Geology Reference
In-Depth Information
Table 12.1 Degrees of aridity defined by an aridity index
Table 12.2 Landforms assemblages in deserts of the
south-west USA and the Sahara
Aridity type
Aridity index
Land area in
category (per cent)
Landform
South-west USA
(per cent)
Sahara
(per cent)
Hyperarid
<0.05
7.5
Desert mountains
38.1
43.0
Arid
0.50-0.20
12.1
Playas
1.1
1.0
Semi-arid
0.20-0.50
17.7
Desert flats
20.5
10.0
Dry subhumid
0.50-0.65
9.9
Bedrock fields
(including hamadas)
0.7
10.0
Regions bordering
through-flowing rivers
1.2
1.0
3
semi-arid lands have a mean annual precipitation of
between 250 and 500 mm.
Dry washes (ephemeral
stream beds)
3.6
1.0
Arid and extremely arid land are deserts; semi-arid grass-
lands mostly prairies or steppes. UNEP uses a different
index of aridity, defined as
Alluvial fans and
bajadas
31.4
1.0
Sand dunes
0.6
28.0
Badlands
2.6
2.0
AI
=
PE
/
P
Volcanic cones and
fields
0.2
3.0
where
PE
is the potential evapotranspiration and
P
is the average annual precipitation (Middleton and
Thomas 1997). Four degrees of aridity derive from this
index (Table 12.1).
Although wind action is an important process in shap-
ing desert landforms, desert landform assemblages vary in
different tectonic settings. These regional differences are
brought out in Table 12.2, which shows the proportion
of landforms in the tectonically active south-west USA
and in the tectonically stable Sahara.
Source:
Adapted from Cooke
et al.
(1993, 20)
of rock and coarse sand that acts as a protective blanket.
Such thin veneers of gravel, or coarser material, that over-
lie predominantly finer materials are called
lag deposits
(Plate 12.1). Lag deposits cover a significant proportion
of the world's deserts, but they also occur in other envi-
ronments with little vegetation, including mountains
and periglacial zones. The coarse material has several
local names - gibber in Australia, desert armour in
North America, and
hammada
,
serir
, and
reg
in the Arab
world.
Lag deposits may result from the deflation of poorly
sorted deposits, such as alluvium, that contain a mix of
gravel, sand, and silt. The wind removes the finer sur-
face particles, leaving a blanket of material too coarse
to undergo deflation. The blanket shields the under-
lying finer materials from the wind. However, other
processes can lead to the concentration of coarse par-
ticles on bare surfaces - surface wash, heating and
cooling cycles, freezing and thawing cycles, wetting and
drying cycles, and the solution and recrystallization of
salts.
Where the stone cover is continuous (and the parti-
cles generally flat), surfaces covered by lag deposits are
called
stone pavements
, but they go by a variety of local
AEOLIAN EROSIONAL FORMS
Landforms resulting from wind erosion are seldom pre-
served except in arid areas. In alluvial plains and beaches,
subsequent action by rivers and by waves erases traces of
aeolian erosion. In arid areas, other denudational agents
are often weak or absent and fail to destroy erosional
landforms. The chief erosional forms in drylands caused
by wind erosion are lag deposits, desert pavements, ven-
tifacts, yardangs, and basins (see Livingstone and Warren
1996; Breed
et al
. 1997; Goudie 1999).
Lag deposits and stone pavements
Deflation winnows silt and fine sand, lowering the level
of the ground surface and leaving a concentrated layer
