Other Dunes and Other Sand Deposits - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 88

7

Other Dunes and Other Sand Deposits

In this section we discuss some non-dune sand deposits
(sheets and streaks), as well as those dune forms that are
defined by topographic interaction or by vegetation. We
furthermore discuss the superposition of multiple genera-
tions of dunes to create compound dunes.

more obvious parabolic ejecta features (Fig. 2.7 ) seen around
some recent craters are formed by upper atmospheric winds
and are not of particular interest in this topic except perhaps
as a source of sand). Sand or dust streaks from the Victoria
crater on Mars are shown in Fig. 8.2 .

7.1 Sheet 7.3 Shadow Dune (Lee Dune)

A sand sheet is basically a planar (flat) sand deposit lacking
large individual accumulations of sand. Sand sheets are
generally broad in extent, consisting of a flat surface except
where sand accumulations are associated with vegetation or
large blocks that are present on the sheet. No slip faces are
present because topographically large sand deposits are
lacking. Sand sheets are among the most aerially extensive
sand deposits on Earth, and this may also be true for sand
deposits on other planets. However, they of course are not
prominent in remote sensing data.

Linear ridges are generated by sand blowing over the top of
an obstacle, where the ridges represent the paths followed
by the majority of the sand as it moves into the wind sha-
dow downwind of the obstacle. Usually no slip faces are
present, and the ridges often break into barchan chains with
increasing distance from the primary obstacle (Fig. 7.3 ).
This type of formation had been suggested by Rubin and
Hesp (2009) as a possible origin of Titan's linear dunes, but
the scale of the lee dunes in China they point to is orders of
magnitude smaller than for Titan, and most Titan dunes lack
an obvious obstacle to 'nucleate' lee forms.
An interesting variant of this dune type is in the Navajo
areas of northern Arizona, where winds blow from the
southwest across the Painted Desert and are driven into the
steep scarp of the Moenkopi Plateau (Fig. 7.4 ). Linear dunes
appear at the edge of the cliff, fed by sands from headward
erosion of the cliff, or blown up gullies cut into it. In some
places, climbing dunes form at the base of the cliff. Inter-
estingly, the linear dunes form behind the headlands rather
than behind the gullies, presumably because wind coming up
the gullies sets up a possibly helical flow pattern that colli-
mates the sand there. The dunes are typically a few meters
thick, of order 10 m across, and in some cases about 1 km
long. They generally lack slip faces. The dunes have some
sparse vegetation but are evidently still active, having ripples
orthogonal to the long axis. The linear dunes (as well as some
parabolic and barchanoid forms on the plateau itself) and the
vegetation associated with them are documented by Hack
(1941). He notes that the longitudinal dunes are not found in
areas with more than 25 cm of rainfall per year.

7.2 Streak

Sand deposition and/or erosion can produce thin, elongate
strips of sand, often (but not always) anchored to a topo-
graphic obstacle on the upwind end of the streak (Fig. 7.1 ).
No slip faces are present along the elongate strips. Wind
streaks are quite common on Earth, but their almost ubiqui-
tous presence across the surface of Mars has stimulated
considerable interest in studying their mechanisms of for-
mation here on Earth. Wind streaks can result from either
enhanced deposition or enhanced erosion, depending on the
size of the particles involved in the creation of the brightness
contrast that distinguishes the wind streak from its sur-
roundings. Greeley et al. (1974a, b) studied such processes in
the wind tunnel, noting a characteristic horseshoe-shaped
vortex flow in the lee of circular obstacles. A few wind streaks
have been observed in radar images of the Venus surface
(Fig. 7.2 ) which are indicative of near-surface winds (the