Basic Types of Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 76

1973). Morphologically, there may be challenges in dis-
criminating this from superposed generations of linear or
transverse dunes.
Transverse ridges are found in several sand seas on
Earth. One set of dunes on Tital (Fig. 6.15 , where a nor-
mally-bimodal wind regime may be 'straightened' by a
topograhic obstacle) was identified (Lorenz et al. 2006) as
possibly transverse. Transverse ridges are common on
Mars, e.g., Figs. 6.16 and 6.17 , as might be expected given
the abundance of the rather similar barchanoid ridge form.

6.5 Linear or Longitudinal (Sief)

Fig. 6.11 Dark dunes (an irregular dome and a pudgy barchan) over
bright mega-ripples. HiRISE image PSP_008679_1905, 2 m/p, credit
NASA/JPL/U.Arizona

A generally symmetrical ridge (Figs. 6.18 , 6.19 , 6.20 and
6.21 ) is sometimes topped by a crest that undulates in height
(Fig. 6.20 ).
Tsoar (1989) made a review of linear dunes, although
much new work has been stimulated on them by the dis-
covery of almost invariably linear dunes over vast areas of
Titan. Notably, numerical simulations and water tank
experiments (e.g., Reffet et al. 2010) have explored the
variation in wind direction and its effect on morphology, an
issue first systematically explored by Rubin and Hunter
(1987) and Rubin and Ikeda (1990).
A certain type of linear dune can form in the lee of
obstacles (see next section) but these appear to only exist on
rather small scales, 10-100 m rather than the 10-100 km
scale typical of the giant linears that define major sand seas
on Earth and Titan. A wavy appearance often develops
along the crest of linear dunes, and leads to their being
referred to by the Arabic term 'seif' (sword). In north Africa
especially, large linear dunes are sometimes referred to as
'draa' (arm), although this term—applied most particularly
to the largest scale bedform in compound linear dunes—is
not formally defined. In the Arabian desert, the word 'uruq'
is more often used for large linears (Arabic is, after all, a
family of regional dialects).
The McKee (1979) diagram shows winds converging at
about 90 forming linear dunes in the vector-average wind
direction. The experiments of Rubin and Hunter and Reffet
et al. show that when the convergence is less than 90,a
transverse dune (or a row of barchans) develops, only
above 90 does the longitudinal form appear. Geomor-
phologists often insist on caution in calling these 'longi-
tudinal' dunes, in that it may not be certain from
morphology alone that the dune is indeed aligned along the
direction of net sand transport and thus the term 'linear'
(without genetic connotations) is to be preferred. It should
also be noted that, while in many cases the dune is indeed
longitudinal in terms of the sand transport, the wind never

the case for barchan dunes formed under the same wind
regime (see Fig. 6 of McKee 1979).
Dunes in barchanoid fields are sometimes referred to as
barchanoid (crescentic, with concave slip face downwind)
and linguoid ('tongue-shaped', with convex slip face

downwind) .
Barchanoid forms are very common on Mars (Figs. 6.13
and 6.14 ) and Earth. Among terrestrial examples are White
Sands and the northern part of the Rub' Al Khali in the
Arabian desert.

6.4 Transverse Ridge

A series of essentially linear crests that are oriented per-
pendicular to the unimodal wind direction. Transverse rid-
ges tend to be asymmetric in cross-sectional profile, with
the shallower face on the upwind side, in contrast to linear
dunes that are more symmetric in overall shape. A single
slip face is present, indicating a single dominant wind
direction. In general, transverse ridges form where sand
supply is greater than would be the case for barchanoid
ridges formed under the same wind regime (see Fig. 6 of
McKee 1979).
Because (unlike barchans) transverse dunes can be con-
sidered as purely two-dimensional structures, sectioned
vertically downwind, they are popular for modeling (e.g.,
Melo et al. 2012) and field measurement studies (e.g. Wang
et al. 2002) since results are relatively free of the complexity
of 3-dimensional effects, although some secondary airflows
may give 3-D effects (e.g., Walker and Nickling 2002).
Secondary airflow effects may be responsible for a grid
pattern that may sometimes develop, referred to in the
Sahara and French literature as 'aklé' (Cooke and Warren