominant geomorphic processes are surface and subsurface weathering, eolian reworking,
development of desert pavements with varnish or special soils such as pedocals.
For some authors (mainly Australian ones), the subsurface weathering plays an
important role in pediment development, mainly on those generated on crystalline bedrock
(Ruxton, 1958; Mabbutt, 1966, 1977; Twidale, 1967, 1983). The upper regolith cover can
be easily removed by overland flows. This fact facilitates the exposure of the basal surface
of weathering, generating an etched plain or etch surface. This kind of process is common
in savannah areas, but they also occur in the semi-arid environment. The zones subjected
to deeper weathering are localized at the piedmont junction along the contact between
the bedrock and the regolith, which constitutes a linear weakness zone.
In softer bedrock the overland flows have higher erosive power. In the case of
calcareous rocks, planation is favoured and assisted by karstic corrosion (Nicod, 1992).
In the Iberian Range of Central Spain, mainly constituted of calcareous rocks, pediment
surfaces have an excellent development, giving rise to extensive pediplains.
The study of micro-topographic changes, carried out in small experimental parcels by
Schumm (1962), provides very valuable data about the variations of the micro-pediments
over an 8 year time-series. They were monitored with erosion pins in the gentle hillslopes
developed on the siltstones of the Badland National Monument, South Dakota. The micropediment surfaces underwent a progressive lowering that increased with their slope
gradient, and the pediment junction experienced parallel retreat. On the other hand, in some
of the cases, active deposition took place in the distal areas (Figure 15.8). In that paper,
Schumm pointed out that the formation of such miniature pediments did not have to be
necessarily similar to the real case. Chorley et al. (1984), however, indicated that this
process may occur in many places at larger scales, suggesting that the analysis of badlands
can improve our understanding about the development of some type of pediments.
1.4. Hypothesis on the origin of pediments
Pediment surfaces occur preferentially in zones of low-relief mountain catchments.
When the catchments are more modest, the bigger will be the surface area occupied by
pediments. The main proposed hypothesis for pediment origin is based on the explanation
of those processes governing slope degradation and piedmont generation. The different