Geology Reference
In-Depth Information
Active ergs
Relict ergs
Figure 12.8
World distribution of active and relict ergs.
Sources:
Adapted from Sarnthein (1978) and Wells (1989)
or by airflow being forced to converge. By this process,
whole ergs and dunefields may migrate downwind for
hundreds of kilometres from their sand sources.
of rocks provided the quartz-dominated silt needed for
loess formation. It is now known that several other
processes produce silt-sized particles - comminution by
rivers, abrasion by wind, frost weathering, salt weath-
ering, and chemical weathering. However produced,
medium and coarse silt is transported near the ground
surface in short-term suspension and by saltation. Veg-
etation, topographic obstacles, and water bodies easily
trap materials of this size. Fine silt may be borne fur-
ther and be brought down by wet or dry deposition.
This is why loess becomes thinner and finer-grained
away from the dust source. To accumulate, dust must
be deposited on rough surfaces because deposits on a dry
and smooth surface are vulnerable to resuspension by
wind or impacting particles. Vegetation surfaces encour-
age loess accumulation. Even so, for a 'typical' loess
deposit to form, the dust must accumulate at more than
0.5 mm/year, which is equivalent to a mass accumulation
Loess
Loess
is a terrestrial sediment composed largely of wind-
blown silt particles made of quartz. It covers some 5-10
per cent of the Earth's land surface, much of it forming
a blanket over pre-existing topography that may be up
to 400 m thick (Figure 12.9; Plate 12.10). Loess is easily
eroded by running water and possesses underground pipe
systems, pseudo-karst features, and gullies. In areas of
high relief, landslides are a hazard.
To form, loess requires three things: (1) a source of
silt; (2) wind to transport the silt; and (3) a suitable
site for deposition and accumulation (Pye and Sherwin
1999). In the 1960s, it was thought that glacial grinding
