Page 328 - Fundamentals of Geomorphology

AEOLIAN LANDSCAPES 311

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