Geoscience Reference
In-Depth Information
study of these characteristics. The degree to which clasts
are embedded appears to vary considerably. Sharon (1962)
refers to well-embedded clasts comprising 5-10 % of all
clasts within his study area in Israel. Wood, Graham and
Wells (2005) note that within a single area of desert pave-
ment, different mosaics form, within which some clasts
lie loosely on the surface and others are more deeply
embedded.
Pleistocene surfaces were subtler. Rubification may de-
velop rapidly and was observed on surfaces whose ages
were inferred to be less than 500 years old (Helms, McGill
and Rockwell, 2003).
9.7.7
Development of ventifacted surfaces
On pavements that are subject to blowing sand, ventifacts
may form (Peel, 1968; Cooke, 1970; Li
et al.
, 2006).
There have been few studies of this process, and it is
infrequently recorded. One aspect of wind abrasion is the
removal of desert varnish, which may complicate studies
of the relative ages of surfaces.
9.7.5
Clast orientation
There are relatively few studies of the alignment or fabric
of clasts. Adelsberger and Smith (2009) found no relation-
ship between pavement clast orientation and other charac-
teristics, such as slope, indicating little effect from gravity-
driven creep or overland flow. Whereas some pavements
showed a small degree of orientation, others contained
clasts whose orientation appeared to be completely ran-
dom. However, Abrahams
et al.
(1990) observed that
pavement clasts on slopes had their long axes aligned
parallel to the downslope direction.
9.8
Secondary modifications to
pavement surfaces
9.8.1
Patterns in pavement
Several authors have noted the development of patterns in
pavement. These may be related either to frost heave under
earlier climatic conditions or to the effects of swelling
and shrinking clays associated with wetting and drying
(Peel, 1968). In modern high-altitude gobi on the Qinghai-
Tibet Plateau, freeze-thaw polygons are present (Li
et al.
,
2006).
9.7.6
Clast rubification
Clast rubification refers to the development of an orange
coating on the undersides (ventral sides) of stones in a
desert pavement. The undersides of clasts are potentially
more stable than the upper sides, being protected from sur-
face weathering and sand blasting, and Helms, McGill and
Rockwell (2003) found that the degree of ventral varnish
was more strongly correlated with age than either dorsal
varnish or soil properties. Both the percentage of clasts
rubified (McFadden, Wells and Jercinovich, 1987) and
the degree of reddening of a pavement clast's underside
are considered to be indicative of the relative age of the
pavement (Helms, McGill and Rockwell, 2003; Valentine
and Harrington, 2006), although it is likely that such fea-
tures vary considerably according to location. In southern
Nevada, Valentine and Harrington (2006) noted that pave-
ment clasts on a young (75-80 ka) volcanic cone showed
much less rubification than on an older (ca. 1 Ma) cone. On
the Cima volcanic field, California, only 10-20 % of clast
undersides on flows younger than 0.4 Ma were reddened,
whereas all clasts showed rubification on flows older than
0.4 Ma. The degree of reddening, as determined by hue,
also increased on the older flows (McFadden, Wells and
Jercinovich, 1987). For Holocene surfaces in southeastern
California, Helms, McGill and Rockwell (2003) noted that
9.8.2
Animal burrowing, vegetation and
stone displacement
Several biological processes disrupt the formation of
pavements. These include animal burrowing, plant growth
and cryptobiotic crust formation. The disruption of pave-
ment by cryptobiotic crusts has received little attention,
but Quade (2001) noted this effect on pavement in the
Mojave Desert at altitudes between 1300 and 1500 m.
Vegetation growth affects pavement development and,
in turn, is affected by the presence of pavements. As al-
titude increases in deserts, rainfall and vegetation cover
also rise. For the Mojave Desert, Quade (2001) deter-
mined three divisions of pavement formation related to
altitude, climate change and potentially disruptive veg-
etation cover: (1) surfaces at elevations
1900 m that
lack pavement owing to vegetation cover in both glacial
and interglacial periods; (2) surfaces between
>
∼
1900 and
400 m that show strongly developed pavement, but may
