Other Dunes and Other Sand Deposits - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 96

Fig. 7.16 Megabarchans in the
Rub' Al Khali desert, seen from
the International Space Station
(south is up). These dunes have a
spacing of about 2 km, with
*100 m superposed barchanoid
forms. Credit NASA JSC EOL

Fig. 7.17 Complex dunes of the
Algodones in southern
California, seen from the
International Space Station. For a
view of the same dunefield from
an airliner, see Fig. 24.6 ; for a
radar view of these dunes, see
Fig. 18.18 . West is up. Irrigated
fields in Mexico are at left, just
beyond the 'sand dragon' border
fence (see Fig. 23.9 ) . The Pan
American canal, and Interstate-10
cut across the dunefield, which
grades from some narrow linears
and small barchanoids at the top,
to massive (*1 km-spaced)
compound ridges, and a sand
sheet at the bottom. Credit
NASA/JSC/EOL

of the dune itself. Some examples on Namib linears can be
seen in Fig. 18.24 . A field example with small incipient
transverse ridges is shown in Fig. 7.13 .
Dong et al. (2010) recognize this form with subsidiary
ridges perpendicular to the main linear dunes in the Kum-
tagh desert in China as 'raked' linear dunes. It may be that
these are the result of a change in wind regime—dunes built
up as linears in a bidirectional regime and winds from one
of those directions have since diminished, resulting in the

accumulated sand now blowing in a more unidirectional
fashion (e.g., Fig. 7.14 ). This leads to barchan-like slip
faces periodically along the former crest; a similar effect is
seen in water tank experiments by Reffet et al. (2010). An
alternative idea is that ridges may grow as an 'orthogonal
mode', creeping upwind in the manner that the arms of star
dunes grow. More work is needed—likely exploiting
numerical models predominantly ( Chap. 19 ) to elucidate
these evolutionary mechanisms.