The arid region piedmonts: glacis and alluvial fans
345
which give rise to an extensive terminological and conceptual misunderstanding (Tator,
1952, 1953; Dumas, 1967; Tricart, 1969; P6csi and Sailfird, 1970; Cooke and Warren,
1973; Whitaker, 1973, 1979; Zuidam, 1976; Demangeot, 1981; Cooke et al., 1993). The
English writers usually use the word pediment, which was first used by Gilbert (1877), and
latter defined by Mabbutt (1977), as a flat piedmont surface cut in the bedrock and
separated from the backing hillslope by an abrupt knick-point or piedmont junction
(Figure 15.1). Where a thin veneer of debris coats the erosive surface of the pediment, it is
called mantled pediment. In the German literature the equivalent term is the so-called
fussfliiche (Mensching, 1958).
French authors make an extensive use of the term glacis, but either for clarification or
descriptive purposes, they label it with different adjectives generating a large list of terms
(Tricart, 1969). L6pez-Bermfidez (1973) compiled 28 different denotations, but Whitaker
(1973) managed a larger list of terms. Those glacis directly carved in the bedrock without
associated deposits (equivalent to pediments) have been denoted as bare glacis (Birot
and Dresch, 1966), ablation glacis (Dresch, 1970), erosion glacis (Dresch, 1938), or
denudation glacis (Tricart, 1969; Tricart et al., 1972). Different terms are also used
depending on the thickness of the associated detrital deposits. In consequence, Tricart
(1969) discriminated among denudation glacis (glacis de denudation) with a thin and
discontinuous sedimentary cover, sheet-flood glacis (glacis d'dpandage) with a
continuous and apparent (1 to 3 m thick) veneer of sediments and finally accumulation
glacis (glacis d'ennoyage) where the sedimentary cover is over 5 to 10 m thick. This
terminological overabundance joins with a significant conceptual confusion. For some
French authors the erosion glacis are only developed in soft rocks, like some limestones
and marls; Dresch (1957), for example, set aside the term "pediment" to characterize only
erosion surfaces in crystalline bedrock (Birot and Dresch, 1966). Birot (1960) considered
the definition of glacis as the main problem of arid zone geomorphology. Therefore, here
we propose that, to avoid any kind of terminological confusion, the term pediment should
be used for pure erosive surfaces carved in the bedrock, and mantled or covered pediment
for erosive surfaces fossilized by a thin veneer of alluvial sediments.
Pediments develop in a wide variety of climatic, lithologic, and tectonic contexts
(Dohrenwend, 1994). They occur from subpolar latitudes to arid or humid-tropical climates
(Whitaker, 1973, 1979) and therefore they can be considered plurizonal landforms
(Figure 15.4). The dictionary of Whitaker (1973) even quotes the term cryopediments in
relation to nivation processes occurring in periglacial areas. Pediments, however, develop
extensively throughout arid and semi-arid regions where they are characteristic landforms
(Figure 15.2). In arid zones, vegetation is scarce, thus reducing the impediments to the
erosive work of overland flows that generate the pediments. We understand that pediment
is synonymous for erosion glacis, as proposed by Dresch (1957). In consequence, we think
that most of the previously- listed terms are usually referred to arid and semi-arid climatic
conditions, or they can constitute ancient arid palaeo-landforms.
1.1. Pediment morphology
Pediments are commonly fed by small mountain catchments with limited relief. They can
emerge from mountain fronts, which constitute the junction between the catchment areas
345
which give rise to an extensive terminological and conceptual misunderstanding (Tator,
1952, 1953; Dumas, 1967; Tricart, 1969; P6csi and Sailfird, 1970; Cooke and Warren,
1973; Whitaker, 1973, 1979; Zuidam, 1976; Demangeot, 1981; Cooke et al., 1993). The
English writers usually use the word pediment, which was first used by Gilbert (1877), and
latter defined by Mabbutt (1977), as a flat piedmont surface cut in the bedrock and
separated from the backing hillslope by an abrupt knick-point or piedmont junction
(Figure 15.1). Where a thin veneer of debris coats the erosive surface of the pediment, it is
called mantled pediment. In the German literature the equivalent term is the so-called
fussfliiche (Mensching, 1958).
French authors make an extensive use of the term glacis, but either for clarification or
descriptive purposes, they label it with different adjectives generating a large list of terms
(Tricart, 1969). L6pez-Bermfidez (1973) compiled 28 different denotations, but Whitaker
(1973) managed a larger list of terms. Those glacis directly carved in the bedrock without
associated deposits (equivalent to pediments) have been denoted as bare glacis (Birot
and Dresch, 1966), ablation glacis (Dresch, 1970), erosion glacis (Dresch, 1938), or
denudation glacis (Tricart, 1969; Tricart et al., 1972). Different terms are also used
depending on the thickness of the associated detrital deposits. In consequence, Tricart
(1969) discriminated among denudation glacis (glacis de denudation) with a thin and
discontinuous sedimentary cover, sheet-flood glacis (glacis d'dpandage) with a
continuous and apparent (1 to 3 m thick) veneer of sediments and finally accumulation
glacis (glacis d'ennoyage) where the sedimentary cover is over 5 to 10 m thick. This
terminological overabundance joins with a significant conceptual confusion. For some
French authors the erosion glacis are only developed in soft rocks, like some limestones
and marls; Dresch (1957), for example, set aside the term "pediment" to characterize only
erosion surfaces in crystalline bedrock (Birot and Dresch, 1966). Birot (1960) considered
the definition of glacis as the main problem of arid zone geomorphology. Therefore, here
we propose that, to avoid any kind of terminological confusion, the term pediment should
be used for pure erosive surfaces carved in the bedrock, and mantled or covered pediment
for erosive surfaces fossilized by a thin veneer of alluvial sediments.
Pediments develop in a wide variety of climatic, lithologic, and tectonic contexts
(Dohrenwend, 1994). They occur from subpolar latitudes to arid or humid-tropical climates
(Whitaker, 1973, 1979) and therefore they can be considered plurizonal landforms
(Figure 15.4). The dictionary of Whitaker (1973) even quotes the term cryopediments in
relation to nivation processes occurring in periglacial areas. Pediments, however, develop
extensively throughout arid and semi-arid regions where they are characteristic landforms
(Figure 15.2). In arid zones, vegetation is scarce, thus reducing the impediments to the
erosive work of overland flows that generate the pediments. We understand that pediment
is synonymous for erosion glacis, as proposed by Dresch (1957). In consequence, we think
that most of the previously- listed terms are usually referred to arid and semi-arid climatic
conditions, or they can constitute ancient arid palaeo-landforms.
1.1. Pediment morphology
Pediments are commonly fed by small mountain catchments with limited relief. They can
emerge from mountain fronts, which constitute the junction between the catchment areas