Geology Reference
In-Depth Information
Box 12.1
THE ORIGIN OF PANS
A uniquely aeolian origin for pans is disputable. Recent
research indicates that a range of processes may lead to
pan formation. Deflation may top the list, but exca-
vation by animals and karst-type solution may play a
role in some cases. Pan formation appears to run along
the following lines (Goudie and Wells 1995). First, cer-
tain environmental conditions are prerequisites to pan
formation. Low effective precipitation and sparse vege-
tation cover are the main necessary conditions, but salt
accumulation helps as it curbs vegetation growth. Sec-
ond, the local ground surface and sedimentary cover
must be susceptible of erosion. Vulnerable materials
include sands and sandstones, clays and shales, and
marls. These materials are susceptible only where more
than a thin layer of a resistant deposit such as calcrete
does not cap them. Once an initial depression is cre-
ated, several processes may assist its growth. Deflation
is the chief process but it may be enhanced by ani-
mals' overgrazing and trampling the ground and by salt
weathering, which may attack bedrock. A depression
will not continue to grow unless it is protected from
fluvial processes by being isolated from an effective
and integrated fluvial system. Such protection may be
afforded by low slope angles, episodic desiccation and
dune encroachment, dolerite intrusions, and tectonic
disturbance.
Yardangs are carved out of sediments by abrasion
and deflation, although gully formation, mass move-
ments, and salt weathering may also be involved. Yardang
evolution appears to follow a series of steps (Halimov
and Fezer 1989; Goudie 1999). First, suitable sediments
(e.g. lake beds and swamp deposits) form under humid
conditions. These sediments then dry out and are ini-
tially eaten into by the wind or by fluvial gullying. The
resulting landscape consists of high ridges and mesas sep-
arated by narrow corridors that cut down towards the
base of the sediments. Abrasion then widens the corri-
dors and causes the ridge noses to retreat. At this stage,
slopes become very steep and mass failures occur, par-
ticularly along desiccation and contraction cracks. The
ridges are slowly converted into cones, pyramids, saw-
tooth forms, hogbacks, and whalebacks. Once the relief
is reduced to less than 2 m, the whole surface is abraded
to create a simple aerodynamic form-alowstream-
lined whaleback - which is eventually reduced to a plain
surface.
Zeugen
(singular
Zeuge
), also called
perched
or
mush-
room rocks
, are related to yardangs (Plate 12.3). They
are produced by the wind eating away strata, and espe-
cially soft strata close to the ground. Exceptionally, where
sand-laden wind is funnelled by topography, even hard
rocks may be fluted, grooved, pitted, and polished by
sandblasting. An example comes from Windy Point, near
Palm Springs, in the Mojave Desert, California.
Ventifacts
Cobbles and pebbles on stony desert surfaces often
bear facets called
ventifacts
. The number of edges or
keels they carry is sometimes connoted by the German
terms
Einkanter
(one-sided),
Zweikanter
(two-sided),
and
Dreikanter
(three-sided). The pyramid-shaped
Dreikanter
are particularly common. The abrasion of
more than one side of a pebble or cobble does not neces-
sarily mean more than one prevailing wind direction.
Experimental studies have shown that ventifacts may
form even when the wind has no preferred direction.
And, even where the wind does tend to come from one
direction, a ventifact may be realigned by dislodgement.
The mechanisms by which ventifacts form are debat-
able, despite over a century of investigation (see
Livingstone and Warren 1996, pp. 30-2), but abrasion by
dust and silt, rather than by blasting by sand, is probably
the chief cause. Interestingly, the best-developed ven-
tifacts come from polar and periglacial regions, where,
owing partly to the higher density of the air and partly
