Geoscience Reference
In-Depth Information
Table 4.1
Landscape components in different arid regions
(expressed as a percentage of area
a
)
tion. Stony deserts can also occur in structural plateau
locations.
Fluviolacustrine landscapes.
River systems are not an
obvious feature of drylands, but can form significant
landscape components, either through channel systems
inherited from wetter past climates or as systems origi-
nating from neighbouring temperate or tropical regions
or in dryland uplands. Some channel systems reach the
oceans but in a number of important dryland regions
internal drainage leads to lacustrine or salt flat sump
regions.
SW USA
Sahara
Libya
Arabia
Australia
b
Mountains
38.1
43
39
47
16
Low-angle bedrock
surfaces
0.7
10
6
1
14
Alluvial fans
31.4
1
1
4
River plains
1.2
1
3
1
13
Dry watercourses
3.6
1
1
1
−
Badlands
2.6
2
8
1
Playas
1.1
1
1
1
1
Sand seas
0.6
28
22
26
38
Drylands dominated by aeolian deposits.
Aeolian land-
scapes occur in regions beyond the limit of fluvial activ-
ity, but may derive much of their sediment from flood-
plains, lake basins and other water-derived contexts. To
this framework provided by Mabbutt (1969), which ap-
plies to sand seas, can be added loess landscapes, which
tend to blanket plateau top or plateau edge locations.
Desert flats
c
20.5
10
18
16
18
Recent volcanic
deposits
0.2
3
1
2
−
a
Percentages given are only approximate, with the degree of accuracy
differing between areas.
b
From Mabbutt (1976). Categories used by Mabbutt do not necessarily
coincide with those used in other areas: included for comparison only.
The remaining data are from a study by Clements
et al.
(1957) for
the US Army.
c
Undifferentiated: includes areas bordering playas.
Depending on the scale of analysis, individual deserts or
drylands may embrace one or more of the broad physio-
graphic contexts described above, while structural frame-
works, among other factors, may result in the transitions
between different zones occurring over large or small dis-
tances. This is well illustrated, for example, by the rel-
ative lack of mesoscale landscape variations over large
distances in the Kalahari, Saudi Arabian and Australian
arid zones, with their stable, cratonic settings (Rendell,
Chapter 2 in this volume) and the relatively low re-
lief over hundreds of kilometres in these drylands (e.g.
Thomas and Shaw, 1991). By way of contrast, the inter-
cratonic/interorogenic contexts of the deserts of the west-
ern USA and eastern Sahara (in Ethiopia) result in much
greater transitions in relief, and accordingly greater diver-
sity, and over shorter distances, between physiographic
zones.
Goudie (2002) illustrates a further complication in
desert diversity. He notes that desert regions that at first
can appear remarkably similar can often display, on deeper
analysis, markedly contrasting characteristics, as with
the Atacama of South America and Namib of South
Africa:
context for the systematic analysis of geomorphological
processes and landforms that occur in the chapters that
form the heart of this topic. Chapter 1 has already ex-
plained the different climatic and oceanographic mecha-
nisms that led to dryland development, and in Chapter 2
(Table 2.1) specific drylands are linked to specific tectonic
contexts. However, neither of these approaches explains
the resultant landscapes, the geomorphological outcomes.
Mabbutt (1969) provided a summary of physiographic
contexts in deserts (also reproduced in Goudie, 2002),
which can be adapted to provide the following overview:
Upland drylands.
Geology controls relief and bedrock is
widely exposed, resulting in weathering and often ero-
sion - controlled landscapes. Depending on the degree
of relief and on latitudinal setting, temperature may be
as important a control as moisture deficit on plant cover
and on the weathering processes that occur.
Piedmont drylands.
Transition zones between upland and
basin settings. Sediment transport may be dominant
but depositional (e.g. alluvial fan) and erosional (e.g.
pediment) landscapes both occur.
At first sight they might be expected to be very sim-
ilar, [with] narrow coastal fringes in a similar lati-
tude with cold currents offshore and with mountains
bounding them on their eastern sides
Stony deserts
. These often occur at the foot of pediments
or in basin locations where either outwash or defla-
...
they are both
hyper-arid and foggy. However there the similarity
