Slope systems - Arid Zone Geomorphology - page 207

A first-order control of these processes is due to climate
and climate variability. The previous chapter discussed cli-
mate controls on the type and nature of runoff and erosion
processes in drylands. Since the work of Langbein and
Schumm (1958), semi-arid regions have been seen to be
the dominant region where such processes are important,
and more recent studies have shown that ceteris paribus
this importance holds (e.g. Kosmas et al ., 1997). In drier
conditions, less erosion seems to be a result of less cu-
mulative available energy in storm events. More humid
conditions seem to be limited by more complete vegeta-
tion cover. However, the ecogeomorphology, in particular
the distribution and pattern of vegetation, is the other ma-
jor first-order control of these processes in drylands. It is
not possible to consider climate and vegetation separately,
as they covary, certainly at spatial scales of more than a
few tens of kilometres (Wainwright, 2009b). The nature
and type of runoff and erosion processes are very closely
controlled by vegetation quantity and distribution.
In general terms, at short timescales the evolution of
slopes can be considered in relation to the effectiveness
of climatically driven processes as modified by the vege-
tation. The specific behaviour of a particular location will
be controlled by lithological variability and especially the
relative weathering and erosion rates at a point, producing
a first-order response in terms of whether the slopes are
regolith- or rock-dominated. These behaviours are further
controlled by boundary conditions of the responses of
neighbouring slope units. At longer timescales, geologi-
cal structure and context affect slope evolution and these
controls are further modified by climatic changes. Inas-
much as a number of drylands had significantly different
climates through the Pleistocene and earlier (see Chapters
3 and 4), it is also important to consider the inheritance of
past landscape-forming processes on slope forms. These
different controls are now considered in the context of two
types of slope system - badlands and rock slopes - which
are considered as stereotypical of drylands.

and Verstraten, 1982) and in other drylands in Africa (e.g.
Boardman et al ., 2003; Eriksson, Reuterswald and Chris-
tiansson, 2003; Feoli, Gallizia Vuerich and Woldu, 2002;
Achten et al. , 2008), Chile (Maerker et al ., 2008), In-
dia (Joshi, Tambe and Dhawade, 2009) and China (Liu
et al ., 1985). Badland topography has been considered to
be a 'model landscape', but detailed investigations gener-
ally demonstrate complex process-form relationships, in
some cases over small spatial scales (Figure 10.3, but note
that the spatial scales in the figure are caricatures to a cer-
tain extent). Valley-bottom and hillslope gullies, extensive
rilling, piping at a range of scales and mass movements
all accompany unconcentrated overland-flow and splash
erosion as important landscape-forming processes in bad-
lands.
Most badlands are to be found on unconsolidated or
weakly consolidated bedrock, particularly marls and re-
cent alluvium, which can erode rapidly. Caution is re-
quired, though, in interpreting them as areas of ongoing
areas of high erosion, as several studies have suggested
that some badlands - or at least large areas within them -
can be stable over millennia (Wise, Thornes and Gilman,
1982, but cf. Wainwright, 1994; Howard, 1997; Dıaz-
Hernandez and Julia, 2006). Evidence from badlands at
the humid end of the dryland spectrum (or even in fully
humid locations: Lam, 1977; Segerstrom, 1950; Hicks,
Gomez and Trustrum, 2000; Parkner et al ., 2006) sug-
gests that the lack of vegetation (e.g. on former industrial
sites in Wales: Bridges and Harding, 1971; Haigh, 1979;
and in the US: Schumm, 1956; and in unconsolidated
tephras following volcanic eruptions in Japan: Lin and
Oguichi, 1985) or the disturbance of vegetation (e.g. in
subhumid locations in the French Prealps: Ballais, 1996;
and following clearance phases on Easter Island: Mieth
and Bork, 2005) are important in providing triggering
conditions for badland formation. This interpretation is
consistent with the vegetation-erosion interactions con-
sidered conceptually by Thornes (1985). There has been
some suggestion that biological crusts and lichen (see
following chapter) can provide stabilization mechanisms
on some badland slopes (Alexander et al ., 1994; Loppi,
Boscagli and De Dominicis, 2004). Once incision has oc-
curred and slopes have become steep, it is less likely that
vegetation will be able to recolonise. For example, Bo-
chet, Garcıa-Fayos and Poesen (2009) considered plant
covers and species richness on slopes of different aspects
in eastern Spain (mean annual precipitation 373 mm) and
found that north-facing slopes above 63 ◦ were unable to
support vegetation. This threshold decreased to 50 and 46 ◦
for east- and west-facing slopes and was as low as 41 ◦ for
south-facing slopes. These different thresholds can lead

10.2 Badlands

Badlands are heavily dissected, strongly gullied land-
scapes, often with very sparse vegetation cover. The name
apparently derives from early French colonists in northern
America encountering terrain that was bad, or difficult, to
cross. A number of North American badlands have be-
come almost type localities - notably those in Dinosaur
Provincial Park, Alberta, Canada (e.g. Bryan, Campbell
and Yair, 1986) and the Henry Mountains (e.g. Gilbert,
1877; Howard, 2009). However, badlands are also com-