Page 129 - Sedimentology and Stratigraphy

116 Aeolian Environments

accumulations of aeolian dust are relatively uncom-
mon (8.6.2), but airborne material can be literally
carried around the world by winds and be deposited
in all depositional environments.

8.2 DESERTS AND ERGS
A desert is a continental area that receives little
precipitation: they are arid areas that receive less
than 250 mm yr 1 precipitation. (Areas that receive
average precipitation of between 250 and 500 mm
yr 1 are defined as semi-arid and are not usually
considered to be true deserts.) This definition of a
desert does consider temperature to be a factor, for,
although the ‘classic’ deserts of the world today, such
as the Sahara, are hot as well as dry places, there are
also many dry areas that are also cold, including
‘polar deserts’ of high latitudes. The shortage of
water limits the quantity and diversity of life in a
desert: only a relatively limited range of plants and
animals have adapted to live under these dry condi-
tions and large parts of a desert surface are devoid of
vegetation. The lack of vegetation is an important
influence on surface processes because without a
plant cover detritus lies loose on the surface where it
is subject to aeolian activity.
An erg is an area where sand has accumulated as
a result of aeolian processes (Brookfield 1992): these Fig. 8.2 Pebbles in a stony desert with a shiny desert
regions are also sometimes inappropriately referred to varnish of iron and manganese oxides.
as a ‘sand sea’. Ergs are prominent features of some
deserts, but in fact most deserts are not sandy but are
large barren areas known as rocky deserts. Rocky
deserts are areas of deflation, that is, removal of 8.3 CHARACTERISTICS OF
material, and as such are not depositional environ- WIND-BLOWN PARTICLES
ments. However, pebbles, cobbles and boulders that
lie on the surface may subsequently be preserved if 8.3.1 Texture of aeolian particles
they are covered by other sediment, and these clasts
may show evidence of their history in a rocky desert. When two grains collide in the air they do so with
Rocks in a desert are subject to a sand-blasting effect greater impact than they would experience under
as sand and dust particles are blown against the sur- water because air, being a much lower density med-
face by the wind: this erosive effect on the faces pro- ium than water, does not cushion the impact to the
duces a characteristic clast shape, which is called a same extent. The collisions are hence relatively high
zweikanter if two faces are polished smooth, or drei- energy and one or both of the grains may be damaged
kanter if there are three polished faces, with angled in the process. The most vulnerable parts of a grain
edges between each face. Long exposure of a rock are angular edges, which will tend to get chipped
surface in the oxidising conditions of a desert also off, and with multiple impacts the grains gradually
results in the development of a dark, surface patina become more rounded as more of the edges are
of iron and manganese oxides known as a desert smoothed off. Sand grains that have undergone a
varnish (Fig. 8.2). sustained period of aeolian transport therefore