Geoscience Reference
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Maghreb (Morocco,
N. Algeria, Tunisia)
Hyper-arid - arid
Low lakes, active
dunes, etc.
Arid - Semi-arid
Ephemeral lakes
and rivers. Local
dune activity
Humid - Sub-humid
High lakes and
rivers. Cave sinter
development, etc.
Northern Sahara
Africa
- north
of the
Equator
Eastern Sahara
Sudan
Chad
Middle Kalahari
Southern
Africa
Southern Kalahari
Southern/interior
Australia
Australia
North-east Australia
Thar Desert
India
50
45
40
35
30 25
Years BP
20
15
10
5
0
×
10 3
Fig. 5.4 A simplified chronology of late Quaternary expansion and contraction of tropical and subtropical arid zones derived from
geomorphological and sedimentological data. Blank areas in the bars represent periods of uncertain dominant climate (due to climatic
instability or a lack of data). (From Thomas 1989.)
subtropical regions, however, do indicate that the
spatial distribution of arid regions has changed
over this time frame (Fig. 5.4).
ized rates of weathering and sediment production.
This also means that any inherited weaknesses,
for example developed from weathering in past
climate regimes, become more significant as
these features are key to the in situ weathering
development and thus, ultimately, erodibility of
the sediments.
5.2
SEDIMENT SOURCES AND TRANSPORT
This section will examine how the natural arid
landscape functions from source to storage,
following the dominant transport processes and
in situ modification (weathering) that can occur
after deposition. Weathering is highly variable,
depending on localized differences in climate
(mainly temperature and humidity), geology
(mainly lithology and structure) and inherited
weathering artefacts (Smith & Warke 1997).
Weathering is important as it weakens the bed-
rock ready for erosion and transport.
5.2.1.1 Insolation
Large temperature ranges (which can exceed
50°C, Goudie 1997) associated with the diurnal
temperatures of deserts have been attributed
with expansion and contraction of rocks in situ .
Early workers suggested that this was the main
factor behind rock breakdown. Experimental
work, however, has failed to reproduce the same
degree of weathering under laboratory condi-
tions, leading to much debate about the possible
role of insolation (see Goudie (1997) and dis-
cussions therein).
5.2.1 Significance of weathering in arid regions
Arid environments are typified by weathering-
limited sediment supply. Exposed bedrock is
common and the lithological and petrographic
characteristics of the geology are more important
than in humid environments in controlling local-
5.2.1.2 Moisture
Although by their definition arid areas have
limited water availability from rainfall, moisture
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