Deserts of the World - Design with the Desert: Conservation and Sustainable Development - page 16

FIGURE 1.14
Photograph of a part of the Gobi Desert in China. (Courtesy of Jayne Belnap.)
and miscellaneous historical and archaeological evidence. * Of these, the loess profiles give
the longest record of environmental change, for maximum thicknesses of 1100 ft have
been observed. 81 The materials have also proved susceptible to dating by palaeomagnetic
and thermoluminescence methods. The oldest loess in the Central Loess Plateau has been
dated palaeomagnetically at about 2.4 million years, 82 and this confirms that aridity has
a lengthy history, for much of the silt is derived from the Gobi (Figure 1.14) and Ordos
deserts. 83 Seventeen periods of loess sedimentation, separated by periods of nondeposition
and soil development, have been recognized in the classic section of Luochuan for the
period since 1.77 million year BP, and the depositional episodes have been correlated with
glacial periods. It has been suggested that glacial maxima in Tibet, Tien Shan, and the
Kun Lun Mountains were accompanied by a greater frequency of cyclonic depressions
and sandstorms in the Gobi Desert and by more effective easterly transport of dust by a
westerly jet stream centered north of the Tibetan anticyclone. 84
The Turkestan desert of the (former) USSR lies between 36°N and 48°N and between
50°E and 83°E. It is bounded on the west by the Caspian Sea, on the south by the mountains
bordering Iran and Afghanistan, on the east by the mountains bordering Sinkiang, and
on the north by the Kirghiz Steppe. Two great ergs are included: the Kara-Kum (“black
sands”) and the Kyzyl-Kum (“red sands”).
As in China, 85 loess deposits are both extensive and thick (up to 650 ft), and they have
been dated in a similar manner. The loess record in Uzbekistan extends as far back as
2 million years, there are at least nine major soils in loess above the Brunhes/Matuyama
boundary (c. 690,000 year BP), and loess deposition appears to have been relatively slight
during the Holocene. Likewise, in Tajikistan, the loess record goes back to the Pliocene,
and impressive sections contain more than 20 loess units with intervening palaeosols,
many of which show heavy calcification.
The loess horizons themselves appear to have formed under more arid conditions than
today, 86 for they contain large amounts of carbonates and soluble salts, have a xerophilous
mollusc fauna, and show few indications of waterlogging. Rates of accumulation in the
* This archaeological evidence is of the type employed by E. Huntington. 80