Geology Reference
In-Depth Information
Figure 10.10.
Typical grain-size distribution curves for loess (a) and cover sand (b).
Figure 10.11.
A barchan (crescentic) sand dune on the Qinghai-Xizang (Tibet) Plateau south of
Wudoaliang, viewed from the Trans-Plateau Highway in June 1987. By 1991, the dune had migrated
a distance of
∼
350 m and crossed the highway.
10.3.5. Sand Dunes and Sand Sheets
Sand dunes are usually associated with hot dry environments but they also form in cold
dry environments if suitable source material is present (Figure 10.11). Cold-climate dunes,
like their hot counterparts, are transverse, crescentic (with horns pointing in the direction
of movement), or parabolic (with horns pointing away from the direction of movement)
in morphology. Occasionally, linear forms are superimposed on dune fi elds consisting
mainly of transverse types. Sand bodies lacking a dune relief, i.e. without slip faces, are
referred to as sand sheets. Usually, the sand particles are similar in grain size to those of
hot deserts, commonly rounded, and possess subdued micro-relief.
Extensive sand dunes and sand sheets occur in parts of northern Alaska, the Canadian
Arctic, Scandinavia, and the Qinghai-Xizang (Tibet) Plateau (Table 10.8). Tension cracks,
