Weathering processes and resulting forms
255
Another significant aspect of the biological action is related to hydric and aeolian
erosion (Thomas, 1988). It is well known that the vegetation cover affects sediment
mobilization, but the existence of a superficial biocrust reduces considerably erosion.
Where the aeolian activity holds up, dunes can reach stabilization (Veste, 1995)
(Figure 11.13). As the infiltration capacity decreases rilling appears, even though erosion
is made difficult by the biocrust cementation (Figure 11.12).
3. Resultant weathering forms
Rock surfaces in deserts can be affected by differential weathering and this can give rise
to different types of microforms. These appear at the base of vertical walls forming caves
or shelters, in slopes in which develop alveoles and tafonis and in horizontal surfaces or of
low inclination in which are recognized gnammas or weathering pits. All these forms can
occur in the same rock outcropping and although they are very common in deserts they
could occur in other climates (Twidale and Corbin, 1963; Martini, 1978). They mainly
develop over igneous isogranular (granite, rhyolite) and sedimentary rocks (sandstones),
although they can occur in conglomerates, gneisses, and porphyries.
The water content is higher at the base of the rock outcrops, partly due to capillarity
ascent. Therefore, weathering is more important than in the upper zone, where the rock is
drier and practically without weathering (Mabbutt, 1977). These circumstances explain
the basal undercutting of the rock walls, which develops more easily in the south-facing
zones of some areas, generating caves or shelters (Howard and Selby, 1994). Where
undercutting affects isolated hills or inselbergs monoliths of a fungus-like form can be
produced. This morphology can also be generated by subsuperficial weathering and later
exhumation, with the spectacular flared slopes of the Eyre Peninsula, in south Australia
constituting an excellent example of this origin (Twidale, 1962).
On rocky walls of moderate or considerable steepness, rounded hollows can develop
that on occasion can completely cover the rock surface. Also they can be observed on the
detached blocks of the slope. If the size of these hollows is centimetric they are named
alveoles that together form the morphology of a honeycomb. Their origin is discussed and
is attributed to aeolian erosion, exfoliation, frost action, and salt weathering. They are
common in coastal desert environments (Mustoe, 1982).
When dimensions are bigger, decimetric and sometimes metric, they are called tafonis
(Figure 11.14). They usually appear in groups and have circular or elliptic sections with
the bottom covered by detritus that mobilizes with the wind, rain and runoff. They can
grow and join others and can also in their growth towards the inside connect with other
tafoni. Sometimes, like alveoles, they appear orientated following planes of weakness of
the rock, sometimes imperceptibly. Rock surfaces show exfoliation and inside the tafoni
saline efflorescences are common. Their origin is discussed. Some authors differ between
wall tafonis and basal tafonis, related to basal undercutting processes (Bradley et al., 1978;
Smith, 1978). It is in agreement that the dominant processes are those related to water with
salt movement close to the surface, which correspond to salt weathering and wetting and
drying cycles. Also they adduce the insolation weathering, frost action, dissolution of
carbonated cements, and aeolian erosion (Evans, 1969 to 1970; Mustoe, 1983; Young,
1987; Robinson and Williams, 1992). Besides these subaerial processes, weathering under
255
Another significant aspect of the biological action is related to hydric and aeolian
erosion (Thomas, 1988). It is well known that the vegetation cover affects sediment
mobilization, but the existence of a superficial biocrust reduces considerably erosion.
Where the aeolian activity holds up, dunes can reach stabilization (Veste, 1995)
(Figure 11.13). As the infiltration capacity decreases rilling appears, even though erosion
is made difficult by the biocrust cementation (Figure 11.12).
3. Resultant weathering forms
Rock surfaces in deserts can be affected by differential weathering and this can give rise
to different types of microforms. These appear at the base of vertical walls forming caves
or shelters, in slopes in which develop alveoles and tafonis and in horizontal surfaces or of
low inclination in which are recognized gnammas or weathering pits. All these forms can
occur in the same rock outcropping and although they are very common in deserts they
could occur in other climates (Twidale and Corbin, 1963; Martini, 1978). They mainly
develop over igneous isogranular (granite, rhyolite) and sedimentary rocks (sandstones),
although they can occur in conglomerates, gneisses, and porphyries.
The water content is higher at the base of the rock outcrops, partly due to capillarity
ascent. Therefore, weathering is more important than in the upper zone, where the rock is
drier and practically without weathering (Mabbutt, 1977). These circumstances explain
the basal undercutting of the rock walls, which develops more easily in the south-facing
zones of some areas, generating caves or shelters (Howard and Selby, 1994). Where
undercutting affects isolated hills or inselbergs monoliths of a fungus-like form can be
produced. This morphology can also be generated by subsuperficial weathering and later
exhumation, with the spectacular flared slopes of the Eyre Peninsula, in south Australia
constituting an excellent example of this origin (Twidale, 1962).
On rocky walls of moderate or considerable steepness, rounded hollows can develop
that on occasion can completely cover the rock surface. Also they can be observed on the
detached blocks of the slope. If the size of these hollows is centimetric they are named
alveoles that together form the morphology of a honeycomb. Their origin is discussed and
is attributed to aeolian erosion, exfoliation, frost action, and salt weathering. They are
common in coastal desert environments (Mustoe, 1982).
When dimensions are bigger, decimetric and sometimes metric, they are called tafonis
(Figure 11.14). They usually appear in groups and have circular or elliptic sections with
the bottom covered by detritus that mobilizes with the wind, rain and runoff. They can
grow and join others and can also in their growth towards the inside connect with other
tafoni. Sometimes, like alveoles, they appear orientated following planes of weakness of
the rock, sometimes imperceptibly. Rock surfaces show exfoliation and inside the tafoni
saline efflorescences are common. Their origin is discussed. Some authors differ between
wall tafonis and basal tafonis, related to basal undercutting processes (Bradley et al., 1978;
Smith, 1978). It is in agreement that the dominant processes are those related to water with
salt movement close to the surface, which correspond to salt weathering and wetting and
drying cycles. Also they adduce the insolation weathering, frost action, dissolution of
carbonated cements, and aeolian erosion (Evans, 1969 to 1970; Mustoe, 1983; Young,
1987; Robinson and Williams, 1992). Besides these subaerial processes, weathering under