Geoscience Reference
In-Depth Information
21.2.5.8
The role of climate change
with a well-defined keel and two opposing facets are
thought to be formed by parallel winds, whereas wind-
perpendicular keels are associated with ventifacts with
only one abraded face. These statements are contradicted
by other studies (Maxson, 1940), including those of con-
temporary ventifacts in the Mojave Desert (Laity, 1995).
A weather station maintained for 17 years in the Little
Cowhole Mountains, combined with a series of anemome-
ters placed around the hillslope, clearly demonstrated that
ventifacts with a sharp keel and two facets develop from
opposing winds, which flow perpendicular to the rock.
Nonetheless, the mapping of keel orientation to determine
wind direction produces a much greater scatter of results
than the use of grooves or flutes. Keels produce a good
result when the ventifact is very mature, with almost no
remnant of the original face or rock shape remaining. In
earlier stages of development, the keel orientation tends to
be rather varied. By contrast, linear features on rocks de-
velop early in the evolution of the ventifact and are much
easier to map accurately (although caveats apply). In all
instances, however, care must be taken and the work must
not be performed uncritically.
If a large region is mapped, palaeocirculation patterns
can be reconstructed with reference to fossil ventifacts
(Powers, 1936; Sharp, 1949; Tremblay, 1961; Nero, 1988;
Smith, 1984; Laity, 1992, 1995; Knight, 2008). Large sta-
ble rocks provide the best medium for mapping, as smaller
ventifacts may be overturned or rotated by animal move-
ment, flooding, earthquakes or other events. In modern
environments, the wind direction derived from ventifact
mapping may be compared with that attained from other
features, such as vegetation growth patterns, to understand
the impact of topography on prevailing and erosive winds
(Griffiths
et al.
, 2009). In areas of bidirectional flow, one
direction may dominate in terms of erosional energy, and
this is reflected in the greater development of features on
that side (Corbett, 1993).
Several studies have suggested that a reverse eddy flow,
caused by flow separation along the keel, may explain
abrasion on the lee side of ventifacts (Breed, McCauley
and Whitney, 1989; Fisher, 1996). These observations
were made based on studies of relict ventifacts, for which
the wind regime is unknown. The understanding of di-
rectional relationships is important in the reconstruction
of wind flow fields. There are several potential problems
with the concept of reverse eddy flow erosion: (1) wind
velocity to the lee of rocks appears insufficient to support
sand flow of this nature; (2) it is not supported by target
studies either in the field or in the laboratory (Laity and
Bridges, 2009); and (3) for most ventifacts, the lee of the
ventifact is covered in a protective layer of sand during
The role of climate change in ventifact formation is not
well understood because it is very difficult to study and
the ages of the rocks are poorly constrained. It has the
potential to alter: (1) the surface of the rock, through case-
hardening processes and varnish deposition, thereby af-
fecting subsequent episodes of erosion; (2) wind strength,
direction and the supply of particles, as well as the amount
of stabilising vegetation; and (3) the surface level of the
ground, through erosional or depositional processes. In the
Mojave Desert, for example, much of the sand at lower
elevations is stable. Active sand at hillcrests maintains
modern ventifacts, whereas varnished fossil forms mantle
the lower slopes and plains.
The impact of chemical and physical changes to the sur-
face of rocks during periods between erosion episodes has
not been studied. In arid environments, the formation of
coatings on stabilised ventifacts (Dorn, 1995) may provide
a surface that is more resistant to abrasion than the natural
rock, requiring additional erosive energy to remove in sub-
sequent aeolian episodes. Hardened surfaces are observed
on ventifacts to the lee of the Sierra Nevada, California, at
altitudes of
2500-2700 m. The ventifacts were probably
formed in periglacial regimes, although the environment
is semi-arid today. In numerous instances, abrasion can be
seen to have worked its way
under
a heavily fluted outer
carapace of granite or tuff, exploiting the softer material
beneath the case-hardened surface. Observations such as
these support the idea that rates of erosion are not uniform
over time, but fluctuate with climate and changes to the
surface properties of the rock.
∼
21.2.5.9
The use of ventifacts to determine
wind direction
Ventifacts and abraded outcrops are an excellent proxy
for wind direction. Wind direction can be determined by
reference to the keel (which forms perpendicular to the
wind), to pitting (perpendicular to the wind) or to grooves
or flutes (parallel to the wind on low-angle facets) (Max-
son, 1940; Selby, 1977; Laity, 1987; Nero, 1988; Don-
ner and Embabi, 2000; Laity and Bridges, 2009). Where
yardangs are also present, they will have the same orienta-
tion as flutes and grooves on ventifacted surfaces (Donner
and Embabi, 2000).
The use of keels to map wind direction is more problem-
atic than pits or surface lineations. Knight (2008, p. 96)
questions the use of keels in the mapping of ventifacts,
claiming that such features may be formed either perpen-
dicular
or
parallel to wind flow; low elongate ventifacts
