Page 318 - Fundamentals of Geomorphology

AEOLIAN LANDSCAPES 301

Box 12.1

THE ORIGIN OF PANS

A uniquely aeolian origin for pans is disputable. Recent marls. These materials are susceptible only where more
research indicates that a range of processes may lead to than a thin layer of a resistant deposit such as calcrete
pan formation. Deflation may top the list, but exca- does not cap them. Once an initial depression is cre-
vation by animals and karst-type solution may play a ated, several processes may assist its growth. Deflation
role in some cases. Pan formation appears to run along is the chief process but it may be enhanced by ani-
the following lines (Goudie and Wells 1995). First, cer- mals’ overgrazing and trampling the ground and by salt
tain environmental conditions are prerequisites to pan weathering, which may attack bedrock. A depression
formation. Low effective precipitation and sparse vege- will not continue to grow unless it is protected from
tation cover are the main necessary conditions, but salt fluvial processes by being isolated from an effective
accumulation helps as it curbs vegetation growth. Sec- and integrated fluvial system. Such protection may be
ond, the local ground surface and sedimentary cover afforded by low slope angles, episodic desiccation and
must be susceptible of erosion. Vulnerable materials dune encroachment, dolerite intrusions, and tectonic
include sands and sandstones, clays and shales, and disturbance.

Yardangs are carved out of sediments by abrasion rocks may be fluted, grooved, pitted, and polished by
and deflation, although gully formation, mass move- sandblasting. An example comes from Windy Point, near
ments, and salt weathering may also be involved. Yardang Palm Springs, in the Mojave Desert, California.
evolution appears to follow a series of steps (Halimov
and Fezer 1989; Goudie 1999). First, suitable sediments Ventifacts
(e.g. lake beds and swamp deposits) form under humid
conditions. These sediments then dry out and are ini- Cobbles and pebbles on stony desert surfaces often
tially eaten into by the wind or by fluvial gullying. The bear facets called ventifacts. The number of edges or
resulting landscape consists of high ridges and mesas sep- keels they carry is sometimes connoted by the German
arated by narrow corridors that cut down towards the terms Einkanter (one-sided), Zweikanter (two-sided),
base of the sediments. Abrasion then widens the corri- and Dreikanter (three-sided). The pyramid-shaped
dors and causes the ridge noses to retreat. At this stage, Dreikanter are particularly common. The abrasion of
slopes become very steep and mass failures occur, par- more than one side of a pebble or cobble does not neces-
ticularly along desiccation and contraction cracks. The sarily mean more than one prevailing wind direction.
ridges are slowly converted into cones, pyramids, saw- Experimental studies have shown that ventifacts may
tooth forms, hogbacks, and whalebacks. Once the relief form even when the wind has no preferred direction.
is reduced to less than 2 m, the whole surface is abraded And, even where the wind does tend to come from one
to create a simple aerodynamic form–alowstream- direction, a ventifact may be realigned by dislodgement.
lined whaleback – which is eventually reduced to a plain The mechanisms by which ventifacts form are debat-
surface. able, despite over a century of investigation (see
Zeugen (singular Zeuge), also called perched or mush- LivingstoneandWarren1996,pp.30–2),butabrasionby
room rocks, are related to yardangs (Plate 12.3). They dust and silt, rather than by blasting by sand, is probably
are produced by the wind eating away strata, and espe- the chief cause. Interestingly, the best-developed ven-
cially soft strata close to the ground. Exceptionally, where tifacts come from polar and periglacial regions, where,
sand-laden wind is funnelled by topography, even hard owing partly to the higher density of the air and partly