Geology Reference
In-Depth Information
et al., 1995). When considered collectively, these limited data explain why J. R. Mackay
was unable to detect signifi cant abrasion over his 50-year period of observation at
Paulatuk.
Case hardening and so-called “desert varnish” are problematic phenomena that may
be wind-related and which are especially well developed in the ice-free areas of Antarc-
tica. These terms are used to describe a poorly-understood process by which the exterior
of a rock is made more resistant to weathering, probably by the evaporation of a mineral-
bearing solution, leaving a thin cementation layer (Campbell and Claridge, 1987, pp.
124-129; Glasby et al., 1981) (see Chapter 4). Some argue that a biogeochemical origin
should be considered (Dorn and Oberlander, 1982; Dorn et al., 1992). Even more so than
ventifact formation, the development of rock varnish is thought to be extremely slow.
Referring specifi cally to Antarctica, Campbell and Claridge (1987, pp. 127-129) state that
“surfaces on which pitted and stained rocks are found are always old, of the order of a
million years or more.”
10.3.2. Wind Defl ation
Defl ation, the second aspect of wind activity, is the winnowing out of fi ne particles by
wind and their transportation. Because vegetation is a major controlling factor, defl ation
reaches its greatest intensity on unvegetated surfaces and in the arid polar deserts.
An obvious indicator of defl ation is the presence of a lag gravel or desert pavement on
the ground surface. This refl ects the removal, by wind, of fi ner particles. Another is the
presence of shallow depressions or blow-outs, ranging from a few centimeters in width
and depth to troughs many meters wide and deep (Figure 10.9B).
The amount of sediment removed by defl ation depends largely upon the strength of the
wind. For example, strong winds during the winter of 1990-91 on the Fosheim Peninsula
of Ellesmere Island, in the Canadian High Arctic, resulted in an estimated soil loss of
∼
4-5 kg/m
−2
(4 -5 mm), an amount equivalent to more than 20 years of denudation by wash
and other processes (Lewkowicz, 1998). The maximum size (45 mm long) and weight (25 g)
of particles transported during winter demonstrate that eolian transportation in the
Canadian Arctic, like the Antarctic, is not confi ned to sand- and silt-sized materials.
Elsewhere, M. Seppälä (1974) measured sand transport of 0.15 g/cm/hr during a 4-month
period in a blow-out approximately 75 m wide. This translates into 3.2 tons of sand moving
through this “gateway.” Vegetation is the obvious inhibitor of defl ation activity. For
example, A. Pissart et al. (1977) describe defl ation upon a sandy glacial outwash surface
on southern Banks Island, Canada, where clumps of willow (
salix
) have led to the pres-
ervation of sandy mounds, 1.0-3.0 m high, separated by defl ation blow-outs or troughs.
As with wind abrasion, the majority of defl ation activity probably occurs during the
winter when wind speeds are highest and when, typically, the upper few centimeters of
frozen ground are desiccated and relatively friable. However, defl ation may also occur in
summer when dust storms, brought on by surface heating and instability of sandy areas
during periods of intense solar radiation, may rise several hundreds of meters into the air
blanketing the surrounding terrain with a thin cover of fi ne sand particles. Braided stream
channels and outwash plains at low fl ow are particularly suited to this eolian activity.
10.3.3. Niveo-Eolian Sediments
The combination of wind-transported sediment and snow has long been noted (Fristrup,
1952; Pissart, 1966b). The resulting sediments, predominantly laminar in nature, are com-
