Geoscience Reference
In-Depth Information
hydrology (surface runoff and infiltration of water), aeo-
lian inputs (of dust, fine sand and salts) and time. As time
passes and the climate changes, the morphological, phys-
ical, chemical, mineralogical and biological properties of
the pavement alter. It is believed that the characteristics of
a mature pavement, including smooth surfaces, low relief,
lack of vegetation and closely knit particles, require an
extended period, typically thousands of years, to develop.
rule, however, and may vary with location. For example, in
Israel, Amit
et al.
(1996) described 'weak' gravel covers
of 10-20 %, 'moderately developed' desert pavements
covering
∼
50 % of the surface and a 'well-developed'
cover of
85 %. Clast coverage varies with slope gra-
dient, with increases in slope leading to size sorting and
variations in clast coverage (Poesen
et al.
, 1998). For the
Mojave Desert, Quade (2001) found that pebble densities
were greatest at lower altitudes (
>
90 %) and decreased
systematically with altitude at a rate of
>
∼
3 % per 100 m.
At pebble densities
60 %, characteristics common to a
mature pavement are lost, such as surface smoothness and
the interlocking of clasts.
The gravel layer may consist of angular to rounded
clasts, with the form dependent on the origin of the parti-
cles, their age and degree of weathering. Young pavements
on beach ridges may have rounded particles (Figure 9.7),
whereas those on alluvial fans (particularly near sources
of salts such as playas) often have more angular clasts.
Cooke (1970) divided the clasts into
primary particles
,
which are similar in size, form or lithology to the orig-
inal surface, and
secondary particles
, derived from the
primary particles by disintegration (splitting or granular
disintegration) or added later as human artefacts (Casana,
Herrmann and Qandil, 2009). The presence of secondary
Neolithic or Palaeolithic artefacts on pavement surfaces
provides evidence of their stability (Peel, 1968).
Splitting of particles creates smaller fragments, tends
to increase their angularity and increases surface parti-
cle density. The degree of splitting is a function of time:
in this respect, older surfaces often show greater break-
down (Cooke, 1970). However, splitting is also related
to the concentration of aerosolic salts that are deposited
on surfaces and, when alluvial fans are adjacent to saline
playas, may be greater at the distal than proximal ends.
Recently split fragments may lack the varnish that charac-
terises older clasts. Where granitic boulders are present,
granular disintegration produces coarse grus at the surface
(Figure 9.5(b)). In this case, some fragments, which were
originally very angular in form, may actually show some
rounding with time (Al-Farraj, 2008).
The physical characteristics of pavements are often
more variable than they first appear, even across a sin-
gle surface of an alluvial fan. A spatially complex pat-
tern exists, wherein pavement units differ in the percent
clast cover, degree of packing, particle size, angularity,
degree of varnish cover, sorting and the degree to which
the particles are embedded (Wood, Graham and Wells,
2002). These differing characteristics can be used to cre-
ate desert pavement mosaic map units (Wood, Graham
and Wells, 2002), with boundaries that range from very
<
9.4
Stone pavement characteristics
9.4.1
Setting
Physical factors that affect the development of pavements
include surface stability, slope and elevation. Pavements
develop where surfaces are stable, i.e. are not subject
to significant water erosion (Peel, 1968; Cooke, 1970;
Ugolini
et al.
, 2008; Adelsberger and Smith, 2009; Mat-
mon
et al.
, 2009). Associated with the stability of the
surface are low slope angles. Pavements in Jordan, for
example, have developed ona1%slope(Ugolini
et al.
,
2008). Although the relationship is not yet well studied,
it appears that elevation limits pavement formation, as
a result of altitudinally related increases in rainfall and
disruptive vegetation cover (Quade, 2001; Marchetti and
Cerling, 2005).
Pavements are built on two different types of surfaces:
(1) those of an alluvial character and (2) those of a resid-
ual character (Cooke, 1970). The first is characterized
by gravel of mixed composition and distant origin, in-
cluding pavements formed on beach ridges, alluvial fans
and stream terraces. The second incorporates clasts from
residual deposits (e.g. volcanic pyroclastic deposits) or
bedrock surfaces (e.g. basalt flows).
9.4.2
Surface clast concentration
and characteristics
On surfaces where stones are highly concentrated, the
term stone or desert pavement is used, particularly in
North America. The pavement usually consists of closely
packed gravel in a surface layer a few centimetres thick
(one- to two-particle-thick layer) that rests on, or is em-
bedded in, a soil that may contain dispersed stones, or
may be relatively free of stones.
With respect to percent clast cover, desert pavements
have been described as distinctive features in which at
least 65 % of the surface is covered in stones (Wood,
