Chapter 15
The arid region piedmonts: glacis and alluvial fans
Piedmonts constitute the transition between the mountain areas, where erosion is the
predominant process, and low-lying areas where transport and sedimentation are the
dominant processes. Aridity controls weathering on the hillslopes and in this context being
the particle production from rock fragmentation, it is normally small. Nevertheless, within
mountain and basin deserts (Mabbutt, 1977), the climatic conditions are usually variable in
the uplands, but the contribution of rainfall or snow melting is important. In this way, as a
consequence of such external supply the piedmont zones will receive water flows with a
higher sediment load than those occurring in typical arid areas. The mobilized materials
within the mountain catchments are transported by confined streams, which emerge into
piedmont zones where the water flow is abruptly unconfined and then transported
sediments spread out radiating downslope. The unconfined water flow decreases its
velocity and transport capacity abruptly, giving rise to progressive sedimentation. On the
other hand, in the backing hillslopes the development of erosive-accumulative ramps is
typical, which constitute another characteristic landform of arid zone mountain fronts.
The landform assemblage generated within the piedmonts, therefore, can hold either an
erosive-nature pediment (glacis) or a depositional one (alluvial fans). Pediments and
alluvial fans can be intimately related, and sometimes may be difficult to differentiate
(Figure 15.1). Bull (1977) suggested that fans can be distinguished from pediments when
the thickness of the associated deposits is up to 1/100 times the axial length of the
depositional area. In addition, a fan can be levelled in response to an external base-level fall,
giving place to pediment-like surfaces, which in turn can be buried by renewed alluvial-fan
deposition. On the other hand, pediments tend to develop in low-relief areas (low and
narrow ranges and modest escarpments), whereas alluvial fans are usually linked to more
energetic topographic configurations (mountain fronts, valley junctions, and so forth).
1. Glacis and pediments: definition, terminology and geographic distribution
According to Ldpez-Bermfidez (1973) who carried out a good updated revision on glacis;
this term (which means the gentle sloping ramp of the ancient Roman fortifications) was
first used by Dresch (1938), to designate specific landforms with a gently concave
longitudinal profile and with slopes down to 6 ~ Tricart (1969) considered that glacis
constitute vast piedmont plains with a low gradient, and coated by a thin sedimentary
veneer, which eventually connected with the desert axial fluvial systems constituted by the
ephemeral stream channels of ouadis (wadis) (Figure 15.2). Important terminological
problems arise with the progressive incorporation of this term into different languages,
The arid region piedmonts: glacis and alluvial fans
Piedmonts constitute the transition between the mountain areas, where erosion is the
predominant process, and low-lying areas where transport and sedimentation are the
dominant processes. Aridity controls weathering on the hillslopes and in this context being
the particle production from rock fragmentation, it is normally small. Nevertheless, within
mountain and basin deserts (Mabbutt, 1977), the climatic conditions are usually variable in
the uplands, but the contribution of rainfall or snow melting is important. In this way, as a
consequence of such external supply the piedmont zones will receive water flows with a
higher sediment load than those occurring in typical arid areas. The mobilized materials
within the mountain catchments are transported by confined streams, which emerge into
piedmont zones where the water flow is abruptly unconfined and then transported
sediments spread out radiating downslope. The unconfined water flow decreases its
velocity and transport capacity abruptly, giving rise to progressive sedimentation. On the
other hand, in the backing hillslopes the development of erosive-accumulative ramps is
typical, which constitute another characteristic landform of arid zone mountain fronts.
The landform assemblage generated within the piedmonts, therefore, can hold either an
erosive-nature pediment (glacis) or a depositional one (alluvial fans). Pediments and
alluvial fans can be intimately related, and sometimes may be difficult to differentiate
(Figure 15.1). Bull (1977) suggested that fans can be distinguished from pediments when
the thickness of the associated deposits is up to 1/100 times the axial length of the
depositional area. In addition, a fan can be levelled in response to an external base-level fall,
giving place to pediment-like surfaces, which in turn can be buried by renewed alluvial-fan
deposition. On the other hand, pediments tend to develop in low-relief areas (low and
narrow ranges and modest escarpments), whereas alluvial fans are usually linked to more
energetic topographic configurations (mountain fronts, valley junctions, and so forth).
1. Glacis and pediments: definition, terminology and geographic distribution
According to Ldpez-Bermfidez (1973) who carried out a good updated revision on glacis;
this term (which means the gentle sloping ramp of the ancient Roman fortifications) was
first used by Dresch (1938), to designate specific landforms with a gently concave
longitudinal profile and with slopes down to 6 ~ Tricart (1969) considered that glacis
constitute vast piedmont plains with a low gradient, and coated by a thin sedimentary
veneer, which eventually connected with the desert axial fluvial systems constituted by the
ephemeral stream channels of ouadis (wadis) (Figure 15.2). Important terminological
problems arise with the progressive incorporation of this term into different languages,