Geoscience Reference
In-Depth Information
usually composed of a succession of short scoop-like de-
pressions only a few centimetres in length. Groove trends
are typically parallel on near-horizontal surfaces and re-
flect the flow direction of the highest velocity winds (Laity,
1987). Maxson (1940) suggested that grooves and flutes
are initiated by vortices and then modified by saltating
sand grains, whereas Schoewe (1932) proposed that the
skidding action of grains on hard, smooth surfaces might
be important. Whitney (1978) put forward the idea that
vortex pits coalesce into flute pits and pit chains, but pit
chains have not been observed in the field. Scanning elec-
tron micrographs of groove interiors (Laity and Bridges,
2009) show impact-generated cleavage fractures, suggest-
ing that direct sand grain impact and rebounding grains
are responsible for groove and flute formation.
Etching and fretting are processes of differential erosion
that occur when there are strongly developed hardness dif-
ferences in a rock.
Etching
develops in layered rocks such
as ignimbrites, where the wind erodes away the weaker
material. Etching can be replicated in layered target ma-
terials subject to sand abrasion in the field (Laity, 2009)
or in the wind tunnel (Laity and Bridges, 2009; Bridges
et al.
, 2010) and subject to sand abrasion (Figure 21.14).
Fretting
occurs when there are small, hard inclusions in a
rock, such that projecting points, knobs or ridges develop.
The inclusions resist erosion, while the softer matrix is
eroded away. The result may be finger-like projections
called
dedos
or
demoiselles
(Hobbs, 1917) (Figure 21.15).
When there are large, hard inclusions within the rock,
a
knobby texture
may develop on high-angle windward
faces (Figure 21.16). In granitic rocks, for example, large
Figure 21.15
Dedos are finger-like projections that result
when small, hard inclusions occur in a rock. This dedo is about
15 cm in length.
Figure 21.16
A knobby texture can develop on high-angle
windward faces when large, hard inclusions occur within the
rock. This andesite boulder is located in the Mono Basin, Cali-
Figure 21.14
Etching, the erosion of weaker material in lay-
ered rocks, can be replicated by placing layered target mate-
rials (formed of plaster in this photograph) among a field of
