352
Climatic Geomorphology
disposition. Each individual channel has decimetric widths and depths. These little
channels can find obstacles in their trajectory, such as bushes and large blocks that have to
be bypassed. The braided pattern of these channels is the same one as those developed in
the big rivers, which implies that lateral planation occurs in response to the continuum
divergence of these small channels. The erosive work exerted by the lateral shifting of the
channels gradually removes all the small existent reliefs and, eventually it develops the
pediment. The radial systems of glacis developed at the foot of the inselbergs can be
explained in this way, otherwise it is difficult to understand from the sheet-flood
hypothesis alone (Tricart, 1969). Some authors consider that lateral planation is more
effective when it affects the less resistant rocks (Sharp, 1940; Denny, 1967), whereas in the
granites or crystalline rocks the denudation is smaller (Howard, 1942). Although the
activity of the diffuse overland flows is verified in field studies, this hypothesis should not
be considered as a general explanation (Mabbutt, 1977).
Twidale (1962) and Corbel (1963) considered that the processes of chemical
weathering have some impact on pediment development. In shield and platform deserts,
the denudation of ancient weathering profiles can generate etchplains that later are
transformed pediment in response to planation processes (Mabbutt, 1977). On the other
hand, weathering is especially intense at piedmont angle locations (Twidale, 1962, 1967,
1976a,b), which collects all the overland flows generated on the adjacent hillslopes. This
author relates the origin of both pediment and flared slopes with this kind of selective
weathering. During humid periods the weathering front progresses downslope,
while during the dry periods active denudation removes the previously weathered
material (Figure 15.9).
1.5. Pediment evolution. Pediplains
The overland flows running downslope throughout the surface of the glacis eventually
pour into endoreic depressions, seas, oceans or, more frequently to a desert stream. If the
initial base level of the pediment changes, then different geomorphological processes
reshape the primitive pediment surface. In the first case, the endoreic basins undergo a
continuous aggradation, in response to the erosion of the basin borders which give rise
to a progressive reduction of the pediment slope. When sea-level falls, just as happens in
glacial periods, headward erosion propagates upslope dissecting, separating and isolating
the previous pediment surfaces, as is evidenced in numerous examples on the Mediterranean fringe. On the other hand, when sea-level rises the area occupied by the pediments
decreases, being able to even be completely covered by the marine waters.
The most common cases of pediment evolution are those related to the wadis (desert
streams). The materials transported by the overland flows generate an erosion pediment in
the proximal areas, and accumulation ones in the mid and distal areas, where deposits left
by the desert streams may merge with the alluvial plain. In these zones of coalescence an
interfingering between both kinds of deposits occurs. Pediment deposits are generally
formed by angular clasts of generally the same composition, due to the shared sediment
source of the glacis channels, whereas the alluvial plain deposits are polymictic and
more rounded due to their longer longitudinal transport (Guti~rrez and Pefia, 1976). When
base-level falls in response to climatic or tectonic causes, the lateral alluvial systems incise
Climatic Geomorphology
disposition. Each individual channel has decimetric widths and depths. These little
channels can find obstacles in their trajectory, such as bushes and large blocks that have to
be bypassed. The braided pattern of these channels is the same one as those developed in
the big rivers, which implies that lateral planation occurs in response to the continuum
divergence of these small channels. The erosive work exerted by the lateral shifting of the
channels gradually removes all the small existent reliefs and, eventually it develops the
pediment. The radial systems of glacis developed at the foot of the inselbergs can be
explained in this way, otherwise it is difficult to understand from the sheet-flood
hypothesis alone (Tricart, 1969). Some authors consider that lateral planation is more
effective when it affects the less resistant rocks (Sharp, 1940; Denny, 1967), whereas in the
granites or crystalline rocks the denudation is smaller (Howard, 1942). Although the
activity of the diffuse overland flows is verified in field studies, this hypothesis should not
be considered as a general explanation (Mabbutt, 1977).
Twidale (1962) and Corbel (1963) considered that the processes of chemical
weathering have some impact on pediment development. In shield and platform deserts,
the denudation of ancient weathering profiles can generate etchplains that later are
transformed pediment in response to planation processes (Mabbutt, 1977). On the other
hand, weathering is especially intense at piedmont angle locations (Twidale, 1962, 1967,
1976a,b), which collects all the overland flows generated on the adjacent hillslopes. This
author relates the origin of both pediment and flared slopes with this kind of selective
weathering. During humid periods the weathering front progresses downslope,
while during the dry periods active denudation removes the previously weathered
material (Figure 15.9).
1.5. Pediment evolution. Pediplains
The overland flows running downslope throughout the surface of the glacis eventually
pour into endoreic depressions, seas, oceans or, more frequently to a desert stream. If the
initial base level of the pediment changes, then different geomorphological processes
reshape the primitive pediment surface. In the first case, the endoreic basins undergo a
continuous aggradation, in response to the erosion of the basin borders which give rise
to a progressive reduction of the pediment slope. When sea-level falls, just as happens in
glacial periods, headward erosion propagates upslope dissecting, separating and isolating
the previous pediment surfaces, as is evidenced in numerous examples on the Mediterranean fringe. On the other hand, when sea-level rises the area occupied by the pediments
decreases, being able to even be completely covered by the marine waters.
The most common cases of pediment evolution are those related to the wadis (desert
streams). The materials transported by the overland flows generate an erosion pediment in
the proximal areas, and accumulation ones in the mid and distal areas, where deposits left
by the desert streams may merge with the alluvial plain. In these zones of coalescence an
interfingering between both kinds of deposits occurs. Pediment deposits are generally
formed by angular clasts of generally the same composition, due to the shared sediment
source of the glacis channels, whereas the alluvial plain deposits are polymictic and
more rounded due to their longer longitudinal transport (Guti~rrez and Pefia, 1976). When
base-level falls in response to climatic or tectonic causes, the lateral alluvial systems incise