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of duricrusts (silcretes and calcretes, and hybrid forms
predominate), which locally can contribute markedly to
variability in surface geomorphological expressions, no-
tably in the context of river valley systems (Nash, Shaw
and Thomas, 1994).
Overall, geomorphic variability within the Kalahari is
accountable in terms of three primary factors in order of
ascending significance. First are local tectonic influences
(e.g. Shaw and Thomas, 1993; McFarlane and Eckardt,
2007) that include, via a subtle subsiding graben on the
northern edge of the Kalahari proper, the context for the
development of the Okavango Delta, which is in reality
a very-low gradient alluvial fan. Second are variations in
sediment availability and supply, impacting upon the op-
portunities for depositional landform development. Third,
and perhaps most important for the resultant overall land-
scape, is the role of climatic changes during the Late
Quaternary Period. Grove (1969) prophetically suggested
the importance of the latter, but it has only been with the
advent and application of suitable geochronometric tech-
niques (see Chapter 3) that the complexity of landscape
and geomorphic evolution in the region is being recog-
nised (e.g. Burrough, Thomas and Bailey, 2009; Stone
and Thomas, 2008).
4.3
Within-dryland diversity
The extensive Prairie and Great Plains regions of North
America are, like the Kalahari described above, regions
that today are arid to semi-arid, flat and extensive. These
'Greater American Deserts' also require careful geomor-
phological interpretation in order to reveal their inherent,
but sometimes subtle, complexity. Also, like the Kalahari,
they owe many of their present characteristics to the influ-
ences of Late Quaternary climatic changes. The remaining
drylands of North America (Figure 4.2) have markedly
contrasting physiographic settings to the plains, being lo-
cated in intraorogenic basin settings (see Table 2.1), where
tectonics have set the scene for more obvious geomorpho-
logical variability than occurs in more stable locations.
This is well illustrated by the Great Basin Desert. This
covers almost a third of the North American arid zone
(over 400 000 km
2
) and has an altitudinal range from over
4300 m a.s.l. in western Nevada to 86 m below sea level in
Death Valley. Within this region, climatic conditions range
from hyper-arid to semi-arid, with rain shadow effects
created by the mountains themselves locally enhancing
aridity to extreme levels (e.g. in Death Valley, California).
The Rocky Mountains mark the eastern margin of the
Great Basin and the Sierra Nevada and Cascade Range
Figure 4.2
Drylands of the USA.
the setting, for in between lie over 150 separate basins that
themselves are separated by 160 mountain belts that trend
north-south (Dohrenwend, 1987; Grayson, 1993; Goudie,
2002).
This setting gives rise to marked, and obvious, geo-
morphological diversity within the total system. Some
elements of the totality are, however, similar in context
to the otherwise contrasting Kalahari: fans, sand seas and
the remnants of playa lakes inherited from wetter climatic
periods are all present. Because of the basin-and-range
setting, however, spatial variability is marked and con-
trasts within the landscape arise over shorter distances
that in drylands with more flat terrain. Erosional land-
scape components, on mountain slopes and on pediments,
predominate over depositional settings, which are con-
fined to basin floors or the alluvial fans of the interface
zone. Thus sand seas, where sediment derived from slope
and lake basin systems has been reworked by the wind,
are smaller and more localised (e.g. within Death Valley),
