Geoscience Reference
In-Depth Information
(a)
(b)
(c)
Figure 21.20
(a) The surface of a groove in an actively forming, sand-abraded marble ventifact at
×
100 magnification. (b) At a
scale of
×
2000, cleavage fractures of marble crystals are evident. The surface is much rougher at the macroscale than suggested
by field specimens. (c) At
×
50 000, microcleavage of marble crystal grains and fractures in the process of propagation occur.
Angular marble debris are present.
surface is clearly caused by impact damage from sand and
is not reflective of the smoother surface that might be an-
ticipated from uniform wearing by dust (Figure 21.20(c)).
Such impact features are observed both within grooves
and on ridges.
speed and direction, particle supply and burial/exposure
of rocks vary over both short and long timespans. The
exposure to abrasion is potentially very long, spanning
thousands of years and several episodes of climatic
change. Owing to changing environmental conditions
and to changes to the rock surface itself, the erosion rate
of ventifacts is nonlinear.
At the local scale, particle movement is affected by the
size and spacing of rocks, resulting in areas of greater
abrasion where flow is more turbulent or where grains
are propelled upward from a ventifact facet into the high-
velocity air flow, where they travel many metres before
21.2.5.6 Rates of abrasion
Calculating rates of wind abrasion associated with ven-
tifact formation is difficult. It is problematic to infer field
rates based on laboratory studies, because field conditions
are infinitely more complex. Wind abrasion in the field
