Other Dunes and Other Sand Deposits - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 89

Fig. 7.1 Dark wind streaks downwind of conical obstructions, Mars
and Earth. Left is from the Daedalia Planum region of Mars (MOC);
right is near Amboy in southern California (USGS); both views are
from a portion of MOC2-206, 2/7/00, NASA/JPL/MSSS

Fig. 7.2 This Magellan radar image shows a bright wind streak
associated with a small volcano. The streak is bright (rough) implying
it is rough, and the dual 'horns' are characteristic of a 'horseshoe
vortex' downing of an obstacle, suggesting that rather than the
deposition of material from the volcano, the feature is actually due to
the removal of fine material (roughening what is left) by scouring by
winds. The volcano is about 5 km in diameter, and the wind streak is
35 km long and 10 km wide, located at 9.4S, 247.5E at the western
end of Parga Chasma, Venus. Magellan press release P-38810

7.4 Climbing Dunes (and Falling Dunes)

This type of dune is essentially a sand deposit on the upwind
side of a hill which interrupts a sand transport pathway. The
sand accumulates forming a ramp (e.g., Fig. 7.5 ). The
formation of these dunes has been simulated in wind tunnel
experiments (e.g., Xianwan et al. 1999). Sand climbing a hill
may accumulate on the downwind side, forming a 'falling'
dune (Fig. 7.6 ). Obviously, a falling dune could grow to the
point where it might become more properly classified as a
lee dune (see Sect. 7.3 ) and a continuum of examples can be
seen where sand transport pathways cross irregular topog-
raphy (e.g., Figs. 1.12 and 7.7 ).

a reverse airflow—essentially the same effect as the lee
vortex discussed in Chap. 20 ). This separation may occur
between about half and double the obstacle height (e.g.,
Tsoar 1983) and leads to sand accumulation in this range.

7.6 Lunette

7.5 Echo Dune (Reflection Dune) A crescent-shaped dune downwind of a dry lake-bed (often

referred to by the Spanish word 'playa'; coastal salt flats are
also sometimes called by the Arabic term 'sabkha'). As the
name suggests, the overall shape is similar to that of a narrow
crescent moon. Often lunettes are comprised of sand-sized
aggregates of silt and clay-sized particles derived from the
lakebed. The close proximity of a lunette to a playa is really
all that distinguishes this crescent-shaped feature from the
more common barchan dune.

A ridge of sand that has accumulated near the base of a cliff
or wall, where wind diverts upwards over the cliff and sand
tends to pile up at a standoff distance from the base of the
cliff. This standoff distance is larger the steeper the cliff
(e.g., Qiam et al. 2011). This sort of deposit may be familiar
when blowing snow encounters buildings and is often also
seen at the small scale around boulders (e.g., Fig. 7.8 ).
Clearly, there is a transition between climbing dunes and
echo dunes: this transition depends on the steepness of the
obstacle (perhaps also on its scale). When the obstacle is
sufficiently shallow, the streamlines merely divert over it,
and the sand may form a shallow ramp. When the obstacle is
steep, the streamlines of the flow must depart from horizontal
well before the obstacle, and a separation bubble forms (with

7.7 Nebka

Nebka is sand (typically displaying some degree of soil
development) around large plants, where erosion has low-
ered the surroundings more than that of the materials