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hypothesis that pavement clasts have been continuously
exposed since their formation (Liu and Broecker, 2008).
their initial shape. The weathering processes that fracture
clasts are sensitive to climatically induced variations in
the flux of aeolian fines. Aerosolic salts are a significant
component of clast fractures and are found in the soils
beneath many stone pavements. Their abundance is indi-
cated by high soil electroconductivities and the presence
of secondary gypsum. Weathering is conspicuous near
the margins of saline playas (Figure 9.5(b)) and along the
late Pleistocene shorelines of pluvial lakes. Medium- to
coarse-grained plutonic and metamorphic rocks are par-
ticularly susceptible to this process (McFadden, Wells and
Jercinovich, 1987).
The degree of salt weathering varies spatially, both
within a given region (e.g. closer to or farther from a playa)
and from region to region. Moreover, salt can affect rocks
both at the surface and within the soil. In the Negev Desert,
Amit, Gerson and Yaalon (1993) documented gravel shat-
tering by salts within reg soils. Not all pavement surfaces
are shattered by salt. In Jordan, relatively young pave-
ments of low soil salinity lacked salt-shattered clasts, al-
though salts increased clast porosity, allowing calcite to
infill voids (Ugolini et al. , 2008). Although salt weath-
ering is often associated with an increase in angularity
of the particles, the reverse may occur. In South Aus-
tralia, silcrete boulders, which have been subject to many
pre-weathering processes involving brecciation and rece-
mentation, break up along previous fracture lines rather
than modern stress patterns. The break-up produces small,
very angular and uniformly sized clasts that are not subject
to further diminution. The highly saline Lake Eyre envi-
ronment chemically weathers the clast edges, decreasing
their angularity over time (Al-Farraj, 2008).
9.6 Processes of clast size reduction
in pavements
The comminution of boulders and gravel over time pro-
duces smaller particles that eventually interlock to form
a tightly knit pavement. The reduction in clast size re-
sults from two principle processes - splitting and granu-
lar disintegration. Splitting reduces rock size and tends to
produce angular fragments that form the basic framework
of the clast monolayer. Granular disintegration provides
even smaller particles, which commonly infill the spaces
between the split fragments, leading to a tightly packed
veneer (Figure 9.5(b)). In a very mature pavement, frac-
turing of clasts near the surface may be absent, suggesting
that there is a point in time after which there is no further
reduction in clast size (Amit, Gerson and Yaalon, 1993;
Matmon et al. , 2009).
The production of secondary particles by splitting is
common in pavement development. The principal pro-
cesses invoked are insolation, frost and salt weathering.
Insolation weathering refers to the disintegration of rocks
by volumetric changes associated with seasonal or diurnal
changes in temperature. In the American southwest, many
boulders have cracks with a north-south (N-S) orienta-
tion, suggesting they may be genetically related to thermal
stresses caused by the differential heating and cooling of
the rock associated with the daily movement of the sun
across the sky (McFadden et al. , 2005). A pronounced
N-S orientation of vertical cracks was also recorded for
dark chert cobbles on the Libyan Plateau in central Egypt
(Adelsberger and Smith, 2009). Solar-controlled break-
down of chert may be a function of the dark colour or
other physical properties of the rock, which aid in form-
ing initial cracks that may be further propagated by other
processes, such as salt weathering. Crack growth may
also be enhanced by hydration, and crack tips with min-
imal solar exposure and greater moisture retention will
tend to propagate (Moores, Pelletier and Smith, 2008).
Frost weathering results from the volumetric expansion
of ice in confined spaces. Some deserts experience sub-
freezing conditions in the winter, the extent of which may
have been even greater during past climate regimes. It is
possible, therefore, that this process may be active in some
deserts or have produced some of the split fragments on
older pavement surfaces.
The role of salts in desert pavement formation is well
9.6.1
Pavement soils
Most stone pavements are underlain by soils (see also
Chapter 7), many of which share common characteristics.
Over time, the thickness and qualities of the pavement
soils change. However, even young soils show some de-
gree of evolution.
The fine-grained pavement substrate is derived largely
from the windblown suspended load, and is composed
of clays, silts and sandy silts (McFadden, Wells and Jer-
cinovich, 1987; Reheis et al. , 1995; Sauer, Schellmann
and Stahr, 2007). It is generally free of coarse parti-
cles. The A-horizon is commonly characterised at the
surface by a small-scale polygonal pattern of cracks,
which form a three-dimensional ped framework of ver-
tical cracks (Symmons and Hemming, 1968; Wood, Gra-
ham and Wells, 2005; Meadows, Young and McDonald,
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