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Table 3.4
Examples of morphological evidence of arid and semi-arid zone extensions.
Landform
Indicative of
Location
References
Slopes
Pediments in humid
environments
Former extension of arid slope systems
Brazil
Bigarella and de Andrade (1965)
Colluvial mantles
Former devegetation due to drier
climates
Swaziland
Watson, Price Williams and
Goudie (1985)
Dissected alluvial fans
Former high sediment-water ratios due
to aridity
Southwestern
USA
Lustig (1965)
Alluvial fan construction
Reduction of vegetation in source
areas due to aridity
Death Valley
Southern
Australia
Hunt and Mabey (1966)
Williams (1973)
Rivers
Clogged drainage
Former river blockage by dunes
Niger
Talbot (1980)
Incised channels
Devegetation and lower sea levels
Amazonia
Tricart (1975, 1984)
Aggraded rivers
Lower and more seasonal discharge
Nile
Adamson, Gillespie and
Williams. (1982);
Williams et al. (2010)
Dunes
Vegetation linear dunes
Former drier climates and past
circulation patterns
Australia
Southern Sahara
Kalahari
Bowler (1976); Fitzsimmons
et al. (2007)
Grove (1958);
Lancaster et al. (2002)
Thomas (1984); Stone and
Thomas (2008)
Drowned barchans
Former aridity
Botswana
Cooke (1984)
Vegetated parabolic
dunes
Drier conditions in past
Colorado
Muhs (1985); Muhs et al. (1986)
Gullied dunes
Former aridity
Sudan
Talbot and Williams (1978)
Lithifield dunes
Former aridity
India
Sperling and Goudie (1975)
3.2.8
Arid zone contraction
expansion of desert conditions since the work of Sarn-
thein (1978). Such systems are not, however, as simple to
interpret in palaeoclimatic terms as was once thought, with
a number of issues arising in recent work (e.g. Hesse and
Simpson, 2006; Stone and Thomas, 2008; Chase, 2009).
The dune form most widely found in the 'relict' state,
linear dunes, is perhaps the most stable of all major desert
dune types, even when undergoing sediment transport,
which both leads to their widespread preservation and
inheritance from more arid times past, but which also
complicates their clear palaeoclimatic interpretation, es-
pecially in situations where dunes lie close to the margins
of present day aeolian activity. The palaeoclimatic sig-
nal that can be gained from relict dunes, therefore, to
some degree depends on their location relative to contem-
porary environmental conditions (Thomas and Burrough,
2011). Because of their importance, reconstructions based
on sand sea evidence are considered in greater depth in
Various geomorphological sources have yielded infor-
mation evidencing past periods of greater humidity in
present day arid areas (Table 3.5). Studies of closed-
basin lake-level fluctuations from shoreline evidence (Fig-
ure 3.6) have become significant indicators of environ-
mental changes in many areas of the tropics and subtropics
(see reviews by Street and Grove, 1979; Street-Perrott
and Roberts, 1983; Street-Perrott, Roberts and Metcalfe,
1985; Currey, 1990; Burrough and Thomas, 2009).
Closed basins are important because their levels adjust
to changes in inputs, whereas open lake-basin levels are
more likely to remain stable, as input increases can be
balanced by changes in overflow. Palaeolake shorelines
in the form of terraces or depositional beach ridges oc-
curring within today's arid zone suggest a disequilibrium
between these landforms and present day environmental
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