Geoscience Reference
In-Depth Information
Figure 9.3
Extensive piedmont gravel plains cover inland Quaternary basins in China. As shown here, they commonly have
∼
100 % gravel cover, extend over vast expanses and lack vegetation. This surface is largely unmodified, with rounded fluvial
clasts.
environments (Figure 9.3). Pavements with angular in-
terknit particles, dark varnish and a stone-free silty sub-
strate appear to be uncommon, but are observed in areas
protected from fluvial erosion (Figure 9.4).
Gobi may develop secondary characteristics. The clasts
can be coated on their undersides by calcium carbonate
and on their upper surfaces by varnish. Ventifacts may
form, with abrasion features such as polished faces or
pits (Li
et al.
, 2006). At high altitudes in periglacial arid
regimes, freeze-thaw polygons occur.
The percentage of gravel coverage necessary to con-
stitute a gobi is not clearly defined. Gobi surfaces on
a clifftop above the Mogao Grottoes, China, have clast
cover values ranging from a low of 9 % near the Mingsha
Mountains to a high of 66 %. For the wind regime of this
region, a value of 65 % is thought to eliminate erosion of
sand in the substrate (Wang
et al.
, 2006). The gravel pro-
tects the underlying sediments by increasing the aerody-
namic roughness of the surface boundary layer (McKenna
Neuman, 1998). The greater the local wind velocity, the
larger the percentage of gravel cover is needed to pre-
vent erosion. In wind tunnel experiments where gravels
were initially veneered by a 50 mm layer of sand, the
air, at speeds from 2 to 30 m/s, was allowed to blow un-
til no additional sand was removed ('equilibrium gravel
coverage') (Wang
et al.
, 2006). The equilibrium coverage
increased with wind speed (e.g. 25-35 % at 8 m/s and
Figure 9.4
This pavement in western China differs consider-
ably from the gravel plains shown in Figure 9.3. A monolayer
of angular, heavily varnished pebbles sits on top of a thick
(25
+
cm) layer of wind-derived silt. It strongly resembles the
pavement from the Mojave Desert shown later in Figure 9.9.
